So Ma to Sensory Notes 2010

8/6/2019 So Ma to Sensory Notes 2010

1/29

BIOMEDICAL SCIENCE

PSYCHOLOGY/PSYCHOPHYSIOLOGY

HET227 Neurophysiology

SOMATOSENSORY SYSTEM

LECTURES


2/29

2010 HET227 i

BIOMEDICAL SCIENCE

PSYCHOLOGY/PSYCHOPHYSIOLOGY

HET227 Neurophysiology

SOMATOSENSORY SYSTEM LECTURES

CONTENTS

INTRODUCTION TO SOMATOSENSORY SYSTEM _____________________________________________1

NERVOUS SYSTEM OVERVIEW______________________________________________________________1

STRUCTURE OF NEURONS________________________________________________________________1

THE SYNAPSE____________________________________________________________________________2

CLASSIFICTION OF NEURONS ____________________________________________________________2INTRODUCTION TO NEUROGLIA_________________________________________________________3

MEMBRANE PROPERTIES OF NEURONS___________________________________________________3

RECEPTORS & RECEPTIVE FIELDS__________________________________________________________6

INTRODUCTION TO REFLEXES _____________________________________________________________7

CLASSIFICATION OF REFLEXES ____________________________________________________________7

REVIEW OF CLASSIFICATION OF NEURONS _________________________________________________8

EMBRYOLOGICAL DEVELOPMENT _________________________________________________________9

EMBRYOLOGICAL DEVELOPMENT OF NERVOUS TISSUE ___________________________________11

SPINAL CORD _____________________________________________________________________________12

DIVISIONS OF THE SPINAL CORD __________________________________________________________12

PERIPHERAL NERVE ORGANISATION______________________________________________________14

DORSAL ROOT ORGANISATION____________________________________________________________14

SPINAL CORD ORGANISATION_____________________________________________________________14

ORGANISATION OF SECONDARY SPINAL CORD NEURONS __________________________________15

ASCENDING PATHWAYS___________________________________________________________________16

DORSAL (POSTERIOR) COLUMN PATHWAY______________________________________________16

DORSAL-MEDIAL LEMNISCUS PATHWAY________________________________________________16SPINOTHALAMIC TRACTS_______________________________________________________________17

LOCALISATION OF SENSATION ____________________________________________________________17

DETERMINATION OF PAIN_________________________________________________________________18

THALAMIC NUCLEI _______________________________________________________________________18

SOMATOSENSORY CORTEX________________________________________________________________19

POSITION SENSES _________________________________________________________________________22

CLINICAL CONSIDERATIONS ______________________________________________________________23

STIMULUS INTENSITY _____________________________________________________________________25


3/29

2008 HET227 1

INTRODUCTION TO SOMATOSENSORY SYSTEM

Our sensory responses rely on our perception (both conscious & unconscious) of sensory information.

Afferent information below conscious awareness = (mainly) visceral information - essential fordetermining the appropriate efferent output to maintain the bodys homeostasis

Afferent information above conscious awareness = perception information: Somatic sensation - arising from body surface.

o soma = body

o somatic = of the body Special senses - including vision, hearing, taste & smell, balance.

Perception is the conscious interpretation of the external world created by the brain from a pattern of nerve

impulses delivered to it from sensory receptors.

Afferent neurons have receptors at their peripheral endings which respond to stimuli in both the internal and

external world.

What we perceive is different to what is really there. Receptors detect only a limited number of existing forms. There are between- & within individual differences in sensory responses. In types of receptors & sensitivities. In neural processing of /stimulus/responses.

Pathology of the senses results in deprivation or excessive stimulation.

Appropriate neurological treatment depends on specific diagnosis.

Diagnosis can include simple tests using direct stimuli or electrical stimuli.

Evoked potentials visual, auditory, somatosensory

Automated and semi-automated EP testing equipment.

NERVOUS SYSTEM OVERVIEW

Includes all the neural tissue in the body.

Neurons - Basic functional units.

Neuroglia or glial cells - Connective tissue of the nervous system.

Basic support cells (separate & protect neurons)

Outnumber neurons (~ half volume of whole nervous system)But number about ~90% of cells within CNS (less branching smaller volume)

Central Nervous System (CNS):

Spinal cord and brain. Responsible for integrating, processing, coordinating sensory data and motor commands. Sensory information about conditions inside and outside body. Motor commands to control or adjust activities of peripheral organs (eg skeletal muscles). Brain is also seat of higher cortical function (memory, learning, emotion etc).Peripheral nervous system (PNS):

All neural tissue outside the CNS. Carries information between the CNS and other parts of the body.

o TO the CNS = sensory information afferent ad = to, ferre = carry.o FROM the CNS = motor commands efferent effero = to bring about.

STRUCTURE OF NEURONS

Neurons have a variety of shapes.

Most common type in CNS: multipolar neuron.

(Large cellbody or soma, connected to single, elongated axon, several short, branched dendrites).

Cell body:

Relatively large, round nucleus. Perikaryon: the cytoplasm surrounding the nucleus

o Contains organelles to provide energy & synthesize organic materials.Dendrites:

Variable number, extending from cell body. Typically highly branched


4/29

2008 HET227 2

Each branch with dendritic spineso carry information from other neuronso represent 80-90 % neurons total surface.

Axon:

Long, cytoplasm process capable of propagating electrical signals (action potentials) Base or initial segmentof the axon is attached to the cell body at a thickened region or axon hillock. Telodendria: fine extensions at the end of the axon trunk which end at synaptic terminals.

THE SYNAPSE

Synapses are junctions between 2 neurons.

Axon terminal ofpresynaptic neuron sends the message

Dendrites or cell body ofpostsynaptic neuron receives the message.

Action potentials trigger the release ofneurotransmitters by the synaptic terminal.o Neurotransmitters are packaged in the synaptic vesicles.o Released at the presynaptic membraneo Received by receptors in the postsynaptic membrane.

Presynaptic cell is usually a neuron. Postsynaptic cell can be either a neuron or another type of cell.

o Synapse between a neuron & a neuron can occur on a dendrite, on a cell body, or along the axonof the receiving cell.

o Synapse between a neuron & muscle: neuromuscular junction.o Synapse between a neuron & gland: neuroglandular junction.

Structure of the synaptic terminal varies with the type of postsynaptic cell.

A synaptic knob occurs when the postsynaptic cell is a neuron.Synaptic cleft- Narrow cleft between pre- & postsynaptic cells which the neurotransmitter diffuses across to bind to

postsynaptic membrane receptors.

CLASSIFICTION OF NEURONS

Neurons can be grouped by structure or by function.

Structural classification:

Classification based on of the relationship of the dendrites to the cell body and the axon.

1. Anaxonic neurons. - Small, no distinguishing features that distinguish dendrites from axons. Located in brain & special sense organs.

2. Bipolar neurons. - Smaller than uni- and multipolar neurons 2 distinct processes: dendritic process branching extensively at its distal tip and axonal process, with cell

body between. Rare, occur in special sense organs.

3. Unipolar neurons. - Most common sensory neurons of PNS; End at synapses in the CNS Dendrites and axons are continuous, with cell body lying off to one side.

o axons may be 1 m or more; longest from tips of toes to spinal cord.4. Multipolar neurons. - ,Most common neuron in CNS

2 or more dendrites and single axonFunctional classification:

1. Sensory neurons or afferentneurons. About 10 million in the body. Somatic sensory neurons =- monitor outside world & our position within it.

Exteroreceptors: eg touch, temperature, pressure sensations, & more complex senses of sight, smell,hearing.

Proprioceptors monitor position & movement of muscles & joints. Visceral sensory neurons monitor internal conditions & status of other organ systems.

Interoceptors monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive)and sensations of taste, deep pressure & pain.

2. Motor neurons or efferentneurons. About 1/2 million in the body. Somatic motor neurons =- innervate skeletal muscles.

You can have conscious control over this activity. Visceral motor neurons = innervate all peripheral effectors other than skeletal muscles (smooth muscle,

cardiac muscle, glands, adipose tissue). Monitor internal conditions & status of other organ systems.

3. Interneurons or association neurons.

20 billion (most within brain & spinal cord). Responsible for distribution of sensory information & coordination of motor activity.


5/29

2008 HET227 3

INTRODUCTION TO NEUROGLIA

Neuroglia or glial cells make up about 90% if the cells within the CNS.

Connective tissue of the CNS. They do not initiate or conduct nerve impulses. They physically support the interneurons, metabolically and functionally.

Major types of glial cells in the CNS:

1. Astrocytes - Star shaped, most abundant. Important functions: Main glue of CNS ie create a 3D framework for CNS to hold neurons together in proper spatial

relationships.

Form scaffold to guide neurons to correct destination during fetal brain development. Help maintain the blood-brain barrier. Help repair damage neural tissues to stabilise the tissue and prevent further injury. Help control the interstitial environment (volume of blood flow through capillaries; transport

nutrients/ions/dissolved gases between capillaries & neurons; regulate concentration of sodium &

potassium ions & CO2)

Enhance synaptic formation & transmission, & play a role in neurotransmitter activity (absorb &recycle some neurotransmitters; release chemicals to enhance or suppress communication across

synaptic terminals).2. Oligodendrocytes - Form insulative myelin sheaths around axons in CNS; Provide structural framework. Myelin insulation increases the nerve conduction velocity. Effectively tie clusters of axons together in same sheath segment (or internodes). Appear glossy white (lipids present) white matter in CNS = dominated by myelinated axons. grey matter = dominated by unmyelinated axons.

3. Microglia Immune defence cells of the CNS (cousins of monocytes white blood cells). Smallest and least numerous. Capable of migrating through neural tissue move to affected areas to remove foreign invaders or tissue debris. Remain stationary until activated by infection or injury.

4.

Ependymal cells - Line internal cavities of the CNS. Contribute to formation of cerebrospinal fluid (have cilia which contribute to the flow of the fluid through the

ventricles).

Serve as neural stem cells with the potential to form new neurons & glial cells.Major types of glial cells in the PNS:

Cell bodies of PNS clustered in ganglia.

Neuronal cells in PNS are insulated from their surroundings by two types of neuroglia:

1. Satellite cells Surround the neuron cell bodies in ganglia. Similar to astrocytes Regulate internal environment

2. Schwann cells Forms a segment of myelin sheath around one segment of a single peripheral axon. Series of Schwann cells needed to form a sheath along entire length of axon.

MEMBRANE PROPERTIES OF NEURONS

The important membrane processes:

o Resting potential the transmembrane potential of a resting cell.o All neural activities begin with a change in the resting potential.

o Graded potential a temporary localised change in the resting potential.o Decreases with distance from the stimulus.

o Action potential occurs if the graded potential is sufficiently large.o Propagates across the surface of the membrane.o Does NOT decrease with distance from the stimulus.

o Synaptic activity produces the graded potentials in the postsynaptic cell membrane.o Involves release of neurotransmitters (eg Ach) that bind to receptors on postsynaptic cell membrane


6/29

2008 HET227 4

o Information processing in its simplest form, integrates stimuli at the individual cell level.TRANSMEMBRANE or RESTING POTENTIAL

Membrane potential= a separation of charges across the membrane or to a difference in the relative number of

cations and anions in the intra- and extra-cellular fluid. Measured in mV.

Transmembrane potential= membrane potential of a resting or undisturbed cell.

At the resting potential, ion movement occurs through passive and active mechanisms. Passive forces: Uses chemical and electrical forces.o Chemical gradients:

Potassium ions move OUT, driven by concentration gradient. Sodium ions move IN, driven by concentration gradient.

o Electrical gradients: Cell membrane more permeable to potassium than to sodium. potassium leaves more rapidly than sodium enters. results in excess of negatively charged proteins inside & greater positive charge

outside near the outer surface of the cell.

Active forces:o The sodium-Potassium Exchange pump.o The cell must expend energy (ATP) to bail out sodium ions that leak in and to recapture

potassium ions that leak out (passive forces).

o ATP is used by an ion exchange pump balances the passive forces of diffusiono resting potential remains stable.

CHANGES IN THE TRANSMEMBRANE POTENTIAL:

MEMBRANE CHANNELS

The transmembrane potential is dynamic, rising & falling in response to temporary changes in membrane

permeability. changes result from opening or closing of specific channels.

OPENING of gated channels alters the rate of ion movement across the cell membrane changes transmembrane

potential.

Chemically-regulated channels

open or close when they bind specific chemicals. eg receptors that bind ACh at the neuromuscular junction.Voltage-regulated channels open or close in response to changes in the transmembrane potential. characteristic of areas of excitable membrane (capable of generating and conducting an action potential). Sodium, potassium & calcium channels.

o Sodium channels have an activation gate (opens on stimulation to let sodium ions in) & aninactivation gate (closes to stop the entry of sodium ions).

Mechanically-regulated channels

open or close in response to physical distortion of the membrane surface. Important in sensory receptors (touch, pressure, vibration).

At the resting potential most gates are CLOSED.

GRADED or LOCAL POTENTIALS

o A temporary localised change in the resting potential.o Decreases with distance from the stimulus. Changes in the transmembrane potential that CANNOT SPREAD far from the area surrounding the site of

stimulation.

o Any stimulus that opens a gated channel will produce a graded potential.o STEP 1: Arrival & spreading of additional positive charge which shifts the transmembrane potential toward 0 mV.

o Increase in positivity of resting potential (-70 mV) toward 0 mV == depolarisationo Includes smaller negative values (-65 mV, -10 mV) or above 0 mV (+10mV etc).

o STEP 2:LOCAL current the movement of positive charges parallel to the inner & outer surfaces of amembrane.

o Sodium ions are released from its outer surface.o They move toward open channels replacing ions that have already entered the cell.

o STEP 3: The degree of polarisation decreases with distance. At some distance from the entry point, the effectsof the transmembrane potential are undetectable.


7/29

2008 HET227 5

Repolarisaiton = the process of restoring the normal resting potential after depolarisation.

Involves both ion movement through membrane channels + activities of ion pumps.

Hyperpolarisation = loss of positive ions resulting in an increase in negativity of resting potential from 70 mV to

80 mV or more.

o Opening a gated potassium channel potassium rate of outflow increases == interior loses positive ions.ACTION POTENTIALS

o Initiation of an action potential requires a depolarisation large enough to open voltage-gated channels.o Opening occurs at the threshold for the axon,

o THRESHOLDS: typically between 60 mV & -55 mV==depolarisation of 10-15 mV.

o Below threshold eg a shift to 62 mV graded potential.o Above threshold eg a shift to 60 mV action potential.

o The properties of an action potential are independent of the relative strength of the depolarising stimulus, aslong as that stimulus exceeds threshold

the ALL-OR-NONE principle. Depolarisation to threshold

Activation of sodium channels & rapid depolarisation

Inactivation of sodium & activation of potassium channels

Return to normal permeabilityRefractory periods

o Absoluterefractory period (0.4-1.0 sec) the period from when an actin potential begins until the normalresting potential has stabilised during which the membrane will not respond at all to additional depolarising

stimuli.

o The smaller the axon diameter, the longer the period.o Relativerefractory period begins when the sodium channels regain their normal resting potential and

continues until the transmembrane potential stabilises at resting levels.

o Will only respond if the membrane is sufficiently depolarised.o Usually requires larger than normalstimulus.

Propagation of Action Potentials

Continuous Propagation of unmyelinated axons:

Continuous chain reaction along axon. Action potential in initial segment results in depolarisation. Local current depolarises adjacent portion. Action potential develops in this segment, & previous segment enters refractory period. Cycle is repeated along length of axon. Action potentia always proceeds AWAY from site of generation cannot reverse. Occurs at approx 1 m/sec

Saltatory Propagation of myelinated axons:

Complete myelin sheath increases resistance to flow of ions across the membraneo prevents continuous propagationo Ions can only cross the cell membrane at the nodes only nodes depolarise

Action potentialjumps from node to nodesaltatory propagation (saltare == leaping)

Less surface area is involved & fewer sodium ions need to be pumped out (uses less energy)More rapid about 50 x faster than unmyelinated fibres of comparable size.


8/29

2008 HET227 6

RECEPTORS & RECEPTIVE FIELDS

Receptors for general senses scattered throughout the body.

Classification system divides into four types of receptors by nature of the exciting stimulus:

Nociceptors Pain (JP TO DISCUSS LATER)

Chemoreceptors Chemical concentration (CO TO DISCUSS NEXT SEMESTER)

Thermoreceptors TemperatureMechanoreceptors Touch, pressure, proprioception

Thermoreceptors free nerve endings located in dermis of skin, in skeletal muscles, in liver, in hypothalamus.

Cold receptors: 3 4 times more numerous than Warm receptors

No structural differences identified between Cold and warm receptors.

Use same pathways that carry pain sensations.

Phasic receptors: very active when temperature is changing, but quickly adapt to stable temperature.

Mechanoreceptors sensitive to stimuli that distort their cell membranes.

Tactile receptors: Closely related sensations of touch, pressure, vibration.

Baroreceptors: Detect pressure changes in walls of blood vessels, portions of digestive, reproductive, and

urinary tracts.

Proprioreceptors: Monitor positions of joints and muscles. Most structurally and functionally complex ofgeneral sensory receptors.

Tactile receptors:

Free nerve endings sensitive to touch & pressure. Situated between epidermal cells. Tonic receptors with small

receptive fields.

Root hair plexus nerve endings of hairs, monitoring distortions and movements across the body surface. Adapt

rapidly.

Merkels discs fine touch and pressure receptors. Extremely sensitive tonic receptors, with very small receptive

fields.

Meissners corpuscles perceive sensations of fine touch and pressure and low-frequency vibration. Adapt to

stimulation within a second of contact. Most abundant in eyelid, lips, fingertips, nipples, external genitalia.

Pacinian corpuscle sensitive to deep pressure. Fast-adapting, most sensitive to pulsing or high frequency

vibration.Ruffinian corpuscles sensitive to pressure and distortion of the skin, located in reticular (deep) dermis. Tonic,

show little adaptation.

Detection of stimuli

Receptor specificity: Each receptor has a characteristic sensitivity. Eg touch receptor sensitive to pressure but

relatively insensitive to chemical stimuli

May result form structure of receptor cell or from presence of accessory cells or structures that shield the

receptor cell from other stimuli.

Receptors may be quite localised eg in muscle spindles and tendon organs or may be more diffuse eg bare

nerve endings in the skin.

Free nerve endings extend through the tissue the way plant roots extend through the soil. Can be stimulated

by different stimuli, and exhibit little receptor specificity. Eg responding to tissue damage by providing

pain sensation may be stimulated by chemical stimuli, pressure, temperature changes or trauma.

Knowledge of the location of a stimulus is termed topognosis.

RECEPTIVE FIELD: The spatial region throughout which stimulation causes an afferent neuron to respond

That is: The area monitored by a single receptor cell.

The centre of a receptive field is generally the most sensitive to stimuli

Response decreases as the stimulus is moved outward to the periphery of the field.

With some receptors, particularly skin pain, temperature and mechanoreceptors, as stimulus intensity increases it

may then start to stimulate an adjacent receptive field producing overlap.

ACUITY The precision with which a stimulus is located.Different sensory modalities and different regions of the body vary in the level of acuity.

The larger the receptive field the poorer your ability to localize a stimulus.


9/29

2008 HET227 7

Touch on tongue or fingertips Highly localised touch. receptive fields less than a millimetre. Very acute.

Touch on general body surface receptive field of 7 cm diameter. Less acute

Heart pain is even less acute and tends to be widespread and diffuse.

TWO-POINT DISCRIMINATION or TWO-POINT THRESHOLD is one way of determining the acuity for a given

stimulus in various parts of the skin.

Over the whole body, two-point discrimination varies from less than 10 mm to as much s 65 mm.

Acuity is dependent on several aspects;

Receptive field size Innervation density: relates to spacing or density of the receptors Central convergence: relates to the possibility of several signals form various receptive fields all

eventually converging on one central neurone; this will decrease acuity.

Lateral inhibition: occurs when stimulation of the periphery of the receptive field causes inhibition in higherorder neurones.

o This can increase acuity by sharpening up the edges of the perceived fields and reducing overlap.

INTRODUCTION TO REFLEXES

Reflexes are built-in, automatic responses triggered by stimuli in the skin/joints. Simple or basic reflexes are unlearned responses (knee jerk). Acquiredor conditionedreflexes are the result of learning/practice (somersault).Reflexes are usually controlled by the spinal cord, but most reflexes do not travel up to the brain to be processed

which is why they occur so quickly.

The reflex response is predictable - they always travel the same pathway. Reflex arc - the very simple nervous pathway involved in the reflex action, typically involving 5 basic

components:

Receptor which responds to a stimulus. Afferentor sensory pathway. Integrating centre CNS (particularly spinal cord &/or brain). Efferentor motor pathway. Effector muscle or gland which carries out the response.

Excite the muscle to respond (shorten) & inhibit the antagonist muscle.CLASSIFICATION OF REFLEXES

Classified based on the basis of:

1. Their development: Innate reflexes Acquired reflexes learned, enhanced by repetition. MAY be modified over time or suppressed through conscious effort.

2. The site of information processing Spinal reflexes processing in the spinal cord.

Monosynaptic reflexes: simplest, little delay (with a latency to response onset of 15-25 msec); Polysynaptic reflexes: more complicated responses. Can involve several muscle groups.

Cranial reflexes processed in the brain eg reflex movements in response to loud noises.3. The nature of the resulting motor response

Somatic reflexes provide mechanism for involuntary control of muscle system Superficial reflexes triggered by stimuli at the skin or mucous membrane. Stretch reflexes (deep tendon reflexes or myotactic reflexes)triggered by sudden elongation of

a tendon.

Visceral reflexes automatic reflexes control activities of other systems.4.

The complexity of the neural circuit involved. Monosynaptic reflexes - simple Polysynaptic reflexes - complex


10/29

2008 HET227 8

These are NOT mutually exclusive categories (different ways of describing a simple reflex).

Monosynaptic reflexes: simplest, little delay (with a latency to response onset of 15-25 msec);

Stretch reflex automatic regulation of skeletal muscle length. Eg knee jerk reflex. Action potentials carried to & from spine along large myelinated Type A fibres; Complete reflex takes 20-40 msec. Include postural reflexes to help maintain normal upright posture.

Muscle spindles are the sensory receptors involved in the stretch reflex. When a skeletal muscle is stretched, the muscle spindles elongate muscle tone increases provides

automatic resistance reduces chance of damage due to overstretching.

Polysynaptic reflexes:more complicated responses. Can involve interneurons which control several muscle groups.

Tendon reflex: - Monitors external tension produced during a muscular contraction, prevents tearing or breakingof tendons.

Separate receptors from muscle spindles & proprioceptors in tendons. Withdrawal reflex: - moves affected parts of the body away form a source of stimulation.

Strongest withdrawal reflexes are triggered by pain. Eg. Flexor reflex affects muscles of a limb.

o Involves reciprocal inhibition one set of motor neurons are stimulated, those controllingantagonistic muscles are inhibited.

Crossed extensor reflex: - the motor response occurs ON THE OTHER SIDE OF THE BODY. Complements the flexor reflex they happen simultaneously.

ALL polysynaptic reflexes share the same characteristics:

1. Involvepools of interneurons.2. Are intersegmentalin distribution (extend across spinal segments, may activate muscles in different parts

of body).

3. Involve reciprocal inhibition.4. Have reverberating circuits to prolong the reflexive motor response.5. Several reflexes may cooperate to produce a coordinated controlled response.

DIAGNOSTIC TESTING

Many somatic reflexes can be tested for assessment of neurological function.

Superficial reflex:Babinski response or plantar reflex.

Stretch reflexes: patellar, biceps, triceps & Achilles reflexes.

REVIEW OF CLASSIFICATION OF NEURONS

Unmyelinated fibres impulse conduction is continuous (uninterrupted).

Maximum speed is 15 m/s.

Myelinated fibres excitable membrane is confined to nodes of Ranvier (myelin is electrical insulator)

Impulse conduction is saltatory (interrupted or jumping) jumping from node to node.Greater speed of conduction, with maximum of 120 m/s.

Larger myelinated fibre = more rapid conduction.

Larger fibres have longer internodal segments and the nerve impulses take a longer stride between nodes.

A rule of six to express the ratio between size and speed:

10m external diameter = conduct at 60 m/s

15 m diameter = conduct at 90 m/s

Peripheral nerve fibres are classified in accordance with conduction velocities and other criteria.

Motor fibres are classified into Groups A, B, C, in descending order.

Sensory fibres classified into Types I IV.

BUT there is some interchange in practice:

Unmyelinated sensory fibres are usually called C fibres rather than Type IV.


11/29

2008 HET227 9

EMBRYOLOGICAL DEVELOPMENT

During first 2-3 weeks of development amnion, yolk sac, allantois chorion.

Amnion surrounds the developing embryo/fetus. Sac becomes filled with amniotic fluid (initially derived from maternal blood) Eventually extends all the way around the embryo.

Buoyant environment; protects against physical trauma, helps maintain constant homeostatic temperature Helps prevent growing embryonic parts from adhering and fusing together Allows the embryo freedom of movement, aiding musculoskeletal development.

Yolk Sac

Inner layer of endoderm and outer layer of mesoderm. Hangs from the ventral surface of the embryo. Forms part of the gut (digestive tract) Produces the earliest blood cells Source of primordial germ cells that migrate into the embryos body to seed the gonads.Allantois

Essential to placenta formation. Forms as a small out-pocketing at the caudal end of the yolk sac Structural base for constructing the umbilical cord that links the embryo to the placenta, and becomes part of the

urinary bladder.

When fully formed, the umbilical cord contains a core of embryonic connective tissue (Whartons jelly), theumbilical arteries and vein, and is covered externally by amniotic membrane.

Chorion helps form the placenta Encloses the embryonic body and all other membranes.Gastrulation: Germ Layer Formation

Lays down the basic structural framework of the embryo The transformation of the 2-layered embryonic disc into the 3-layered embryo in which the primary germ layers

are present.

cellular rearrangement and widespread cell migrations. Shortly after the amnion forms

Embryonic disc elongates and broadens anteriorly (becomes a pear-shaped plate of cells) Primitive streak (a raised groove) appears on its dorsal surface and establishes the longitudinal axis of the

embryo

Cell migration follows, resulting in the division of the cells into the 3 primary germ layers. The embryonic tissues from which all tissues and organs of the body develop.

Ectoderm forms from the top layer of cell mass separates to form the amniotic cavity.

Endoderm forms from another layer also separates to form theyolk-sac.

Mesoderm The 3rd layer forming between these 2 layers The 3 layers attached to the trophoblast(now called the chorion) by a structure formed by the mesoderm called

the body stalk(future umbilical cord).

Early in embryonic life the yolk sac merges with the body stalk and becomes non-functional.Ectoderm outer skin develops into All nervous tissue Epidermis of skin Cornea, lens of eye and internal eye muscles Internal and external ear Neuroepithelium of sense organs Epithelium of pineal gland, pituitary gland, and adrenal medulla Epithelium of oral and nasal cavities, paranasal sinuses, salivary glands, and anal cavity.

Endoderm inner skin becomes epithelial liningof


12/29

2008 HET227 10

gastrointestinal tract (except oral and anal cavities) and epithelium of its glands urinary bladder, gallbladder and liver pharynx, auditory (Eustachian) tubes, tonsils, larynx, trachea, bronchi and lungs thyroid, parathyroid, pancreas, thymus gland a number of other organs including the vestibular and lesser vestibular glands, prostate, vagina

Mesoderm middle skin forms

All skeletal, most smooth and all cardiac muscle; Cartilage, bone and other connective tissue; Blood, bone marrow, lymphatic tissue; Endothelium of blood vessels and lymphatic vessels; Dermis of skin Middle ear Epithelium of kidneys and ureters, adrenal cortex, gonads and genital ducts.Organogenesis: Differentiation of the Germ Layers

Formation of body organs and organ systems.

Cells of the embryo continue to rearrange themselves, forming clusters, rods, or membranes that differentiate into

definitive tissues and organs.

Each of the primary tissues formed during gastrulation proceed to undergo growth, differentiation and

morphogenesis


13/29

2008 HET227 11

EMBRYOLOGICAL DEVELOPMENT OF NERVOUS TISSUE

NEURULATION: formation of the neural tube

The entire nervous system originates from the neural plate, an ectodermal thickening in the form of the amniotic sac.

During the 3rd week after fertilisation, the plate forms paired neural folds, which unite to create the neuraltube and neural canal.

Union of the folds commences in the future neck region and proceeds rostrally (towards the head) andcaudally (towards the tail) from there.

Neuropores - the open ends of the tube are closed off before the end of the 4 th week. Cells at each edge of each neural fold escape from the line of union and form the neural crestalongside the

tube. Cell types derived from the neural crest include spinal and autonomic ganglion cells and the Schwann

cells of peripheral nerves.

Anterior end of the neural tube becomes the brain The rest becomes the spinal cord. By end of 1st month of development, 3 primary brain vesicles are obvious: fore-, mid- and hindbrain By end of 2nd month, all brain flexures are evident, cerebral hemispheres cover the top of the brain stem and

brain waves can be recorded.

SPINAL NERVES

The dorsal(posterior) part of the tube is called the alar plate;

Neurons developing in the alar plate are predominantlysensory in function and receive dorsal nerve roots

growing in from the spinal ganglia.The ventral(anterior) part of the tube is the basal plate:

Neurons in the basal plate are predominantly motorand give rise to ventral nerve roots.

At appropriate levels of the spinal cord the ventral roots also contain autonomic fibres.

Dorsal and ventral roots unite to form the spinal nerves.

The spinal nerves emerge from the vertebral canal in the interval between the neural arches being formed

by the mesenchymal vertebrae.

BRAIN VESICLES

Rostrally, the closed neural tube expands in the form of three brain vesicles:

the prosencephalon, (forebrain)

the mesencephalon (midbrain)

the rhombencephalon (hindbrain).

Alar plate expands on each side to form the telencephalon (cerebral hemispheres)

Basal plate remains in place as the diencephalonAn optic outgrowth from the diencephalon is the forerunner of the retina and opticnerve.

Embryonic brainstem: the diencephalon, mesencephalon, and rhombencephalon.The brainstem buckles as development proceeds.

Results in the mesencephalon being carried to the summit of the brain.

The rhombencephalon folds upon itself, causing the alar plates to flare and creating the rhomboid (diamond

shaped) fourth ventricle of the brain.

DEVELOPMENT OF THE SPINAL CORD AND PERIPHERAL NERVES

Enlargement of the intermediate zone of the alar plate creates the dorsal horn of grey matter.The dorsal horn receives the central processes ofdorsal root ganglion cells which are derived from the neural crest.Partial occlusion of the neural canal by the developing dorsal grey horn gives rise to the dorsal median septum and

to the definitive central canal of the cord.

Enlargement of the intermediate zone of the basal plate creates the ventral grey horn and the ventral median fissure.Axons emerge from the ventral horn and form the ventral nerve roots.

NEURAL ARCHES

During the 5th

week, the mesenchymal vertebrae surrounding the notochord give rise to neural arches to protect thespinal cord.

Initially bifid (split), later fusing around the midline to form the vertebral spines.

Spina bifida when the two halves of the neural arches fail to unite.Spinal nerves develop as the muscle/skin etc develop.

Dermatomes are the strips of skin and muscles supplied by an individual spinal nerve.

Dermatomes are orderly in the embryo, but as the body develops the dermatome is distorted by the continuing growth.

Dermatomes are clinically important, because damage or infection or a spinal nerve or dorsal root ganglion will

produce characteristic loss of sensation in the skin.

EGshingles a virus infects dorsal root ganglia, causing a painful rash whose distribution corresponds to that of the

affected sensory nerve.


14/29

2008 HET227 12

SPINAL CORD

Spinal cord in man is bout 45 cm long and weighs about 30 g.

The spinal cord and nerve roots are sheathed bypia materand float in cerebrospinal fluidcontained in thesub-arachnoid space.Cord is partially divided by ventral (anterior) sulcus.Central canal.

Small dorsal (posterior) sulcus.Spinal cord terminates approximately at the lower end of the second lumbar vertebra. During growth of the fetus and young child, the spinal cord does not continue to lengthen as the vertebral

column lengthens, so the cord becomes located progressively more superiorly in the vertebral canal.

The lumbar and sacral nerves arise from higher up the vertebral canal and then course downward through thelower canal as a large bundle of nerves until they emerge at the appropriate lumbar or sacral region.

Like the brain, the spinal cord is composed of grey (deep) and white (surface) matter.

White matter: Mainly tracts of axonal processes coursing up and down the cordGREY MATTER

Contains the nerve cell bodies, along with many short nerve fibres, and can be the site of very extensivesynaptic exchanges.

Has appearance of multiple horns. Arranged in two halves. Central region around the central canal. Two ventral (anterior) horns project from the central area. Location of anterior motor neurons efferent Stops some distance from the perimeter. Two dorsal (posterior) horns project from the central area. Location of sensory neurons afferent Project almost tot eh perimeter Two lateral horns - small projections of grey matter lateral to the central region in some regions of spinal cord.Proportions of grey and white matter vary in different regions of the cord

Cord overall size varies in different regions largest in cervical region (lower half of the neck)

most white matter in the upper reaches of the cord (sensory and motor pathways serving all four limbs)

most grey in the lumbar region (lower end of the spinal cord).Over the dorsal limit of the column is the dorso-lateral sulcus.

DIVISIONS OF THE SPINAL CORD

Between the dorso-lateral (postero-lateral) sulcus and the small dorso (posterior) sulcus is the dorso-intermediate sulcus (postero-intermediate sulcus).

Dorsal (posterior) funiculus the white matter between the dorsal column and dorso-lateral sulcus. Ventral (anterior) funiculus the white matter between the ventral column and the ventral sulcus. Grey commissure the grey matter between the dorsal and ventral columns. Substantia gelatinosa the tip of the dorsal grey column. Dorsolateral tract between the substantia gelatinosa and the perimeter of the cord.The dorsal grey column can be divided into:

Substantia gelatinosa Nucleus proprius Nucleus tertius (ventral and medial) Visceral grey (lateral) Intermediomedial Intermediolateral

In the thick sections of the spinal cord, the nerve cells exhibit a laminar or layeredarrangement.

True lamination is confined to the posterior horn.

10 laminae of Rexedhave been defined in the grey matter.


15/29

2008 HET227 13

WHITE MATTER ASCENDING AND DESCENDING TRACTS

The white matter can also be divided into columns: Two dorsal(posterior) white columns lying between dorsal grey horns Two lateralwhite columns lying on each side of the cord lateral to the grey matter Two ventral(oranterior) white columns lying between and anterior to the ventral grey horns.All these contain fibre tracts that run lengthwise along the cord.

Propriospinal tracts travel for only a few segments of the cord

Connect separate cord segments of grey matter with one another to help in performance of cord reflexes(segment is portion of cord that corresponds to a single pair of spinal nerves).

Remainder of white matter contains: Ascendingtracts - carrysensory information to the brain Descendingtracts carry motorsignals from the brain to the cord.NOTE: Text books usually illustrate ascending and descending tracts on opposite sides of the spinal cord. This is for

ease of illustration only.

ASCENDING (SENSORY) TRACTS

Fasciculus gracilis andfasciculus cuneatus Both medial to the dorsal grey column Together make up most of the dorsal white columns

carrying signals directly from spinal sensory roots all the way to the gracile and cuneate nuclei in thelower end of the medulla

Mainly fine, discriminatory touch Allows recognition of surface locations of sensory stimuli on the skin or the positions of the different

parts of the body. Ventraland dorsal spinocerebellar tracts lateral margin of cord relay signals from the posterior grey horns upward to the cerebellum Spino-olivary tract Lateral margin of the cord and ventral to ventral spinocerebellar tract from the posterior grey horns of the cord to the inferior olive of the medulla. Sensory signals transmitted inspinocerebellar tracts and also inspino-olivary tract signals from muscles and joints that appraise the cerebellum at all times about the movements and positions of

different body parts. Ventraland lateral spinothalamic tracts Carrying signals relayed to posterior grey horn, then through the anterior white commissure and finally upward

through these tracts on the opposite side of the cord to the brain stem and thalamus.

crude touch, pain, temperatureDESCENDING (MOTOR) TRACTS Lateral corticospinal tract From the motor cortex of the brain Ventral corticospinal tract Also from the motor cortex of the brain Rubrospinal tract From the red nucleus of the mesencephalon Reticulospinal tracts From the reticular substance of the mesencephalon, pons, and medulla. Olivospinal tract From the inferior olive of the medulla Vestibulospinal tract From the vestibular nuclei of the medulla and pons Tectospinal tract From the tectum of the mesencephalon.

Excitatory tracts stimulation of most motor tracts causes either increased muscle tone or actual muscle contraction.

Inhibitory tracts stimulation of some tracts can decrease muscle tone

In the cervical region of the cord, the grey commissure is invaded by white fibres leading to the appearance of the

reticular formation

Five different classes of fibres:

afferent fibres extending short or long distances in the cord


16/29

2008 HET227 14

long ascending fibres (from supra-spinal levels) long descending fibres (from supra-spinal levels) intra-segmental and inter-segmental fibres efferent fibresIn addition there are

decussating fibres (crossing the median plane to unlike structures commissural fibres crossing the median to like structuresThese different fibres allow the sensory information to be integrated to allow both simple and complex processing ofstimuli:

Simple processingof sensation - such as flutter, contact, vibration, and pressure (such sensations can beexplained by activity of primary peripheral afferent fibres)

Complex processingof sensation such as tactile recognition of unseen objects and the determination ofdirection of movement of cutaneous stimuli.

Peripheral nerves allow the determination of the position of the stimuli and its modality.

Central integrative functions are required to create perceptions and the remainder of our somaesthetic experience.

PERIPHERAL NERVE ORGANISATION

Somatosensory signals travel in fibres in peripheral nerves whose diameters range from the smallest unmyelinated

fibres to the largest and fastest fibres.These neurons are pseudo-unipolar cells with their cell bodies in the dorsal root ganglion.

The neurons are randomly organised within the nerves.

Each nerve collects signals from a region of skin called a dermatome: Dermatomes overlap so that adjacent dermatomes are innervated by more than one spinal nerve. Overlap is more extensive for pain and temperature than for discrimination and touchDamage to a peripheral nerve may produce a limited anaesthetised area.

Such anaesthetised regions may still have preserved pain sensibilities while very reduced or absent othermodalities (termed intermediate zones).

Pathological conditions:

Axotomy (transection) of a peripheral nerve

Causes axonal degeneration to the cell body or degeneration of myelin sheath by Schwann cells regeneration may occur at up to 4 mm per day. Demyelination (common form of neuropathy; chronic alcoholism, diabetes, toxins) Can cause slowing or failure of conduction Ischaemias lasting longer than 7 hours May lead to permanent impairment. Acute distortion may not affect smaller fibres, only the large myelinated ones.

DORSAL ROOT ORGANISATION

Afferent fibres enter the CNS by the dorsal (posterior) root ganglion of the spinal cord.

There is a loose correlation between spinal nerve locations and the dermatomes innervated by them.

Pathological conditions: Dorsal root lesions

Similar effects to damage of peripheral nerves (eg herniation of intravertebral disc) Irritation can cause paraesthesia (tingling) or hyperaesthetic regions (heightened sensitivity).

SPINAL CORD ORGANISATION

Afferent neurons enter the dorsal root ganglion and: Synapse with secondary neurons within the same segment of the cord

OR

Ascend or descend a few segments of the cord before synapsing with a secondary neuronOR

Enter a major ascending spinal tractDivergence: a single neuron may influence many other neurons through contact made via a highly branched axon

The output from a single neuron then spreads onto many neurons.


17/29

2008 HET227 15

Convergence: the opposite, with a single neuron receiving the outputs from a large number of other neurons The inputs from many cells converge on one.

Smallerdiameter, unmyelinatedneurons aggregate into a lateral mass (mainly nociceptive and thermoceptive).Largerdiameter, myelinated neurons aggregate in a medial mass (proprioceptive and discriminative touch).Small diameter neurons which branch and ascend or descend only 1 or 2 segments in the dorsal funiculus synapse

on secondary neurons in lamina 1 and the outer zone of lamina 2.

Larger neurons also branch with many ascending in the dorsal funiculi to terminate in the nucleus gracilis andnucleus cuneatus of the medulla.

They may also ascend or descend only a few segments to synapse on secondary neurons in the outer portions of

lamina 2 and in laminae 3 to 5.

Usually afferent fibres entering a single dorsal root synapse over a number of spinal cord segments.

Neurons with receptors with receptive fields on the proximal limbs or the dorsal body surface tend to synapse inlaterally placed grey matter neurons.

This imposes a topographical mapping of the skin surface in a medio-lateral way in the cord axial columns.

ORGANISATION OF SECONDARY SPINAL CORD NEURONS

A spinal cord neuron can be identified by its location, morphology and response to natural stimuli.

While its cell body is restricted to an individual lamina, the dendritic trees can extend over several laminae.

This means neurons can be influenced by many other inputs.

The dendritic trees of Rexeds laminae are arranged as follows:

Lamina 1 (marginal zone) disk-shaped dendritic trees confined to lamina 1;Mostly nociceptive A and C fibres.

Lamina 2 (substantia gelatinosa) outer stalked cells and inner islet cells; both are mainly restricted tolamina 2 with some extension to lamina 3; islet cells terminate mainly in L2 while stalk cells terminate in

L1 or project intersegmentally in the dorsal funiculus too the substantia gelatinous of nearby laminae;

Mostly C fibres and so nociceptive and strong mechanical outputs; L2 cells habituate to repeated stimuli.

Lamina 3-6 contain most of the axons of cells which ascend into main ascending somatic sensory pathways;contain large, complex dendritic arbor which can extend along many laminae and so mixing with other

inputs.

As a generality, moving ventrally from lamina 3 to lamina 6, the neurons change from being biased towards

light mechanical stimuli to responding to several modalities ie they become multi-modal.Cells found in the more ventral parts of the dorsal column have more complex receptive fields as a consequence of

their more extensive dendritic trees.

EG. Cells in lamina 4 have simple excitatory receptive fields while those in lamina 5 have both excitatory

and inhibitory sub-regions.

In some cases, for axons which ascend in the dorsal columns and excitatory field may be surrounded by an

inhibitory field. Thissurround inhibition may sharpen the spatial acuity of the somatic sensations.

Cells in any given region may be excited by one modality and then inhibited by a less noxious light stimulus to the

same site; or intense excitatory stimulus may be inhibited by activating other sites in the body. Such a

response may explain the subjective relief obtained by rubbing an injury.

EXAMPLE: Inhibition of reference response to stimuli applied to DIFFERENT locations.

EXAMPLE: Inhibition of reference response (background response at approx 25 impulses/s) to different stimuli

applied to SAME location.

Brush depresses the response (lower than original background signal). Pressure has initial increase in activation and then returns to a slightly depressed response (lower than

original background signal).

Pinch and Squeeze both have stronger initial responses and then return to enhanced levels result ofdamage to the area caused by the pinching and squeezing.

INHIBITION AND EXCITATION

Influences from higher centres, mainly the cerebral cortex via the corticospinal tract, can cause inhibition or

excitation of many of the dorsal column neurons.These are DESCENDING influences.


18/29

2008 HET227 16

ASCENDING PATHWAYS

The thalamus is an integral part of the relaying of ascending signals from the spinal cord to the somatosensoryregions of the cerebral cortex.

Such communications can occur over a variety of pathways, including the dorsal (posterior) column, spinothalamic,

spinocervicothalamic and spinoreticulothalamic.

The dorsal (posterior) column pathway is the most extensive (by 10 to 100 fold).

DORSAL (POSTERIOR) COLUMN PATHWAY

Carries sensations of highly localised (fine) touch, pressure, vibration and proprioception (position sense).

Pathway begins at a peripheral receptor.

Processes ascending in the dorsal column of the spinal cord terminate in the dorsal column nuclei of the medulla:the nucleus gracilis and the nucleus cuneatus.There is one relay neuron between the primary afferent fibre travelling in the dorsal columns and the thalamic

nuclei.

These dorsal columns have reached the peak of their development in primates in which they occupy about 40% of

the total cross section of the sub-mudullary cord.

The axons ascending within the dorsal column are organised according to the region innervated.

Axons carrying sensations from the inferior half of the body ascend within thefasciculus gracilis and synapsein the nucleus gracilis of the medulla oblongata. Axons carrying sensations from the superior half of the trunk, upper limbs and neck ascend in thefasciculus

cuneatus and synapse in the nucleus cuneatus.

Axons of the second-order neurons of the nucleus gracilis and nucleus cuneatus CROSS OVER to the opposite side

of the brain stem and ascend to the ventral nuclei of the thalamus.Decussation: the crossing of an axon from the left side to the right (or vice versa).

At the level of the cortex, the sensory modalities are arranged in a simple gross topographical map (much less welldefined than the visual map).

Some of this topographical mapping is evident in the dorsal columns with the more caudal spinal nerves represented

by more lateral positions in the dorsal columns of the lumbar region of the spinal cord.

This dorsal column topography is re-sorted in higher levels of the cord so as to allow a proper somatotopicorganisation.

Two major pathways are involved in somatic sensory perception:

Dorsal (posterior)-medial lemniscus pathwaySpinothalamic pathway

They have the following features in common:

Both comprise 1st-order, 2nd-order and 3rd-order sets of sensory neurons. The somas of the 1st-order neurons (primary afferents) occupyposterior root ganglia. The somas of 2nd-order neurons occupy CNS grey matter on the SAMEside as the 1st-order neurons. The 2nd-order neurons cross the midline and then ascend to terminate in the thalamus. The 3rd-order neurons project from the thalamus to thesomatic sensory cortex. Both pathways aresomatotopic: an orderly map of body parts can be identified experimentally in the grey

matter at each of the three loci of fibre termination. Synaptic transmission from primary to secondary neurons and from secondary to tertiary, can be modulated

(inhibited or enhanced) by other neurons.

DORSAL-MEDIAL LEMNISCUS PATHWAY

In the dorsal column nuclei of the medulla nucleus gracilis and nucleus cuneatus the column of influence ofreceptive fields can be shown to be less for more caudal parts of the body.

Nucleus gracilis receives from the hind limb and trunk(cat)Nucleus cuneatus receives fromforelimb (cat)

The most dorsalcell in these nuclei receives from the most distalsensory receptors.Ventrally situated neurons receive from moreproximally located receptors.Cluster neurons: there are regions in these nuclei which receive direct dorsal column inputs providing a direct

pathway to the thalamic nuclei via the lemniscus.

Receive signals from such receptors as the hair receptors in the cat which respond to light touch or thedisplacement of only a few hairs


19/29

2008 HET227 17

Specially involved in rapidly changing, spatial, tactile stimuli. May be a highly suitable way of ensuring that information about spatial pattern so rapidly changing

tactile stimuli are not retarded in any detrimental way.

Extra-clusterneurons are of mixed modality.

The characteristics of the dorsal column transmissions are that it involves thefastest propogating fibres and that it

can handle rapidly changing signals up to at least 400 Hz in follow mode, up to 700 Hz in detection mode.

Such high frequency signals can only travel in the dorsal column. Use of a tuning fork to test dorsal column functionby neurologists is a reflection of this feature.

SPINOTHALAMIC TRACTS

There is only one relay neuron between the primary afferent fibre and thalamic cells.

Provide conscious sensations of poorly localised (crude) touch, pressure, pain and temperature.

Begins at the peripheral receptors and ends at the primary sensory cortex of the cerebral hemispheres.

Axons of first-order sensory neurons enter the spinal cord and synapse on second-order neurons within the posterior

grey horns.

Axons of these interneurons cross to the opposite side of the spinal cord and ascend within the anterior

or lateral spinothalamic tracts.

Spinocervicothalamic tract:Originates from the dorsal column nuclei in lamina 4 (cat) and lamina 4 to 5 (monkey).

Axons from these neurons form thespinocervical tractwhich ascends in the ipsilateraldorsolateral column to thelateral cervical nucleus in C1 and C2 regions of the spinal cord.

Lateral cervical neurons project axons to the thalamus by first crossing in the anterior commissure, then travelling

to the inferior olivary nucleus then to thepons and then via the medial lemniscus to the thalamus.Spinothalamic tracts :Anterior spinothalamic tracts: Carry crude touch and pressure sensations.Lateral spinothalamic tracts: Carry pain and temperature sensations.These tracts end in the third-order neurons in the ventral nucleus group of the thalamus.

Ventral posterolateral nucleus (VPL)

Posterior group of nuclei (PO)

Intralaminar nuclei (IM).

After sensations are sorted and processed, they are relayed to the primary sensory cortex.Spinocerebellar pathway :

Sends proprioceptive information about the position of skeletal muscles, tendons and joints to the cerebellum.

This information does NOT reach our conscious awareness.

First-order neurons synapse on interneurons in the dorsal grey horn.

The axons of the second-order neurons ascend in one of the spinocerebellar tracts.

Axons that enter the R or Lposterior spinocerebellar tractDO NOT CROSS OVER they reach the cerebellarcortex via the inferiorcerebellar peduncle of that side.

Axons that DO CROSS OVER enter the L or Ranterior spinocerebellar tract. And reach the cerebellar cortex viathesuperiorcerebellar peduncle.

LOCALISATION OF SENSATION

Information arriving at the thalamus is sorted by thalamic nuclei according to The nature of the stimulus The region of the body involved.Processing by the thalamus determines whether we perceive a given sensation as fine touch, or pressure or

vibration.

Localisation of sensation depends on the projection of information from the thalamus to theprimary sensory cortex.

Sensory information from the toes arrives at one end of the primary sensory cortex Information from the head arrives at the other end.Electrical stimulation of primary sensory cortex has resulted in functional mapping of the primary sensory cortex.

SENSORY HOMUNCULUS: little man.

Proportions are different to that of an individual.

Represents the area of sensory cortex devoted to a particular body region, relative to the number of

sensory receptors the region contains.


20/29

2008 HET227 18

DETERMINATION OF PAIN

In the dorsal column and the spinothalamic tracts, determination that a stimulus is painful (rather than hot, cold, or

vibrating) is determine by the projection of the sensation to different populations of second-order and

third-order neurons.

Any abnormality along the pathway can lead to inappropriate sensations: Phantom limb pain: following amputation, pain may be experienced in the missing limb, caused by activity in

the sensory neurons or interneurons along the spinothalamic pathway. The neurons involved once monitored theintact limb.

Referred pain: when pain is felt in an uninjured part of the body, when the pain actually originates atanother locations. Eg heart attack (pain typically in the left arm); appendicitis (generally felt in lower right

quadrant); liver and gallbladder pain felt in right shoulder.

THALAMIC NUCLEI

The thalamus is the largest nuclear mass in the entire nervous system.

The afferent and efferent connections of the 12 main nuclear groups are so diverse that the thalamus cannot be said

to have a unitary function.

Thalamic nuclei can be categorised into 3 functional groups: specific or relay nuclei; association nuclei; non-specific nuclei.

Specific relay nuclei:

Thalamic nuclei responsible for handling most of the afferent signals.

Reciprocally connected to specific motor or sensory areas of the cerebral cortex. Ventral anterior nucleus (VA): projects to prefrontal cortex. Ventral lateral nucleus (VL):

Anteriorpart projects to supplementary motor area. Posteriorpart projects to motor cortex.

Ventral posterior nucleus (VP): projects to somatic sensory cortex (SI) and smaller projection to secondsomatic sensory area (SII). Somatotopically arranged:

Ventroposteromedial nucleus (VPM) devoted to face and head. Ventroposterolateral nucleus (VPL) devoted to trunk and limbs. Modality segregation is a feature of both these nuclei

Proprioceptive neurons most anterior

Tactile neurons in mid-region Nociceptive neurons at back (posterior nucleus)

Medial geniculate nucleus: (hearing): projects to primary auditory cortex. Lateral geniculate nucleus (vision): projects to primary visual cortex.Association ornon-specific nuclei: other thalamic nuclei containing thalamic interneurons or diffuse projections to

the cortical and sub-cortical structures.

Association nuclei:Reciprocally connected to the association areas of the cerebral cortex.

Anterior nucleus: projects to cingulate cortex. Involved in limbic circuit; function related to memory. Mediodorsal nucleus: inputs form olfactory and limbic systems; reciprocally connected with prefrontal cortex.

Some functions in relation to cognition, judgement, mood. Lateral posterior nucleus and pulvinar: belong to a single nuclear complex. Project to entire visual association

cortex and entire parietal association cortex.

Non-specific nuclei: (not specific to any one sensory modality) Intralaminar nuclei: project widely to cerebral cortex. Reticular nucleus: all thalamo-cortical projections pass through the reticular nucleus and give collateral

branches to it.

Somatosensory information reaches all of these thalamic nuclei.

Contralateral somatosensory signals provide the sources for the four thalamic terminations.

Dorsal (posterior)-medial lemniscus pathway: cells terminate in VPL with a few in PO.Spinothalamic pathway: cells go to VPL, PO and IM nucleus

Ventral posterolateral nucleus (VPL):

Core of cutaneous receptors


21/29

2008 HET227 19

Shell of deep receptors Relays discriminative touch, pressure and joi9nt position. Small percentage of VPL neurons show surround inhibition where they are inhibited by stimulation of

areas just outside their excitatory receptive fields.

Posterior group of nuclei (PO):

Large receptive fields

Multi-modal Not somatotopic Convergent inputs.

Intralaminar nuclei (IM):

Mainly convergent inputs (including pain)DAMAGE TO SOMATOSENSORY CORTEX:

Thalamic centres provide a slight degree of crude tactile sensibility even when the somatosensory cortex is

destroyed.

This suggests that there are perceptions in thalamic or lower centres.

Pain and temperature are not affected.

SOMATOSENSORY CORTEX

Varies in thickness from 2 to 4 mm,

thinnest in primary sensory areas; thickest in motor and association areas.

More than half is hidden from view in the walls of the sulci.

Cortex has both a laminarand a columnarstructure. Varies in detail from one region to another Permits the cortex to be mapped into different histologically different areas.

LAMINAR ORGANISATION - Six layers of cortex:

1. PLEXIFORM LAMINA (molecular) layer: contains the most dendritic branches of pyramidal cells, and the most distal branches of axons

projecting from the intralaminar nuclei of the thalamus. Signals from lower brain centres, excitability adjustment.

2. EXTERNAL GRANULAR (outer granular) layer: Contains small pyramidal and stellate cells Signals from lower brain centres, excitability adjustment. Signals to other related cortical layers

3. PYRAMIDAL (outer pyramidal) layer Contains medium-sized pyramidal cells projecting to other parts of the cortex. Signals to other related cortical layers

4. INTERNAL GRANULAR (inner granular) layer Contains stellate cells receiving afferents from the thalamic nuclei Entry level for most sensory signals

5. GANGLIONIC LAMINA (large, inner pyramidal) layer Contains large pyramidal cells projecting to corpus striatum, brainstem, and spinal cord. Signals to more distant parts of CNS.

6. MULTIFORM LAMINA (FUSIFORM or polymorphic cell) layer. Contains modified pyramidal cells projecting to the thalamus. Signals to more distant parts of CNS.

MAIN CELL TYPES within the laminae:

1. PYRAMIDAL CELLS:

Bodies ranging from 20-30 m in laminae II and III, more than twice that height in lamina V. Apical and basal dendrites branch freely and are studded with spines. The axon gives off recurrent branches before leaving the grey matter. ALL are excitatory.

2. SPINY STELLATE CELLS:

Have spiny dendrites Are generally excitatory Receive most of the afferent input from the thalamus and other areas of the cortex


22/29

2008 HET227 20

Synapse upon pyramidal cells.3. SMOOTH STELLATE CELLS:

Have non-spiny dendrites Are generally inhibitory Receive recurrent collateral branches from pyramidal cells Synapse upon other pyramidal cells. Inhibitory GABA-secreting neurons make up 25% of all neurons in the cerebral cortex.

Some synapse on bases of dendritic spines, some synapse upon somas, some synapse on initialaxonal segments.

COLUMNAR ORGANISATION

Neurons arranged functionally in terms of columns 50-100 m in diameter, extending radially through the laminae.

Within each column, all of the cells are modality-specific. Eg, a given column may respond to movement of a particular joint but not to stimulation of the overlying

skin.

Somatosensory modalities are conveyed by anatomically different pathways. Cells making up these pathways have distinctive response properties. Sensory receptors and primary sensory neurons responsive to one sub-modality (such as pressure or

vibration) are connected to clusters of cells in the dorsal column nuclei and thalamus that receive inputsonly for that sub-modality.

These relay neurons in turn project to modality-specific cells in the cortex.Columns are also organised by receptive field.

Receive inputs from the same local area of skin and respond to a single class of receptors. Receptive fields of a column share a common centre (although are not precisely congruent).

Columns therefore provide an anatomical structure that preserves the properties of location and modality.

Somatic sensory cortex has 3 major divisions:

1. Primary (S I)2. Secondary (S II)3. Posterior parietal cortex.

PRIMARY SOMATOSENSORY AREA (S-I)

In post-cental gyrus

Lesions in the area cause impairment of contralateral senses.

Stimulation provides somatosensory experiences. Receives projections from ventrobasal thalamus. Responds to touch, pressure, pain, proprioceptive modalities, BUT NOT TO ANY OTHERS. S-1 neurons make point-to-point connections with caudal portion of VPL. Topographical mapping of S1 which is replicated in at least areas 1 and 3b.

Vertical columns respond to some modality-columnar organization.

The S-I in primates has 60% of the cells which respond to somatic stimulation.

Caudal to central sulcus receptive fields become larger and more neurons are driven by non-cutaneous stimuli.

Contains four cytoarchitectural areas (defined by Brodmann) which differ functionally:Brodmanns areas 3a, 3b, 1, and 2.

Thalamic fibres mostly terminate in 3a and 3b, which project axons to areas 1 and 2. Thalamic neurons also send small projections directly to areas 1 and 2.

Topographical organisation: All four areas receive input from all areas of the body surface, but one modality dominates in each area.

Area 1 receives information from receptors it in the skin rapidly adapting cutaneous receptors (withreceptive fields often covering several adjacent fingers).

Area 2 receives convergent input from slowly and rapidly adapting cutaneous receptors or fromcutaneous receptors and proprioceptors in the underlying muscles and joints.

Area 3a also receives proprioceptive information from receptors in muscles and joints, with thedominant input from proprioceptors signalling muscle stretch.

Area 3b receives information from receptors it in the skin primarily from cutaneousmechanoreceptors.

The four areas are extensively interconnected, with serial and parallel processing involved in higher-order

elaboration of sensory information.Lesions of S-1: Critical to discrimination of size and form

Impaired kinaesthesis Pain and temperature not affected.


23/29

2008 HET227 21

SECONDARY SOMATOSENSORY AREA (S-II)

At level of Sylvian Fissure under temporal lobe.

Innervated by neurons from each of the four areas of S-I. The projections of S-I are required for the function of S-II. EG. If neural connections from the hand area of S-I are removed, stimuli applied to the skin do not activate

neurons in S-II.

Removal of parts of S-II has no effect on the response of neurons in S-I.Projects to the insular cortex, which in turn innervates regions in the temporal lobe important for tactile memory.Modality properties of S2 differ to those of S-1.

Most activated by light mechanical stimulation of the hairs or of the skin surface.Adapt rapidly to maintained stimuli.

Shows similar columnar organization and spatial distribution of receptive fields.

POSTERIOR PARIETAL CORTEX (BRODMANNS AREAS 5 AND 7)Immediately posterior to S-I.

This area receives input from S-I as well as input from the pulvinar and therefore has an associational function.

Projects to motor areas of the frontal lobe, and plays an important part in sensory initiation and guidance of

movement.

Also connected bilaterally through the corpus callosum.

Area 5: Integrates tactile information from mechanoreceptors in the skin with proprioceptive inputs from the

underlying muscles and joints. Also integrates information from the two hands.

Area 7: Receives visual information as well as tactile and proprioceptive inputs

Allows integration if stereognostic and visual information.PLASTICITY OF SOMATIC SENSORY CORTEX

[Monkeys] cortical sensory representations of the individual digits of the hand can be defined very exactlyby recording the electrical response of cortical cell columns to tactile stimulation of each digit in turn.

These digital maps can be altered by peripheral sensory experience: The median nerve supplies to ventral surface of the outer 3 digits of the hand, and the

radial nerve supplies the dorsal surface. If the median nerve is crushed, representation of dorsal

surface on digital map increases at the expense of the ventral representation.

If the middle (3rd) digit is denervated, the corresponding cortical area is unresponsive fora few hours and then becomes progressively (over weeks) taken over by expansion of therepresentations of the 2nd and 4th digits.

These experiments show that the somatic sensory maps areplastic being modified by peripheral events. Lesion studies (in monkeys) have shown the reorganisation of topographical allocations following

peripheral lesions

Experimental conditions (cats) have shown the number of cortical columns responding to a particularthalamocortical input can be increased by local infusion of a GABA antagonist drug which suppresses

lateral inhibition. The effect of removal of a peripheral sensory field is comparable:

If one set of thalamocortical neurons falls silent due to loss of sensory input, it no longerexerts lateral inhibition and cortical columns within its territory are taken over by neighbouring

active sets.

EG: in the human somatosensory body map, the digits are represented next tot eh face. Inseveral well-documented cases of upper limb amputation, patients had later experiences of

phantom finger sensations on touching their face on that side with an implement such as a combheld in the other hand. This illusion can occur within 2 weeks of amputation [explained on the basis

of unmasking of pre-existing over-lap of thalamocortical neurons.

Cortical signals are transmitted to thalamic, medullary and cord relay centres which can control the sensitivity of the

sensory input.

These inhibitory signals are termed corticofugal. If stimulus intensity becomes too high, the corticofugal signals will decrease the transmission through the

relay centres.

Such control capacity ensures that the operating range of the somatosensory system is always appropriateand never too low or too high.


24/29

2008 HET227 22

POSITION SENSES

Also called proprioceptive senses

The sense of position and movement of ones own limbs and body without using vision.Divided into 2 categories:

STATIC POSITION limb position sense DYNAMIC or KINAESTHETIC PROPRIOCEPTION limb movement sense.

Important for controlling limb movements, manipulating objects that differ in shape and mass, maintaining anupright posture.

Receptors involved in propriuoceptioon:

Loose connective tissue and fascial receptors Fasciae: connective tissue layers supporting and surrounding organs

Skin receptors. Pacinian corpuscles (sensitive to deep pressure) also occur in the superficial and deep

fasciae, in joint capsules particularly suitable for dynamic proprioception.

Direct proprioceptors: Types ofmechanoreceptors in muscle and joints which signal the stationaryposition of the limb and the speed and directions of limb movement:

Muscle spindle receptors Golgi tendon organs

Receptors located in joint capsules.

DIRECT PROPRIOCEPTORS

Muscle spindle receptors - specialised stretch receptors in muscle; monitor skeletal muscle length and trigger stretchreflexes.

Golgi tendon organs - receptors in the tendon that sense contractile force or effort exerted by a group of musclefibres.

Similar in function to Ruffini corpuscles (sensitive to pressure and distortion of the skin) Located at junction between skeletal muscle and its tendon. Dendritic branches repeatedly branch and wind around densely packed collagen fibres of the tendon. Stimulated by tension in the tendon; they monitor the external tension developed during muscle

contraction.

Receptors located in joint capsules - sense flexion or extension of the joint.

Joint capsules are richly innervated by free nerve endings that detect pressure, tension, and movement atthe joint.

Sense of body position results from the integration of information from these receptors with theinformation provided by muscle spindles, Golgi tendon organs and the receptors of the inner ear.

Proprioceptors do not adapt to constant stimulation

Each receptor continuously sends information to the CNS.Most proprioception is processed at sub-conscious level:

A relatively small proportion of the arriving information reaches your conscious awareness over theposterior column pathway.

SPINOCEREBELLAR PATHWAYto the cerebellum outside our conscious awareness.Delivers proprioceptive information about the position of skeletal muscles, tendons and joints to the cerebellum.

This information does NOT reach our conscious awareness.

First-order neurons synapse on interneurons in the dorsal grey horn. Second-order neuron axons ascend in one of the spinocerebellar tracts.

Axons that enter the R or Lposterior spinocerebellar tractDO NOT CROSS OVER they reach thecerebellar cortex via the inferiorcerebellar peduncle of that side.

Axons that DO CROSS OVER enter the L or Ranterior spinocerebellar tract. And reach thecerebellar cortex via thesuperiorcerebellar peduncle.

Proprioceptive information from each part of the body is relayed to a specific position of the cerebellar cortex.

PROPRIOCEPTIVE PATHWAYSDorsal column-lemniscus system

The dorsal column-lemniscus system is the principal pathway for perception of touch and proprioception.

Large-diameter axons with fast conduction velocities to the dorsal horn of the spinal cord then to the brainstem and thalamus.

The proprioception and tactile axons in this column are segregated anatomically.


25/29

2008 HET227 23

Proprioception axons more ventrally in the dorsal column than those of tactile receptors, which arelocated dorsally.

Proprioceptors terminate more rostrally in the gracile and cuneate nucleus. Proprioceptive afferents terminate in the Lamina VII, on interneurons in laminae V and VI and on motor

neurons in lamina IX.

Thalamic processing of proprioception

Thalamic neurons responses occur selectively at either one or two extents of joint rotation: Maximum when joint is at full rotation Maximum when joint at minimum rotation. This contrasts with the actual joint receptors which are maximally stimulated at specific degrees of rotation

of the joint.

Ventral posterior nucleus (VP): projects to somatic sensory cortex (SI) and smaller projection to secondsomatic sensory area (SII). Somatotopically arranged:

Ventroposteromedial nucleus (VPM) devoted to face and head. Ventroposterolateral nucleus (VPL) devoted to trunk and limbs. Modality segregation is a feature of both these nuclei

Proprioceptive neurons most anterior Tactile neurons in mid-region Nociceptive neurons at back (posterior nucleus)

CLINICAL CONSIDERATIONS

Loss of sensory function is one of our most sever of disabilities.

Removal of sensory input (sensory deprivation) can have severe effects on normal behaviour.

Loss of sensory function is generically termed agnosia

Specific loss or number of sensory modalities is given a combining form so that we have terms such as

astereognosia loss of sense of ones body.

Loss of any sensory modality is not always easy to definitively prove: A subject may not be aware of the loss, they may never experience the sensation and so are not then

aware of its absence.

A subject may have subconscious reception but have not conscious awareness ie, they may not knowthey can sense something.

This situation arises through the site of a lesion or dysfunction, if it is at the primary receptor or affects the pathways

to the CNS it is usually clear that a problem is present.

If the damage is in one of the target areas of the CNS for the information, then only one level of the sense may be

destroyed, including perceptual aspects.

Any loss of somatosensory function is suggestive of damage to the post central gyral regions of the cortex

(Brodmanns areas 3a, 3b, 1 and 2) and also the adjacent cortex (areas 5, 7 and S2).

Lesions of the somatosensory system results in a variety of deficiencies: Reduced capacity or loss of tactile localisation Reduced capacity or loss of tactile discrimination Reduced capacity or loss of tactile recognition of objects Reduced capacity or loss of limb position sense

SOMATOSENSORY LESIONS

In 1920, Henry Head proposed that lesions of the somatosensory cortex cause three mainly independent effects:

1. Increased somatosensory thresholds for tactile, thermal, painful stimuli poor detection poor two-point discrimination poor sense of limb position clumsy hand/finger movements

2. Impaired tactile appreciation of object qualities impaired touch recognitione.g. impaired object identification or recognition by touch

Astereognosis inability to match or identify shapes by touch.

3. Impaired perception of limb position in space ie, impaired spatial recognitione.g. impaired limb placement (reaching)

loss of knowledge of own body (illness, pain, body parts)


26/29

2008 HET227 24

These categories are still in use today.

Many studies on the changes to somatosensory processing have been undertaken on subjects with cortical head

wounds or who had surgery to reduce epilepsy.

Damage to the post-gyrus region

Any damage to the post-central gyrus region has been seen to result in abnormally high sensory thresholds and

impaired stereognosis.

Examples of patient somatosensory changes:

difficulty in detecting light touch to the skin poor two-point discrimination difficulty locating point touch on skin contralateral to side where lesion was present. If blindfolded difficulty in knowing if the contralateral hand was passively moved.

Another symptom of lesions has been called A symptom of lesionsin post-gyrus region efferent paresis

Patients have clumsy movements of the fingers as a result ofloss of positional feedback.This consequence occurs whether the lesion is in the right or left hemisphere.

If the lesion is in the region of theface representation area in the somatosensory cortex a symptom termed motor aphasia occurs in which the patient has difficulty in

controlling the lips and tongue to pronounce desired sounds. These symptoms do not occur if the lesion is in the right hemisphere. Even though the thresholds for somatosensory perceptions may be normal, this does not preclude disorders

of the somatosensory system.

Normally we process several stimuli simultaneously.

We distinguish between them and process accordingly.

Damage to the secondary somatosensory region

Simultaneous extinction: Another somatoperceptual disorder

Failure to distinguish between simultaneous stimuli One or more of the simultaneous stimuli are not appreciated and therefore are extinguished. Commonly produced by damage to thesecondary somatosensory cortex, particularly in the right parietal lobe

Damage to the posterior parietal region (including secondary somatosensory areas 5 and 7) Often results in patients unable to accurately position their limbs. Specially pronounced if the use of visual feedback clues is prevented. Impaired kinaesthesia or disruption of the coordination between tactile sensory and visual information

processing in the cortex.

SOMATOSENSORY AGNOSIAS

Astereognosis: Loss of tactile appreciation of objects. Asomatognosia: Loss of knowledge about ones own body

Astereognosis:

Rarely demonstrated by itself in the absence of other somatosensory disorders. Other spatial deficiencies are not uncommon. Associated with damage to more posterior portions of the somatosensory cortex. Primary Agnosias: Result in loss of the ability to recognise the tactile qualities of an object

Because the tactile image is incomplete or absent. Secondary Agnosias or asymbolia

Occur when the tactile image is preserved but is disconnected from other sensoryrepresentations

The subject is confused about the full significance of the object.Asomatognosis:

Most distressing condition. Unaware of illness or problems caused by neglect of their body. Several variations of the condition:

Anosognosia: lack of awareness of illness Anososiaphoria: indifference to illness Autopagnosia: inability to locate or name body parts Asymboliafor pain: absence of normal response to pain.

May affect both sides or only one side.


27/29

2008 HET227 25

SENSORY ILLUSIONS AND HALLUCINATIONS

Sensory illusions

More likely to involve the visual sense than tactile sense Phantom touches and tickles are experienced in normal and abnormal patients.

Sensory hallucinations

Can be considered to occur when the cortex is stimulated BUT the receptors are not In normal conditions the frontal cortical lobes control this discrepancy.

Absence of frontal lobe control can produce a pathological state Visual hallucinations more common than tactile.

Normal processing of sensory hallucinations is thought to be the result of the cortex resolving theambiguity between cortical responses and absence of any receptor signal.

Schizophrenics or people under the influence of psychogenic drugs may fail to resolve the ambiguity andso experience the sensations as if they were actually occurring.

STIMULUS INTENSITY

Sensory quality depends on which nerve is actuated and where in the brain it leads.

But can the brain distinguish betweensensory quantity ormagnitude of the sensation (stimulus intensity) as well as

quality? Laws have been devised in an attempt to define the perception of stimulus intensity.

THE WEBER-FESCHNER PRINCIPLE:An approximate psychological law relating the degree of response or sensation of a sense organ and the intensity of

the stimulus.

The law asserts that equal increments of sensation are associated with equal increments of the logarithm of the

stimulus, or that the just noticeable difference in any sensation results from a change in the stimulus which bears aconstant ratio to the value of the stimulus.

Thejust-noticeable difference is the smallest difference perceivable between two similar stimuli

FOR EXAMPLE:

In the bright midday sun you light a candle. Does anyone notice it getting brighter? Will you identify my voice if I call you on your mobile phone at a concert? You're carrying the downside of a refrigerator up a flight of stairs and someone puts a hammer on the

fridge, do you sense the difference?Mostly, the Fechner Weber Principle or Law holds that you won't notice a difference.

WEBERs LAWBackground: Ernst Heinrich Weber (1795-1878) was the German anatomist and physiologist who firstintroduced the concept of the just-noticeable difference. Weber was a professor at the University of Leipzig from

1818 until 1871. He is known chiefly for his work on sensory response to weight, temperature, and pressure.Weber determined that there was a threshold of sensation that must be passed before an increase in the intensity of

any stimulus could be detected; the amount of increase necessary to create sensation was the just-noticeabledifference.He further observed that the difference was a ratio

Date post:	08-Apr-2018
Category:	Documents
Upload:	rachel-kate-graham
View:	220 times
Download:	0 times

So Ma to Sensory Notes 2010

Documents