THE NERVOUS
SYSTEM: NEURAL
TISSUE
• Nervous system
– Swift, brief responses to stimuli
• Endocrine system
– Adjusts metabolic operations
– Directs long-term changes
Two organ systems coordinate and
direct activities of body
Nervous system includes all neural
tissue in body
• Central Nervous System
– Brain and spinal cord
• Peripheral Nervous System
– All neural tissue outside CNS
Histology of Neural Tissue
• Neurons
Cells in Nervous Tissue
• Neuroglia
• about half the volume of cells in the CNS
• smaller than neurons
• 5 to 50 times more numerous
• do NOT generate electrical impulses
• divide by mitosis
• Four types in the CNS
– Astrocytes
– Oligodendrocytes
– Microglia
– Ependymal cells
Neuroglia (Glia)
Neuroglia (Neuroglial Cells)
Central Neuroglia
Astrocyte protoplasmic astrocyte
fibrous astrocyte
Oligodendrocyte perineuronal satellite cell
interfascicular cell
Microglia
Ependymal Cell
Peripheral Neuroglia
Schwann Cell
in peripheral nerve
and ganglion
Capsular (Satellite) Cell
in ganglion
• Largest of glial cells
• Most numerous
• Star shaped with many processes
projecting from the cell body
• Help form and maintain blood-brain barrier
• Provide structural support for neurons
• Maintain the appropriate chemical
environment for generation of nerve impulses/action potentials
• Regulate nutrient concentrations for neuron survival
• Regulate ion concentrations - generation of action potentials by neurons
• Take up excess neurotransmitters
• Assist in neuronal migration during brain development
• Perform repairs to stabilize tissue
Astrocytes
Oligodendrocytes
• Most common glial cell
type
• Each forms myelin sheath
around the axons of
neurons in CNS
• Analogous to Schwann
cells of PNS
• Form a supportive
network around CNS
neurons
• fewer processes than astrocytes
• round or oval cell body
Microglia
• Small cells found near blood vessels
• Phagocytic role - clear away dead cells
• protect CNS from disease through phagocytosis of microbes
• migrate to areas of injury where they clear away debris of
injured cells - may also kill healthy cells
• few processes
• derived from mesodermal cells
that also give rise to monocytes
and macrophages
Ependymal Cells
• Form epithelial membrane lining cerebral cavities (ventricles) & central canal
- that contain CSF
• Produce & circulate the cerebrospinal fluid (CSF) found in these chambers
• CSF = colourless liquid that protects the brain and SC against
chemical & physical injuries, carries oxygen, glucose and other necessary
chemicals from the blood to neurons and neuroglia
• epithelial cells arranged in a
single layer
• range in shape from cuboidal
to columnar
• Flat cells surrounding PNS axons
• Support neurons in the PNS
PNS: Satellite Cells
PNS: Schwann Cells
• each cell surrounds multiple unmyelinated PNS axons with a single
layer of its plasma membrane
• Each cell produces part of the myelin sheath surrounding an axon in
the PNS
• contributes regeneration of PNS axons
Neurons
•have the property of electrical excitability - ability to produce
action potentials or impulses in response to stimuli
•what is the main defining characteristic of neurons?
Representative Neuron
1. cell body or soma -single nucleus with prominent nucleolus
-Nissl bodies
-rough ER & free ribosomes for protein
synthesis
-proteins then replace neuronal cellular
components for growth
and repair of damaged axons in the PNS
-neurofilaments or neurofibrils
give cell shape and support -
bundles of
intermediate filaments
-microtubules move material
inside cell
-lipofuscin pigment clumps
(harmless aging) - yellowish
brown
2. Cell processes =
dendrites (little trees)
- the receiving or input
portion of the neuron
-short, tapering and
highly branched
-surfaces specialized
for contact with other
neurons
-cytoplasm contains
Nissl bodies &
mitochondria
Neurons
3. Cell processes = axons
• Conduct impulses away from cell body-
propagates nerve impulses to another neuron
• Long, thin cylindrical process of cell
• contains mitochondria, microtubules &
neurofibrils - NO ER/NO protein synth.
• joins the soma at a cone-shaped elevation =
axon hillock
• first part of the axon = initial segment
• most impulses arise at the junction of the
axon hillock and initial segment = trigger
zone
• cytoplasm = axoplasm
• plasma membrane = axolemma
• Side branches = collaterals arise from the
axon
• axon and collaterals end in fine processes
called axon terminals
• Swollen tips called synaptic end bulbs
contain vesicles filled with neurotransmitters
Functional Classification of Neurons
• Sensory (afferent) neurons
– transport sensory information from skin, muscles,
joints, sense organs & viscera to CNS
• Motor (efferent) neurons
– send motor nerve impulses to muscles & glands
• Interneurons (association) neurons
– connect sensory to motor neurons
– 90% of neurons in the body
• Afferent division of PNS
• Deliver sensory information from sensory receptors to CNS
– free nerve endings: bare dendrites associated with pain, itching, tickling,
heat and some touch sensations
– Exteroceptors: located near or at body surface, provide information about
external environment
– Proprioceptors: located in inner ear, joints, tendons and muscles, provide
information about body position, muscle length and tension,
position of joints
– Interoceptors: located in blood vessels, visceral organs and NS
-provide information about internal environment
-most impulses are not perceived – those that are,
are interpreted as pain or pressure
Sensory Neurons
Sensory Neurons
• Sensory receptors cont…
– mechanoreceptors: detect pressure, provide sensations of touch, pressure,
vibration, proprioception, blood vessel stretch, hearing and equilibrium
– thermoreceptors: detect changes in temperature
– nociceptors: respond to stimuli resulting from damage (pain)
– photoreceptors: light
– osmoreceptors: detect changes in OP in body fluids
– chemoreceptors: detect chemicals in mouth (taste), nose (smell)
and body fluids
-analgesia: relief from pain
-drugs: aspirin, ibuprofen – block formation of prostaglandins that
stimulate the nociceptors
-novocaine – block nerve impulses along pain nerves
-morphine, opium & derivatives (codeine) – pain is felt but not perceived in
brain (blocks morphine and opiate receptors in pain centers)
• Efferent pathways
• Stimulate peripheral structures
– Somatic motor neurons
• Innervate skeletal muscle
– Visceral motor neurons
• Innervate all other peripheral effectors
• Preganglionic and postganglionic neurons
Motor Neurons
-both divisions use two neurons and one ganglion
-first neuron has its cell body within the CNS
(Pre-ganglionic)
-second neuron has its cell body within the
ganglion (Post-ganglionic)
-sympathetic division:
-preganglionic fibers arise from middle
of the cord and are very short
-long post-ganglionic neurons
-flight or fight response
-NT = norepinephrine
-parasympathetic division:
-cranial nerves: vagus
-preganglionic fibers arise from bottom
of the cord and are very long
-short post-ganglionic neurons
-housekeeper division - relaxed state
e.g. pupil dilation, food digestion, slows
heartbeat
- NT = acetylcholine
Motor Units • Each skeletal fiber has only ONE
NMJ
• MU = Somatic neuron + all the
skeletal muscle fibers it innervates
• Number and size indicate precision of
muscle control
• Muscle twitch
– Single momentary contraction
– Response to a single stimulus
• All-or-none theory
– Either contracts completely or not at
all
• Muscle fibers of different motor units are intermingled so that net distribution of force
applied to the tendon remains constant even when individual muscle groups cycle
between contraction and relaxation.
• Motor units in a whole muscle fire asynchronously
some fibers are active others are relaxed
delays muscle fatigue so contraction can be sustained
Structural Classification of Neurons
• Based on number of processes found on cell body
– multipolar = several dendrites & one axon
• most common cell type in the brain and SC
– bipolar neurons = one main dendrite & one axon
• found in retina, inner ear & olfactory
– unipolar neurons = one process only, sensory only (touch, stretch)
• develops from a bipolar neuron in the embryo - axon and dendrite fuse and
then branch into 2 branches near the soma - both have the structure of axons
(propagate APs) - the axon that projects toward the periphery = dendrites
• Named for histologist that first described them or
their appearance
Structural Classification of Neurons
•Purkinje = cerebellum
•Renshaw = spinal cord
• others are named for shapes
e.g. pyramidal cells
Classification of neurons by cell size
• 1. golgi type I :
– Neurons have a long axon and large soma
• 2. Golgy type II :
– Neurons have short axon undergoes extensive
terminal aeborization and small soma
The Nerve Impulse
Continuous versus Saltatory Conduction
• Continuous conduction
(unmyelinated fibers)
– An action potential spreads
(propagates) over the surface of
the axolemma
– as Na+ flows into the cell
during depolarization, the
voltage of adjacent areas is
effected and their voltage-gated
Na+ channels open
– step-by-step depolarization of
each portion of the length of
the axolemma
Saltatory Conduction
• Saltatory conduction
-depolarization only at nodes of
Ranvier - areas along the axon
that are unmyelinated and
where there is a high density of
voltage-gated ion channels
-current carried by ions flows
through extracellular fluid from
node to node
• Properties of axon
• Presence or absence of myelin sheath
• Diameter of axon
Rate of Impulse Conduction
Synaptic Communication
• Synapse
– Site of intercellular communication between 2 neurons or between a neuron and an effector (e.g. muscle)
• Originates in the soma
• Travels along axons
• Permit communication between neurons and other cells
– Initiating neuron = presynaptic neuron
– Receiving neuron = postsynaptic neuron
• Most are axodendritic axon -> dendrite
• Some are axoaxonic – axon > axon
Synapse
Tipes of synapses
• Axodendritic:
– Between an axon and a dendrite
• Axosomatic:
– Between an axon and a soma
• Axoaxonic:
– Between two axon
• Dendrodendritic:
– between two dendrites
Synaptic morphology
• Presynaptic membrane:
– Contains metochondria, a few elements of SER,
and an abundance of synaptic vesicles.
• Synaptic cleft
• Postsynaptic membrane:
– Contains neorotransmitter receptors
SYNAPSE
Impuls transmission at synapse can
occur:
• Electrically
• Chemically
• Chemical
– Membranes of pre and postsynaptic neurons do not touch
– Synaptic cleft exists between the 2 neurons – 20 to 50 nm
– the electrical impulse cannot travel across the cleft – indirect method is required – chemical messengers (neurotransmitters)
– Most common type of synapse
– The neurotransmitter induces a postsynaptic potential in the PS neuron – type of AP
– Communication in one direction only
– Is the conversion of an electrical signal (presynaptic) into a chemical signal back into an electrical signal (postsynaptic)
• 1. nerve impulse arrives at presynaptic end bulbs
• 2. fusion of synaptic vesicles to PM and release of NTs
• 3. binding of NT to receptors on postsynaptic neuron
• 4. opening of channels in PM of postsynaptic neuron (e.g. sodium)
• 5. postsynaptic potential develops – depolarization
• 6. triggering of AP in postsynaptic neuron
Synapses
• Electrical
– Direct physical contact between cells required
– Conducted through gap junctions that permit
free movement of ions from one cell to another
– Two advantages over chemical synapses
• 1. faster communication
• 2. synchronization between neurons or
muscle fibers
– E.g. heart beat, brain stem, retina, and
cerebral cortex
Synapses
• axon terminal swell to form synaptic end bulbs or form swollen bumps called varicosities
• release of neurotransmitters from synaptic vesicles
– multiple types of NTs can be found in one neuron type
Synapse
Neurotransmitters
• Are signaling molecules that are released at
the presynaptic membranes and activate
receptors on postsynaptic membranes.
• More than 100 identified
• Some bind receptors and cause channels to open
• Others bind receptors and result in a second
messenger system
• Results in either excitation or inhibition of the
target
• Represented by three groups:
– Small molecules transmitters
– Neoropeptides
– Gases
Neurotransmitters
Neorotransmitters
Small molecules
1. Acetylcholine (ACh)
• -All neuromuscular junctions use ACh
• -ACh also released at chemical synapses in the PNS and by
some CNS neurons
• -Can be excitatory at some synapses and inhibitory at others
• -Inactivated by an enzyme acetylcholinesterase
• -Blockage of the ACh receptors by antibodies = myasthenia
gravis - autoimmune disease that destroys these receptors and
progressively destroys the NMJ
– -Anticholinesterase drugs (inhibitors of acetylcholinesterase)
prevent the breakdown of ACh and raise the level that can
activate the still present receptors
2. Amino acids: glutamate & aspartate & GABA
– Powerful excitatory effects
– Stimulate most excitatory neurons in the CNS (about ½ the
neurons in the brain)
– Binding of glutamate to receptors opens calcium channels = EPSP
– GABA (gamma amino-butyric acid) is an inhibitory
neurotransmitter for 1/3 of all brain synapses
Neurotransmitters
Valium is a GABA agonist - enhancing its inhibitory effect
Neurotransmitters 3. Biogenic amines: modified amino acids
– catecholamines: norepinephrine (NE), epinephrine, dopamine (tyrosine)
– serotonin - concentrated in neurons found in the brain region = raphe nucleus
• derived from tryptophan
• sensory perception, temperature regulation, mood control, appetite, sleep induction
• feeling of well being
– NE - role in arousal, awakening, deep sleep, regulating mood
– epinephrine (adrenaline) - flight or fight response
– dopamine - emotional responses and pleasure, decreases skeletal muscle tone
Other types:
a. ATP - released with NE from some neurons
b. Nitric oxide - formed on demand in the neuron then release (brief lifespan)
-role in memory and learning
-produces vasodilation - Viagra enhances the effect of NO
Neuropeptides • widespread in both CNS and PNS
• excitatory and inhibitory
• act as hormones elsewhere in the body
-Substance P -- enhances our perception of pain
-opoid peptides: endorphins - release during stress, exercise
enkephalins - analgesics
(200x stronger than morphine)
-pain-relieving effect by blocking the release of
substance P
dynorphins - regulates pain and emotions -Hypothalamic-releasing hormones (thyrotropin-releasing hormone and
somatostatin)
-Hormones stored in and teleased from the neurohypophysis (ADH and
oxytocin)
**acupuncture may produce loss of pain sensation because of
release of opioid-like substances such as endorphins or
dynorphins
Neurotransmitters
• Gases
– May act as neuromodulators. The ones that do
are nitric oxide (NO) and carbon monoxide
(CO).
Removal of Neurotransmitter
• Diffusion
– move down concentration gradient
• Enzymatic degradation
– acetylcholinesterase
• Uptake by neurons or glia cells
– neurotransmitter transporters
• NE, epinephrine, dopamine, serotonin
Peripheral Nerve
Nerve Fiber
Myelinated Nerve Fiber
Axon, Myelin sheath, Schwann cell
Unmyelinated Nerve Fiber
Axon, Schwann cell
Connective Tissue Sheath
Endoneurium
Perineurium – blood vessels
Epineurium
Composition of Peripheral Nerve
Connective tissue investment
• Connective tissue investments of peripheral
nerves include the:
– Epineurium
– Perineurium
– Endoneurium
Epineurium
• Is the outermost layer
• Is composed of dense irregular, collagenous
connective tissue containing thick elastic
fibers that completely ensheathe the nerve.
Collagen fibers within the sheath are
aligned and oriented to prevent damage by
overstretching of the nerve bundle.
Perineurium
• The middle layer of connective tissue
investments, covers each bundle of nerve
fibers (fascicle) within the nerve.
• Composition:
– Dense connective tissue but is thinner
than epineurium.
Endoneurium
• The innermost layer connective tissue
investment of a nerve, surrounds individual
nerve fibers (axons).
• Is a loose connective tissue composed of a
thin layer of reticular fibers (produced by
Schwann cells), scattered fibroblasts,
macrophages, and mast cells.
• The endoneurium is in contact with the
basal lamina of the Schwann cells.
Somatic motor and autonomic
nervous systems
• Functionally, the motor component is divided into
the somatic and autonomic nervous systems
• The somatic nerves systems provides motor
impulses to the skeletal muscles
• The autonomic nerves systems provides motor
impulses to the smooth muscles of the viscera,
cardiac muscle and secretory cells of the exocrine
and endocrine glands.
Motor component of the somatic
nervous system
• Motor innervation to skeletal muscle is
provided by somatic nerves from spinal and
selected cranial nerves.
• The cell bodies of these nerve fibers
originate in the CNS
Autonomic nervous system = ANS
(involuntary , visceral)
• Is generally defined as a motor system.
• Controls the viscera of the body by supplying the general visceral efferent (visceral motor) component to smooth muscle, cardiac muscle, and glands.
• The autonomic nervous system possesses two neurons between the CNS and the effector organ.
• Cell bodies of the first neuron lie in the CNS and their axons are usually myelinated.
• These preganglionic fibers (axons) seek an autonomic ganglion located outside the CNS, where they synapse on multipolar cell bodies of postganglionic neurons.
• Postganglionic fibers usually unmyelinated although they always are enveloped by Schwnn cells, exit the ganglion to terminate on the effector organ.
• The ANS is subdivided into two
functionally deferent divisions:
– The sympathetic nervous system
– The parasympathetic nervous system
Ganglia
• Are aggregations of cell bodies of neurons
located outside the CNS, there are two types
of ganglia:
– Sensory
– autonomic
Sensory ganglia
• Sensory ganglia house cells bodies of
sensory neurons.
• Cell of the sensory ganglia are
pseudounipolar which enveloped by
cuboidal capsule cells. These capsule cells
are surrounded by connective tissue capsule
composed of satellite cells and collagen.
Autonomic ganglia
• Autonomic ganglia house cells bodies of
postganglionic autonomic nerves.
• Nerve cells bodies of autonomic ganglia are
motor in function.
Central nervous system
• The CNS, composed of the brain and the
spinal cord, consist of white matter and gray
matter without intervening connective tissue
elements ; therefore, the CNS has the
consistency of a semifirm gel.
Continued
• White matter is composed mostly of myelineted fibers a long with some unmyelineted fibers and neoroglial cells.
• Gray matter is consist of aggregation of neuronal cells bodies, dendrites, and unmyelineted portion of axons as well as neuroglial cells.
• Gray matter in the brain is located at the periphery (cortex) of the cerebrum and cerebellum. Whereas the white matter lies deep to the cortex and surrounds the basal ganglia.
continued
• Spinal cord:
– White matter is located in the periphery,
whereas grey matter lies deep in the spinal
cord, where it forms an H shape in cross
section.
– Central canal lined by ependymal cells.
Meninges
• Are three connective tissue covering the
brain and spinal cord.
• Meninges consist of:
– Dura mater : the outermost layer
– Arachnoid : the intermediate layer
– Pia mater : the innermost layer
Dura mater • The dura mater is the dense outermost layer
of the meninges.
• Cerebral dura:
– Is a dense, collagenous CT composed of two
layers that are closely apposed in the adult.
– 1. Periosteal dura mater, the outer layer, is
composed of osteoprogenitor cells, fibroblast
and collagen fibers. Periousteal dura mater
serves as the periosteum of the inner surface of
the skull, and as such it is well vascularized.
continued
2. Meningeal dura :
– Inner layer of the dura is composed of fibroplast and
collagen fibers.
– This layer contains small blood vessels
– Internally meningeal dura covered by a layer of cells
called border cell layer, is composed of fibroblast.
Spinal dura mater
Does not adhere to the walls of the vertebral canal.
The epidural space : the space between the dura and the
bony walls of the vertebral canal, is filled with epidural
fat and a venous plexus.
Dura mater
Strongest
2 layers :
- Periosteal
- Meningeal
Layers fuse
except at dural
sinuses
Dura mater
Layers fused except at sinuses
Forms :
- Falx cerebri
- Falx cerebelli
- Tentorium cerebelli
Arachnoid mater
• Spaces
– Subdural
• Between dura and arachnoid
• Little CSF
– Subarachnoid
• between arachnoid and pia
• CSF and blood vessels
Arachnoid
• Is the intermediate layer of the meninges.
• Is avascular although blood vessels course
through it.
• It consist of fibroblast, collagen, and some
elastic fibers.
• Subdural space located between dura and
arachnoid, is a potential space because it
appears only after injury resulting subdural
hemorrhage
Arachnoid mater
* Arachnoid Villi
Projections through dura
Pass into superior sagittal
sinus
Passage of CSF
* Web-like attachments to pia
continued
• In certain regions the arachnoid extend
through the dura to form arachnoid villi,
which protrude into the dural venous
sinuses. The function of the arachnoid villi
is transporting CSF from the subarachnoid
spaces into the venous system.
Pia mater
• Is the innermost highly vascular layer of the
meninges, is in close contact with the brain,
following closely all of its contours.
• The pia mater does not contact with the
neural tissue because a thin layer of
neuroglial processes is always interposed
between them.
continued
• Composition : a thin layer of flatened, modified fibroblast.
• Blood vessels, abundant in this layer, are surrounded by pia cells interspersed with macrophage, mast cells, and lymphocytes.
• The pia mater is completely separated from the underlying neural tissue by neuroglial cells.
• Blood vessels penetrate the neural tissue and are covered by pia mater until they from the continuous capillaries characteristic of the CNS.
• Pedicels of the astrocytes, cover capillaries within the neural tissue.
Pia mater
* Delicate
* Vascular
* Clings to surface of brain
Blood-brain barrier
• Endothelial cells of CNS capillaries prevent
the free passage of selective blood-borne
substances into the neural tissue.
• This barrier is established by the endothelial
cells lining the continuous capillaries that
course through the CNS.
• These endothelial cells form zonula
occludentes with one another, retarding the
flow of material between cells.
continued
• These endothelial cells have relatively few
pinocytotic vesicles and vesicular traffic is
almost completely restricted to receptor
mediated transport.
Choroid plexus
• Is composed of folds of pia mater within the
ventricles of the brain, produces CSF.
• Are formed by folds of pia mater countain
abundant of fenestrated capillaries and
invested by the simple cuboidal
(ependymal) lining extend into the third,
fourth, and lateral ventricles of the brain.
• Are produced CSF.
Cerebrospinal fluid
• Cerebrospinal fluid bathes, nourishes, and
protects the brain and spinal cord.
• Is produces by the choroid plexus.
Cerebral cortex
• Is responsible for learning, memory,
sensory integration, information analysis,
and initiation of motor responses.
• Is divided into six layers as follows:
1. Molecular layer : contains horizontal cells and
neuroglia
2. External granular layer : contains mostly
granule(stellate) cells and neuroglial cells
continued
3. External pyramidal layer : contains
pyramidal cells and neuroglial cells.
4. Internal granual layer contains small
granule cells (stelate cells), pyramidal
cells, and neuroglia.
5. Internal pyramidal layer contains larges
pyramidal cells and neuroglia
6. Multiform layer consist of various shapes
(Martinotti cells), and neuroglia.
Cerebellar cortex • Is responsible for balance, equilibrium,
muscle tone, and muscle coordination.
• Is divided into three layers:
1. Molecular layer, lies directly below the pia mater.
2. Purkinje cell layer, contains the large, flask-shaped Purkinje cells, which are present only in the cerebellum.
3. Granular layer, consist of small cells and glomeruli (cerebellar islands).
Neural Regeneration
Nerve regeneration
• Nerve cells, unlike neuroglial cells, cannot
proliferate but can regenerate their axons,
located in the PNS.
• When a traumatic event destroy neurons,
they are not replaced because neurons
cannot proliferate ; therefore the damage to
the CNS is permanent.
continued
• However, if a peripheral nerve fiber is
injured or transected, the neurons attempts
to repair the damage, regenerate the
process, and restore function by initiating a
series of structural and metabolic events,
collectively called the axon reaction.
Axon reaction
• The reactions to the trauma are
characteristically localized in three regions
of the neurons:
1. Local changes: at the site of damage.
2. Anterograde changes: distal to the site of
damage
3. Retrograde changes: proximal to the site of
damage.
Local reaction • Local reaction to injury involves repair and
removal of debris by neuroglial cells.
• The severed ends of the axon retract away from
each other, and the cut membrane of each stump
fuses to cover the open end, preventing loss of
axoplasm.
• Macrophages and fibroblast infiltrate the damaged
area, secrete cytokines and growth factors, and up-
regulate the expression of receptors.
• Macrophages invade the basal lamina and assisted
by Schwann cells, phagocytose the debris.
Anterograde reaction
• In the anterograde reaction process, that portion of the axon distal to an injury undergoes degeneration and is phagocytosed
• The axon undergoes anterograde changes as follows:
1. The axon terminal becomes hypertrophied and degeneretes within a week. Schwwan cells prolivered and phagocitose the remnants of the axon terminal, and the newly formed Schwann cells occupy the synaptic space.
Continued
– 2. The distal portion of the axon undergoes
wallerian degeneration, distal to the lesion, the
axon and the myelin disintegrate, Schwann
cells dedifferentiate and myelin synthesis is
discontinued. Macrophages and Schwann cells
phagocytose the disintegrated remnants
– 3. Schwann cells proliferate, forming a column
of Schwann cells ( Schwann tubes ) enclosed
by the original basal lamina of the
endoneurium.
Retrograde reaction and regeneration
• In these process, the proximal portion of the
injured axon undergoes degeneration followed by
sprouting of a new axon whose growth is directed
by Schwann cells.
• The portion of the axon proximal to the damage
undergoes the following changes :
– 1. the perikaryon of the damaged neuron becomes
hypertrophied, its Nissl bodies disperse, and its nucleus
is displaced ( these events called chromatolysis). The
soma is actively producing free ribosomes and
synthesizing proteins and various macromolecule.
continued
– 2. Several “sprouts” of axon emerge from the
proximal axon stump, enter the endoneurium,
and are guided by the Schwann cells to their
target cell. For regeneration to occur, the
Schwann cells, macrophages, and fibroblasts as
well as the basal lamina must be present. These
cells manufacture growth factors and cytokines
and up-regulate the expression for the seceptors
of these signaling molecules.
continued
– 3. the sprout is guided by the Schwann cells
that redifferentiate and either begin to
manufacture myelin around the growing axon
or, in nonmyelinated axons, form a Schwann
cell sheath. The sprout that reaches the target
cell first form a synapse, whereas the other
sprout degenerate.
• Limited ability in PNS
• Severed peripheral nerve successfully regenerates a
fraction of the axons
– Function is permanently impaired
– Schwann cells participate
• Wallerian degeneration
– Loss of axon distal to damage
Regeneration
• More complicated than PNS regeneration
• Far more limited
• More axons involved
• Astrocytes produce scar tissue preventing axonal regrowth
• Astrocytes release chemicals blocking regrowth
Regeneration in CNS
Nerve ending – nerve terminal
• Two structural type :
– 1. Motor ending ( terminal of axon )
• Transmit impulses from the CNS to skeletal &
smooth muscle & to glands ( secretory ending)
– 2. Csensory ending = sensory receptor =
terminal of dendrites :
• Perceive various stimuli and transmit this input to
the CNS
continued
• These sensory receptor are classified into
three type depending on the source of the
stimulus, and are components of the general
or special somatic and visceral afferent
pathway :
– Exteroceptors
– Proprioceptors
– interoceptors
Exteroceptors
• Location : near the body surface
• Specialized to perceive stimuli from the external
environment
• These receptors sensitive to :
– Temperature
– Touch
– Pressure and
– Pain
• Are component of the general somatic afferent
continued
• Special somatic afferent :
– Specialized for light ( sense of vision) and
sound (sense of hearing)
• Special visceral afferent modality :
– Specialized for smell and taste
Proprioceptors
• Are specialized receptor located in joint
capsules, tendon and intrafusal fibers within
muscle.
• These general somatic afferent receptors
transmit sensory input to the CNS, which
translated into information that relates to an
awareness of the body in space and
movement
continued
• Vestibular (balance) mechanism, located
within the inner ear, are specialized for
receiving stimuli related to motion vectors
within the head.
Interoceptors
• Are specialized receptors that perceive
sensory information from within organs of
the body.
Specialized peripheral receptors
• Certain peripheral receptors, specialized to
receive particular stimuli, include
mechanoreceptors, thermoreceptor, and
nociceptors
• The dendritic ending located in various
regions of the body, including muscles,
tendons, skin, fascia and joint capsules
continued
• These receptors are classified into three
types :
– Mechanoreceptors, which respond to touch
– Thermoreceptors,which respond to cold and
warmth
– Nociceptors, which respond to pain due to
mechanical stress, extremes temperature
differences and chemical substance
Mechanoreceptors
• Mechanoreceptors respond to mechanical
stimuli that may deform the receptor or the
tissue surrounding the receptor.
• Stimuli that trigger the mechanoreceptors
are touch, stretch, vibration and pressure
Nonencapsulated mechanoreceptors
• Are simple unmyelinated receptors present
in the skin, connective tissues and
surrounding hair follicle
– Peritricial nerve ending, located in the
epidermis of the skin, especially in the face and
cornea of the eye
– Merckel’s disks, specialized for perceiving
discriminatory touch, located in non hairy skin
and regions of the body more sensitive to touch.
Encapsulated mechanoreceptors
• Encapsulated Mechanoreceptors exhibit characteristic
structure and are present in specific location
– 1. Meissner’ corpuscles :
• Specialist for tactile
• Location : dermal papillae of the non hair portin of
the hand, eyelids, lip, tongue, nipples, skin of the
foot and forearm.
• Each corpuscle is formed by three or four nerve
terminals and their associated Schwann cells, all
which are encapsulated by connective tissue.
continued
– 2. Pacinian corpuscles
• Location : in the dermis and hypodermis in the digits of the hand, breast, connective tissue of the joint, periosteum and the mesentery
• Spezialied to perceive pressure, touch and fibration
• Morphology :
– ovoid & large receptor
– Single unmyelinated fiber as a core and its Schwann cell
– Surrounded by approximately 60 layers of modified fibroblast
– Each layer separated by a small fluid-filled space
Ruffini’s corpuscles
• Location : in the dermis of skin, nail beds,
periodontal ligament and joint capsules
• Composition :
– branched nonmyelinated terminals interspersed
with collagen fibers
– Surrounded by four to five layers of modified
fibroblast
Krause’s end bulb
• Morphology :
– Spheris
– Unmyelinated nerve ending
• Location : papilla dermis, joints, conjunctiva, peritoneum, genital regions, subendothelial c.t. of the oral and nasal cavities
• Function : unknown, they were thought to be receptors sensitive to cold
Muscle spindles and Golgi tendon organs
• Muscle spindles provide feedback concerning the
changes and the rate alteration of the muscle
length
• Golgi tendon organs monitor the tension and the
rate at which the tension is being produced during
movement
• Information from these two sensory structures is
processed at the unconscious level within the
spinal cord; the information also reaches the
cerebellum & cerebral cortex, so that individual
may sense muscle position.
Thermoreceptor
• Which respons to temperature differences of
about 2° C, are three types: warmth
receptors, cold receptors and temperature-
sensitive nociceptors.
• Specific receptors have not been identified
for warmth
• Cold receptors are derived from naked
nerve ending in the epidermis
Nociceptors
• Are receptors sensitive to pain caused by
mechanical stress, extreme of temperature, and
cytokines as bradykinin, serotonin and histamin.
• Are naked ending of myelinated nerve fibers that
branch freely in the dermis before entering the
dermis
• Divided into three groups :
– Those that respond to mechanical stress or damage
– Those that respond to extremes in heat or cold
– Those that respond to chemical compound such as
bradykinin, serotonin and histamin
Peripheral Nerve Endings: Afferent Endings
Receptor Neurons of Craniospinal Ganglion
pseudounipolar neurons of dorsal root ganglia
trigeminal (semilunar, Gasserian ganglion),
geniculate (VII), superior IX, superior X ganglia
(GSA)
geniculate (VII), inferior IX, inferior X ganglia (VA)
Morphological Classification
free nerve endings
expanded tip endings
encapsulated endings ----- CT envestment
Afferent Endings
Free Nerve Endings
- Nerve endings without special structural
organization
- pain and temperature receptor
Expanded Tip Endings
- Merkel’s Touch Corpuscle
Merkel cells in basal layer of epidermis
- Type I Hair cells of Vestibular Labyrinth
Afferent Endings
Encapsulated Endings
- Meissner’s Corpuscle
- Pacinian Corpuscle
(Corpuscle of Vater-Pacini)
- Genital Corpuscle
- Ruffini’s Ending
- End Bulb of Krause
- Golgi tendon organ: Proprioceptor
Receptor
Endings
Free nerve
ending
Expanded
tip ending Encapsulated
ending
Merkel’s
Touch Corpuscle
expanded tip ending
Merkel cell
- clear cell located in the
basal layer of epidermis
- membrane bound electron
dense granules resembles
synaptic vesicle
Meissner’s Corpuscle
Pacinian Corpuscle
Efferent Endings Somatic Efferent Endings
Neuromuscular Junction
(Myoneural Junction, Motor End
Plate)
Autonomic Efferent
Endings
Endings on smooth muscle
and blood vessels
Neuromuscular Junction
(Myoneural Junction,
Motor End Plate)
NMJ
M
N
Autonomic Efferent Endings
Neuromuscular Spindle
• Both receptor and effector
• Structure
1. Capsule
2. Intrafusal Muscle Fibers
- Nuclear Bag Fiber
- Nuclear Chain Fiber
3. Receptor and Effector Nerve
Endings
- Afferent Ending
- Efferent Ending
NB: nuclear bag fiber IF: intrafusal muscle fiber
CA: capsule EF: extrafusal muscle fiber
Afferent Endings
Encapsulated Endings
- Meissner’s Corpuscle
- Pacinian Corpuscle
(Corpuscle of Vater-Pacini)
- Genital Corpuscle
- Ruffini’s Ending
- End Bulb of Krause
- Golgi tendon organ: Proprioceptor