Notes on CNS Physiology by Medical Study Center(2)

8/9/2019 Notes on CNS Physiology by Medical Study Center(2)

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Notes On CNS Physiology

Gray Matter of Spinal Cord: in the spinal cord gray matter is in the form of H shaped pillars which can be

divided into three types of columns i.e. anterior horn or ventral horn, posterior horn or dorsal horn and

in segments from T1 to L2 there is lateral horn.

Neurons in these horns are

Ventral Horn: Two groups of neurons

Alpha motor neurons which are large multi polar neurons and their nerve fibers are alpha efferents

which innervate skeletal muscle. Gamma neurons present in ventral horn are small and multi polar

neurons and there nerve fibers are gamma efferents which innervate the intrafusal fibers of the muscle

spindles. Both these alpha and gamma efferents come out of the spinal cord through ventral root of

spinal nerves.

Dorsal Horn: there are 4 groups of neurons

1.

Substantia gelatinosa: this group of neurons is present at the apex of the posterior gray column.

These neurons receive afferent nerve fibers carrying impulses of pain, temperature and crude

touch.

2.

Nucleus Proprius: this group is located anterior to the first group. These neurons receiveafferent nerve fibers carrying impulses of proprioception and two point tactile discrimination.

3.

Clarkes column or nucleus dorsalis: this group is present at the base of the posterior gray

column. These neurons are present in segments from T1 to L3, 4. These neurons are part of

spinocerebellar tract and these receive afferent nerve fibers from spinocerebellar tract.

4.

Visceral Afferent Nucleus: Present at the base of the posterior horn lateral to the clarkes

column. It is also present in segments only from T1 to L3, 4. It receives afferent nerve fibers

carrying impulses from various viscera.

Lateral Horn: in this horn, there are Autonomic preganglionic neurons. Preganglionic sympathetic in T1

to L2. Similar neurons are also present in sacral segments which are parasympathetic preganglionic.

Nowadays we divide the gray matter of spinal cord into 9 laminae and in the sensory tract we use these

laminae to describe the synapses.
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FUNCTIONS OF SPINAL CORD:

1.

It is a part of central nervous system and it contains ascending and descending tracts.

2.

In the spinal cord there are neuronal circuits and centers for various reflexes and these reflexes

are involved in posture maintenance, movements, and withdrawal of parts, local regulation of

blood flow, regulation of GIT motility and secretions, micturition, defecation and other reflexes.

3.

Initial integration of various sensory informations.

REFLEX ACTION: an automatic response to a stimulus without involvement of will or consciousness. The

purpose of this is to have quick response and to save time because this action has a protective value.

For the reflex action a specific organization is required known as reflex arc.

The reflex arc consists of 5 subcomponents.

1.

Receptor: it responds to the stimulus and generates impulses

2.

Sensory or afferent neuron: it carries impulses from the receptor to the centre.

3.

Centre: it is always present in CNS and this is integrating station. In the centre there is one or

more synapse between the afferent and efferent neuron.

4.

Efferent neuron or motor neuron: it carries impulses from the centre to the effector or target

tissue.

5.

Effector or target tissue: it is a responding tissue. These include skeletal muscle, smooth muscle,

cardiac muscle and glands.

For the reflex action, it is essential to have integrity of reflex arc. We can classify reflexes in different

ways.


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Depending upon the number of synapse in the centre i.e. monosynaptic, disynaptic and multi or poly

synaptic.

The other way of classifying is unconditioned or inborn reflexes and acquired or conditioned reflexes

Unconditioned Reflexes: Salivation, micturition, light reflex etc

Conditioned reflex: these are acquired in response to condition stimuli with the help of unconditioned

stimulus. Pavlovs experiment and salivary secretion.

05 August, 2010

Reflexes can be clinically classified into four types i.e.

1.

Superficial: receptors are present in skin or mucus membrane like conjunctival, corneal, plantar,

cremesteric etc

2.

Deep: receptors are present in deep tissues i.e. muscles or tendons like biceps, triceps jerk.

3.

Visceral: receptors are present in the viscera i.e. light reflex, baro receptor, bain bridge reflex

4.

Pathological: present in diseased state i.e. babinskis sign and clonus

PROPERTIES OF REFLEX ACTION:

In reflex arc there is one or more than one synapse, so most of the properties of reflex arc are properties

of synapses.-

1.

Central Delay: delay in the centre of reflex arc. If it is monosynaptic reflex arc then delay is 0.5

milli sec that is the time taken by one synapse to conduct impulse. By finding out central delay

we can say that the reflex arc is monosynaptic, disynaptic or polysynaptic.

Central delay can be calculated by

Central Delay = Reaction time (time in which the response occurs after application of stimulus)

peripheral time (Time taken by the impulses to be conducted by afferent and efferent neurons)

2.

Summation: adding up of the effect of stimuli especially if the stimuli are subthreshhold. It is of

two types i.e.

a.

Spatial Summation: Impulses are conducted along increased number of synaptic knobs

or afferent nerve fibers to the post synaptic neurons simultaneously.

b.

Temporal Summation: impulses are conducted along the same number of presynaptic

knobs or afferent nerve fibers repeatedly to the post synaptic neurons and this result

into the adding up of the effect of repeated stimuli leading to excitation of postsynaptic

neuron. The rate of repetition of stimuli should be such that the effects of previous

stimulus is there when the second stimulus comes.

3.

Facilitation: when response of post synaptic neuron or motor neuron is more than expected.

Suppose there are two afferent nerve fibers i.e. A and B. Afferent A and B makes synapse with


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two types of motor neurons or post synaptic neurons i.e. subliminal fringe and discharge zone or

excited zone neurons. In facilitation the affect of subliminal fringe neurons is added to the affect

of discharge zone neurons when both A and B neurons are stimulated simultaneously.

When each motor neuron is excited in the muscle one G tension is produced.

4.

Occlusion: Response of the post synaptic or motor neurons is less than expected. Mechanism is

partial sharing of motor neurons by both the afferents.

5.

Recruitment and after discharge: Recruitment means more and more motor neurons become

excited upon continuous repeated stimulation in same muscle. After discharge is that the

response of postsynaptic neurons continues after the stoppage of stimulus. After discharge is

shown by the poly synaptic reflexes only. After discharge is because of two mechanisms i.e.

a.

Long or Delayed Path: because of presence of more inter neurons.

b.

Presence of Reverberating Circuits

6.

Irradiation: Excitation spreads from motor neurons of one muscle to the motor neurons of the

other muscles. Example is that of flexor withdrawal reflex. With stronger stimulus there is more

stimulation.


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7.

Reciprocal Inhibition: For a smooth reflex action, when there is contraction of agonist there

must be relaxation of antagonist. This is because of reciprocal innervations.


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8.

Fatigue: if a reflex is elicited repeatedly, after sometimes the response decreases which is

because of fatigue of synaptic transmission.

9.

RENSHAW CELLS: these are the inhibitory inter neurons

MONOSYNAPTIC REFLEXES

STRETCH REFLEX: Most vastly studied reflex.

Whenever a skeletal muscle is stretched, it contracts. Example is that of knee jerk.


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Receptors for the stretch reflex are muscle spindles. Stretch reflex controls the increase in the length.


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Muscle Spindles are present in between the muscle fibers.

Length is 3-10 mm. it is along the long axis of muscle. Ends are fused with the sides of the muscle fiber.

So whenever there is change in the length of muscle, same change occurs in muscle spindle. Each

muscle spindle is encapsulated and contains modified muscle fibers which are called intrafusal muscle

fibers. Number of muscle spindles in the muscle varies. There is greater number of muscle spindles in

the muscles concerned with fine skilled movements like those of hand. There are 4-12 intrafusal fusal

fibers in each muscle spindle. There are 2 types of intrafusal fibers i.e. Nuclear bag fibers and nuclearchain fibers.

1.

Nuclear Bag fibers: number is 1-3. it has a central dilated portion which contains the nuclei.

2.

Nuclear Chain fibers : shorter, number 3-9 and these contain nuclei in the form of a chain

throughout the length of fiber. Ends are attached to the sides of nuclear bag fibers.

These fibers has got two portions i.e. end portion which is contractile and contains actin myosin

filaments. The central portion is without actin myosin filaments and this is the receptor portion. Muscle

spindles are excited whenever the central portion of intrafusal fibers are stretched.

Nerve Supply of Muscle Spindle: There are two types of nerve endings present along the intrafusal fibers

1.

Primary or annulospiral: these are wrapped around the central portion of both nuclear bag and

chain fibers. These are class 1a having diameter of 17 micrometer and velocity of conduction 70-

120 m/sec.

2.

Secondary Or Flower Spray: these are present only around nuclear chain fibers. these are class 2

having diameter of 8 micrometer and velocity of 30-70 m/sec.


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Motor nerve supply is through the gamma efferent which supply the end portion of intrafusal fibers in

muscle spindle which is contractile. The diameter is 3-6 micrometer and velocity of conduction is 15-30

m/sec. gamma efferents come out through ventral root and forms 30% of nerve fibers in ventral root.

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Muscles Spindles are stimulated by:

1.

Stretching of Muscle spindles i.e. Lengthening of Muscle Spindles.2.

Internal stretching which means the spindle can be stimulated without the stretching of muscle.

It is involved in muscle tone. Whenever impulses comes through the gamma efferents to the

end portion of the intrafusal fibers, end portion contracts and central portion is stretched

leading to stimulation of muscle spindle.

There are two types of stretch i.e.

1.

Steady or static stretch

2.

Dynamic or quick stretch.

When muscle is subjected to steady or continuous stretch, discharge of impulses from both primary andsecondary nerve endings increases and this discharge remains increased as long as the muscle remains

stretched. So nuclear chain fibers are involved in the response to the static stretch. When a dynamic

stretch is applied to the muscle, there is tremendous increase in the discharge of impulses from only

primary nerve endings which remains as long as the muscle is rapidly stretched.


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Primary nerve endings respond to both increase in the length of the muscle and also rapid increase or

rate of increase in the length of muscle. Secondary nerve endings respond only to the increase in the

length of the muscle and not the rate.

Whenever muscle contracts there is shortening of muscle and along with that there is shortening of

muscle spindle so discharge of impulses from the muscle spindle decreases and this results intorelaxation of muscle.

FUNCTIONS OF MUSCLE SPINDLE:

1.

Regulate the length of muscle and prevent the length to go beyond limits.

2.

Involved in the muscle tone mechanism.

3.

There are the receptors for the clinically tested reflexes.

4.

To have smooth voluntary movements, there is coactivation of alpha and gamma motor

neurons. For voluntary movements, impulses are initiated from motor cortex and comes to the

alpha motor neurons for contraction of muscle. At the same time impulses also come to the

gamma motor neurons.

Gamma efferents to the intrafusal fibers are of two types i.e. static and dynamic.

Gamma efferents facilitates the response of muscle spindle to two types of responses i.e. dynamic and

static.

FUNCTIONS OF THE STRETCH REFLEX:

1.

It is involved in the muscle tone mechanism and so it is also called as myotatic reflex.

2.

It is also involved in the maintenance of posture.

3.

Helps in lifting of weight and load.4.

It also prevents the unsteady and jerky movements and this is the dampening function of the

stretch reflex.

MUSCLE TONE: Continuous state of partial muscle contraction is called muscle tone. It involves the

muscle spindle and the stretch reflex.

There is some continuous gamma efferent discharge to the end portion of the intrafusal fibers of the

muscle spindle and this leads to the contraction of end portion resulting into the stretching of central

portion of intrafusal fibers leading to internal stretching. So there is discharge of impulses to the spinal

cord where these excite the alpha motor neurons and from there impulses come to the muscles to

cause their contraction.

Gamma efferent discharge is controlled basically by a bulboreticular facilitatory area in the brain stem.

In the reticular formation of the brain stem, in the upper pons. From this area, impulses come along the

reticulo spinal tract to facilitate the gamma motor neurons.


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The normal influence of the cerebral cortex and basal ganglia on the muscle tone is inhibitory. That is

why in their lesions there is hypertonia. The normal influence of cerebellum on the muscle tone is

faciliatatory. The effect of cerebellum is through both alpha and gamma motor neurons. In lower motor

neuron lesion there is atonia or hypotonia.

DISYNAPTIC REFLEX: example is Inverse Stretch reflex also called the golgi tendon organ reflex also

called lengthening reaction. It is also called autogenic inhibition. Receptors are golgi tendon organs.

When a strong stretch is applied to a muscle, it undergoes relaxation. With increased intensity of

stretch, muscle contracts with more force. So at extremes muscle damage can occur however this is

prevented by the relaxation of muscle at high muscle tension caused by stimulation of golgi tendon

organs. These receptors are encapsulated, their size is 0.5 to 1mm. these are located at the junction of

the muscle fibers and tendons. 10 -15 muscle fibers are connected to each tendon organ. From these

tendon organs, impulses are carried along 1 b nerve fibers. Velocity of conduction is 70 m/sec. these 1b

fibers go to the spinal cord first synapse with inter neuron which in turn synapse with the motor

neurons. Through these inter neurons there is inhibition of motor neurons. So the synaptic delay will be

1 milli/sec. this is a protective reflex. It prevents tearing of the muscle and detachment of the tendons

from the bones. Golgi tendon organs are involved in the regulation of muscle tension. Inverse stretch

reflex helps to regulate muscle tension.

CLASP KNIFE RIGIDITY: in a patient of upper motor neuron lesion, due to involvement of extra pyramidal

tracts, there is clasp knife rigidity.


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11 August, 2010

Both of the monosynaptic and disynaptic reflexes dont show recruitment and after discharge.

POLYSYNAPTIC REFLEX: Superficial Reflexes & flexor withdrawl reflex.

FLEXER WITHDRAWL REFLEX: whenever a noxious stimulus is applied to a part of the body, the part

becomes flexed and withdrawn away from the source of stimulus. In this reflex, receptors are

nociceptors which are free nerve endings and stimuli are noxious which are of different types i.e.

mechanical like pin prick, chemical or thermal.


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In this reflex, response depends upon the intensity of stimulus and it is a graded reflex.

In this reflex when one leg is flexed, the other leg becomes extended and it is crossed extensor reflex

and it is a part of withdrawl reflex. Purpose of this reflex is to support the body.

A number of interneurons are involved in the centre of this reflex some of which may be inhibitory and

other may be excitatory. Central delay is more than 1.5 seconds.

This reflex involves reciprocal inhibition because of reciprocal innervation. Motor neurons supplying the

flexers on the same side are excited and motor neurons supplying the extensors on the same side are

inhibited. This is opposite on the other side.

Recruitment, after discharge and irradiation are present in this reflex.


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SENSORY RECEPTORS: these are specialized cells or nerve endings in the body which responds to various

stimuli. Receptors generate impulses which inform the CNS about the changes in the external and

internal environments. These are transducers which convert different forms of energy into electrical

energy that is nerve impulse or action potential.

Becoming aware of presence of a particular stimulus is called sensation.

Stimulus is the change in environment.

Sensory receptors are classified in different ways.

1.

Exteroreceptors: these are present in the superficial surface of body i.e. skin and mucus

membranes and respond to changes in external environment.

2.

Interoreceptors: present inside the body and respond to changes in internal environment like

baroreceptors, osmoreceptors, chemoreceptors.

3.

Teloreceptors: these respond to stimuli at a distance like receptors for hearing and vision.

4.

Proprioceptors: these receptors respond to changes in position and movement.

Other classification

1.

Mechanoreceptors: respond to mechanical deformation of receptor membrane or of the

membrane of the cells adjacent to the receptors. These include a large number of receptors i.e.

free nerve endings, merkels discs, meisners corpuscles, pacinian corpuscles, hair follicle

receptos, ruffinis nerve endings, muscle spindle, golgi tendon organs, receptors in the vestibular

apparatus and cochlea, baroreceptors.

2.

Thermoreceptors: these respond to temperature changes i.e. cold receptors and warmth

receptors.

3.

Nociceptors: these respond to noxious stimuli. These detect tissue damage.

4.

Chemoreceptors: respond to chemical changes include chemoreceptors in the carotid and aortic

bodies. Receptors in the chemosensitive area in the medulla oblongata. Olfactory receptors in

the olfactory membrane. Taste receptors. Osmoreceptors in the hypothalamus. Glucostat cells

in the satienty center of the hypothalamus.

5.

Electromagnetic Receptors: respond to electromagnetic rays and these include rods and cones.

Other classification

1.

Non encapsulated

2.

Encapsulated

FREE NERVE ENDINGS: non encapsulated, widely distributed in different tissues of the body i.e.

epidermis and dermis of skin, mucus membrane of GIT, EPITHELIUM OF RESPIRATORY TRACT, fascia,

cornea, periosteum, perichondrium, tympanic membrane dental pulp etc.


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Impulses are carried by both myelinated and unmyelinated nerve fibers. These are involved in the

sensation of crude touch, tickle, itch, pressure, pain, cold and warmth. These undergo slow adaptation.

MERKELS Discs: Non encapsulated, present in the form of clusters in the deeper part of the epidermis of

skin. Respond to continuous touch like holding a pen. Under go slow adaptation.

HAIR FOLLICLE RECEP TORS: non encapsulated, these along with their nerve fibers are wrapped around

the hair follicles and these are stimulated by bending of hair. These undergo rapid adaptation.

MEISNERs CORPUSCLES: Encapsulated. Present in dermal papillae of skin of palm, sole, nipple and

external genitalia. Each corpuscle has a capsule and in the capsule there are flattened cells arranged

transversely along the long axis of corpuscle. Endoneurium of nerve fibers which enter the corpuscle

becomes fused with the capsule of the receptor. Number of meisners corpuscles decreases with age.

These are involved in fine touch, two point tactile discrimination and also low frequency vibration i.e.

frequency upto 80 Hz. These undergo rapid adaptation.

PACINIAN CORPUSCLES: these are ovoid shape receptors. Size is 2 mm long and 0.5 mm thick and are

visible to naked eye. These are present in deeper part of skin, fascia, ligaments, joint capsules,

periosteum, peritoneum and many other tissues. These have a thick capsule. On cut section, capsule has

got onion like appearance. The capsule consists of concentric compressed lamellae of flattened cells.

These may be modified schwan cells. In the core of the corpuscle, there are the nerve endings. These

receptors respond to touch and also high frequency vibrations. These undergo rapid adaptation.

RUFFINIs END ORGANS: these are present in the dermis of skin. Encapsulated. Inside there is a bundle

of collagen fibers along with the nerve endings. These respond to heavy pressure or stretching of the

skin. These undergo slow adaptation.

KRAUSEs END BULB: Encapsulated and are present in the skin of nipple and external genitalia. There

structure resembles that of meisners corpuscles. They respond to two point tactile stimulation.

Undergo rapid adaptation.


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PROPERTIES OF SENSORY RECEPTORS:

1.

SPECIFITY: each receptor responds to specific stimulus or specific type of energy. Stimulus which

is specific for each type of receptor is called adequate stimulus. Receptors may respond to

stimuli other than adequate if the intensity is much greater than that of adequate stimulus.

Example: light rays are the specific stimulus for photo receptors. But when we apply deeppressure over the eye balls, light halos are perceived.

2.

Specific pathways carry impulses from specific type of receptors and these pathways terminate

into a specific part of the cerebral cortex e.g. visual pathway starts from the photoreceptors and

terminate in visual cortex. This is called labeled line principle.

3.

The sensation evoked is that for which the receptor is specialized no matter how or where along

the pathway the activity is initiated. This principle, first enunciated by Mller in 1835, has been

given the rather cumbersome name of the law of specific nerve energies.For example, if the

sensory nerve from a Pacinian corpuscle in the hand is stimulated by pressure at the elbow or by

irritation from a tumor in the brachial plexus, the sensation evoked is touch. Similarly, if a fine

enough electrode could be inserted into the appropriate fibers of the dorsal columns of thespinal cord, the thalamus, or the postcentral gyrus of the cerebral cortex, the sensation

produced by stimulation would be touch. The general principle of specific nerve energies

remains one of the cornerstones of sensory physiology.

4. No matter where a particular sensory pathway is stimulated along its course to the cortex,

the conscious sensation produced is referred to the location of the receptor. This principle

is called the law of projection.Cortical stimulation experiments during neurosurgical

procedures on conscious patients illustrate this phenomenon. For example, when thecortical receiving area for impulses from the left hand is stimulated, the patient reports

sensation in the left hand, not in the head.

Phantom limb: patient complains of pain and proprioceptive sensation in the limb which is

amputated. Reason being in the stump of amputated limb, cut end of nerve fibers for the

mentioned receptors are stimulated by the pressure.

16 August, 2010

5.

Receptor Potential or Generator Potential: Whenever a stimulus is applied to a receptor a

localized change which is hypo polarization occurs in the membrane of receptor. This is because

of influx of sodium. This generator potential resembles EPSP. When receptor generator

potential reaches the threshold of excitation, then there is generation of action potential. So the

purpose of this generator potential is to bring the membrane potential to the threshold of

excitation.


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Amplitude of receptor potential depends upon the intensity of stimulus.

Rate of discharge of impulses from the receptor along the sensory nerve fibers depends upon

the amplitude of receptor potential. So we can say that greater the intensity of stimulus more

will be the amplitude of receptor potential and greater will be the rate of discharge of impulses

from the sensory fibers arising from the receptor.

Opening of sodium channels occurs due to different types of stimuli.

6.

ADAPTTAION: When a stimulus of constant intensity is applied to a receptor continuously,

discharge of impulses along a sensory nerve fibers, decreases after some time. Depending upon

the degree of adaptation we can divide the receptors into0 two types

a.

Phasic Receptors: these undergo rapid adaptation. These include the touch receptors

like meisners corpuscles, hair follicle receptors and pacinian corpuscles.

b.

Tonic Receptors: Slow and incomplete adaptation. These include pain receptors, barro

receptors, muscle spindles, receptors for the cold.

These both types of adaptations are physiologically important. Rapid adaptation enables us to

perceive new stimuli or new events, also to perceive change in the intensity of stimuli. E.g. in the

morning when we take bath and wear clothes, we feel the presence of clothes for a very short

period of time and then we dont feel it. However if an insect crawls on the skin we can feel that

also. The slow and incomplete adaptation of certain receptors is also physiologically beneficial.

Pain sensation is a warning or protective sensation. As long as a damaging stimulus is there, we

can perceive the stimulus and so we can seek treatment. Muscle spindles which undergo slow

adaptation are involved in tone and postural regulation.

Mechanism of adaptation of receptors: Suppose a pacinian corpuscle is to be adapted. Suppose

a touch stimulus is applied to a part of the pacinian corpuscle. This slight pressure causes the

indentation of the receptor membrane. Under this indentation the pressure increases. This

increased pressure will be exerted on the core where there are nerve endings and so these

nerve endings will be stimulated and there will be discharge of impulses along the nerve fibers.

After sometimes, the increased pressure under the area of indentation becomes dispersed. So

this will lead to less discharge of impulses after some time.

Other mechanism involved is accommodation. After some time there is inactivation of sodium

channels.


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7.

Intensity Discrimination: Brain is able to discriminate the intensity of stimuli. Two types of

feedbacks go to the brain, to have intensity discrimination. First feedback is the rate of

discharge of impulses from the receptors. Other feedback is the number of receptors

stimulated.

8.

Sensory Unit: A sensory unit consists of single sensory nerve fiber and all its peripheral nerve

branches. The number of peripheral branches varies. Receptive field of a sensory unit is the area

from where sensory impulses are carried by the sensory unit. Receptive fields of different

sensory units also vary in size. E.g. receptive field of sensory unit in the cornea and sclera is 5-

200 square mm.

SENSATIONS:

There are two types of sensations i.e. somatic sensations and special sensations.

SOMATIC SENSATIONS: we divide the somatic sensations into three main types i.e.

1.

Mechanoreceptive: these can be further divided into

a.

Tactile: these include

i.

Touch

ii.

Pressure


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iii.

Tickle

iv.

Itch

v.

vibration

b.

Position

2.

Pain

3.

Thermal

TACTILE SENSATIONS:

Receptors involved are: Free nerve endings, merkels disks, meisners corpuscles, pacinian corpuslces,

hair follicle receptors, ruffinis end prgans and krauses end bulbs. From the tactile receptors, 3 types of

sensory nerve fibers carry impulses i.e.

A beta nerve fibers carry impulses from the encapsulated receptors and velocity is 30-70 m/sec

A delta nerve fibers carry impulses from both encapsulated and non encapsulated receptors and velocity

is 5-30 m/sec.

C type fibers which are unmyelinated and they carry impulses from non encapsulated and the velocity of

conduction is 0.52 m/sec.

Touch Sensation: it is a light pressure when applied produces indentation of the receptor membrane or

membrane of the cell adjacent to the cell and this indentation may be upto 10 micrometer. Sensitivity to

touch varies in different parts of the body and it depends upon the number of toch receptors present.

So the part which are more sensitive carry greater number of receptors. In descending order of

sensitivity

1.

Tip of tongue2.

Finger tips

3.

Lip

4.

Palm

5.

Forhead

6.

Back of hand

7.

Neck

8.

Back

Two point tactile discrimination is the ability to distinguish two very near stimuli of touch. Minimum

distance varies in different parts of the body. This distance is least in most sensitive parts and i.e. 1-3mm and greatest in least sensitive parts i.e. back 20-50 mm.

Touch sensation can be checked by asthesio meter which consists of metallic handle to which metallic

hair can be attached. These metallic hair or brisles are of different thicknesses.

One aspect of touch sensation is stereognosis which is the ability to identify the object by touching with

hands and eyes closed. This property is highly developed in blind people.


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Pressure Sensation: it is perceived when there is deformation of deeper tissue. Almost the same

receptors carry the pressure sensation i.e. free nerve endings and ruffinis nerve endings.

Tickle and Itch: these are perceived when there is light local repetitive mechanical stimulation. Tickle

gives pleasure. Itch is annoying. Receptors involved are C Mechanoreceptors which are free nerve

endings and impulses are carried by C type of Nerve fibers. Itching is also produced by the chemicalstimuli such as histamine and bradykinin. When there is itching it leads to scratch reflex. Scratching

results into relief of itching by two ways 1) by scratching irritant is removed. 2) when there is severe

scratching, it results into pain and pain impulses suppress itch impulses by inhibition in the spinal cord.

Itching can occur in skin or certain mucus membranes like that of eye. Impulses of tickle and itch are

carried by ventral spinothalamic tract.

Light touch, two point tactile stimulation ad Phasic pressure are carried by dorsal column medial

lamniscal system.

Crude touch and pressure are carried by ventral spinothallamic tract.

Vibration: it is a thrill or buzzing sensation perceived, when a vibrating tuning fork is placed over a part

of body especially bony part. Repetitive rhythmic pressure stimuli are perceived as vibration. There are

two types of vibrations i.e. Low frequency vibrations (80 hz) which stimulates the meisners corpuscles.

High frequency vibrations (500 Hz or More) stimulates pacinican corpuslces. Impulses of vibration are

carried by dorsal column medial laminiscal system

Position Sensation: divided into two

1.

Static Position: conscious perception of different parts of body with respect to each other

2.

Kinesthetic: conscious perception of rate of movement of different parts of body.

In these sensations the receptors involved are proprioceptors. These receptors are present in deeper

parts of skin, ligaments, joint capsules, muscles and tendons. Proprioceptors include muscle spindles,

golgi tendon organs, pacinian corpuscles and ruffinis end organs. Impulses of proprioception are carried

by dorsal column medial laminiscal system.

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PAIN SENSATION:

It is a protective sensation. It is a warning for a disease or something damaging in the body.

Whenever a noxious stimulus is applied to a body, there is pain and the part is flexed and withdrawn

from the source of noxious stimulus. If the pain is in viscera, it draws attention of the individual to seek

treatment. Pain receptors are nociceptors and these are free nerve endings. These are widely

distributed in different parts of the body. Two types of nerve fibers carry pain impulses i.e.


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A Delta and C nerve fibers carry pain. A delta having velocity of conduction 5-30 m/sec and these carry

fast pain impulses. C fibers are un myelinated and these conduct slow pain impulses and velocity of

conduction is 0.5 to 2 m/sec. there are two types of pain i.e.

1.

First / Fast pain: it is sharp, short and pricking pain. It is more localized.

2.

Second/ Slow pain: it is diffuse, dull and burning pain. It occurs after the first pain and it isassociated with tissue damage.

Pain impulses are carried by lateral spinothalamic tracts. Pain occurs when there is application of

noxious stimulus which could be mechanical, chemical and thermal.

Mechanical: it can be pin prick, or cut with a sharp object, in viscera distension or increased pressure,

raised intracranial pressure.

Thermal: Extremes of temperature, freezing cold and burning hot.

Chemical: these include histamine, bradykinin, serotonin, substance p and insect bites.

Some tissues are without pain receptors. In compact pain receptors are in periosteum. In bone diseases

unless it involves periosteum, there is no pain. In the brain receptors are in meninges. So in brain tumor

there is no pain unless meninges are involved. Similarly in lungs pain receptors are in pleura. In

intestines it is in peritoneum. In liver and spleen pain receptors are only in their capsules. In the muscles

tissue itself, there are no pain receptors but in the connective tissue covering.

Pain sensation has got two aspects i.e. pain perception and reaction to pain or emotional aspect of the

pain. Reaction to the pain varies with the individuals. In severe pain there is vocalization, clenching of

hands, contraction of facial muscles, narrowing of palpebral fissure, change in blood pressure and heart

rate. There may be skin flushing or pallor. There may be violent movements of the body. Pain sensitivityvaries with different individuals and in different races. Pain receptors undergo very slow and incomplete

adaptation.

Pain can also be classified into superficial pain and deep pain.

Superficial pain: involves superficial viscera and localized.

Deep pain: also known as visceral pain involves viscera and is not so localized but diffuse in nature. It is

colicky in nature. Visceral pain is felt in the area of the viscera and is also referred to different distant

structures. E.g cardiac pain refers to the left shoulder, left arm neck. Ureteric colic radiates to testicles.

Basis of referred pain is dermatomal rule. Structure where pain originates and the structure to which the

pain radiates receive the sensory nerve supply from same segment of spinal cord.

Pain sensitivity can be tested by pinching of skin. Pin prick by measured pressure. Application of heat

above 45 degree centigrade.


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THERMAL SENSATION: Human beings can perceive different grades of thermal sensation.

In thermal sensation three types of receptors are involved i.e. cold receptors, warmth receptors and

pain receptors. Pain receptors are stimulated at the extremes of temperature i.e. freezing cold and

burning hot. Thermal receptors are located beneath the skin and mucus membrane. These thermal

receptors are in the form of aggregations or clusters called spots i.e. cold spots and warm spots. Cold

spots are much more. There is 4-10 times greater number of cold spots than warmth spots. From the

cold receptors impulses are carried by A delta fibers but some also from C type. From the warmth

receptors impulses are carried by C type of fibers. Temperature receptors are non encapsulated.

Discharge of impulses from different receptors at different temperatures is mentioned in above

diagram.

Thermoreceptors undergo adaptation to some degree in neutral or indifferent zone of 29-36 degree

centigrade and above and below this there is not so much adaptation.

Mucus membrane of the mouth is relatively insensitive to high temperature. We can take hot drinks like

tea and coffee. Exposed areas of the body are relatively less sensitive to temperature changes as

compared to covered areas. The covered areas can detect a temperature change of 0.2 degree

centigrade. While the exposed areas can detect change of 0.5 to 1 degree centigrade.

Thermoreceptors can respond to change in the temperature and also respond to changing temperature.

Response is much more to changing temperature.

Thermoreceptors are stimulated by change in the rate of metabolism.

ASCENDING TRACTS OR SENSORY TRACTS:

These tracts are divided into different groups i.e.


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DORSAL COLUMN MEDIAL LAMINISCAL SYSTEM

ANTEROLATERAL SYSTEM / SPINOTHALAMIC TRACT

SPINOCEREBELLAR TRACTS

MISCELLANEOUS

SPINOTACTAL

SPINO RETICULAR

SPINO OLIVARY

VISCERAL SENSORY TRACTS.

DORSAL COLUMN MEDIAL LAMINISCAL SYSTEM: This dorsal column has got certain features

1.

It consists of rapidly conducting myelinated nerve fibers and velocity of conduction may be upto

110 m/sec

2.

Sensations carried by this tract requires perception of fine grades of intensity like sensation

carried are light touch, tactile discrimination.

3.

Sensations carried by this tract require accurate and precise localization like in two point tactile

discrimination.

Sensation carried by this tract include fine touch, tactile discrimination Phasic pressure, vibration,

proprioception.

First order nerve fibers carrying impulses enter the spinal cord through the dorsal nerve root. Without

synapse they enter the posterior white column to ascend as dorsal column medial leminiscal system. As

the tract ascends, fibers from the upper part of the body are added laterally. From the 6ththoracic

vertebra a septum appears in the tract to divide it into two fasciculi i.e. medial fasciuls gracilus and

lateral one is fasciulus cuneatus. These two fascicule ascend to enter the medulla oblongata where they

synapse with the second order neuron in the nucleus gracillis and nucleus cuneatus. Second order nerve

fibers arise from these two nuclei and these are called internal arcuate fibers. Internal arcuate fibers

cross over to the opposite side to form medial laminiscus. This crossing over in the medulla oblongata is

called sensory decussation in the lower part. Medial laminiscus ascends through the brain stem. In the

brain stem medial laminiscus is joined by trigeminal pathway carrying same sensations and then the

medial laminiscus synapse in the thalamus with the VPL. the fibers coming from trigeminal pathway

synapse in the VPM.


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From the VPL third order nerve fibers arise which pass through the internal capsule to terminate in the

somatosensory area in the poscentral gyrus.

Some fibers arising from the cuneate nucleus go to the cerebellum through the internal cerebellar

peduncle. These are also called cuneocerebellar fibers. These fibers carry proprioceptive impulses to the

cerebellum.

If the lesion of this tract is in spinal cord, effect will be ipsilateral and if the lesion is above upper part of

medulla then the effect will be on the contralateral side.

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ANTEROLATERAL SYSTEM / SPINOTHALAMIC TRACT

There are 3 features of this tract

1.

Small myelinated nerve fibers and relatively slow conducting. The velocity is 40 m/sec.

2.

Sensations carried by these tracts do not require perception of fine grades of intensity. These

sensations are crude.

3.

Sensations carried by these tracts do not require spatial orientation and accurate localization.

There are two types of spinothalamic tract i.e.

1.

anterior or ventral spinothalamic tract: these carry sensations of crude touch, pressure, tickle

itch and sexual sensations. First order fibers enter the spinal cord through the dorsal nerve root

and these synapse with the neurons in the lamina V and VI. From these laminae 2ndorder

neurons arise and from here they cross over to the opposite side through the anterior gre and

white commisure and from the opposite side ventral spinothalamic tract forms. This tract forms

in the lower part of the body and as the fibers ascend the fibers from the upper part of thebody are aggregated medially to the lower fibers. Tract enters the medulla and reaches spinal

laminiscus which is made up of three tracts i.e ventral spinthalamic tract, lateral spinothalam ic

and spinotactal tract. From here fibers ascend through the pons, midbrain and then it synapse in

the neurons in the ventrobasal complex of thalamus i.e. VPL. from here 3rdorder nerve fibers

arise and from here fibers pass through the posterior limb of internal capsule to terminate into

the post central gyrus of the somatosensory area.

2.

Lateral Spinothalamic tract: this tract carries impulses of pain and temperature. First order

nerve fibers enter the spinal cord through the dorsal nerve root and these synapse with the

neurons in the lamina I, II and III including substantia gelatinosa. From these neurons 2ndorder

nerve fibers arise which cross over obliquely to the opposite side through anterior grey andwhite commisure. After crossing over they form the lateral column of the tract on the opposite

side. As the tract ascends fibers from the upper part of the body are added on the anteromedial

aspect. Fibers from the lower part are on the posterolateral aspect of the tract. The tract

ascends through the spinal cord to enter the medulla oblongata where it joins the spinal

laminiscus which ascends through the brain stem. Temperature and fast pain fibers synapse

with the neurons in the ventro basal complex of thalamic nuclei. From here 3rdorder nerve

fibers arise which pass through the posterior limb of internal capsule to terminate into the

postcentral gyrus in the somatosensory area. Most of the Slow pain fibers do not go to the

ventrobasal complex of thalamic nuclei. They go to other areas including reticular formation,

tactum of midbrain and periaqueductal grey matter. Some fibers also go to the intra laminar and

midline nuclei of the thalamus. From here 3rdorder neurons arise which go to the sensory

cortex. In some books pathway of fast pain fibers is discussed as neospinothalamic tract and

that of slow pain fibers is discussed as paleospinothalamic tract. In the neospinothalamic tract

the neurotransmitter between the 1storder A delta fibers and 2ndorder neuron in lamina I, II

and III is Glutamate. In paleospinothalamic tract the neurotransmitter between the 1storder C

fibers and 2ndorder neurons is substance P.


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The effect of lesion of these tracts is on the contralateral side.


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SPINOCEREBELLAR TRACTS:

Spinocerebellar tracts consists of fast fibers having velocity of conduction 120m/sec.

These are of two types

1.

Posterior spinocerebllear tracts.: first order neurons enter the spinal cord through the dorsal

nerve root and synapse with the second order neurons in the clarkes nucleus or nucleus

dorsalis present in the segment T1 to L3 and L4 at the base of posterior grey column. 2ndorder

nerve fibers arise which without crossing over enter the lateral white column to form the tract.

The tract ascends in the spinal cord to enter the medulla oblongata. And from here it passes

through the inferior cerebellar peduncle to go the cerebellum where they terminate in the

vermis and intermediate zone on the same side. Fibers from the lumbar and sacral segment


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ascend to join the clarkes column in the L3 and L4. So in this tract fibers come from trunk and

lower limbs, not from the upper limb. This tract carry impulses from the proprioceptors about

the change in the length, muscle tension, position and movements. These impulses are used by

cerebellum to coordinate movements of lower limbs and to also maintain the posture.

2.

Ventral/Anterior Spinocerebellar tract: 1storder nerve fibers enter the spinal cord through the

dorsal nerve root and synapse with the 2ndorder neurons located near the clarkes column.

From these neurons second order nerve fibers arise which cross over to the opposite side to

enter the lateral white column on the opposite side. The tract ascends in the spinal cord to enter

the medulla oblongata and passes through the pons and in the midbrain it passes through the

superior cerebellar peduncle to enter the cerebellum. In the cerebellum the fibers cross back to

terminate into the vermis and intermediate zones of both sides. Here Some fibers may remain

uncrossed but mostly they cross in cerebellum. This tract also carries impulses from the

proprioceptors to the cerebellum and are used for coordination of movements. This tract

provides information to the cerebellum about the motor impulses which have reached the

ventral horn of spinal cord along the cortico spinal and rubrospinal tracts.


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MISCELLANEOUS GROUP OF TRACTS:

SPINOTACTAL TRACTS: First order nerve fibers enter the spinal cord through the dorsal nerve root, these

synapse with 2ndorder neurons in the posterior grey column of the spinal cord. 2ndorder nerve fibers

arise which cross over to the opposite side to enter the lateral white column of the opposite side. The

tract ascends through the spinal cord to enter the medulla oblongata. Here it joins the spinal laminiscus.

The tract fibers terminate into the superior colliculus in the tactum of the midbrain. This tract provides

afferent pathway for spinovisual reflexes which are involved in the control of movement of head andeyes towards the source of stimulation. Superior colliculus is also involved in the reflex pathway upon

stimulation by light.

SPINORETICULAR TRACT: 1storder nerve fibers enter the spinal cord through the dorsal nerve root.

These synapse with the second order neuron in the posterior grey column in spinal cord. 2ndorder nerve

fibers arise which without crossing over ascends and form the tract on the same side. It ascends to

eneter the medulla oblongata and terminate into the reticular formation in the midbrain, pons and


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medulla. This tract provides afferent impulses to the reticular formation and is involved in the control of

level of consciousness. The sensory input to the reticular formation maintains excitation in the reticular

activating system.

SPINOOLIVARY TRACT: 1storder nerve fibers enter the spinal cord through the dorsal nerve root and

synapse with the 2nd

order neurons in the posterior grey column from where 2nd

order nerve fibers arise.Most of the fibers cross over to the opposite side to enter the lateral white column to form this tract.

The tract ascends through the spinal cord to enter the medulla oblongata. Here the tract fibers synapse

with the neurons in the inferior olivary nucleus. From this nucleus 3rdorder nerve fibers arise which

cross over to the opposite side to enter the inferior cerebellar peduncle to go to the cerebellum. This

tract carries impulses from proprioceptors to the cerebellum.

SENSORY VISCERAL TRACTS: in the viscera mostly the receptors present are stretch receptors and pain

receptors. Impulses from the viscera of the thorax and abdomen enter the spinal cord through dorsal

nerve root and then these fibers join the spinothalamic tract to be carried to the sensory cortex.

Impulses about fullness of urinarty bladder before micturition and impulses about the fullness of rectum

before defecation are carried by dorsal column medial laminiscal system.

Dorsal column carries conscious proprioception and rest of the tracts carry unconcsiouc propriocetion

26 August, 2010

ANALGESIA SYSTEM

It is pain control system in CNS. Pain sensitivity in different individuals vary which is partly due to this

system.

Components of Analgesia system are:

1.

Periventricular nuclei surrounding the third ventricle in the hypothalamus

2.

Periaqueductal grey matter present in the midbrain and upper pons and is the grey matter

surrounding the aqueduct of sylvius

3.

Raphe magnus nucleus in the lower pons and upper medulla oblongata. It is a thin midlinenucleus.

4.

Pain inhibitory complex in the dorsal horn of the spinal cord. Incoming pain fibers i.e. A delta

and C fibers synapse here.

Some authors also include a group of nuclei in pons and medulla known a reticularis para giganto

cellularis.


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Neurotransmitters in analgesia system are

1.

Enkephalin is the main neurotransmitter in the analgesia nucleus. Both fibers of 1 and 2 secrete

enkaphalin.

2.

Fibers from 3 secrete serotonin in the dorsal horn.

3.

Enkephalin is secreted from 4.

When any component of analgesia system is stimulated it leads to suppression of pain sensation.

Analgesia system when stimulated suppresses both fast and slow pain. This system also inhibits the

reflexes initiated by pain like fexor withdrawal reflex.

In the brain there are receptors with which morphine bind to produce analgesia. And these receptors

are called opiate receptors. Enkephalin binds with opiate receptors to produce analgesia. There are

different types of enkephalin like endorphins, dynorphins, met enkephelin and leuenkephelin.

How the analgesia system get stimulated, it is not clear. In acupuncture there is evidence of release of

endorphins.

The enkephelin secreted in the dorsal horn can cause inhibition of the pain carrying fibers in the spinal

cord through pre synaptic inhibition.

CEREBRAL CORTEX:

It is composed of grey matter and forms complete covering of the cerebral hemispheres. It is about 2 -5mm thick. It is thickest at the crest of the gyrus and thinnest at the bottom of sulcus. The total surface

area of the cerebral cortex is about one quarter meter.

Cerebral cortex has got 6 layers

1.

Outermost is molecular layer composed of dense network of nerve fibers and these nerve fibers

are dendrites and axons which come from the deeper layers.

2.

External granular layer which is composed of pyramidal cells and stellate cells and dendrites

from these go to the molecular layer and axons go to the deeper layer.

3.

External pyramidal layer which consists of small pyramidal cells. There dendrites pass to the

molecular layer and axons to the white matter.

4.

Internal granular layer which contain stellate cells, small pyramidal cells. In this layer there is a

horizontal band of nerve fibers which is called the band of Baillarger.

5.

Internal pyramidal layer also called the ganglionic cell layer which consists of large and medium

sized pyramidal cells. There axons pass into the white matter. In the precentral gyrus this layer

contains very large or giant pyramidal cells called Betz cells which give rise to nerve fibers which

constitute 3 percent of the total nerve fibers in the corticospinal tracts. And these terminate


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directly on to the alpha motor neurons in the ventral horn of the spinal cord. In this layer there

is also inner band of Baillarger.

6.

Multiform or polymorphic layer which conisits of cells of different morphology.

Incoming sensory fibers to the cerebral cortex first go to the layer number 4. From here the spread can

be to the outer layers or to the inner layers.

In the somatic sensory area, neurons are arranged in the form of vertical columns and each vertical

column has a diameter of 0.3 to 0.5 mm and each column contains about 10,000 neurons and each

vertical column of sensory neurons is concerned with one modality of sensation.

I f all the six layers are distinct and developed then the cerebral cortex is called homotypical. If all the six

layers are not distinct and not well developed then it is called heterotypical cerebral cortex which is

again of two types i.e. granular and agranular.

In granular type of heterotypical cerebral cortex the granular layers i.e. 2 and 4 are well developed while

the pyramidal layers i.e. 3 and 5 are not well developed. This type of cortex is present in postcentralgyrus, superior temporal gyrus and hippocampal gyrus. Most of these areas are sensory areas.

In agranualr type pyramidal layers i.e. 3 and 5 are well developed and distinct while the granular layers

are not well developed. This type of cortex is present in the precentral gyrus and other parts of frontal

lobe which are mainly motor areas.

FUNCTIONAL AREAS IN THE CEREBRAL CORTEX:

The functional areas in the cerebral cortex have been numbered after broadman. These areas can be

divided into two types i.e. sensory and motor.

SENSORY AREAS:

Methods to study or invesitigate sensory areas:

1.

Ablation method: in this method a part of cerebral cortex is damaged and the sensory deficit

produced is studied,

2.

Evoked potential method: stimuli are applied on different parts of the body and potentials from

cerebral cortex are recorded.

3.

Clinicopathological study: the patient is having sensory deficit and after death of patient

different parts of cerebral cortex are studied.

Different sensory areas in the cerebral cortex include

1.

Somatic sensory area S1: it is located in the post central gyrus on the lateral surface of cerebral

hemisphere. It also extends to the medial surface into the paracentral lobule. It is brodmans

area 1,2 and 3. In this area there is contralateral representation of different parts of the body

and body is represented upside down. By point to point sensory stimulation of different parts of


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body a figure of body can be formed in this area and this is called sensory homunculus or

sensory figuring in the area S1. Different parts of the body in the sensory homunculus are not

represented according to size but according to their sensory function. Parts of the body having

greater number of sensory receptors have greater representation in sensory homunculus like

jaw, lips tongue and fingers. Thorax thigh and limbs occupy smaller area. The genital organs are

on the medial surface of the cerebral hemisphere. Face has got some bilateral representation.

The S1 area recives fibers from the VPL and VPM nucleus of thalamus.

Functions: this area is involved in the perception of all the somatic sensations from the contra

lateral side of the body. This area is also concerned with the perception of fine grades of

intensity. It is also involved in stereognosis. When there is lesion of this area, the sensation of

fine touch, propriocepteion, tactile discrimination and vibration is mostly affected while pain is

least affected. Thalamus is subcortical centre for pain.

WERNICKES AREA: Also called general interpretive area, sensory speech area located in the

posterior part of the superior temporal gyrus behind the auditory cortex. It is brodmans area

22. This area is highly developed in the dominant hemisphere so well developed in left

hemisphere. This area receives impulses or signals from the visual association area and also

from the auditory area. In this area visual and auditory informations are completely understood.

Thoughts are formulated. What to be expressed or answered is decided. Words are chosen.

Words are arranged into sentences. Signals are sent to the brocas area i.e. motor speech area

along the arcuate fasciculus. This area is concerned with high intellectual functions. This is also

called sensory speech area. When this area is damaged there is sensory aphaisa or fluent

aphasia.

VISUAL AREAS:

1.

Primary visual area: it is located around the calcarine fissure. It is brodmans area 17. It

receives impulses from the visual pathway. Secondary visual area or visual association area

which is located anterior and superior to primary visual area.

2.

ANGULAR GYRUS: It is located between wenickes area and visual area. It is brodmans area

39. There is involved in language, mathematics and cognition.

Primary auditory area: located in superior temporal gyrus. Brodmans area 41. It receives impulses from

auditory pathway and there is interpretation of auditory impulses.

TASTE AREA: located at the inferior end of post central gyrus, superior wall of brodmans area 43.

OLFACTORY AREA. Divided into two parts medial and lateral. Medial consints of septal nusclei anterior

to hypottahlams. Pyriform cortex, prepyriform cortex and cortical portion of amygdila.


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MOTOR AREAS OF CEREBRAL CORTEX:

METHODS OF INVESTIGATION OF MOTOR AREAS:

1.

ABLATION METHIOD: part of cerebral cortex is damaged and loss of motor activity is noted.

2.

POINT TO POINT electrical stimulation of motor areas is carried out and movement

produced is noted.

3.

CLINICOPATHOLOGICAL STUDY: the patient having paralysis when he dies, his brain is

inspected for damage.

Motor area can be divided into four types:

1.

PRIMARY MOTOR AREA: Located in the precentral gyrus on the lateral surface of cerebral

hemisphere. It also extends to the medial surface into the paracentral lobule. It is brodmans

area 4. It is composed of agranular type of cerebral cortex. In this area there is also

topographical representation of various parts of the body and there is motor homunculus in

the primary motor area and there is contralateral and upside down representation. Motor

homunculus is the mirror image of sensory homunculus. The area of representation in the

motor homunculus is according to functional importance and not on anatomical size. Parts

of the body involved in fine skilled movements occupy the much larger portion of

homunculus. Generally there is contralateral representation but Face has some bilateral

representation. Blood flow to different parts of the motor area depends upon the activity of

the body part.

Connections of primary motor area:

i.

It receives afferent from the premotor area, supplementary area and primary

motor area of opposite side.

ii.

Afferents from the somatosensory area, visual area and also from auditory area.

iii.

Afferents from basal ganglia, thalamus and cerebellum also red nucleus.

iv.

Efferents to the primary motor area of opposite side.

v.

Efferents to the spinal cord as corticospinal or pyramidal tracts.

vi.

Cortico bulbar fibers. Cortico striate fibers which go to basal ganglia. Cortico

thalamic fibers going to thalamus. Corticoponto cerebellar fibers going topontine nuclei and then to cerebellum

Electrical stimulation of this area leads to coordinated movements involving contralateral

side of the body. Movements are initiated in the primary motor area.

2.

PREMOTOR AREA: it is located anterior to the primary motor area on the lateral surface of

hemisphere. It is brodmans area 6. It is broad at the top and narrow below. It occupies


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superior, m idle and inferior frontal gyri. There is also topographical representation of

various body parts and this resembles that in the primary motor area.

When this area is stimulated there is movement involving groups of muscles. Gross rotation

of head eyes and trunk to the opposite side. This area stores programs of motor activity in

the light of the past experience or past information. It programmes the activity of primarymotor area. It is connected to the primary motor area directly or through basal gangli and

then through thalamic nuclei to the primary motor area.

3.

SUPPLEMENTARY MOTOR AREA: it is located anterior and superior to the premotor area on

the medial surface of the cerebral hemispheres. It occupies the medial frontal gyrus. It

extends upto the cingulate sulcus on the medial surface of hemisphere. In this area different

parts of body are also topographically represented and the pattern is that the face is

anterior, feet are posterior and trunk is towards the cingulated sulcus. This area has

connections with the primary motor area and also with both sides of the body. When this

area is stimulated, there are movements involving both sides of the body. This area is

involved in the control of positional or attitudinal movements.

4.

SPECIAL MOTOR AREAS:

a.

Motor speech area or brocas area: located in inferior frontal gyrus, anterior to face

representation in the primary motor area. It is brodmans area 44. It receives signals

from the Wernickes area along the arcuate fasciullus. This area forms detailed

motor pattern for the contraction of muscles involved in phonation and articulation.

Signals are sent to the primary motor area to initiate muscle contraction for

phonation and articulation. When there is damage to the brocas area there is

motor aphasia or non fluent aphasia. This patient has difficulty in uttering words.Speech is limited to only few words.

b.

Frontal Eye Field: it is located anterior to the premotor area in the middle frontal

gyrus. It is brodmans area 8. When this area is electrically stimulated, there is

conjugate deviation of eyes to the opposite side. This area also controls eyelid

movements or blinking. It also controls voluntary fixation. We are able to fixate eyes

on different objects. When this area is damaged there is inability to fixate eyes from

one object to the other.

c.

Area for head rotation: it controls the head rotation, number is not given but it is

present above frontal eyefield.

d.

Area for hand skills which controls the hand skills. When there is damage to this

area, it leads to motor apraxia. In motor apraxia movements become incoordinate

and non purposeful.


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DESCENDING TRACTS:

CORTICOSPINAL TRACT: Originates from cerebral cortex. Contains about 1 million neurons and contains

only about 3% of betz cells. 30 % fibers orginates from the primary motor area and another 30% from

premotor and supplementary motor areas. 40% of fibers originate from somatic sensory area. After

origin from cerebral cortex, these fibers pass through corona radiate. Then they converge towards the

genu and anterior 2/3rdof the posterior limb of internal capsule. Fibers for the upper parts of the body

are in genu and fibers for the lower part are lateral. After this the fibers enter the midbrain. Here the

fibers occupy the middle 3/5thof the cerebral peduncle or crus cerebri. Here fibers for the upper parts

are medial and for lower parts are lateral. From the midbrain tract fibers enter the pons. In the pons,

tract is broken into bundles due to transverse ponto cerebellar fibers. After this the tract enters themedulla oblongata. In the upper part the bundle of fibers again group together to form pyramidal

shaped swelling on the anterior aspect of medulla oblongata. This name pyramidal is given because of

these pyramids formed in the upper part of medulla. In the lower part of the medulla oblongata, 80% of

fibers cross over to the opposite side to form motor decussation. After crossing over the crossed fibers

enter the lateral white column to form lateral corticospinal tract. Remaining uncrossed 20 % enter the

anterior white column to form anterior cortico spinal tracts. Fibers of the anterior cortico spinal tract

cross over in the spinal cord to terminate on to the motor neurons in the cervical and upper thoracic

segments of spinal cord. These fibers are thought to be from the supplementary motor areas. Fibers of

the corticospinal tract, when they terminate in the spinal cord, they first synapse with interneurons and

which in turn synapse with the motor neurons in the ventral horn of the spinal cord. Only fibers fromfrom the betz cells have direct termination with the motor neurons of spinal cord. Fibers of the lateral

corticospinal tract terminate onto the motor neurons in the ventral horn of spinal cord at various levels.

The termination is indirect. If we take the tract overall, 45% fibers of this tract terminate into in the

cervical segments, 20 % into thoracic segments and remaining 35% terminate into the lumbar segments

of spinal cord. There are certain collateral branches which go to different parts of brain i.e. caudate

nucleus, lentiform nucleus, red nucleus, olivary nuclei and also reticular formation.

FUNCTIONS OF CORTICOSPINAL TRACT:

It controls fine skilled movements of limbs particularly the distal parts of limbs.

CORTICO BULBUR TRACT: it has got same pathway as the corticospinal tract but the tract fibers cross

over in the brain stem to terminate on to the motor neurons in the nuclei of cranial nerves.

EXTRACORTICOSPINAL OR EXTRAPYRAMIDAL TRACTS:


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RUBROSPINAL TRACT: it originates from the red nucleus which is present in the tegmentum of mid

brain. Then these cross over to the opposite side and descend through the pons, medulla oblongata to

enter the lateral white column of spinal cord and it terminates onto the motor neurons in the ventral

horn of spinal cord at various level of spinal cord. Termaintion is indirect through interneurons. Red

nucleus has connections with cerebral cortex and cerebellum. Rubrospinal tract forms an alternate or

accessory pathway through which cerebral cortex and cerebellum can control the motor activity.

Rubrospinal tract is fascillitatory for flexers and inhibitory for extensors or antigravity muscles.

TACTOSPINAL TRACT:

It originates from the superior colliculus in the tactum of mid brain. Then it descends without crossing

over, descend through the pons and medulla oblongata to reach spinal cord where it terminates on to

the motor neurons in the ventral horn in the upper cervical segments of the spinal cord. In the brain

stem, this tract is near the medial longitudinal bundle and in the spinal cord it is near the anterior

column. This tract is involved in the control of reflex postural movement of head and neck in response to

visual stimulation. Here from superior colliculus impulses also go to the extraocular muscles to control

the movement of eyes.

RETICULOSPINAL TRACT: it arises from reticular formation. Reticular formation consists of groups of

scattered neurons along with nerve fibers. It is present in midbrain, pons and medulla. Superiorly

reticular formation is connected to cerebral cortex and inferiorly to the spinal cord. One component of

reticular formation is called reticular activating system.

Two components of reticulospinal tract

1.

Pontine reticulo spinal tract: arise from the reticular formation of pons. Most of the fibers

remain uncrossed. Tract enters the anterior white column of spinal cord. Then it terminates

onto the motor neurons in the ventral horn at various levels. This tract is fascillitatry to the

extensors or the antigravity muscles. It controls the activity of both alpha and gamma motor

neurons. In the control of muscle tone it plays an important role.

2.

Medullary reticulospinal tract: it originates from the reticular formation of medulla oblongata.

Most of the fibers cross over to the opposite side and these descend to enter the lateral white

column of spinal cord. This tract is inhibitory to the extensors.

VESTIBULOSPINAL TRACT: vestibular nuclei are present in the floor of 4trh ventricle in the lower ponsand upper medulla oblongata. Vestibular nuclei receive fibers from the inner ear and also cerebellum.

There are two components of this tract. The major component is lateral vestibulospinal tract and minor

component is medial vestibulospinal tract.

LATERAL PART: remains uncrossed and terminate on the motor neurons of spinal cord. Facilliatatory to

the extensors of the body.


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Medial part: it arises from the medial vestibular nucleus. This nucleus, receives fibers mainly from the

semicircular canals. After the origin, the tract fibers remain uncrossed and they terminate onto the

motor neurons in the cervical segments of the spinal cord. This tract is also facilitatory to the extensors.

OLIVOSPINAL TRACT: it originates from the inferior olivary nucleus in the medulla oblongata. After the

origin, it crosses over to the opposite side and descends into the lateral white column of spinal cord.This tract is also involved in the control of motor activity. Inferior olivary nuclei receive fibers from the

motor cortex, corpus striatum, red nucleus and spinal cord.

DESCENDING AUTONOMIC PATHWAYS: These pathway arise from the higher centres which control the

autonomic nervous system and these include, cerebral cortex, hypothalamus, amygdala and reticular

formation. These fibers cross over to the opposite side and join the reticulospinal tracts to enter the

spinal cord in the lateral white column. The tract fibers terminate onto the preganglionic sympathetic

neurons in the spinal cord segments from T1 to L2 and also to the preganglionic parasympathetic

neurons in the sacral segments mainly S2-4.

02 September, 2010

Motor system consists of two types of neurons

1.

Lower motor neurons: neurons which innerveate skeletal muscles. These form final common

pathway to skeletal muscles. If any impulse has to pass to the sk. Muscle, it has to pass through

LMN. These are located in the anterior grey column of spinal cord. These are alpha motor

neurons. The motor neurons in the nuclei of cranial nerves in the brain stem are also LMN.

2.

Upper Motor Neurons: these are located above the level of lower motor neurons. These are

supraspinal neurons. These are located in the cerebral cortex and brain stem. These neurons

along with their pathways are the UMN. These include the corticospinal and the

extracorticospinal system which control the motor activity through separate pathways.

LMN LESIONS:

Causes: Injury or trauma to the alpha motor neurons or motor fibers in the ventral root of spinal cord.

Infection as in polio mellitus. Vascular like thrombosis, hemerrohage. Degeneration like in motor neuron

disease. Tumor involving the LMN.

Features:

1.

Only few muscles are affected.

2.

In the affected part there is flaccid paralysis. Loss of voluntary movements along with loss of

muscle tone.

3.

Loss of all superficial and deep reflexes.


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4.

Muscle tone is because of myotatic reflex so there is Atonia or Hypotonia.

5.

Muscle atrophy due to loss of trophic action of motor nerves supplying the muscles.

6.

Intact motor nerve supply maintains the growth and structure of the muscle.

7.

There are fasciulations and fibrillations. Fsiculations are spontaneous contractions of group of

muscles fibers and fibrillations are contractions of individual muscle fibers. These appear when

there is slow degeneration of LMN.

8.

Contractures of paralysed muscles (Shortening)

9.

Reaction to degeneration.

10.

Response to faradic electrical stimulation stops after 7 days while the response to galvanic

stimulation stops after ten days of lower motor neuron lesion. Normal muscle can respond to

both type of stimulation. Faradic stimulation is interrupted current stimulation and galvanic is

direct current stimulation

11.

Babinskis sign is not present in LMN.

UPPER MOTOR NEURON LESION:

Causes: Trauma, vascular lesions like thrombosis hemerrhoge in brain, brain tumor or abscess.

In the UMN there are two types of tracts i.e. Cortico Spinal and Extra cortico spinal tract.

Features of Cortico Spinal tract lesions:

1.

Loss of fine skilled voluntary movements especially of the distil parts of limbs.

2.

Babinskis sign is Positive i.e. abnormal plantar reflex. It is also present in other conditions like in

infants (because of incomplete myelination of tract), during sleep and intoxication.

3.

Loss of superficial abdominal reflex. T7T12. This reflex is lost due to facillitatry effect of the

corticospinal tract on the interneurons involved in reflex arc.

4.

Loss of cremesteric reflex. L1 Again the same reason.

Features due to lesions of Extracortico Spinal Tract:

1.

There is spastic paralysis. Loss of voluntary movements along with hypertonia.

2.

There is involvement of large number of muscles i.e. there may be hemiplegia.

3.

Deep reflexes become exaggerated or brisk.


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4.

There may be ankle or knee clonus.

5.

Clasp knife rigidity.

6.

There are no fasciulations and no fibrillations.

7.

There is slight muscle atrophy and this is due to disuse.

In clinical practice we always find the mixture of lesions of Corticospinal and Extracorticospinal tracts.

Rigidity involves both agonists and antagonists

LESIONS of SPINAL CORD:

1.

Lesion of Posterior Nerve root:

a.

if only one posterior nerve root is damaged, there is no significant sensory loss, because

of overlapping of adjacent dermatomes. To produce significant sensory loss, atleast

three posterior nerve roots should be damaged.

b.

Loss of all superficial and deep reflexes as reflex arc is not completed.

c.

Loss of muscle tone in the affected part.

d.

Movement of affected part is also difficult because of loss of proprioceptive input to the

brain.

2.

LESION OF Ventral root: it contains both somatic motor nerve fibers and autonomic fibers.

a.

There is flaccid paralysis.

b.

All the superficial and deep reflexes are lost.

c.

If the lesion of ventral root is in segment from T1to L2 then there is inhibition of

sympathetic outflow and this leads to vasodilation, fall in peripheral resistance and

blood pressure and also there is loss of sweating.

3.

Hemi Section of Spinal Cord: one half is damaged and other is intact. Cause is fracture

dislocation of vertebral column due to stab wound, bullet wound or expanding tumor.

a.

First there is stage of spinal shock, then there is stage of reflex activity.

b.

At the Level of Hemisection:

i.

There is Ipsilateral LMN type of paralysis. No motor loss on the opposite side.


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ii.

Complete anesthesia (Complete loss of Sensations) at the level of hemisection

which is ipsilateral. On the contralateral side there is no significant sensory loss

c.

Below the Level of Hemisection:

i.

There is Ipsilateral UMN type of Paralyis. No motor loss on contralateral side.

ii.

Ipsilateral loss of sensations carried by Dorsal column medial Laminiscal System

iii.

Loss of sensations carried by spinothalamic tract on the contraletral side i.e.

crude touch, tickle, itch, pressure, pain and temperature. Sensory loss on the

contralateral side 1 to 2 segments below the section because of oblique

crossing.

d.

Above the Level of Hemisection:

i.

Hyperasthesia because of stimulation of cut section of nerves.

If the hemisection involves the segments T1 to L2, then there is disturbance of Sympathetic outflow with

fall in the peripheral resistance and blood pressure and loss of sweating Ipsilateral.

Brown Sequard Syndrome: Ipsilaterally there is predominant motor loss and contralaterally there is

predominantly sensory loss. Features are same as that of the Hemisection.

COMPLETE TRANSECTION OF SPINAL CORD:

Cause is fracture dislocation of vertebral column, stab wound, bullet wound or expanding tumor.

If transaction is above C3 level, life is not possible without artificial respiration.

If transaction is in the lower cervical segments then there is quadriplegia.

If transaction is in lower thoracic level, then there is paraplegia.

The effects of spinal cord transaction can be discussed in three stages

1.

Stage of Spinal Shock or Stage of Flacidity: Immediately after transaction, there is complete

flaccid paralysis, loss of all sensations below the level of transaction. All superficial and deep

refelexes are lost. Loss of muscle tone in smooth muscle and skeletal muscle. Even the sphinctertone is lost. There is urinary and fecal incontinence. If the transaction is in the segments of T1 to

L2 then there is disturbance in sympathetic outflow i.e. loss of peripheral resistance and fall of

BP. Legs become cold and blue. The cause of this stage is loss of tonic faciliatatry impulses from

higher centers to neurons. This stage remains for 2 to3 weeks.

2.

Stage of Reflex Activity: this stage starts after 2 to 3 weeks. Muscle tone begins to return 1STin

the smooth muscle. Sphincters regain their tone which leads to urinary and fecal retention. Tone


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also returns to the smooth muscles of the blood vessels which improves the blood pressure. The

spinal cord sympathetic neurons learn to function independently without facilitatory impulses

from the higher centers. This improves the blood pressure. Tone also begins to return in the

skeletal muscle. First it returns in the flexors of the body. They become less flabby. This return is

reflex in nature but tone as not as much as normal because the myotatic reflex is weak in the

absence of facilitatory impulses from higher centers. As tone returns first in the flexors so legs

become flexed. So there is paraplegia in flexion. Muscles start contraction during reflex activity

but not for voluntary movements. Spontaneous muscle contractions appear especially in the

flexors. Flexor reflex appears. Initially the response is weak but gradually it increases. A Mass

reflex or Mass response appears which is when skin over the legs or anterior abdominal wall is

scratched, there is contraction of flexors of both legs and anterior abdominal wall muscles along

with evacuation of urine from bladder even if it is less in amount which may be partly due to

contraction of muscles of anterior abdominal wall. There is profuse sweating below the level of

transaction especially if the segments involved are below T1,2. In males erection can can occur

when there is mechanical stimulation of skin of glans penis or inner side of thigh. After 36

months of recovery, below the transection there is upper motor neuron type of lesion.

Sensations dont recover at all. There is automatic bladder and there is automatic defecation

reflex. The patients learn how to elicit defecation and micturition by scratching the skin of

perianal area or inner side of thigh. This stage of reflex activity continues if the patient has good

nursing care.

3.

Failure of Reflex Activity: If the patient dosent have good nursing care then the patient goes in

to this stage. This stage starts if there is general infection or toxemia. Response during reflexes

decreases, wasting of muscles. Bed sores appear.

INCOMPLETE TRANSECTION OF SPIN AL CORD: there is severe damage to the spinal cord but notcomplete transaction. In it first there is spinal shock and then the stage of reflex activity. Stage of reflex

activity has certain peculiar features.

1.

Muscle tone first appears in extensors leading to paraplegia in extension. Reason is in

incomplete transaction, reticulospinal and vestibulospinal tract which are faciliatatory to

extensors escape the damage.

2.

Extensor thrust reflex: the patient is on his back and his one leg is flexed at hip and knee and

foot is resting on the bed. The examiner with the palm of hand exert upward pressure on the

sole of foot of flexed leg. This results into contraction of triceps and posterior calf muscles,

resulting into extension of legs.

3.

When flexor reflex is elicited, there is more strong crossed extensor reflex.

4.

Phillipsons reflex: when one leg of the patient is flexed passively, the other leg becomes

extended and after some time, the flexed leg become extended and extended leg becomes

flexed. So there are steppage movements. This means in this type of transection locmotory

movements can occur but walking is not possible.


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5.

After the recovery in these patients there will be UMN type of paralysis below the level of

transection.

6.

At the level of transection, there will be LMN type of paralysis.

7.

Sensations dont recover.

8.

There are disturbances of micturition and defecation.

LESIONS OF SPINAL CORD:

SYRINGOMYELIA: Excessive over growth of neuroglia with cavity formation in the grey matter around

the central canal of spinal cord. If this disease affects the brain stem, then this disease is called

syringobulbea. There is dissociated anesthesia. Sensations carried by lateral spinothalamic tract are lost

while those carried by ventral spinothalamic tract and dorsal column tract remains intact. There is loss

of pain and temperature with intact touch sensation. As there is loss of pain and temperature so the

reflexes which can be elicited due to pain and temperature are also lost. If this disease affects smokers

then they have burned finger tips. Whe

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Notes on CNS Physiology by Medical Study Center(2)

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