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SENSORY SYSTEM 1. PAIN2. Somatosensory cortex

Lecture - 5DR. ZAHOOR ALI SHAIKH

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PAIN It is mainly a protective mechanism

of the body, to bring conscious awareness of the fact that tissue damage is occurring.

Response may be Motor – e.g. withdrawal Emotional – e.g. anxiety, crying, depressionAutonomic reaction e.g. tachycardia, rise in

B.P., sweating,

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Classification of pain1. Fast pain- carried by A delta fibers Receptor-- Nociceptor

It is felt within 0.1 sec. after stimulation.▪ e.g. pricking, cut with knife.

2. slow pain – carried by C Fiibers - Recptor-- Polymodal

Felt in 1 sec. or more following painful stimulus.

It is associated with tissue damage & can be referred to as , aching pain or chronic pain

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Pain receptors Free nerve endings -Nociceptors,

Polymodal. Pain receptors do not adopt at all or very

slowly. They are found in largest no. & density in

skin, periostium joint surface, arterial wall & duramatar.

pain receptors are activated by 3 types of stimuli;

1. Mechanical (cutting,pinching)– they elicit fast pain.

2. Thermal (Heat, very cold)- they elicit also fast pain.

3. Chemical (Tissue injury, prostaglandin)- they produce slow pain.

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Chemical agents that produce pain Bradykinin, serotonin, Histamin, K+

ion, Acids, acetyl choline, & proteolytic enzymes.

Prostaglandins & substance – P enhance the sensitivity of pain receptors.

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Characteristics of A delta fibersFAST PAIN Transmitted by Aδ fibers in the peripheral

nerves Characteristics of Aδ fibers Myelinated - Diameter fine 2 - 5 μm 12 - 30 m/sec. conduction velocity Terminate at I and V layer Dorsal horn cell Fast pain, rapid, pricking and well localized Neurotransmitter - Glutamate ( excitatory) 20% pain conduction

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Characteristics of C fibers

SLOW PAIN Chronic type of pain, transmitted by c

fibersCharacteristics of C fibers Non-Myelinated Diameter 0.4 – 1.2 μm conduction velocity 0.5 - 2 m/s Terminate in layer II and III of dorsal horn

(substantia gelatinosa) Slow, diffuse, dull, aching Neurotransmitter - P-Substance (excitatory) 80% of pain conduction

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Applied

1. What will happen if sensory area SI is removed.

Ans. persons ability to interpret the quality of pain & precise location of pain will be affected.

2. Why patient with chronic pain syndrome have difficulty in sleeping?

Ans. Paleospinothalamic (chronic slow pain) pathway sends information to reticular formation and thalamic nuclei which are part of brain activating / alerting system, therefore it may explain why chronic pain syndrome causes difficulty in sleep.

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Visceral Pain

It is produced by stimulation of pain receptors in viscera.

Pain receptors in viscera are sparsely distributed.

Afferent from viscera reach CNS via sympathetic & parasympathetic pathway.

Visceral pain travel along the same pathway as somatic sensation i.e. Spinothalamic tract.

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CHARACTERSTICS OF VISCERAL PAINPoorly localized Associated with nausea and

autonomic disturbance Often referred to another part

of the body Cutting, crushing are not

painful ,when applied to viscera Pain in viscera is caused by

distension, ischemia and inflammation

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Reffered Pain Pain that is not felt in the diseased

structure itself, but at another place in the body far away from the site of origin.

Visceral and deep somatic pain are often referred, but superficial pain is not.

Mechanism of reffered pain Dermatomal rule Convergence of peripheral & visceral

pain on the same second order neuron that project to brain

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IMPORTANT EXAMPLES OF REFEERED PAIN

ORGAN HEART

APPENDIX SMALL INTESTINE PLEURA TONGUE TEETH UTERUS

SITE OF ACTION PRECORDIUM, INNER

ASPECT OF LEFT ARM, EPIGASTRIUM

UMBILICUS UMBILICUS ABDOMEN EAR HEAD LOW BACK,

RADIATING TO LOWER ABDOMEN

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GATE CONTROL THEORY OF PAIN The dorsal horn of spinal cord , in

particular the neurons of substantia gelatinosa, form the gate through which pain impulses must pass in order to reach the brain.

Impulses coming along the C fibers cause the release of substance P & open the “gate” in the dorsal horn.

Glutamate is another excitatory neurotransmitter released by A-delta fiber at the dorsal horn cell

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GATE CONTROL THEORY OF PAIN Impulses coming along the large

diameter Aβ fibers close the “gate” at the dorsal horn.

The “gate” is also under control of higher centers in brain, by means of analgesic system of corticospinal & reticulospinal fibers.

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Substance P pain pathway

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Analgesic Pathway

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Brain Analgesic System

Brain has built in analgesic system. Brain can suppress pain by

descending analgesic pathways (nuclei in medulla and reticular formation).

How it works? By sending message through

descending pathway to the inhibitory neuron in the Dorsal horn cell of spinal cord.

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Brain Analgesic System

Brain descending pathways release Enkephalin which bind with opiate receptors at afferent pain fiber terminals in Dorsal horn of spinal cord and work like Morphine (powerful analgesic).

Endorphin, Enkephalin and Dynorphin are endogenous or natural analgesic system. They suppress release of substance P.

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Other Factor That Modify Pain Exercise, Stress modify pain. How they work? By release of Endorphin. How ACUPUNCTURE works for pain? By release of Endorphin, Endogenous

opiates & neurotransmitters like serotonin, Norepinephrine, Cortisol.

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Somatosensory Cortex

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Somatosensory Areas Somatosensory Area I – S I.(Brodmann area 1,2,3) – post central

gyrus parietal lobe. Somatosensory area II – S II.(Brodmann area no. 40) in the wall of

sylvian fissure which separate temporal lobe from frontal & parietal lobes.

Sensory Association area (broadmann area 5 & 7) located in parietal lobe behind S I.

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Somatosensory Areas (cont) Brodmann was a histologist, he

studied and made map of human cortex and divided it into about 50 distinct areas called brodmann’s areas based on histological, structural differences.

Many neurophysiologist and neurologist refer by number to many different functional areas of human cortex.

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SENSORY SYSTEM

From specific sensory nuclei of thalamus, neurons carrying sensory information project into two somatic sensory areas of the cortex, SI & SII.

In addition SI project to SII.Generally when we use the term

somatosensory cortex we mean SI area.

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Somatosensory cortex (SI area)

Somatosensory cortex is a site of perception of 1. Somasthetic [touch, pain, temperature,

pressure] 2. Properioception [awareness of body

position] The arrangement of thalamic fibers in SI is

such that parts of body are represented in order, along the post central gyrus with the legs on the top & head at the lower end of the gyrus.

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Representation of the different areas of the body in somatosensory area I

of the cortex

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Somatosensory cortex (cont) In the sensory cortex – there is detailed

localization of the fibers from various parts of the body in the post central gyrus.

In sensory homunculus [little man] in the sensory cortex , different body parts are not represented equally

Size of cortical receiving area for impulses from a particular part of the body is proportionate to the no. of receptors.

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Somatosensory cortex (cont) In the cortical areas for sensation –

very large area is occupied by impulses coming from lips, face, and hand (thumb) also parts of mouth concerned with speech.

Trunk & back has small area of presentation in sensory cortex.

Each side of the cortex receives information from opposite side of the body.

Representation of the different areas of the body in somatosensory area I of the cortex

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Layers of somatosensory cortex and their functions Somatosensory area on each side of brain

receives information from the opposite side of body as ascending sensory pathway which cross to the opposite side

Sensory cortex contain 6 separate layers of neuron arranged in vertical columns.

Layer I is at the surface & layer VI is deep. Neurons in each layer perform different

functions.

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SOMATOSENSORY CORTEX [CONT] The incoming sensory signals excites

neuronal layer IV first, then the signals spreads both towards the surface of the cortex & towards deep layer.

Layer II & III send axons to cerebral cortex on the opposite side of the brain through corpus callosum.

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Sensory cortex has vertical columns of neurons, each column detects a different sensory spot on the body with a specific sensory modality

From anterior portion of post central gyrus many of signals spread directly to motor cortex, (specially muscles, tendons joint receptors) these signals play a major role in controlling motor signals that activate muscle contraction.

SOMATOSENSORY CORTEX [CONT]

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Functions of somatosensory area I Ablation (damaging) of SI area in

animals causes loss of following types of sensory judgment;

1. Loss of localization but still touch is felt.

2. Loss of stereognosis ( inability to judge size or shape of the object.) it is called Astereognosis.

3. Loss of fine touch, two point discrimination.

4. Loss of proprioception.

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Somatosensory area II

SII is located in the superior wall of the sylvian fissure, the fissure that separate the temporal lobe from the frontal & the prietal lobe.

Face is presented anteriorly, arms centrally & legs posteriorly.

The presentation of the body parts on sylvian fissure is not as complete & detailed as in post central gyrus

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Somatosensory area II Little is known about somatosensory

area II (SII). Signals enter into SII from brain

stem, also SI area and other areas of brain visual & auditory.

Projection from SI are required for function of SII.

Removal of parts of SII has no apparent effect on neurons in SI. Therefore SI is more important.

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Somatosensory association area

Located in parietal lobe behind area SI.

It receives signals from ;1. Somatosensory area I2. Thalamus3. Visual cortex4. Auditory cortex

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Effect of removing somatosensory association area.

Person looses the ability to recognize objects felt on the opposite side of the body, he looses the sense of form of his own body on the opposite side also. He forget it is there.

This complex sensory deficit is called Amorphosynthesis.

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Important Note Damage to somatosensory cortex in left

hemisphere produces sensory loss on the right side of the body and vice versa

If there damage to somatosensory cortex , thalamus can give awareness of touch, pain, temperature, pressure, but thalamus can not localize the area and the intensity

Localization, level of intensity of stimulus, and Stereognosis [recognition of object without looking at them] is function of somatosensory cortex

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Clinical abnormalities of pain

Hyperalgesia: Excessive Pain due

to tissue damage because the

threshold of pain

receptor is decreased

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Applied 1-Peripheral Neuropathy Polyneuritis or

Polyneuropathy(When many peripheral

nerves are affected)All forms of sensations

are impaired in distal parts of limbs (Glove & stocking anesthesia)

Usually symmetrical

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Polyneuritis or Polyneuropathy Causes : Diabetes Mellitus, Vit. B

deficiency (B1, B6, B12) Drugs e.g. INH (anti T.B.)

Patient complaints of, numbness, sometimes pain in the feet

On examination: loss of position & vibration sense.

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2- Brown –Sequard Syndrome (Hemisection of spinal cord) Causes: due to stab injury , gunshot ( bullet )

wound, or tumor . The example shown here is a lesion on the left

side at the thoracic level of the spinal cord : Ipsilaterally ( on the same side of lesion ) (1) At the level of the lesion : Loss of all sensations.(2) Below the level of the lesion : loss of vibration ,

position and two-point discrimination . Why ? Contra laterally (on the opposite side ) : loss of

pain and temperature sensibility Why ?

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Brown-Sequard Syndrome

Site of Lesion

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Brown –Sequard Syndrome (cont..)

There is motor weakness ( lower motor neuron type at the level of the lesion.

Below the lesion- Spastic lower limb (with upper motor neuron type of lesion on the same side). Why?

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