lecture 5 dr zahoor ali shaikh n.
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SENSORY SYSTEM 1. PAIN 2. Somatosensory cortex

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Lecture - 5 DR . ZAHOOR ALI SHAIKH. SENSORY SYSTEM 1. PAIN 2. Somatosensory cortex. 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

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  • 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, depression
      • Autonomic reaction e.g. tachycardia, rise in B.P., sweating,
classification of pain
Classification of pain
  • Fast pain- carried by A delta fibers

Receptor-- Nociceptor

    • It is felt within 0.1 sec. after stimulation.
      • e.g. pricking, cut with knife.
  • 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
pain receptors
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;
    • Mechanical (cutting,pinching)– they elicit fast pain.
    • Thermal (Heat, very cold)- they elicit also fast pain.
    • Chemical (Tissue injury, prostaglandin)- they produce slow pain.
chemical agents that produce pain
Chemical agents that produce pain
  • Bradykinin, serotonin, Histamin, K+ ion, Acids, acetyl choline, & proteolytic enzymes.
  • Prostaglandins & substance – P enhance the sensitivity of pain receptors.
characteristics of a delta fibers
Characteristics of A delta fibers


  • 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
characteristics of c fibers
Characteristics of C fibers
  • Chronic type of pain, transmitted by c fibers

Characteristics 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 (substantiagelatinosa)
  • Slow, diffuse, dull, aching
  • Neurotransmitter - P-Substance (excitatory)
  • 80% of pain conduction
  • 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.

  • 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.

visceral pain
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.
characterstics of visceral pain
  • Poorly 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
reffered pain
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
important examples of refeered pain




  • EAR
  • HEAD
gate control theory of pain
  • The dorsal horn of spinal cord , in particular the neurons of substantiagelatinosa, 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
gate control theory of pain1
  • 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.
brain analgesic system
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.
brain analgesic system1
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.
other factor that modify pain
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.
somatosensory areas
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.
somatosensory areas cont
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.
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.
somatosensory cortex si area
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.

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

somatosensory cortex cont
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.
somatosensory cortex cont1
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

layers of somatosensory cortex and their functions
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.
somatosensory cortex cont2
  • 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.
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.
functions of somatosensory area i
Functions of somatosensory area I
  • Ablation (damaging) of SI area in animals causes loss of following types of sensory judgment;
    • Loss of localization but still touch is felt.
    • Loss of stereognosis ( inability to judge size or shape of the object.) it is called Astereognosis.
    • Loss of fine touch, two point discrimination.
    • Loss of proprioception.
somatosensory area ii
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
somatosensory area ii1
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.
somatosensory association area
Somatosensory association area
  • Located in parietal lobe behind area SI.
  • It receives signals from ;
      • Somatosensory area I
      • Thalamus
      • Visual cortex
      • Auditory cortex
effect of removing somatosensory association area
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.
important note
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
clinical abnormalities of pain
Clinical abnormalities of pain
  • Hyperalgesia: Excessive Pain due to tissue damage because the threshold of pain

receptor is decreased

applied 1 peripheral neuropathy
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
polyneuritis or polyneuropathy
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.
2 brown sequard syndrome hemisection of spinal cord
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 )
  • At the level of the lesion : Loss of all sensations.
  • 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 ?
brown sequard syndrome cont
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?