Lecture - 5
DR. ZAHOOR ALI SHAIKH
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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,
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Fast pain- carried by A delta fibers
1.
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
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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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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.
2.
3.
Mechanical (cutting,pinching)– they elicit fast pain.
Thermal (Heat, very cold)- they elicit also fast pain.
Chemical (Tissue injury, prostaglandin)- they produce
slow pain.
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Bradykinin, serotonin, Histamin, K+ ion,
Acids, acetyl choline, & proteolytic enzymes.
Prostaglandins & substance – P enhance the
sensitivity of pain receptors.
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FAST 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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SLOW PAIN
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
(substantia gelatinosa)
 Slow, diffuse, dull, aching
 Neurotransmitter - P-Substance (excitatory)
 80% of pain conduction
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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.
1.
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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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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
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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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ORGAN
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HEART
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APPENDIX
SMALL INTESTINE
PLEURA
TONGUE
TEETH
UTERUS
SITE OF ACTION
 PRECORDIUM, INNER
ASPECT OF LEFT ARM,
EPIGASTRIUM
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UMBILICUS
UMBILICUS
ABDOMEN
EAR
HEAD
LOW BACK, RADIATING
TO LOWER ABDOMEN
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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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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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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 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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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 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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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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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 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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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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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.
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Representation of the different areas of the body
in somatosensory area I of the cortex
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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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The incoming sensory signals excites
neuronal layer IV first, then the signals
spreads both towards the surface of the
cortex & towards deep layer.
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Layer II & III send axons to cerebral cortex on
the opposite side of the brain through corpus
callosum.
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SOMATOSENSORY CORTEX
[CONT]
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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.
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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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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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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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Located in parietal lobe behind area SI.
It receives signals from ;
1.
2.
3.
4.
Somatosensory area I
Thalamus
Visual cortex
Auditory cortex
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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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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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Hyperalgesia: Excessive Pain due to tissue
damage because the threshold of pain
receptor is decreased
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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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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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Causes: due to stab injury , gunshot ( bullet ) wound, or
tumor .
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The example shown here is a lesion on the left side at the
thoracic level of the spinal cord :
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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 ?
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Contra laterally (on the opposite side ) : loss of pain and
temperature sensibility Why ?
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BrownSequard
Syndrome
Site of Lesion
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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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