Nervous System
Physiology
By
Dr. SHAHAB SHAIKH
PhD MD MBBS
Lecture – 4: Sensory system - 2
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Faculty of Medicine
Al Maarefa Colleges of Science & Technology
PAIN
• The International Association for the Study of Pain defines
Pain as :
“an unpleasant sensory and emotional experience
associated with actual or potential tissue damage, or
described in terms of such damage”
• Derived from Latin “Poena” meaning punishment from God
• It is mainly a protective mechanism of the body. makes us
aware of actual or impending damage to the body.
• Pain is an intensely subjective experience, and is therefore
difficult to describe.
• 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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Classification of pain
1. Fast pain
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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
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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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Classification of pain
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Pain receptors
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Free nerve endings –Nociceptors.
Pain receptors do not adopt at all or adapt very
slowly.
They are found in largest no. & density in skin,
periosteum, 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
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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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Characteristics of A delta fibers
FAST PAIN
 Transmitted by Aδ (delta) 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
– 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 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 (substantia gelatinosa)
– Slow, diffuse, dull, aching
– Neurotransmitter - Substance - P
– 80% of pain conduction
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Sensory Pathways
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Sensory Cortex Homunculus
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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 PAIN
• Poorly localized
• Associated with nausea and autonomic
disturbance
• Often referred to another part of the body
• Pain in viscera is caused by distension,
ischemia and inflammation
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REFERRED 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
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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 a neurotransmitter released by Adelta fiber at the dorsal horn cell.
• 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.
• By sending message through descending
analgesic pathway to the inhibitory neuron in the
Dorsal horn cell of spinal cord.
• 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 Areas
1. Somatosensory Area I – S I
–
(Brodmann area 1,2,3) – post central gyrus parietal lobe.
2. Somatosensory area II – S II.
–
(Brodmann area no. 40) in the wall of sylvian fissure which
separate temporal lobe from frontal & parietal lobes.
3. Sensory Association area
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(broadmann area 5 & 7) located in parietal lobe behind S I.
• Brodmann a histologist, studied and made map of human cortex and
divided it into about 50 distinct areas called Brodmann’s areas based on
histological, structural differences.
• Neurophysiologist and neurologist refer by number to many different
functional areas of human cortex.
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Somatosensory cortex (S1 area)
• It is located in post central gyrus of the cerebral
cortex.
• There is detailed localization of the fibers from
various parts of the body.
• Somatosensory cortex is a site of perception of
– Somasthetic sensation [touch, pain, temperature, pressure]
– Proprioception
• The arrangement of thalamic fibers in S1 is such
that parts of body are represented in order,
along the post central gyrus with the feet on the
top & head at the lower end of the gyrus.
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Somatosensory cortex (S1 area)
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Somatosensory cortex (S1 area)
• Here 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.
• Thus 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.
• From here many of signals spread directly to motor
cortex, play a major role in controlling motor signals that
activate muscle contraction
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Somatosensory Cortex
• 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.
• 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.
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Somatosensory Cortex
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Functions of Somatosensory Area 1
•
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.
• Little is known about role of 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.
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Somatosensory Association Area
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Located in parietal lobe behind area SI.
It receives signals from ;
1.
2.
3.
4.
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Somatosensory area I
Thalamus
Visual cortex
Auditory cortex
Effect of removing …
– 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
• Mononeuropathy or Polyneuropathy
• In Polyneuropathy 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
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It is Hemi-section of spinal
cord
•
Can occur due to stab injury ,
gunshot ( bullet ) wound, or tumor .
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Effects:
– Ipsilaterally
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At the level of the lesion :
»
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Below the level of the lesion :
»
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Loss of all sensations.
loss of vibration , position and
two-point discrimination
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Contra laterally
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loss of pain and temperature
sensibility
There is motor weakness
(LMN type) at the level of the
lesion.
Below the lesion- Spastic
lower limb with UMN type of
lesion on the same side.
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References
• Human physiology by Lauralee Sherwood,
8th edition
• Text Book Of Physiology by Guyton & Hall,
11th edition
THANK YOU
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