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SOMATIC SENSATIONS:
Tactile
(touch, pressure,tickle & itch, vibration,
stereognosis)&proprioception (position &
movement)
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SENSORY UNIT:
A sensory nerve fiber & all its peripheral branchessensory unit.
Receptive field of a sensory unit is the area of skin from
which impulses are carried by a single sensory unit. Extent of receptive fields of sensory units is variable.
Receptive fields of adjacent sensory units overlap.
In cornea & sclera, receptive field of a sensory unit is50200 mm2.
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SENSATIONS:
2 TYPES:
SOMATIC
SPECIAL
SOMATIC SENSATIONS:
Mechanoreceptive:Tactile: (touch, pressure, tickle &itch, vibration) & position & movement /
proprioception. Thermal
Pain
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TACTILE SENSATION:
Include:
Touch
PressureTickle & itch
vibration
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Receptors involved in tactile
sensation:
Free nerve endings
Merkels discs
Hair follicle receptors Meissners corpuscles
Krauses corpuscles
Pacinian corpuscles Ruffinis end organs
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3 types of nerve fibers carry impulses
of tactile sensation:
A beta: 30-70 m/sec (myelinated)
A delta: 5-30 m/sec (myelinated)
C fibers: 0.5-2 m/sec (unmyelinated & slowest)
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TOUCH & PRESSURE
SENSATION:
A touch stimulusslight indentationof receptormembrane. (fine touch test by cotton wool / pencil tip)
Pressure stimulusdeformationof deeper tissues.
Sensitivity to touch varies in different parts of body &depends upon number of tactile receptors present.
Most sensitiveparts: tip of tongue, lips, finger tips, then
hands, forearm, arm. Least sensitiveto touch is back.
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In touch sensation, one component is 2 point tactilediscrimination.
Ability to discriminate 2 very near pointed ends.
Minimum seperable distance for it is 1-3 mm(on tip of tongue &
finger tips). On the back, the distance is 20-50 mm.
Tested by divider or esthesiometer(it consists of metallic handleto which metallic hair of variable thickness can be attached, we
start from thickest hair for crude touch & go to finest hair forfine touch).
Touch localization (Topognosis) is highly developed (even at theback of body).
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OSPE: 2 point Tactile
discrimination:
Q.a. Test the sense of Two-point tactile
discrimination on the big toe of
the given subject. 04
b. Minimal separable distance
decreases in presence of which 2factors?
01
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Key:a. The student introduces himself, briefs the subject & seeks consent. 0.5
Asks the subject to lie down comfortably on a couch & remove the socks fromboth feet. 0.5
Demonstrates the subject by separating apart (more than 3 mm) 2 blunt ends ofWebers compass or a blunt divider and touching to the skin of subjects fingertips simultaneously, with his eyes open, & asking the subject to tell whether he
was touched at two points simultaneously or just at one point. 01
Then asks the subject to close his eyes & places the 2 blunt ends of the compassapproximated ( less than 1 mm) on the tip of the plantar aspect of his right bigtoe & asks the subject to tell that he has felt the touch at one point or at 2points. 01
If the subject does not identify 2 separate points being touched simultaneously,
then gradually increases the distance until 2 separate points are identified &measures that distance (normally 1-3 mm) & compares on the other side.
01 b. i) Increased number of receptors present in some area of skin and ii) Bigger representation of that area in sensory cortex of brain. 01
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Qa. Test the sense of Topognosis (Tactile
Localization) & Extinction in the forearm
of the given subject. 04b. i) Which term is given to the absence of this
ability and
ii) The lesion of which tract may result in it?01
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Key a. The student introduces himself, briefs about the procedure &
seeks consent. 01
Asks the subject to sit down comfortably on a chair with eyesclosed & exposes both forearms. 01
To test Topognosis (Tactile localization) touches one point at atime on the patients skin of a forearm with the finger tip andasks the subject to open his eyes & point the place where he
perceived the touch. 01 To test Extinction, touches simultaneously with the tips of both
the index fingers with subjects eyes closed at 2 identical pointson the skin of both the forearms gently with a finger tip & askshim to open his eyes & tell where he was touched. 0.5
Tells the subject to tell here or here & here. 0.5 b. i) Topagnosia ii) Lesion of Dorsal Column Medial Lemniscal System. 01
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In touch sensation, a special sensation isSTEREOGNOSIS(ability to identify the objectwith eyes closed through judgment based on: its
shape, size, weight & texture by touch sensation,holding a small object in the palm of one hand).It is highly developed in blind(brail system).
This is an adaptation.
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OSPE: Sense of stereognosis:
Q.a. Test the sense of Stereognosis in the given
subject. 04
b. Which term is given to the absence of thisability and the lesion of which tract may
result in it? 01
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Key.a.
The student introduces himself, briefs about theprocedure & seeks consent. 01
Asks the subject to sit down comfortably on a chair
with his eyes closed. 01 Asks the subject to identify a small object placed in his
right palm. 01
Compares it in the left palm by placing another small
but different object 01b. Astereognosis
Lesion of Dorsal Column Medial Lemniscal System.
01
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TICKLE & ITCH SENSATION:
TICKLEPleasure ITCHAnnoying Tickle & itch: A light, local, mechanical stimulation
(insect crawling on skin). These may be due tochemicals like histamine.
Itching occurs on skin & certain mucus membranes(like that of eye). In conjunctiva, there is irritation.
Receptors involved are free nerve endingscalled
C-Mechanoreceptors. Impulses from these receptors are carried by C type of
nerve fibers& are slowest conducting & unmyelinated.
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VIBRATION SENSE:
Rhythmic repetitive pressure stimuliare perceived asvibration.
When vibrating tuning fork is placed on skin, specially
over bonesthrill / buzzing sensation perceived. Receptors involved: Meissners(low frequency: 60-80
cycles / sec ) & Pacinian corpuscles(high frequency: 80600 cycles / sec ). May be Merkels discs.
Carried by Dorsal Column Tract.
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Qa. Test the sense of vibration in the ankle of the
given subject. 04
b. In which physiological condition there may beloss of vibration sense in feet? 01
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Key: a. The student introduces himself & seeks consent. 0.5 Asks the subject to lie down supine on the couch & remove the
socks from both feet. 0.5 Demonstrates the test by vibrating a lower frequency (100 Hz)
tuning fork and pressing its base over the patient's forehead,then repeating without the vibration, with his eyes open. 0.5
Teaches the subject to say yes, when he receives vibration and
say yes, once again when he ceases to feel them. 0.5 Strikes the tuning fork (100 Hz) on a rubber pad / firm object &
places the base of the vibrating tuning fork on the medialmelleolus, with the subjects eyes closed & compares it with themedial melleolus of the other side. 01
Also places the base of the vibrating tuning fork on the lateralmelleolus & compares it with the lateral melleolus of the otherside. 01
b. In old age there may be loss of vibration sense in feet. 01
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Key.a.
The student introduces himself, briefs about the procedure &seeks consent. 01
Asks the subject to sit down comfortably on a chair with his eyesclosed. 01
Asks the subject to identify with eyes closed, the number (0-9)drawn on his right palm, by dragging gently the rubber-tail of apencil. 01
Compares it in the left palm by similarly drawing a differentnumber (0-9). 01
b. Graphanesthesia, Lesion of Dorsal Column Medial Lemniscal System. 01
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PROPRIOCEPTION: Sense of
position & movement.
2 types:
STATIC
KINESTHETIC STATIC: Conscious perception of orientation
of different parts of body with respect to eachother.
KINESTHETIC: Conscious perception of rateof movement of different parts of body.
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Receptors involved are proprioceptors(Musclespindles, Golgi tendon organs, Ruffinis endorgans & Pacinian corpuscles).
These receptorsare present around the joints (inthe muscle, tendon, joint capsule& ligaments).
Dorsal Column tractcarries impulses of
proprioception.
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OSPE: Sense of proprioception
Qa. Test the sense of proprioception in the foot of
the given subject. 04
b. What are the common locations of thereceptors that carry this sensation. 01
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Key: a. The student introduces himself & seeks consent. 0.5
Asks the subject to lie down supine on the couch & remove thesocks from both feet. 0.5
Demonstrates the subject, with his eyes open, the upward &downward position of the big toe first, by holding the big toe atinterphalangeal joint with the help of index finger & thumb, while
keeping the rest of the toes apart with the other hand. 01 Then asks the subject to close his eyes & places the great toe
pointing upwards or downwards & asks the subject to tell up ordown. 01
Repeats the same in the other foot. 01
b. In tendons, joints & ligaments. 01