Lect. 4, Somatic Sensations

8/13/2019 Lect. 4, Somatic Sensations

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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

Lect. 4, Somatic Sensations

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