Features Of Sensory Receptors

Adaptation: When a maintained stimulus of constant strength is applied to a receptor the frequency of the action potentials in its sensory nerve declines over time. This phenomenon is known as adaptation or desensitization.

Ruffini’s corpuscles are Stretch receptors. They are spindle shaped encapsulated structures located in the lower part of the dermis and resembles the golgi tendon organs in the musculotendineous junction. They respond to stimulation over large areas of skin. They are associated with slowly adapting (SA) fibers that respond as long as the stimulus is present.

Classification of the cutaneous mechanoreceptors by function

Ref: Neuroscience, 2nd edition, Edited by Dale Purves, George J Augustine, chapter 9.

Pacinian corpuscles are rapidly acting touch receptors. They are ovoid structures about 1mm in length. They contain the axon terminal which is surrounded by several concentric lamellae made of very thin flat cells separated by narrow gel filled spaces. 

A i.e. Transmitted by the _13- type of sensory nerve

Kinesthetic sensation is ability to perceive the extent, direction or rate of movements. i.e. perception of touch and pressure and is therefore transmitted by Afl type of fibersQ.

C i.e. Degree of overlap of fibres carrying tactile sensation is much less.

–  Degree of overlap of fibres is much lessQ                                   – Greater Overlap of pain sensory units.

–  Lack of collateralization from adjacent tactile fibres                    – Rapid collateral regeneration of pain fibres

• Sensory changes due to interruption of a single peripheral nerve vary, depending on whether the nerve involved is predominantly muscular, cutaneous or mixed.
• Following injury to a cutaneous nerve, the area of sensory loss is always less than its anatomic distribution because of overlap from adjacent nerve. But area of sensory loss for touch > painQ.

B i.e. Follows amputation.

‘Phantom limb’ is a late complication of amputationQ, and is used to describe the feeling that the amputated limb is still presentQ.

Ans. B: Law of projection

• Cortical stimulation experiments during neurosurgical procedures on conscious patients illustrate this phenomenon. For example, when the cortical receiving area for impulses from the left hand is stimulated, the patient reports sensation in the left hand, not in the head.
• Another example is seen in amputees. Some of these patients may complain, often bitterly, of pain and proprioceptive sensations in the absent limb (phantom limb). The ends of the nerves cut at the time of amputation often form nerve tangles called neuromas. These may discharge spontaneously or when pressure is put on them. The impulses generated in them are in nerve fibers that previously came from sense organs in the amputated limb, and the sensations evoked are projected to where the receptors used to be.

Ans. A: Meissner corpuscles

Sensory organs and their responses:

• Meissner corpuscles-Responds to changes in texture and slow vibrations
• Merkel cells-responds to sustained pressure and touch
• Ruffini corpuscles-respond to sustained pressure
• Pacician corpuscles-respond to deep pressure and fast vibration

Ans. is ‘c’ i.e., Intensity of stimulus and sensation felt

Ans. is ‘c’ i.e., Pacinian corpuscle

Tactile (touch) receptors

These are general exteroreceptors for epicritic senses. These are divided into superficial and deep receptors.

Superficial receptors are present in the epidermis or papillary layer of dermis. In glabrous (nonhairy) skin these receptors are Merkel’s disc (slowly adapting) and Meissner ‘s corpuscle (rapidly adapting). In hairy skin there are hair follicle receptors.

Deep receptors are present in deeper dermis or in the subcutaneous tissues. The deep receptors are same in both hair and nonhairy skin and include Ruffini’s end organ (slowly adapting) and Pacinian corpuscle (Rapidly adapting).

Touch, pressure and vibration are different forms of same sensation. Pressure is felt when the force applied on the skin is sufficient to reach the deep receptors, whereas touch is felt when the force is insufficent to reach the deep receptors, therefore detected by superficial receptors (Merkel’s disc, meissner’s corpuscle). Vibrations are rhythmic variations in pressure (i.e. rhymic variations of force that reaches the deep receptors). Whether a tactile receptor senses pressure or vibration depends on whether the receptor is slowly adapting or slowly adaping :‑

i)  Slowly adapting (Ruffini’s end organ) :- Are meant to detect sustained pressure; they are useless for vibrations.

ii) Rapidly adapting (Pacinian corpuscle) :- Stop discharge in response to sustained pressure; they are useful only when the pressure fluctuates rapidly, i.e. during vibrations. The higher the rate of adaptation of a receptor, the greater is the vibration frequency it can detect.

Thus, tactile (touch) sensation can be divided into :‑

A) Superficial (generally considered as touch) :- Detected by Meissner ‘s corpuscle (detect texture of sur­face, i.e. rough or smooth) and Merkel’s disc (detect two point discrimination).

B) Deep

i) Pressure (Deep touch) :- Detected by Ruffini’s end organ.

ii) Vibrations :- Detected by Pacinian corpuscle