# Features Of Sensory Receptors

### FEATURES OF SENSORY RECEPTORS

Q. 1

Weber Fechner Law is:

 A Magnitude of stimulus strength perceived is approximately proportionate to the log of the intensity of stimulus strength B Magnitude of stimulus strength perceived is directly proportional to the intensity of stimulus strength C Threshold of receptor is directly proportional to stimulus strength D Threshold of receptor is inversely proportional to stimulus strength
Q. 1

Weber Fechner Law is:

 A Magnitude of stimulus strength perceived is approximately proportionate to the log of the intensity of stimulus strength B Magnitude of stimulus strength perceived is directly proportional to the intensity of stimulus strength C Threshold of receptor is directly proportional to stimulus strength D Threshold of receptor is inversely proportional to stimulus strength
Ans. A

Explanation:

Magnitude of stimulus strength perceived is approximately proportionate to the log of the intensity of stimulus strength [Ref: Guyton 11/e p594; Ganong 22/e 126]

Weber – Fechner law

• It states that the magnitude of the sensation felt in proportionate to the log of intensity of the stimulus.
• Example – A person holding 30 grams weight in his hand can barely detect a I gm rise in weight (a rise than 1 gm would not be detectable). But when he holds 300 gm of weight. he can rarely detect a 10 gm increase in weight (a rise less than 10 gm would not be detectable to him. So a 300 gm wt. and (Say) 307 gm of weights would appear same to him.)
• This is expressed mathematically as
 Interpreted signal strength = Log x stimulus intensity + constant

Q. 2

The receptor response decline despite of the continued presence of a stimulus is know as:

 A Accomodation B Adaptation C Refractoriness D Electrotonus
Q. 2

The receptor response decline despite of the continued presence of a stimulus is know as:

 A Accomodation B Adaptation C Refractoriness D Electrotonus
Ans. B

Explanation:

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.

Ref: Ganong’s Review of Medical Physiology, 21st Edition, Page 125

Q. 3

Ruffini end organ is associated with sensation of:

 A Pain B Heat C Stretch D Pressure
Q. 3

Ruffini end organ is associated with sensation of:

 A Pain B Heat C Stretch D Pressure
Ans. C

Explanation:

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.

Cutaneous Mechanoreceptors: A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. The encapsulated cutaneous receptors include Meissner corpuscles, Pacinian corpuscles and Ruffini corpuscles. Other cutaneous receptors are unencapsulated and include the free hair endings  and the Merkel complex.

• Low-threshold (or high-sensitivity) mechanoreceptors: because even weak mechanical stimulation of the skin induces them to produce action potentials. All low-threshold mechanoreceptors are innervated by relatively large myelinated axons (type Aβ;, ensuring the rapid central transmission of tactile information.

Classification of the cutaneous mechanoreceptors by function

 Receptor type Anatomy Axon type Location Function Rate of adaptation Threshold of activation Freenerve endings Minimally specializednerve endings C, Aδ All skin Pain, temperature, crude touch Slow High Meissner’s corpuscles Encapsulated Between dermal papillae Aβ 6–12 μm Principally glabrous skin Touch, pressure (dynamic) Rapid Low Pacinian corpuscles Encapsulated Onionlike covering Aβ 6–12 μm Subcutaneous tissue, interosseous membranes, viscera Deep pressure, vibration (dynamic) Rapid Low Merkel’s disks Associated with peptide- releasing cells Aβ All skin, hair follicles Touch, pressure (static) Slow Low Ruffini’s corpuscles Encapsulated Oriented along stretch lines Aβ 6–12 μm All skin Stretching of skin Slow Low

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

Q. 4

Ruffini end organ is associated with sensation of:

 A Sustained Pressure B Heat C Touch D None of the above
Q. 4

Ruffini end organ is associated with sensation of:

 A Sustained Pressure B Heat C Touch D None of the above
Ans. A

Explanation:

Meissner’s corpuscles are dendrites encapsulated in connective tissue and respond to changes in texture and slow vibrations. Merkel cells are expanded dendritic endings, and they respond to sustained pressure and touch. Ruffini corpuscles are enlarged dendritic endings with elongated capsules, and they respond to sustained pressure. Pacinian corpuscles consist of unmyelinated dendritic endings of a sensory nerve fiber, 2 m in diameter, encapsulated by concentric lamellae of connective tissue that give the organ the appearance of a cocktail onion. Theses receptors respond to deep pressure and fast vibration.

Ref :Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 8. Somatosensory Neurotransmission: Touch, Pain, and Temperature. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong’s Review of Medical Physiology, 24e.

Q. 5

Which of the following phrase adequately describes Pacinian corpuscles

 A A type of pain receptors B Slowly adapting touch receptors C Rapidly adapting touch receptors D Located in the joints
Q. 5

Which of the following phrase adequately describes Pacinian corpuscles

 A A type of pain receptors B Slowly adapting touch receptors C Rapidly adapting touch receptors D Located in the joints
Ans. C

Explanation:

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.

Slowly adapting deep tactile receptors are meant for signalling sustained pressure. Rapidly adapting deep receptors stop discharging in response to sustained pressure. They are useful only when the pressure fluctuates rapidly.

Meissner’s corpuscles are dendrites encapsulated in connective tissue and they respond to changes in texture and slow vibrations.

Merkel cells are expanded dendritic endings, and they respond to sustained pressure and touch.

Ref: Principles of Medical Physiology  By Sabyasachi Sarkar page 647, Ganong’s Review of Medical Physiology, 24e chapter 8

Q. 6

Pacinian corpuscles transmit which sensation:

 A Touch B Vibration C Cold D Heat
Q. 6

Pacinian corpuscles transmit which sensation:

 A Touch B Vibration C Cold D Heat
Ans. A

Explanation:A i.e. Touch

Q. 7

Kinesthetic sensation is:

 A Transmitted by the (3-type of sensory nerve) B Located in Merkl’s disc C Transmitted by Meissner’s corpuscles D Means abnormal perception of sensation
Q. 7

Kinesthetic sensation is:

 A Transmitted by the (3-type of sensory nerve) B Located in Merkl’s disc C Transmitted by Meissner’s corpuscles D Means abnormal perception of sensation
Ans. A

Explanation:

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.

Q. 8

If a single spinal nerve is cut, the area of tactile loss is always greater than the area of loss of painful sensations, because:

 A Tactile information is carried by myelinated fast conducting fibres B Tactile receptors adapt quickly C Degree of overlap of fibres carrying tactile sensation is much less D In the primary sensory cortex tactile sensation is represented on a larger area
Q. 8

If a single spinal nerve is cut, the area of tactile loss is always greater than the area of loss of painful sensations, because:

 A Tactile information is carried by myelinated fast conducting fibres B Tactile receptors adapt quickly C Degree of overlap of fibres carrying tactile sensation is much less D In the primary sensory cortex tactile sensation is represented on a larger area
Ans. C

Explanation:

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.

Q. 9

Which of the following is true regarding a phantom limb:

 A Occurs in leprosy B Follows amputation C Follows a psychiatric illness D After filariasis
Q. 9

Which of the following is true regarding a phantom limb:

 A Occurs in leprosy B Follows amputation C Follows a psychiatric illness D After filariasis
Ans. B

Explanation:

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.

Q. 10

Phantom limb is explained by:

September 2007

 A Webers law B Law of projection C Fechners law of degeneration D Pascals law
Q. 10

Phantom limb is explained by:

September 2007

 A Webers law B Law of projection C Fechners law of degeneration D Pascals law
Ans. B

Explanation:

Ans. B: Law of projection

No matter where a particular sensory pathway is stimulated along its course to the cortex, the conscious sensation produced is referred to the location of the receptor. This principle is called the 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.

Q. 11

Sensory organ for responding to texture is:

September 2009

 A Meissner corpuscles B Merkel cells C Ruffini corpuscles D Pacician corpuscles
Q. 11

Sensory organ for responding to texture is:

September 2009

 A Meissner corpuscles B Merkel cells C Ruffini corpuscles D Pacician corpuscles
Ans. A

Explanation:

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

Q. 12

Weber Fechner law is related to â€‘

 A Phantom limb B Force of contraction in heart C Intensity of stimulus and sensation felt D Cortical plasticity
Q. 12

Weber Fechner law is related to â€‘

 A Phantom limb B Force of contraction in heart C Intensity of stimulus and sensation felt D Cortical plasticity
Ans. C

Explanation:

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

Q. 13

Vibrations are felt by â€‘

 A Meissner’s corpuscle B Merkel’s disc C Pacinian corpuscle D Ruffini’s end organ
Q. 13

Vibrations are felt by â€‘

 A Meissner’s corpuscle B Merkel’s disc C Pacinian corpuscle D Ruffini’s end organ
Ans. C

Explanation:

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 :â€‘

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 :â€‘

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