Sensory Receptors

Definition of Sensory Receptors: Receptors are specialized cells / nerve endings which respond to stimuli. These generate impulses which inform CNS about changes in internal & external environment. Receptors are transducers, which convert various forms of energy into action potential or electrical energy.

STIMULUS: A change in environment. TYPES OF STIMULUS: Electrical, Mechanical, Thermal, Chemical & Electromagnetic Sensation: Becoming aware of a particular type of stimulus, e.g., tactile, thermal & pain sensations.

Non-encapsulated & encapsulated receptors: Non-encapsulated: free nerve endings / nociceptors, Merkel’s discs, hair follicle receptors. Encapsulated: Meissner’s corpuscles, Pacinian corpuscles, Ruffini’s end organs, Krause’s corpuscles.

Non-encapsulated: FREE NERVE ENDINGS: Widely distributed. In epidermis, mucus membranes, joint ligaments, tendons, connective tissue, periosteum, peritoneum, pleural membrane, meninges, dental pulp. Involved in crude touch, pressure, tickle & itch, pain & temperature sensation.

Non-encapsulated: MERKEL’S DISCS: Present in clusters / groups in epidermis (deeper part). Stimulated when there is continuous touch or pressure. Example: holding a pen while writing.

Non-encapsulated: HAIR FOLLICLE RECEPTORS: Wrapped around hair follicles. Stimulated by bending of hair (if something is crawling on skin).

HAIR FOLLICLE HAIR FOLLICLE

Adaptation in non-encapsulated receptors: Free nerve endings & merkel’s discs  slow adaptation. We feel the pen as long as we hold it. Hair follicle receptors undergo rapid adaptation. Insect starts crawling  we feel it  if it does not bite  we forget its presence.

Encapsulated: MEISSNER’S CORPUSCLES: Meissner's corpuscles were discovered by the anatomist Georg Meissner (1829-1905). Rapid adaptation. In dermal capillaries in skin of soul & palm Nipple & external genitalia. Under the capsule are flat modified schwann cells arranged concentically along long axis of corpuscles & end in expanded nerve endings. Involved in: fine touch, 2 point tactile discrimination & vibration.

MEISSNER’S CORPUSCLE IN SKIN

Encapsulated: PACINIAN CORPUSCLE: Rapid adaptation. In dermis of skin, C.T, ligaments, joint corpuscles, peritoneum etc. Thick corpuscles, on section  onion like appearance. Flattened cells arranged concentrically (in layers) Involved in vibration & movement.

PACINIAN CORPUSCLE PACINIAN CORPUSCLE

Encapsulated: RUFFINI’S END ORGANS: In dermis of skin. Respond when skin is stretched / heavy pressure applied on skin. Slow adaptation.

RUFFINI’S END ORGANS RUFFINI’S END ORGANS

Encapsulated: KRAUSE’S CORPUSCLES: In the skin of external genitalia. Respond to 2 point tactile discrimination. Rapid adaptation. Structure resembles  Meissner’s corpuscles.

KRAUSE’S END-BULBS / KRAUSE’S CORPUSCLES

OTHER CLASSIFICATION OF RECEPTORS: 4 TYPES: EXTEROCEPTORS: INTEROCEPTORS TELOCEPTORS PROPRIOCEPTORS

EXTEROCEPTORS: In skin Respond to external stimuli.

INTEROCEPTORS: In body tissues. Respond to internal stimuli.

TELOCEPTORS: Respond to stimuli at a distance. EXAMPLE: Receptors in retina: rods & cones. Hearing receptors in ears.

PROPRIOCEPTORS: Receptors involved in sensation of position & movement. When a part of body is moved, they are stimulated.

Classification on the basis of nature of stimulus: 5 types: MECHANORECEPTORS. THERMORECEPTORS. NOCICEPTORS. CHEMORECEPTORS. ELECTRO-MAGNETIC RECEPTORS.

MECHANORECEPTORS: Respond to mechanical deformation of receptor membrane & of tissue, adjacent to receptor. Large number of receptors are included. EXAMPLES: Free nerve endings, Merkel’s disks, hair follicles, Meissner’s, Ruffini’s, Krause’s corpuscles, muscle spindle, golgi tendon organs, receptors in internal ear: in organ of corti, in semi-circular canals, in macula & vestibular apparatus. Baroreceptors are also included.

THERMORECEPTORS: Respond to temperature changes. Include cold & warmth receptors.

NOCICEPTORS: Respond to noxious stimuli. Receptors which detect tissue damage. Nociceptors are free nerve endings.

CHEMORECEPTORS: Respond to chemical changes. Include chemoreceptors in carotid & aortic bodies, chemosensitive area in medulla oblongata, olfactory receptors, taste receptors, osmoreceptors in hypothalamus & glucostat cells in satiety centre in hypothalamus.

ELECTROMAGNETIC RECEPTORS: Respond to electromagnetic rays. EXAMPLE: Rods & Cones in Retina.