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Sensory receptors
Peripheral nervous system (PNS) connects the brain with the outside world.
Its function is dependent on information. PNS has structural components which
are: Sensory receptors, Peripheral nerves and Efferent motor endings. Sensory
receptors are classified according to the nature of stimulus detected into:
a. Mechanoreceptors (touch, vibration, pressure and stretch). Tactile
sensory receptors are found everywhere in the skin. Baroreceptors are
specialized pressure receptors found in aortic arch and carotid bifurcation to
detect changes in blood pressure.
b. Thermoreceptors (temperature changes)
c. Photoreceptors (light energy present exclusively in the retina of eye)
d. Chemoreceptors (detect chemical in solution). Examples are central
chemoreceptors present underneath the hypothalamus near the respiratory
center in midbrain and peripheral chemoreceptors in aortic arch, carotid
bifurcation and some other large arteries.
e. Nociceptors (pain)
Sensory receptors are classified according to location into:
a. Exteroceptors (surface of skin)
b. Interoceptors or visceroceptors (visceral organs and blood vessels)
c. Proprioceptors (musculoskeletal organs)
Sensory receptors are classified according to complexity into:
a. Simple or generalized senses (most sensory receptors)
b. Complex or special senses (vision, audition, olfaction, gustation)
Generalized sensory receptors are further classified into:
a. Free dendritic endings (unencapsulated)
i. Free nerve endings
ii. Merkel discs are tactile receptor located near the border between
the epidermis and dermis
iii. Root hair plexus
b. Encapsulated
i. Meissner’s corpuscles are tactile receptors composed of stack of
flattened disks in the dermis just below epidermis
ii. Pacinian corpuscles are tactile receptors composed of onion-like
capsule located deep in the skin
iii. Ruffini’s corpuscles are tactile receptors composed of branched
fibers inside a cylindrical capsule within dermis
iv. Muscle spindles (detect muscle stretch)
v. Golgi tendon organs (detect tendon stretch)
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Temporal properties of tactile receptors (adaptation)
- Rapidly adapting fibers (RA) found in Meissner receptor and Pacinian
corpuscle - fire at onset and offset of stimulation
- Slowly adapting fibers (SA) found in Merkel and Ruffini receptors - fire
continuously as long as pressure is applied
Spatial Properties of tactile receptors (detail resolution)
- Deep receptors: Pacinian receptors (RA2) and Ruffini receptors (SA2) have
large receptive fields and respond to high vibration rates.
- Surface receptors: Merkel receptors (SA1) and Meissner receptors (RA1)
have small receptive fields and respond to slow vibration rates.
Low vibration
High vibration
Slow adaptation
Merkel receptors
(SA1)
Ruffini (SA2)
Rapid adaptation
Meissner receptors
(RA1)
Pacinian corpuscles
(RA2)
Merkel receptors
Meissner receptors
Ruffini cylinder
Pacinian corpuscles
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Muscle spindle
Muscle spindle is an example of encapsulated simple sensory receptors. It
is composed of contractile region innervated by gamma () efferent (motor)
fibers that maintain spindle sensitivity. Nuclear bag and nuclear chain fibers
are sensory components wrapped by afferent sensory endings types Ia and II
fibers to conduct information about the state of muscle stretch. Extrafusal
muscle fibers are skeletal muscle fibers innervated by alpha () motor neurons.
Sequence of events involves:
a. Stretching the muscle activates muscle spindle
b. Impulse carried by primary sensory fibers (Ia and II) to the spinal cord
c. Activates alpha motor neuron which sends efferent signal to the
extrafusal muscle fibers
d. Stretched muscle contracts
e. Antagonist muscle is reciprocally inhibited
f. Gamma motor neurons contract the intrafusal fibers to maintain
spindle sensitivity
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