Sensory receptors

Sensory receptors:
Structures that respond to environmental changes/stimuli.
Activation of sensory receptor  depolarisation  transmission along afferent fibre

Sensation = awareness of the stimulus by the CNS; perception = interpretation of the stimulus by the CNS

Location of sensory receptors:
• exteroceptors – near or at body surface; sensitive to stimuli outside the body; eg touch pressure, pain, temperature
• interoceptors/visceroceptors – located within body; sensitive to stimuli inside the body; eg tissue stretching
• proprioceptors – located in musculoskeletal structures; sensitive to body movements

Type of stimulus detected:
• mechanoreceptors – respond to mechanical stimuli or deformation; eg touch, pressure, vibrations
• thermoreceptors – respond to changes in thermal stimuli/temperature; different receptors respond to heat and cold
• photoreceptors/electromagnetic receptors – respond to light; eg retina of eye
• chemoreceptors – respond to chemical stimuli/changes; eg taste and smell, oxygen concentrations in blood
• nociceptors – respond to stimuli that cause pain or tissue damage

Types of sensory receptors:
1) Unencapsulated:
a) Free nerve endings (widely distributed - located in epidermis (between epithelial cells), cornea, stomach, ligaments, joint capsules, bone; mechanoreceptors; sensations of pain, temperature, touch maybe pressure; A delta and C nerve fibres)
b) Merkel disc’s/modified free dendritic nerve endings (located on hairless skin near epidermal-dermal junction; mechanoreceptors; sensation of light pressure/touch; A beta nerve fibres)
c) Root hair plexuses (located in hair follicles; mechanoreceptors; sensation of hair movement and touch; A beta nerve fibres)
2) Encapsulated
a) Meissner’s corpuscles (located in dermal papillae of plantar surface of foot and palm; mechanoreceptors; sensations of light pressure, discriminative touch, low frequency vibration; A beta nerve fibres; low activation thresholds)
b) Krause’s end bulb (similar to Meissner’s corpuscles – located in mucosa)
c) Pacinian corpuscles (largest sensory receptor; located in dermis, ligaments, joint capsules; consists of about 70 layers in an onion like formation; mechanoreceptors; sensations of deep pressure, stretch, high frequency vibration; rapidly adapting; A beta fibres)
d) Ruffini’s corpuscles/ending (located in dermis of hairy skin, more superficial than Pacinian corpuscles; receptor is lamellated and fluid filled; mechanoreceptors; sensations of deep pressure, stretch; slowly adapting; A beta fibres)
3) Other
a) Muscle/neuromuscular spindles (fusiform shaped receptor located in skeletal muscle; mechanoreceptors; sensation of muscle stretch  proprioception; A alpha and beta fibres)
b) Golgi tendon organs (GTO)/neurotendinous spindles (spindle shaped receptors located in tendons among collagen fascicles; consist of connective tissue capsule that enclose some tendon strands with sensory endings; mechanoreceptors; sensations of tendon stretch  proprioception/A alpha fibres; muscle contraction generates more nervous activity than passive stretch)
c) Joint kinaesthetic receptors (4 types; one is a non-encapsulated, others resemble Pacinian, Ruffini’s and tendon stretch receptors; located in joint capsule  proprioception and transmission of pain)

Sensory receptor potentials:
If stimulus is of threshold strength  changes in membrane permeability  action potential is generated and propagated

Adaptation:
reduced responsiveness to a stimulus that is unchanging
the receptor membrane becomes less responsive with time  decline in frequency of action potentials (eg less awareness of feeling of clothing against skin)
does not occur to pain or proprioception receptors
receptor types can be classified as rapid adapting (free nerve endings, hair follicle receptors, Pacinian corpuscles, Meissner’s corpuscles), fast adapting (muscles and tendon receptors) or slow adapting (Merkel’s discs, Ruffini’s corpuscles)

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