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Peripheral Nervous System

Receptors And Sensory Fibres
  1. Skin and some mucous membranes (exteroceptors)
  1. Meissner’s corpuscles - common in dermal papillae of fingers, palms, nipple, etc.
  2. Krause’s end-bulbs and Ruffini’s end-organs - in external genitalia, dennis, tongue, joints, etc.
  3. Pacinian corpuscles- large, lamellated bodies, in external genitalia, also lie more deeply under the skin, in tendons, mesentery, joints, etc.
  4. Receptors/endings 1 to 3 are definitely encapsulated.
  5. Merkel’s discs - intra-epithelial in lower layers of the epidermis and oral epithelium.
  6. Free nerve endings - also intra-epithelial.
  7. Palisade/peritrichal endings around a hair follicle.
  1. Muscle, joint and tendon (proprioceptors)
    1. Golgi tendon organ - branching nerve fibres with thickenings between a tendon’s collagen fibres. Joint and ligament receptors are similar, but some take more specialized forms, e.g., Pacinian.
    2. Muscle spindle
      1. A spindle-shaped C.T. capsule encloses several thin specialized intrafusal skeletal muscle fibres. Away from the muscle fiber’s central region with clustered or linear nuclei (nuclear-bag and nuclear-chain fibres) are plate or trail motor endings served by mostly gamma (fusimotor) nerve fibres.
      2. Two types of sensory ending on the fibres - primary annulospiral and secondary flower-spray-increase the combinations of innervations.
      3. The larger extrafusal muscle fibres stimulate the attached spindle by their movements, while its sensitivity is varied by its own alpha and gamma motor control.
      4. The spindle informs on the complexities of muscle contraction, movement and stretch, and gives a ‘muscle sense’.
    3. (Mechano-receptors of the vestibular apparatus inform the CNS independently of the results of the muscles’ actions in terms of changed position and movement of the head. Skin receptors likewise contribute to ‘proprioception’ in the lax sense.
  2. Viscera (interoceptors)  
    1. Carotid body - chemoreceptor for blood O2 tension; has sinusoids with blood passing in close relation to glomus / Type I cells. Clusters of these cells, with their cored vesicles, are innervated by axons of the glossopharyngeal nerve. The intermixed sustentacular / Type II cells are glial, and have no known role in signal traesduction. The aortic body is similar in structure and function, and connects with the vagus nerve.
    2. Carotid sinus and aortic arch - pressoreceptors/baroceptors (for blood pressure) set within the vessel’s wall.
    3. In lung, gut, bladder, and other viscera - measuring distension, motility and chemical irritation.
    4. Brain (other intero-chemoreceptors)
      1. In hypothalamus: for blood osmolarity, glucose, hormones (and for temperature).
      2. In medulla: for CO2 tension of the blood.
    5. Special senses (extero-chemoreceptors)
  1. Olfactory mucosa (smell)
    1. Three cell kinds are in the pseudostratified, columnar neuro-epithelium:
      1. Bipolar sensory cell, with an axon going directly through the ethmoid bone to the olfactory bulb and, at the apical end, several, long, cilium-like processes (olfactory hairs) extending into the surface coat of mucus and lipid.
      2. Sustentacular cell has pigment in its apical cytoplasm and is equipped with microvilli.
      3. Basal cells to divide and replace sustentacular and sensory cells.
    2. All cell types rest on a BL penetrated by axons that become grouped as unmyelinated nerves in the lamina propria.
    3. Bowman’s compound serous glands also contribute secretion to the surface. Unlike other serous cells, they have much smooth ER.
  2. Taste bud (taste)
    1. Barrel-shaped; lying within the stratified squamous epithelium of the tongue’s circumvallate and fungiform papillae
    2. At the apex towards the opening of the taste pore, project processes of two fusiform cell kinds:
      1. thin, neuro-epithelial receptor cells (dark, light, and intermediate), and
      2. paler sustentacular or supporting cells.
    3. Receptor cells have axons from the facial and glossopharyngeal nerves terminating synaptically upon them. (Taste buds in the pharynx, epiglottis and oesophagus are served by the vagus nerve.)
    4. Towards the bottom of the taste bud are basal cells, proliferating slowly to replace receptor cells.
    5. Von Ebner’s glands in the lamina propria send a serous secretion into the trench around the vallate papilla, in whose walls the taste buds lie.

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