Veterinary Histology and Embryology

RECEPTORS

• These are of many kinds those concerned with general bodily sensibility or somaesthetic sensibility, touch, pressure, heat, cold, paid, temperature, position, movement), and those which form organs of special senses (smell, sight, taste, hearing and head position and movement).
• They may also be classified as exteroceptors, the receptors, affected by external stimuli (touch light, pressure cutaneous pain and temperature, smell sight and hearing) proprioceptors, those affected by stimuli arising within the body wall, especially those of movement and posture and visceroceptors, those affected by stimuli arising within the Viscera. All receptors, depending upon their terminal arborizations, come under two classes: Free or encapsulated.

Free or Nonencapsulated endings

• These are found practically in all epithelia in connective tissue, in muscle and serous membranes.
• Here the fibres after losing their myelin form a subepithelial plexus from which the axons or their branches between cells and end in knob like terminal swellings.

Encapsulated endings

• They include end-bulbs, Meissner’s corpuscles, pacinian corpuscles, muscle spindles and muscle tendon spindles.
• These are all characterized by a capsule of more or less flattened connective tissue cells, which enclose the terminations of the axon.

End bulbs (Krause’s end bulbs)

• Spherical or oval in shape consists of a thick lamellated capsule of connective tissue cells and fibres surrounding a central cavity or inner bulb. Within this end bulb, the naked axons of onr or more mylinated fibres and either simply or in the form of skeins.
• End bulbs are found in lips, man of tongue, cheeks, soft palate, epiglottis, nasal cavities lower end of rectum peritoneum and other serous membranes tendons ligaments, connective tissue of nerve trunks, synovial membranes of certain joints, glands penis and clitoris. These are receptors for cold.

Meissner’s corpuscles or tactile corpuscles

• Occur in hairless portion of skin and especially most numerous in finger tips, palms of hands and sales of feet.
• They lie within the connective tissue of dermal papillae. They are oval compose of flattened horizontally arranged connective tissue cells and lamellae surrounded by connective tissue capsule. One myelinated fibre is distributed to each corpuscle.
• After losing the myelin the naked axons pass into the corpuscle branch, and pursue a spiral course among the connective tissue elements. These corpuscles are concerned with the sense of discriminative touch.

Pacinian corpuscles

• These are laminated elliptical structures. They are large and visible to naked eye. Each corpuscle is formed of a large number of concentric lamellae of connective tissue fibres, lined by a single layer of connective tissue cells.
• The spaces between the lamellae filled with the fluid or semi-fluid substance. There is an inner bulb in the centre. Each corpuscle is supplied with a single myelinated naked axon extends through the centre of the inner bulb and ends in a knob like expansion.
• Fine capillary networks are seen within the lamellae but they do not enter the inner bulb, pacinian corpuscles are found in deeper subcutaneous connective tissue of hand and foot peritoneum, pancreas, mesentery penis, clitoris, urethra nipple, mammary gland in connective tissue in the vicinity of tendons, ligaments, joints.
• They are stimulated by deep or heavy pressure.