Page 367 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Receptors and sense organs (organa sensuum) 349

that terminate in small bulbs. Ruffini corpuscles are mech- myelinated nerve endings. Muscle contraction increases the
VetBooks.ir anoreceptors and are located in the joint capsules, in the pressure on the receptor, generating an action potential.
hypodermis, along vessels, in the digital pads and in the
corium of the hoof. They may also function as cold and Muscle spindle
pain receptors. Muscle spindles are large (2–10 × 0.2 mm) proprioceptors
located in striated muscle. Their outer connective tissue
Lamellar receptors capsule is continuous with the perimysium. A second,
Lamellar receptors (Vater’s corpuscles, Pacinian corpus- internal capsule surrounds a fluid-filled cavity contain-
cles) consist of an axon surrounded by 10–60 concentric ing two types of modified muscle fibres: nuclear bag and
lamellae that resemble the layers of an onion (Figure 16.4). nuclear chain fibres. These fibres are supplied by motor
The inner lamellae are composed of flattened Schwann and sensory neurons. The sensory nerve endings serve as
cells; the outer layers are formed by fibrocytes. Spaces stretch receptors. The impulses they generate are trans-
between the lamellae contain interstitial fluid. Capillaries mitted to the central nervous system, which regulates the
and collagen fibre bundles may penetrate the interlamel- activity of the motor neurons supplying the muscle.
lar gaps.
These types of receptors are visible macroscopically, Enteroreceptors
reaching up to 4 mm in diameter. They occur in the deeper Free sympathetic and parasympathetic nerve endings
layers of the hypodermis, the corium of the hoof and ramify within various layers of the viscera to provide
claw, the metacarpal pads of carnivores, the mesenteries, autonomic regulation of organ function. Within the
the nasolabial plate of the ox and the pancreas of the cat. walls of arteries near the heart (carotid body, aortic arch),
Lamellar receptors detect stimuli associated with pressure these receptors modulate blood pressure. Enteroreceptors
and vibration. in the walls of the atria monitor and control filling of the
heart; others detect stretching within the lungs. In the
Proprioceptors gastrointestinal tract, free nerve endings convey impulses
Proprioceptors contribute to fine tuning of movement generated by stretching of smooth muscle and organ
involving joints, tendons and muscles by conveying infor- capsules. These signals are interpreted as pain.
mation about position and movement.
In joints, proprioceptive stimuli are detected primar- Gustatory organs (organa gustus)
ily by free nerve endings, Ruffini corpuscles and modified Gustation is mediated by chemoreceptors located within
lamellar corpuscles within the joint capsule. taste buds (gemmae gustatoriae). Taste buds are found
primarily on the surface of the tongue and in the pharynx.
Golgi tendon organ They are abundant on the side walls of vallate, foliate and,
Golgi tendon organs are found in tendons, towards the in young animals, fungiform papillae. Taste buds contain
muscle belly. They are stretch receptors that monitor ten- secondary sensory cells that develop by modification of
sion within the musculotendinous apparatus. Golgi tendon epithelial cells.
organs (1.0 × 0.1 mm) are enclosed in a connective tissue
capsule that merges with the perineurium of the associ- Taste buds
ated neuron. At their centre, bundles of collagen fibres Taste buds (50–70 μm) consist of intra-epithelial clusters
are closely associated with a branching network of non- of 20–30 individual cells (Figures 16.5 and 16.6). They

16.4 Multilaminar Pacinian corpuscle (pancreas, cat).
Haematoxylin and eosin stain (x100).

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