Page 365 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Receptors and sense organs 16

VetBooks.ir

(organa sensuum)

Living cells are characterised by the ability to detect, trans- are not necessarily associated with a specific type of recep-
mit, store and, in some cases, respond to stimuli. Through tor structure. While many receptors have a high specificity
evolution, mammals have developed a highly differenti- for a particular kind of stimulus, sensory perception occurs
ated system of receptors and sense organs that allows the elsewhere, at the level of the central nervous system.
organism as a whole to adapt to environmental influences. Morphologically, exteroreceptors are divided into:
This sensory system continuously receives external physi-
cal and chemical inputs and transmits these to the central · free nerve endings,
nervous system. The components of the sensory system · simple receptors and
include receptor organs, neural transmission pathways · lamellar receptors.
and processing centres within the grey matter of the
brain. Free nerve endings
Superficially located receptor organs, or exterorecep- Free nerve endings are terminal dendritic branches of
tors, permit detection of stimuli by the skin and other neuronal axons. They lose their Schwann cell sheath as
external surfaces. In the broadest sense, the sensory cells they penetrate the basal lamina and come to lie in the
of the eye, ear, taste buds and olfactory tissues can be epithelium (Figure 16.1). The nerve fibre receives meta-
included in this group. bolic support from adjacent epithelial cells. In this sense,
Receptor cells that detect movement and changes in epithelial cells perform a similar function to glial cells.
tension in muscle, tendons and joint capsules are referred The axon penetrates the deeper layers of the epithelium,
to as proprioceptors. These include sensory cells that regu- branches extensively and ends near the surface. Free
late balance. nerve endings are found in various locations, including
Enteroreceptors convey stimuli from the internal the epidermis, non-glandular mucosa, corneal epithelium
organs. and in the epithelial layers of the external root sheath of
Based on morphological and functional criteria, sen- hair follicles. They convey stimuli associated with touch
sory receptors can be described as primary or secondary. and pain.
Primary receptors are nerve cells in which stimuli are Some free nerve endings are closely associated with
detected by dendrites. Alteration of the potential differ- specialised tactile epithelial cells (Merkel cells) located
ence at the cell membrane elicits an action potential that primarily in the basal layer of the epithelium. These
is transmitted to the central nervous system (e.g. neuro- nerve–epithelial cell complexes (Merkel’s corpuscles)
sensory cells of the olfactory epithelium, photoreceptors are particularly numerous in the external root sheath
of the retina, unmyelinated free nerve endings in the of hair follicles and in the planum rostrale of the nose
epidermis). of the pig. Merkel’s corpuscles are pressure-sensing
Secondary receptors are modified epithelial cells that mechanoreceptors.
synapse with dendritic processes of afferent neurons. They
are activated by mechanical, thermal, chemical and acous- Simple receptors
tic stimuli (e.g. taste buds, hair cells of inner ear). Simple receptors are found in connective tissue. They con-
sist of a thin outer connective tissue layer encircling an
Exteroreceptors irregularly bundled or branched free nerve ending (Figures
External sensory stimuli include touch, pressure, vibra- 16.2 and 16.3). The nerve endings frequently have a bul-
tion, pain and changes in temperature. Reflecting the bous terminal expansion. Their Schwann cell coating ends
diverse nature of these inputs, the morphology of extero- as they enter the connective tissue sheath.
receptors varies widely from simple nerve endings to Simple receptors are numerous in the papillary layer of
complex receptor organs. Particular physiological stimuli the dermis and around hair follicles.

Vet Histology.indb 347 16/07/2019 15:06