Histology of Sensory Receptors I. Introduction A ... - Faculty.rmc.edu

Histology of Sensory Receptors
I. Introduction
A. Sensory receptors always contain a transducing system which converts the stimulus into an
altered transmembrane electrical potential in the receptor cell. In the structurally simplest
types of receptors (free nerve endings) the transducing system is apparently built into the
membrane of dendrite-like branches at the peripheral end of the axon of the sensory neuron.
When the transducing system is stimulated, the altered transmembrane potential causes an
action potential in the axon which transmits the action potential back to the CNS.
B. The transducing system does not have to be in an anatomically recognizable neuron, but
may instead be in an altered epithelial cell (or perhaps other type of cell) which has some
neuron-like properties and which communicates with a sensory neuron through a chemical
synapse (i.e. taste buds, hair cells in the ear).
C. The stimulus may not fall directly on the receptor cell surface, but may first be processed and
altered by an accessory structure before reaching the receptor cell. The accessory structure
may act to increase the chance of a stimulus reaching the receptor cell, restrict the type or
direction of stimuli which reach the receptor cell, or alter the form of the stimulus (sound
waves -> mechanical vibration of hair cells in the ear).
II.
Free Nerve Endings
A. Naked nerve endings (Figure 15.12)
1. Naked nerve endings are highly branched, dendrite-like endings of sensory neurons and
are not covered by Schwann cells. These endings are usually associated with epidermal
cells or with connective tissue fibers.
2. Naked nerve endings function as touch receptors in the cornea and as pain receptors
(probably by chemoreception of bradykinin released by damaged tissues) in skin and
other areas.
B. Merkel's disks (Figure 15.12)
1. Merkel's disks are naked nerve endings which have expanded disk-like termini in contact
with Merkel cells in the basal layer of the epidermis. The Merkel cells do not appear to
be epidermal cells but are probably derived from neural crest (same source as PNS
neurons).
2. Merkel's disks function as touch receptors in the tongue and as slowly adapting
mechanoreceptors which react to skin deformation.
C. Basket arrays
1. Basket arrays are naked nerve endings which spiral around the base of hair follicles.
2. Basket arrays apparently function as touch and movement receptors.
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III. Myoneural Receptors
A. Golgi tendon organs
1. Golgi tendon organs are small spindle-shaped aggregates containing several bundles of
collagen fibers with free nerve endings wrapped around the collagen fibers. The whole
complex is surrounded by a thin capsule of flattened fibroblastic cells.
2. Golgi tendon organs occur in the tendons of skeletal muscles and respond to tension in
the tendon.
B. Neuromuscular spindles (muscle spindles) (Figure 11.12)
1. Neuromuscular spindles contain two types of modified skeletal muscle cells which have
organized myofibrils in their distal ends but contain few organized myofibrils in their
central regions (where their nuclei are located).
a. Nuclear bag fibers contain nuclei clumped in the center of the cell.
b. Nuclear chain fibers contain nuclei in a row in the center region of the cell.
(1) Primary sensory nerve endings spiral around the central region of both types of
modified cells.
(2) Secondary sensory nerve endings are attached as highly branched "flower
spray" endings on the surface of the nuclear chain fibers on both ends just
distal to the central (nuclear) region.
(3) Both fiber types have motor synapses (gamma efferent) on both distal regions
(where the myofibrils are highly organized) which cause the cell ends to
contract, stretching the sensory receptor region in the nuclear region of the cell
and stimulating the receptor.
(4) The receptor complex is surrounded by a capsule of flattened cells continuous
with the Schwann cells of the peripheral nerve supplying the muscle and the
perimysium (the part of the connective tissue framework of the muscle which
surrounds each bundle of muscle cells and the part which attaches to the
tendon at each end of the muscle) of the muscle. The modified muscle cells in
the receptor are attached to the capsule at their ends and are therefore
attached to the perimysium of the muscle.
2. Neuromuscular spindles monitor tension and stretch in the skeletal muscle since either
force pulls on the perimysium which pulls on the muscle cells in the receptor.
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IV. Encapsulated Receptors
A. Pacinian corpuscles (Figure 15.13, Plate 42)
1. Pacinian corpuscles contain a central cylindrical sensory nerve ending surrounded by a
capsule "wall" of concentric layers of flattened "fibroblastic" cells interspersed with
sparse fine collagen fibers and ground substance.
2. Pacinian corpuscles function as pressure and vibration receptors in subcutaneous and
internal connective tissues.
B. Meissner's corpuscles (Figure 15.13, Plate 42)
1. Meissner's corpuscles contain highly branched, frequently spirally arranged nerve
endings winding among transversely stacked flat fibroblastic cells which form an eliptical
aggregate which may be surrounded by a thin capsule of connective tissue (loose to
moderately dense, rich in flattened fibroblasts).
2. The function of Meissner's corpuscles is not rigorously defined, but they are presumed to
function as light touch receptors since they are most numerous in the dermal papillae of
the skin on fingers, lips, and genitalia.
C. Ruffinian corpuscles (Figure 15.12)
1. The structure of Ruffinian corpuscles is very similar to Golgi tendon organs. They
contain a highly branched nerve ending wound around a spindle-shaped bundle of
collagen fibers. The entire complex is wrapped in a connective tissue capsule of fine
collagen fibers plus flattened fibroblasts.
2. Ruffinian corpuscles are located in deep connective tissues below skin, in joint capsules,
and in the walls of some blood vessels. Their function is unclear, but their location and
structure suggests that they may detect vibration or distortion of connective tissues.
D. Krause's end bulbs (genital corpuscles) (Figure 15.12)
1. Krause's end bulbs contain a highly branched nerve ending in a fluid-filled cavity
surrounded by a spherical capsule of collagen fibers and fibroblasts.
2. Krause's end bulbs are located in dermis of skin and in tendons and ligaments. Their
function is unknown, but they are assumed to be mechanoreceptors.
V. Baroreceptors
A. Baroreceptors are highly branched free nerve endings in the tunica adventitia of all large
arteries of the head and neck.
B. Baroreceptors apparently detect blood pressure by monitoring the stretch in the wall of the
vessel (remember that elastic arteries stretch in response to increased blood pressure).
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VI. Chemoreceptors
A. Aortic and carotid bodies
1. Aortic and carotid bodies are 2-3 mm diameter round structures supplied by an
extensive capillary bed arising from a small artery derived directly from the associated
large artery. Aortic and carotid bodies contain naked nerve fibers closely associated
with capillaries. The receptors are apparently the nerve endings.
Aortic and carotid bodies also contain glomus cells which appear to contain
catecholamine or dopamine secretory granules and which may alter the receptor
sensitivity or may have a totally separate function.
2. Aortic and carotid bodies appear to be sensitive to hydrogen ion, bicarbonate ion, and
dissolved oxygen concentrations. Aortic and carotid bodies are apparently most
sensitive to oxygen, but oxygen levels never fall low enough to stimulate them under
normal conditions, so these receptors normally respond to hydrogen ions and
bicarbonate ions produces when CO 2 reacts with water.
B. Olfactory epithelium (Figure 19.3, Plate 65)
1. Olfactory epithelium receptor cells are modified bipolar neurons which have their cell
bodies in the nasal epithelium and which extend a ciliated dendrite up to the apical
surface of the epithelium. The axons of the receptor cells leave the basal surface of the
epithelium and are grouped into bundles to form the olfactory nerve.
The olfactory epithelium also contains columnar "sustentacular" cells with apical
microvillae and small rounded basal cells with more heterochromatic nuclei, forming a
pseudostratified columnar epithelium which is noticeable thicker than the nasal
epithelium in the non-sensory areas.
2. Olfactory sensory cells respond to many different chemical stimuli and are responsible
for most sensations of taste as well as smell.
C. Taste buds (Figure 16.5, Plate 46)
1. At least 3 cell types occur in the taste buds, and some references describe 5-6 types.
a. "dark" cells (supportive or sustentacular cells)
b. "light" or neuroepithelial cells (receptor cells which synapse with sensory neurons)
c. basal cells (stem or replacement cells for the other types)
These cells form oval "islands" of pseudostratified columnar epithelium in the
stratified squamous epithelium of the lateral (circumvalllate papillae) or apical
(fungiform papillae) surfaces of the papillae on the tongue.
2. Taste bud receptor cells sense acids, bases, salts, and sugars in the oral cavity, but the
detailed mechanism is unknown. The mechanism apparently involves cell surface
receptors on the receptor cells.
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