| RECEPTORS |
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Sensory receptors detect stimuli impinging on us from the outside world – for example, our visual input – as well as from within our body itself – for example, we feel the pain of hunger if we have not eaten. This is the receiving or input part of our nervous system.
Sensory receptors are found throughout the body. They detect stimuli either:
i. directly by the peripheral end of a sensory neuron itself (e.g. free nerve endings), or
ii. by the peripheral nerve ending intermingling among special accessory cells and/or extracellular elements (e.g. Meissner and Pacinian corpuscles),
or
iii. by having special, often non-neural, cells detect the stimulus and then activate the peripheral process of the sensory neuron (e.g. cells of taste bud or hair cells of the auditory and vestibular systems).
In nearly all cases, the cell body of the sensory neuron is located in one of the cerebrospinal ganglia. |
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| Here the focus is on sensory receptors in skin and muscle. They are in the category of
somatosensory receptors (i.e. located in structures which develop from/in the body wall), and most are considered mechanoreceptors (i.e. adequate stimulus is mechanical energy). All of the ones shown below are encapsulated. Other neurons end
in body wall tissues without having a special capsule of accessory cells; often
they are the ones that detect noxious or painful stimuli. |
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Meissner corpuscles occur mainly in glabrous or non-hairy skin. They are sensitive to touch. The peripheral ending of the sensory neuron coils among specialized capsule cells in the pegs of dermis just below the epithelial layers (i.e. epidermis) of skin.
These photomicrographs show this receptor in different ways. One is stained to show the laminated pattern of accessory cells of
the corpuscle (left), while in the other the nerve fiber is accentuated
(right). |
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| Pacinian corpuscles occur in many areas of the body including mesentery, skin (dermis), capsules of exocrine glands (e.g pancreas, salivary glands) and fascial layers.
It is an on-off type receptor that responds very well, for example, to vibration. The peripheral ending of a sensory neuron is in the center of a multi-layered arrangement of capsule cells.
The photomicrographs show a whole mount of one
corpuscle (left) and a
histological section through three separate ones (right).
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Muscle spindles are
fusiform in shape and located in the fleshy part of a skeletal muscle, ‘in parallel’ with the extrafusal muscle fibers. They respond to lengthening or stretch of the muscle.
Peripheral processes of sensory neurons
end adjacent to intrafusal muscle fibers along the mid-length of the spindle.
The light micrographs show sections through the
middle (left) and polar (right) parts of spindles.
Muscle spindles are distinctive receptors in that the intrafusal muscle fibers also get a motor innervation (usually via gamma motor neurons)
in the polar region at each end of the spindle.
The electron micrographs show a sensory (left) and a
motor ending (right) on intrafusal fibers. Notice the prominent numbers of
mitochondria in the nerve terminals.
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Tendon organs are located in skeletal muscles where the fleshy part of the muscle joins the
tendon. Thus they have an ‘in series’ arrangement between the muscle fibers and the collagen of the tendon. They respond to tension generated either by an isometric contraction of muscle or by excessive
(i.e. potentially damaging) lengthening of the muscle. The ending of the sensory neuron intermingles among the collagen fascicles within the tendon organ.
In the photomicrograph, collagen is stained red and
the (extrafusal) muscle fibers are a yellow-gold color.
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Neurocytology |