Mechanoreceptors
Sensory receptors are transducers that convert some form of energy into a change in electrical potential across a neuron's membrane. Mechanoreceptors specifically respond to mechanical energy, or movement. Mechanoreceptors are found in many bodily locations, including the hair cells of the inner ear, the stretch receptors within the muscles, and the various types of receptors in the skin. Mechanoreceptors located in the bladder and parts of the alimentary canal alert the body to pressure within these organs.
Despite their varied locations and structures, all mechanoreceptors work in the same general way. Stimulation of these receptors changes the shape of the membranes in which they are embedded, opening mechanosensitive ion channels that either initiate an action potential, or directly cause the release of neurotransmitter molecules that affect the neighboring neuron.
The touch receptors of the skin vary in size and in the extent of their receptive fields. The touch receptors differ also in how long they continue to respond during a long-lasting stimulus and in the frequency of vibrations to which they are sensitive. Two types of skin mechanoreceptors are Pacinian corpuscles located deep in the dermis layer and Meissner's corpuscles located on the ridges of the fingerprints.
Vibrissae and other hairs on the body surface serve as mechanoreceptors. When a hair is disturbed, its bending changes the shape of its follicle and the skin tissue surrounding it. This in turn activates nerve endings in the area, initiating an action potential. Another group of mechanoreceptors are the proprioceptors found in the muscles and between the muscles and tendons. These receptors inform the brain of the position of various body parts in space; and if they are moving, in which direction, and how fast. There are two classes of the proprioceptors, Golgi tendon organs, and muscle spindles. Golgi tendon organs, located between a muscle and a tendon, monitor the force of a muscle contraction. As the force of a muscle contraction increases, the Golgi tendon organ indirectly causes a slowing of the muscle contraction. This adjustment is necessary for fine-motor coordination.
The muscle spindles monitor the degree to which a muscle is stretched, as in having a weight put on the muscle, for example. As the muscles are stretched, the muscle spindles cause the opening of mechanosensitive ion channels in the afferent neuron, depolarizing it and causing synaptic depolarization of the corresponding motor neuron, which shortens the spindle by causing the muscle to contract.
In the inner ear, the stereocilia, or hairs, of the hair cells are bent by vibrations. This bending causes one hair to pull on its immediate neighbor via tiny filaments called tip links. This briefly opens calcium ion channels in the neighboring hair, causing the release of the neurotransmitter glutamate from the base of the hair cell, stimulating the afferent neuron carrying its information.
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