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Sensory receptor cell responding to mechanical pressure or strain From Wikipedia, the free encyclopedia
A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.
Cutaneous mechanoreceptors respond to mechanical stimuli that result from physical interaction, including pressure and vibration. They are located in the skin, like other cutaneous receptors. They are all innervated by Aβ fibers, except the mechanorecepting free nerve endings, which are innervated by Aδ fibers. Cutaneous mechanoreceptors can be categorized by what kind of sensation they perceive, by the rate of adaptation, and by morphology. Furthermore, each has a different receptive field.
Cutaneous mechanoreceptors can also be separated into categories based on their rates of adaptation. When a mechanoreceptor receives a stimulus, it begins to fire impulses or action potentials at an elevated frequency (the stronger the stimulus, the higher the frequency). The cell, however, will soon "adapt" to a constant or static stimulus, and the pulses will subside to a normal rate. Receptors that adapt quickly (i.e., quickly return to a normal pulse rate) are referred to as "phasic". Those receptors that are slow to return to their normal firing rate are called tonic. Phasic mechanoreceptors are useful in sensing such things as texture or vibrations, whereas tonic receptors are useful for temperature and proprioception among others.
Cutaneous mechanoreceptors with small, accurate receptive fields are found in areas needing accurate taction (e.g. the fingertips). In the fingertips and lips, innervation density of slowly adapting type I and rapidly adapting type I mechanoreceptors are greatly increased. These two types of mechanoreceptors have small discrete receptive fields and are thought to underlie most low-threshold use of the fingers in assessing texture, surface slip, and flutter. Mechanoreceptors found in areas of the body with less tactile acuity tend to have larger receptive fields.
Lamellar corpuscles, or Pacinian corpuscles or Vater-Pacini corpuscle, are deformation or pressure receptors located in the skin and also in various internal organs.[8] Each is connected to a sensory neuron. Because of its relatively large size, a single lamellar corpuscle can be isolated and its properties studied. Mechanical pressure of varying strength and frequency can be applied to the corpuscle by stylus, and the resulting electrical activity detected by electrodes attached to the preparation.
Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential. If the generator potential reaches threshold, a volley of action potentials (nerve impulses) are triggered at the first node of Ranvier of the sensory neuron.
Once threshold is reached, the magnitude of the stimulus is encoded in the frequency of impulses generated in the neuron. So the more massive or rapid the deformation of a single corpuscle, the higher the frequency of nerve impulses generated in its neuron.
The optimal sensitivity of a lamellar corpuscle is 250 Hz, the frequency range generated upon finger tips by textures made of features smaller than 200 micrometres.[9]
There are four types of mechanoreceptors embedded in ligaments. As all these types of mechanoreceptors are myelinated, they can rapidly transmit sensory information regarding joint positions to the central nervous system.[10]
Type II and Type III mechanoreceptors in particular are believed to be linked to one's sense of proprioception.
Other mechanoreceptors than cutaneous ones include the hair cells, which are sensory receptors in the vestibular system of the inner ear, where they contribute to the auditory system and equilibrioception. Baroreceptors are a type of mechanoreceptor sensory neuron that is excited by stretch of the blood vessel. There are also juxtacapillary (J) receptors, which respond to events such as pulmonary edema, pulmonary emboli, pneumonia, and barotrauma.
The knee jerk is the popularly known stretch reflex (involuntary kick of the lower leg) induced by tapping the knee with a rubber-headed hammer. The hammer strikes a tendon that inserts an extensor muscle in the front of the thigh into the lower leg. Tapping the tendon stretches the thigh muscle, which activates stretch receptors within the muscle called muscle spindles. Each muscle spindle consists of sensory nerve endings wrapped around special muscle fibers called intrafusal muscle fibers. Stretching an intrafusal fiber initiates a volley of impulses in the sensory neuron (a I-a neuron) attached to it. The impulses travel along the sensory axon to the spinal cord where they form several kinds of synapses:
In somatosensory transduction, the afferent neurons transmit messages through synapses in the dorsal column nuclei, where second-order neurons send the signal to the thalamus and synapse with third-order neurons in the ventrobasal complex. The third-order neurons then send the signal to the somatosensory cortex.
More recent work has expanded the role of the cutaneous mechanoreceptors for feedback in fine motor control.[12] Single action potentials from Meissner's corpuscle, Pacinian corpuscle and Ruffini ending afferents are directly linked to muscle activation, whereas Merkel cell-neurite complex activation does not trigger muscle activity.[13]
Insect and arthropod mechanoreceptors include:[14]
Mechanoreceptors are also present in plant cells where they play an important role in normal growth, development and the sensing of their environment.[20] Mechanoreceptors aid the Venus flytrap (Dionaea muscipula Ellis) in capturing large[21] prey.[22]
Mechanoreceptor proteins are ion channels whose ion flow is induced by touch. Early research showed that touch transduction in the nematode Caenorhabditis elegans was found to require a two transmembrane, amiloride-sensitive ion channel protein related to epithelial sodium channels (ENaCs).[23] This protein, called MEC-4, forms a heteromeric Na+-selective channel together with MEC-10. Related genes in mammals are expressed in sensory neurons and were shown to be gated by low pH. The first of such receptor was ASIC1a, named so because it is an acid sensing ion channel (ASIC).[24]