A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.
Vertebrate mechanoreceptors
Cutaneous mechanoreceptors 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.
By sensation The slowly adapting type 1 (SA1) mechanoreceptor, with the Merkel corpuscle end-organ (also known as Merkel discs), detects sustained pressure and underlies the perception of form and roughness on the skin. They have small receptive fields and produce sustained responses to static stimulation. The slowly adapting type 2 (SA2) mechanoreceptor, with the Ruffini corpuscle end-organ (also known as the bulbous corpuscle), detects tension deep in the skin and fascia and responds to skin stretch but has not been closely linked to either proprioceptive or mechanoreceptive roles in perception. They also produce sustained responses to static stimulation, but have large receptive fields. The rapidly adapting (RA) mechanoreceptor, with the Meissner corpuscle end-organ (also known as the tactile corpuscle), underlies the perception of light touch such as flutter and slip on the skin. It adapts rapidly to changes in texture (vibrations around 50 Hz). It has a small receptive field and produces transient responses to the onset and offset of stimulation. The lamellar corpuscle, also known as the Pacinian and Vater–Pacini corpuscle, in the skin and fascia detects rapid vibrations of about 200–300 Hz. It also produces transient responses but has a large receptive field. A free nerve ending detects touch, pressure, stretching, and the tickle and itch sensations. Itch is caused by stimulation of free nerve endings by chemicals. A hair follicle receptor, or hair root plexus, senses when a hair changes position. Indeed, the most sensitive mechanoreceptors in humans are A hair cell, in the cochlea of the inner ear (no relation to the follicular receptors – they are named for the hair-like mechanosensory stereocilia they possess) detects sound-frequency vibrations, in a range of roughly 20–20000 Hz, for hearing.
By rate of adaptation 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.
Slowly adapting: Slowly adapting mechanoreceptors include Merkel and Ruffini corpuscle end-organs, and some free nerve endings. Slowly adapting type I mechanoreceptors have multiple Merkel corpuscle end-organs. Slowly adapting type II mechanoreceptors have single Ruffini corpuscle end-organs. Intermediate adapting: Some free nerve endings are intermediate adapting. Rapidly adapting: Rapidly adapting mechanoreceptors include Meissner corpuscle end-organs, Pacinian corpuscle end-organs, hair follicle receptors and some free nerve endings. Rapidly adapting type I mechanoreceptors have multiple Meissner corpuscle end-organs. Rapidly adapting type II mechanoreceptors (usually called Pacinian) have single Pacinian corpuscle end-organs.
By receptive field 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 Lamellar corpuscles, or Pacinian corpuscles or Vater–Pacini corpuscles, are deformation, or pressure, receptors located in the skin and also in various internal organs. 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.
Ligamentous mechanoreceptors 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.
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