A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. Normally there are four main types in glabrous skin: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini endings. There are also mechanoreceptors in hairy skin, and the hair cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into nerve signals sent to the brain. In the periodontal ligament, there are some mechanoreceptors, which allow the jaw to relax when biting down on hard objects; the mesencephalic nucleus is responsible for this reflex.
Mechanism of sensation 
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. Single action potentials from RAI and PC afferents are directly linked to activation of related hand muscles, whereas SAI activation does not trigger muscle activity.
Cutaneous mechanoreceptors 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. They can be categorized by morphology, by what kind of sensation they perceive and by the rate of adaptation. Furthermore, each has a different receptive field.
By morphology 
- Ruffini's end organs detect tension deep in the skin.
- Meissner's corpuscles detect changes in texture (vibrations around 50 Hz) and adapt rapidly.
- Pacinian corpuscles detect rapid vibrations (about 200–300 Hz).
- Merkel's discs detect sustained touch and pressure.
- Mechanoreceiving free nerve endings detect touch, pressure and stretching
- Hair follicle receptors are located in hair follicles and sense position changes of hairs.
By sensation 
Cutaneous mechanoreceptors provide the senses of touch, pressure, vibration, proprioception and others.
- The Slowly Adapting type 1 (SA1) mechanoreceptor, with the Merkel cell end-organ, 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) mechanoreceptors respond to skin stretch, but have 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 underlies the perception of flutter and slip on the skin. They have small receptive fields and produce transient responses to the onset and offset of stimulation.
- Pacinian receptors underlie the perception of high frequency vibration. They also produce transient responses, but have large receptive fields.
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.
- 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.
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.
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.
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.
- Type I: (small) Low threshold, slow adapting in both static and dynamic settings
- Type II: (medium) Low threshold, rapidly adapting in dynamic settings
- Type III: (large) High threshold, slowly adapting in dynamic settings
- Type IV: (very small) High threshold pain receptors that communicate injury
Type II and Type III mechanoreceptors in particular are believed to be linked to one's sense of proprioception
Pacinian Corpuscle 
Pacinian corpuscles are 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 Pacinian 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.
Muscle spindles and the stretch reflex 
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 spindle fibers (also called intrafusal fibers). Stretching a spindle 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:
- Some of the branches of the I-a axons synapse directly with alpha motor neurons.These carry impulses back to the same muscle causing it to contract. The leg straightens.
- Some of the branches of the I-a axons synapse with inhibitory interneurons in the spinal cord. These, in turn, synapse with motor neurons leading back to the antagonistic muscle, a flexor in the back of the thigh. By inhibiting the flexor, these interneurons aid contraction of the extensor.
- Still other branches of the I-a axons synapse with interneurons leading to brain centers, e.g., the cerebellum, that coordinate body movements.
See also 
- [Johansson and Flanagan, Nature Reviews Neuroscience, 2009, 10: 345-359]
- [McNulty and Macefield J Physiol. 2001 Dec 15;537(Pt 3):1021-32]
- [Johansson and Vallbo, 1983, Trends in Neurosci. 6: 27-31]
- [Johnson and Hsiao, Annual Review of Neuroscience, 1992; 15:227-50]
- [Toerbjork and Ochoa Acta Physiol Scand. 1980 Dec;110(4):445-7]
- [Talbot et al. J Neurophysiol. 1968 Mar;31(2):301-34]
- [Johansson and Westling Exp Brain Res. 1987;66(1):141-54]
- Michelson, JD; Hutchins, C (1995). "Mechanoreceptors in human ankle ligaments". The Journal of bone and joint surgery. British volume 77 (2): 219–24. PMID 7706334.
- Scheibert J, Leurent S, Prevost A, Debrégeas G. (2009). The role of fingerprints in the coding of tactile information probed with a biomimetic sensor. Science. 323(5920):1503-6. PMID 19179493 doi:10.1126/science.1166467
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