Mechanoreceptor

Overview
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles. There are also mechanoreceptors in the hairy skin, and the hair cells in the cochlea are the most sensitive mechanoreceptors in tranducing air pressure waves into sound.

Mechanism of sensation
Mechanoreceptors are primary neurons that respond to mechanical stimuli by firing action potentials. Peripheral transduction is believed to occur in the end-organs.

In sensory transduction, the afferent neurons transmit the message through a synapse in the dorsal column nuclei, where another neuron sends the signal to the thalamus, where another neuron sends the signal  to the somatosensory cortex.

Feedback
More recent work has expanded the role of the 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.

History
The human work stemmed from Vallbo and Johansson's percutaneous recordings from human volunteers in the late 1970s. Work in rhesus monkeys has found virtually identical mechanoreceptors with the exception of Ruffini corpuscles which are not found in the monkey.

Types
There are two ways to categorize mechanoreceptors; by what kind of sensation they perceive and by the rate of adaption.

By sensation
Cutaneous mechanoreceptors provide the senses of touch, pressure, vibration, proprioception and others.


 * The SAI type mechanoreceptor, with the Merkel cell end-organ, underlies the perception of form and roughness on the skin.


 * The RAI type mechanoreceptor underlies the perception of flutter, and slip on the skin.


 * Pacinian receptors underlie the perception of high frequency vibration. SAII mechanoreceptors respond to skin stretch, but have not been closely linked to either proprioceptive or mechanoreceptive roles in perception.

By rate of adaption
Mechanoreceptors can also be separated into categories based on their rates of adaptivity. 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, vibrations, etc; whereas tonic receptors are useful for temperature and proprioception among others.


 * Slowly adapting type I mechanoreceptors have multiple Merkel corpuscle end-organs.
 * Slowly adapting type II mechanoreceptors have single Ruffini corpuscle end-organs.
 * 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.

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 1 and rapidly adapting type 1 mechanoreceptors are greatly increased. These two types of mechanoreceptors have small discrete receptive fields and are thought to underly 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.