Mechanoreceptor

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.

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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 somatosensory 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,^[1] whereas SAI activatio 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

Mechanoreceptors are mainly cutaneous ones, but there are also other types, e.g. hair cells.

Cutaneous

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 both by morphology, by what kind of sensation they perceive and by the rate of adaption. Furthermore, they have different receptive field.

By morphology

• Ruffini's end organ detects sustained pressure
• Meissner's corpuscle detects changes in texture and slow vibrations
• Pacinian corpuscle detects deep pressure and fast vibrations
• Merkel's disc detects sustained touch and pressure.
• Mechanorecepting Free nerve endings ((touch, pressure, stretch)
• Hair follicle receptors are located in hair follicles and sense the position of hairs.

By sensation

See also: Cutaneous receptor modalities

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.^[2]

• The RAI type mechanoreceptor underlies the perception of flutter,^[3] and slip on the skin.^[4]

• Pacinian receptors underlie the perception of high frequency vibration.^[5] SAII mechanoreceptors respond to skin stretch, but have not been closely linked to either proprioceptive or mechanoreceptive roles in perception.^[6]

By rate of adaption

Cutaneous 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

Slowly adapting mechanoreceptors include Merkel and Ruffini corpuscle end-organs, 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.

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 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.

Others

Other mechanoreceptors than cutaneous ones include the hair cells, which are sensory receptors in the vestibular system in the inner ear, where they contribute to the auditory system and equilibrioception.

Notes

1. ^ [McNulty and Macefield J Physiol. 2001 Dec 15;537(Pt 3):1021-32]
2. ^ [Johnson and Hsiao, Annual Review of Neuroscience, 1992; 15:227-50]
3. ^ [Talbot et al J Neurophysiol. 1968 Mar;31(2):301-34]
4. ^ [Johansson and Westling Exp Brain Res. 1987;66(1):141-54]
5. ^ [Talbot et al J Neurophysiol. 1968 Mar;31(2):301-34]
6. ^ [Toerbjork and Ochoa Acta Physiol Scand. 1980 Dec;110(4):445-7]

See also

• Thermoreceptor
• Nociceptor
• Proprioception