The mechanoreceptors are sensory receptors found in human skin and are sensitive to mechanical pressure.
There are five types of mechanoreceptors in human skin: Pacini’s corpuscles, Meissner’s corpuscles, Krause’s corpuscles, Merkel’s nerve endings, and Ruffini’s corpuscles.
Each of these receptors is responsible for a different function and together they allow to recognize all the possible sensations that are established through the connection between the external stimulus and the internal interpretation that occurs thanks to the central nervous system.
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Viewed from a general perspective, mechanoreceptors are small sensors that translate each electromagnetic, mechanical, or chemical stimulus into nerve impulses that are interpreted by the brain.
Types of mechanoreceptors
Types of mechanoreceptors
In glabrous (hairless) skin, there are four main types of mechanoreceptors, each shaped according to its function:
Tactile corpuscles (also known as Meissner corpuscles) respond to light touch and quickly adapt to changes in texture (vibrations around 50 Hz).
Bulbous corpuscles (also known as Ruffini endings) sense deep tension in the skin and fascia.
Merkel’s nerve endings (also known as Merkel’s discs) detect sustained pressure.
Lamellar corpuscles (also known as Pacini’s corpuscles) in the skin and fascia detect rapid vibrations (approximately 200-300 Hz).
Receptors in hair follicles sense when a hair changes position. In fact, the most sensitive mechanoreceptors in humans are the hair cells of the cochlea of the inner ear, unrelated to follicular receptors, these receptors transduce sound for the brain.
Mechanosensory free nerve endings sense touch, pressure, and stretch.
Baroreceptors are a type of mechanoreceptor sensory neuron that is excited by stretching of the blood vessel.
Cutaneous mechanoreceptors respond to mechanical stimuli that result from physical interaction, including pressure and vibration. They are located on the skin, like other cutaneous receptors.
All of them are innervated by Aβ fibers, except for free mechanoreceptor nerve endings, which are innervated by Aδ fibers.
Cutaneous mechanoreceptors can be classified by morphology, by what type of sensation they perceive, and by the speed of adaptation. Also, each has a different receptive field.
1-The slowly adapting type 1 mechanoreceptor (SA1), with the terminal organ of Merkel’s corpuscle, underlies the perception of shape and roughness in the skin. They have small receptive fields and produce sustained responses to static stimulation.
2-Slowly adapting type 2 mechanoreceptors (SA2), with the terminal organ of Ruffini’s corpuscle, respond to skin stretching, but have not been closely linked to proprioceptive or mechanoreceptive roles in perception. They also produce sustained responses to static stimulation, but have large receptive fields.
3-The “Rapidly Adapting” (RA) or Meissner corpuscle end organ mechanoreceptor, underlies the perception of flapping and glides on the skin. They have small receptive fields and produce transient responses to the initiation and displacement of stimulation.
4-The Pacini corpuscle or Váter-Pacini corpuscles or laminar corpuscles underlie the perception of high frequency vibration. They also produce transient responses, but have large receptive fields.
By adaptation rate
Cutaneous mechanoreceptors can also be separated into categories based on their adaptation rates.
When a mechanoreceptor receives a stimulus, it begins to fire impulses or action potentials at a high frequency (the stronger the stimulus, the higher the frequency).
The cell, however, will soon “adapt” to a constant or static stimulus, and the impulses will decrease at a normal rate.
Receptors that adapt quickly (that is, quickly return to a normal pulse rate) are called “phasic.”
Those receptors that are slow to return to their normal firing rate are called tonic. Phasic mechanoreceptors are useful for detecting things such as texture or vibrations, while tonic receptors are useful for temperature and proprioception, among others.
1- Slow adaptation : Slow adaptation mechanoreceptors include the end organs of the Merkel and Ruffini corpuscles and some free nerve endings.
Slowly adapting type I mechanoreceptors have multiple Merkel corpuscle end organs.
Slowly adapting type II mechanoreceptors have unique Ruffini corpuscle end organs.
2- Intermediate adaptation : Some free nerve endings are intermediate adaptation.
3- Rapid adaptation : Rapid adaptation mechanoreceptors include the end organs of the corpuscle of Meissner, the end organs of the corpuscle of Pacini, the 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 pacinians) have end organs of the corpuscle of Pacini.
Other non-cutaneous mechanoreceptors include hair cells, which are sensory receptors in the vestibular system of the inner ear, where they contribute to the auditory system and balance perception.
There are also Juxtacapillary receptors (J), which respond to events such as pulmonary edema, pulmonary embolism, pneumonia, and barotrauma.
There are four types of mechanoreceptors embedded in the ligaments. Because all of 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 adaptation in static and dynamic configuration.
Type II : (medium) Low threshold, rapid adaptation in dynamic environments.
Type III : (large) High threshold, adapting slowly in dynamic environments.
Type IV : (very small) High-threshold pain receptor that communicates injury.
Type II and type III mechanoreceptors in particular are believed to be associated with the proper sense of proprioception.
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