Cutaneous mechanoreceptors respond to mechanical stimuli that result from physical interaction, including pressure and vibration. They are located in the skin, like othercutaneous receptors. They are all innervated byAβ fibers, except the mechanoreceptingfree nerve endings, which are innervated byAδ 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 differentreceptive field.[citation needed]
The Slowly Adapting type 1 (SA1) mechanoreceptor, with theMerkel corpuscle end-organ (also known as Merkel discs) detect sustained pressure and underlies the perception of form and roughness on the skin.[1] They have small receptive fields and produce sustained responses to static stimulation.[citation needed]
The Slowly Adapting type 2 (SA2) mechanoreceptors, with theRuffini corpuscle end-organ (also known as thebulbous corpuscles), detect tension deep in the skin andfascia and respond to skin stretch, but have not been closely linked to either proprioceptive or mechanoreceptive roles in perception.[2] They also produce sustained responses to static stimulation, but have large receptive fields.[citation needed]
The Rapidly Adapting (RA) orMeissner corpuscle end-organ mechanoreceptor (also known as thetactile corpuscles) underlies the perception of light touch such as flutter[3] and slip on the skin.[4] It adapts rapidly to changes in texture (vibrations around 50 Hz). They have small receptive fields and produce transient responses to the onset and offset of stimulation.[citation needed]
Free nerve endings detect touch, pressure, stretching, as well as the tickle and itch sensations. Itch sensations are caused by stimulation of free nerve ending from chemicals.[7]
Hair follicle receptors called hair root plexuses sense when ahair changes position. Indeed, the most sensitive mechanoreceptors in humans are thehair cells in thecochlea of theinner ear (no relation to the follicular receptors – they are named for the hair-like mechanosensorystereocilia they possess); these receptorstransducesound for the brain.[7]
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 oraction 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 calledtonic. Phasic mechanoreceptors are useful in sensing such things as texture or vibrations, whereas tonic receptors are useful for temperature andproprioception among others.[citation needed]
Cutaneous mechanoreceptors with small, accuratereceptive 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 largerreceptive fields.[citation needed]
Lamellar corpuscles, or Pacinian corpuscles or Vater-Pacini corpuscle, are deformation or pressure receptors located in the skin and also in various internal organs.[8] 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.[citation needed]
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 firstnode of Ranvier of the sensory neuron.[citation needed]
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.[citation needed]
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.[9]
There are four types of mechanoreceptors embedded inligaments. As all these types of mechanoreceptors aremyelinated, they can rapidly transmit sensory information regarding joint positions to thecentral nervous system.[10]
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 ofproprioception.
Theknee jerk is the popularly knownstretch reflex (involuntary kick of the lower leg) induced by tapping the knee with a rubber-headed hammer. The hammer strikes atendon thatinserts anextensor muscle in the front of the thigh into the lower leg. Tapping the tendon stretches the thigh muscle, which activatesstretch receptors within the muscle calledmuscle spindles. Each muscle spindle consists of sensory nerve endings wrapped around special muscle fibers calledintrafusal muscle fibers. Stretching an intrafusal fiber initiates a volley of impulses in the sensory neuron (aI-a neuron) attached to it. The impulses travel along the sensory axon to the spinal cord where they form several kinds ofsynapses:[citation needed]
Some of the branches of the I-a axons synapse directly withalpha 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.[11]
Insect and arthropod mechanoreceptors include:[14]
Campaniform sensilla: Small domes in theexoskeleton that are distributed all along the insect's body. These cells are thought to detect mechanical load as resistance to muscle contraction, similar to the mammalianGolgi tendon organs.
Hair plates: Sensory neurons that innervate hairs that are found in the folds of insect joints. These hairs are deflected when one body segment moves relative to an adjoining segment, they haveproprioceptive function, and are thought to act as limit detectors encoding the extreme ranges of motion for each joint.[15]
Chordotonal organs: Internal stretch receptors at the joints, can have bothextero- andproprioceptive functions. The neurons in the chordotonal organ inDrosophila melanogaster can be organized into club, claw, and hook neurons. Club neurons are thought to encode vibrational signals while claw and hook neurons can be subdivided into extension and flexion populations that encode joint angle and movement respectively.[16]
Bristle sensilla: Bristle neurons are mechanoreceptors that innervate hairs all along the body. Each neuron extends a dendritic process to innervate a single hair and projects its axon to the ventral nerve cord. These neurons are thought to mediate touch sensation by responding to physical deflections of the hair.[17] In line with the fact that many insects exhibit different sized hairs, commonly referred to as macrochaetes (thicker longer hairs) and microchaetes (thinner shorter hairs), previous studies suggest that bristle neurons to these different hairs may have different firing properties such as resting membrane potential and firing threshold.[18][19]
Mechanoreceptor proteins areion channels whose ion flow is induced by touch. Early research showed that touch transduction in thenematodeCaenorhabditis elegans was found to require a two transmembrane,amiloride-sensitive ion channel protein related toepithelial sodium channels (ENaCs).[23] This protein, called MEC-4, forms a heteromeric Na+-selective channel together with MEC-10. Related genes in mammals are expressed insensory neurons and were shown to be gated by lowpH. The first of such receptor was ASIC1a, named so because it is anacid sensing ion channel (ASIC).[24]
^Torebjörk HE, Ochoa JL (December 1980). "Specific sensations evoked by activity in single identified sensory units in man".Acta Physiologica Scandinavica.110 (4):445–7.doi:10.1111/j.1748-1716.1980.tb06695.x.PMID7234450.
^abTalbot WH, Darian-Smith I, Kornhuber HH, Mountcastle VB (March 1968). "The sense of flutter-vibration: comparison of the human capacity with response patterns of mechanoreceptive afferents from the monkey hand".Journal of Neurophysiology.31 (2):301–34.doi:10.1152/jn.1968.31.2.301.PMID4972033.
^Johansson RS, Westling G (1987). "Signals in tactile afferents from the fingers eliciting adaptive motor responses during precision grip".Experimental Brain Research.66 (1):141–54.doi:10.1007/bf00236210.PMID3582528.S2CID22450227.
^Biswas A (2015).Characterization and Modeling of Vibrotactile Sensitivity Threshold of Human Finger Pad and the Pacinian Corpuscle (PhD). Indian Institute of Technology Madras, Tamil Nadu, India.doi:10.13140/RG.2.2.18103.11687.
^Johansson RS, Flanagan JR (May 2009). "Coding and use of tactile signals from the fingertips in object manipulation tasks".Nature Reviews. Neuroscience.10 (5):345–59.doi:10.1038/nrn2621.PMID19352402.S2CID17298704.
^Chamovitz D (2012).What a plant knows: a field guide to the senses (1st ed.). New York: Scientific American/Farrar, Straus and Giroux.ISBN978-0-374-53388-5.OCLC755641050.
