A mechanoreceptor, also called mechanoceptor, is a

sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduc ...

that responds to mechanical

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

or distortion. Mechanoreceptors are located on

sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduc ...

s that convert mechanical pressure into electrical signals that, in animals, are sent to the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

Vertebrate mechanoreceptors

Cutaneous mechanoreceptors

Cutaneous mechanoreceptors respond to mechanical stimuli that result from physical interaction, including pressure and vibration. They 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. Cutaneous mechanoreceptors can be categorized by what kind of sensation they perceive, by the rate of adaptation, and by morphology. Furthermore, each has a different

receptive field The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organisms. Complexity of the receptive field ranges from the unidimensional chemical structure of od ...

By sensation

* The Slowly Adapting type 1 (SA1) mechanoreceptor, with the

Merkel corpuscle end-organ Merkel nerve endings (also Merkel's disks, or Merkel tactile endings) are mechanoreceptors situated in the basal epidermis as well as around the apical ends or some hair follicles. They are slowly adapting. They have small receptive fields measur ...

(also known as Merkel discs) detect sustained pressure and 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, with the

Ruffini corpuscle end-organ The bulbous corpuscle, Ruffini ending or Ruffini corpuscle is a slowly adapting mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are locat ...

(also known as the

bulbous corpuscle The bulbous corpuscle, Ruffini ending or Ruffini corpuscle is a slowly adapting mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are locat ...

s), detect tension deep in the skin and

fascia A fascia (; : fasciae or fascias; adjective fascial; ) is a generic term for macroscopic membranous bodily structures. Fasciae are classified as superficial, visceral or deep, and further designated according to their anatomical location. ...

and 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) or

Meissner corpuscle end-organ Tactile corpuscles or Meissner's corpuscles are a type of mechanoreceptor discovered by anatomist Georg Meissner (1829–1905) and Rudolf Wagner. This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to pressur ...

mechanoreceptor (also known as the

tactile corpuscle Tactile corpuscles or Meissner's corpuscles are a type of mechanoreceptor discovered by anatomist Georg Meissner (1829–1905) and Rudolf Wagner. This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to pressur ...

s) underlies the perception of light touch such as flutter and slip on the skin. 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. * The Pacinian corpuscle or Vater-Pacinian corpuscles or Lamellar corpuscles in the skin and fascia detect rapid vibrations of about 200–300 Hz. They also produce transient responses, but have large receptive fields. *

Free nerve ending A free nerve ending (FNE) or bare nerve ending, is an unspecialized, afferent nerve fiber sending its signal to a sensory neuron. ''Afferent'' in this case means bringing information from the body's periphery toward the brain. They function as cu ...

s detect touch, pressure, stretching, as well as the tickle and itch sensations. Itch sensations are caused by stimulation of free nerve ending from chemicals. *

Hair follicle The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction betwee ...

receptors called hair root plexuses sense when a

hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and ...

changes position. Indeed, the most sensitive mechanoreceptors in humans are the

hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...

s in the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...

of the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

(no relation to the follicular receptors – they are named for the hair-like mechanosensory

stereocilia Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cel ...

they possess); these receptors transduce

sound In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...

for the brain.

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 potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

s 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 Proprioception ( ) is the sense of self-movement, force, and body position. Proprioception is mediated by proprioceptors, a type of sensory receptor, located within muscles, tendons, and joints. Most animals possess multiple subtypes of propri ...

among others. * Slowly adapting: Slowly adapting mechanoreceptors include Merkel and

s, and some free nerve endings. ** Slowly adapting type I mechanoreceptors have multiple

s. ** Slowly adapting type II mechanoreceptors have single

s. * Intermediate adapting: Some free nerve endings are intermediate adapting. * Rapidly adapting: Rapidly adapting mechanoreceptors include

Pacinian corpuscle end-organs The Pacinian corpuscle (also lamellar corpuscle, or Vater–Pacini corpuscle) is a low-threshold mechanoreceptor responsive to vibration or pressure, found in the skin and other internal organs. In the skin it is one of the four main types of cuta ...

, hair follicle receptors and some free nerve endings. ** Rapidly adapting type I mechanoreceptors have multiple

s. ** Rapidly adapting type II mechanoreceptors (usually called Pacinian) have single

By receptive field

Cutaneous mechanoreceptors with small, accurate

s 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.

Lamellar corpuscles

Lamellar corpuscle The Pacinian corpuscle (also lamellar corpuscle, or Vater–Pacini corpuscle) is a low-threshold mechanoreceptor responsive to vibration or pressure, found in the skin and other internal organs. In the skin it is one of the four main types of cuta ...

s, or Pacinian corpuscles or Vater-Pacini corpuscle, are deformation or 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 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. 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 Nodes of Ranvier ( ), also known as myelin-sheath gaps, occur along a myelinated axon where the axolemma is exposed to the extracellular space. Nodes of Ranvier are uninsulated axonal domains that are high in sodium and potassium ion channels co ...

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. 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

micrometre The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...

Ligamentous mechanoreceptors

There are four types of mechanoreceptors embedded in

ligament A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...

s. As all these types of mechanoreceptors are

myelin Myelin Sheath ( ) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be lik ...

ated, they can rapidly transmit sensory information regarding joint positions to the

. * 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

Other mechanoreceptors

Other mechanoreceptors than cutaneous ones include the

s, which are

s in the

vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating motor coordination, movement with balance. Together with the cochlea, a part of the auditory sys ...

of the

, where they contribute to the

auditory system The auditory system is the sensory system for the sense of hearing. It includes both the ear, sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, incre ...

and

equilibrioception The sense of balance or equilibrioception is the perception of balance and spatial orientation. It helps prevent humans and nonhuman animals from falling over when standing or moving. Equilibrioception is the result of a number of sensory sy ...

. Baroreceptors are a type of mechanoreceptor sensory neuron that is excited by stretch of the blood vessel. There are also juxtacapillary (J) receptors, which respond to events such as

pulmonary edema Pulmonary edema (British English: oedema), also known as pulmonary congestion, is excessive fluid accumulation in the tissue or air spaces (usually alveoli) of the lungs. This leads to impaired gas exchange, most often leading to shortness ...

, pulmonary emboli,

pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...

, and

barotrauma Barotrauma is physical damage to body tissues caused by a difference in pressure between a gas space inside, or in contact with, the body and the surrounding gas or liquid. The initial damage is usually due to over-stretching the tissues in ...

Muscle spindles and the stretch reflex

The knee jerk is the popularly known

stretch reflex The stretch reflex (myotatic reflex), or more accurately ''muscle stretch reflex'', is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining the muscle at a constant length but ...

(involuntary kick of the lower leg) induced by tapping the knee with a rubber-headed hammer. The hammer strikes a

tendon A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...

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 receptor Stretch receptors are mechanoreceptors responsive to distention of various organs and muscles, and are neurologically linked to the Medulla oblongata, medulla in the brain stem via Afferent nerve fiber, afferent nerve fibers. Examples include stre ...

s within the muscle called

muscle spindle Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...

s. Each muscle spindle consists of sensory nerve endings wrapped around special muscle fibers called intrafusal muscle fibers. Stretching an intrafusal 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

synapse In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...

s: # Some of the branches of the I-a axons synapse directly with

alpha motor neuron Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar neuron, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for i ...

s. 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.

Mechanism of sensation

somatosensory The somatosensory system, or somatic sensory system is a subset of the sensory nervous system. The main functions of the somatosensory system are the perception of external stimuli, the perception of internal stimuli, and the regulation of bod ...

transduction, the afferent neurons transmit messages through

s in the dorsal column nuclei, where

second-order neuron Second-order may refer to: Mathematics * Second order approximation, an approximation that includes quadratic terms * Second-order arithmetic, an axiomatization allowing quantification of sets of numbers * Second-order differential equation, a d ...

s send the signal to the

thalamus The thalamus (: thalami; from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter on the lateral wall of the third ventricle forming the wikt:dorsal, dorsal part of the diencephalon (a division of ...

and synapse with third-order neurons in the ventrobasal complex. The third-order neurons then send the signal to the

somatosensory cortex The somatosensory system, or somatic sensory system is a subset of the sensory nervous system. The main functions of the somatosensory system are the perception of external stimuli, the perception of internal stimuli, and the regulation of bod ...

. More recent work has expanded the role of the cutaneous mechanoreceptors for feedback in fine motor control. Single action potentials from Meissner's corpuscle, Pacinian corpuscle and

Ruffini ending The bulbous corpuscle, Ruffini ending or Ruffini corpuscle is a slowly adapting mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are locat ...

afferents are directly linked to muscle activation, whereas Merkel cell-neurite complex activation does not trigger muscle activity.

Invertebrate mechanoreceptors

Insect and arthropod mechanoreceptors include: * Campaniform sensilla: Small domes in the

exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...

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 mammalian Golgi tendon organs. *

Hair plate Hair plates are a type of Proprioception, proprioceptor found in the folds of insect joints. They consist of a cluster of hairs, in which each hair is innervated by a single Mechanosensation, mechanosensory neuron. Functionally, hair plates operate ...

s: 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 have

proprioceptive Proprioception ( ) is the sense of self-movement, force, and body position. Proprioception is mediated by proprioceptors, a type of sensory receptor, located within muscles, tendons, and joints. Most animals possess multiple subtypes of propri ...

function, and are thought to act as limit detectors encoding the extreme ranges of motion for each joint. *

Chordotonal organ Chordotonal organs are stretch receptor organs found only in insects and crustaceans. They are located at most joints and are made up of clusters of scolopidia that either directly or indirectly connect two joints and sense their movements relativ ...

s: Internal stretch receptors at the joints, can have both extero- and

functions. The neurons in the chordotonal organ in ''Drosophila 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. * Slit sensilla: Slits in the

that detect physical deformation of the animal's exoskeleton, have

function. *

Bristle sensilla Bristle sensilla are a class of mechanoreceptors found in insects and other arthropods that respond to mechanical stimuli generated by the external world. As a result, they are considered exteroceptors. Bristle sensilla can be divided into two m ...

: 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. 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.

Plant mechanoreceptors

Mechanoreceptors are also present in plant cells where they play an important role in normal growth, development and the sensing of their environment. Mechanoreceptors aid the

Venus flytrap The Venus flytrap (''Dionaea muscipula'') is a carnivorous plant native to the temperate and subtropical wetlands of North Carolina and South Carolina, on the East Coast of the United States. Although various modern hybrids have been created ...

(''Dionaea muscipula Ellis)'' in capturing large prey.

Molecular biology

Mechanoreceptor proteins are

ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by Gating (electrophysiol ...

s whose ion flow is induced by touch. Early research showed that touch transduction in the

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a Hybrid word, blend of the Greek ''caeno-'' (recent), ''r ...

'' was found to require a two transmembrane, amiloride-sensitive ion channel protein related to epithelial sodium channels (ENaCs). This protein, called MEC-4, forms a heteromeric Na⁺-selective channel together with MEC-10. Related genes in mammals are expressed in

s and were shown to be gated by low pH. The first of such receptor was ASIC1a, named so because it is an acid sensing ion channel (ASIC).

References

External links

* {{Somatosensory system Sensory receptors Sensory systems Perception Ethology