Mechanoreceptor
Encyclopedia
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 corpuscle
s, Meissner's corpuscle
s, Merkel's discs
, and Ruffini corpuscles. There are also mechanoreceptors in hairy skin, and the hair cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into nerve signals sent to the brain. In the periodontal ligament
, there are some mechanoreceptors, which allow the jaw to relax when biting down on hard objects; the mesencephalic nucleus is responsible for this reflex.
s. Peripheral transduction
is believed to occur in the end-organs.
In somatosensory transduction, the afferent neurons transmit messages through synapse
s in the dorsal column nuclei
, where second-order neurons send the signal to the thalamus
and synapse with third-order neurons in the ventrobasal complex
. The third-order neurons then send the signal to the somatosensory cortex.
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 by morphology, by what kind of sensation they perceive and by the rate of adaptation. Furthermore, each has a different receptive field
.
and others.
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
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
among others.
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.
s, which are sensory receptor
s in the vestibular system
of the inner ear
, where they contribute to the auditory system
and equilibrioception
.
There are also Juxtacapillary (J) receptors
, which respond to events such as pulmonary edema
, pulmonary emboli
, pneumonia
, and barotrauma
.
Because of its relatively large size, a single Pacinian 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 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 Pacinian corpuscle is 250 Hz, the frequency range generated upon finger tips by textures made of features smaller than 200 micrometre
s.
(involuntary kick of the lower leg) induced by a physician tapping the knee with a rubber-headed hammer. The hammer strikes a tendon
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
s within the muscle called muscle spindle
s. Each muscle spindle consists of sensory nerve endings wrapped around special muscle fibers called spindle fibers (also called intrafusal fibers). Stretching a spindle 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 synapses:
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscle
Pacinian corpuscle
Lamellar corpuscles or Pacinian corpuscles are one of the four major types of mechanoreceptor. They are nerve endings in the skin, responsible for sensitivity to vibration and pressure. Vibrational role may be used to detect surface, e.g., rough vs...
s, Meissner's corpuscle
Meissner's corpuscle
Meissner's corpuscles are a type of mechanoreceptor. They are a type of nerve ending in the skin that is responsible for sensitivity to light touch. In particular, they have highest sensitivity when sensing vibrations lower than 50 Hertz...
s, Merkel's discs
Merkel nerve ending
Merkel nerve endings are mechanoreceptors found in the skin and mucosa of vertebrates that provide touch information to the brain. The information they provide are those regarding pressure and texture. Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal...
, and Ruffini corpuscles. There are also mechanoreceptors in hairy skin, and the hair cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into nerve signals sent to the brain. In the periodontal ligament
Periodontal ligament
The periodontal fiber or periodontal ligament, commonly abbreviated as the PDL, is a group of specialized connective tissue fibers that essentially attach a tooth to the alveolar bone within which it sits...
, there are some mechanoreceptors, which allow the jaw to relax when biting down on hard objects; the mesencephalic nucleus is responsible for this reflex.
Mechanism of sensation
Mechanoreceptors are primary neurons that respond to mechanical stimuli by firing action potentialAction potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
s. Peripheral transduction
Transduction (physiology)
In physiology, transduction is the conversion of a stimulus from one form to another.Transduction in the nervous system typically refers to stimulus alerting events wherein a mechanical/physical/etc stimulus is converted into an action potential which is transmitted along axons towards the central...
is believed to occur in the end-organs.
In somatosensory transduction, the afferent neurons transmit messages through synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
s in the dorsal column nuclei
Dorsal column nuclei
In neuroanatomy, the dorsal column nuclei are a pair of nuclei in the brainstem. The name refers collectively to the cuneate nucleus and gracile nucleus, which are present at the junction between the spinal cord and the medulla oblongata...
, where second-order neurons send the signal to the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
and synapse with third-order neurons in the ventrobasal complex
Ventrobasal complex
-Function:The ventrobasal complex receives sensory input from the medial lemniscus , from spinothalamic tracts, and from the trigeminal nerve. It subsequently relays the inputs to the primary sensory area of the cerebral cortex. Synchronously, it distinguishes sensory inputs from deep sensory...
. The third-order neurons then send the signal to the somatosensory cortex.
Feedback
More recent work has expanded the role of the cutaneous 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
Work on humans 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.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
A delta fiber
A delta fibers, or Aδ fibers, are a type of sensory fiber.They are associated with cold and pressure, and as nociceptors stimulation of them is interpreted as fast/first pain information....
. They can be categorized by morphology, by what kind of sensation they perceive and by the rate of adaptation. Furthermore, each has a different receptive field
Receptive field
The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system....
.
By morphology
- Ruffini's end organs detect tension deep in the skin.
- Meissner's corpuscleMeissner's corpuscleMeissner's corpuscles are a type of mechanoreceptor. They are a type of nerve ending in the skin that is responsible for sensitivity to light touch. In particular, they have highest sensitivity when sensing vibrations lower than 50 Hertz...
s detect changes in texture (vibrations around 50 Hz) and adapt rapidly. - Pacinian corpusclePacinian corpuscleLamellar corpuscles or Pacinian corpuscles are one of the four major types of mechanoreceptor. They are nerve endings in the skin, responsible for sensitivity to vibration and pressure. Vibrational role may be used to detect surface, e.g., rough vs...
s detect rapid vibrations (about 200–300 Hz). - Merkel's discs detect sustained touch and pressure.
- Mechanoreceiving free nerve endingFree nerve endingA free nerve ending is an unspecialized, afferent nerve ending, meaning it brings information from the body's periphery toward the brain. They function as cutaneous receptors and are essentially used by vertebrates to detect pain.-Structure:...
s detect touch, pressure and stretching - Hair follicle receptors are located in hair follicleHair follicleA hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
s and sense position changes of hairHairHair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
s.
By sensation
Cutaneous mechanoreceptors provide the senses of touch, pressure, vibration, proprioceptionProprioception
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
and others.
- The Slowly Adapting type 1 (SA1) mechanoreceptor, with the Merkel cell end-organ, 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 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) mechanoreceptor underlies the perception of flutter and slip on the skin. They have small receptive fields and produce transient responses to the onset and offset of stimulation.
- Pacinian receptors underlie the perception of high frequency vibration. They also produce transient responses, but have large receptive fields.
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
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
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
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
among others.
- Slowly adapting: Slowly adapting mechanoreceptors include Merkel and Ruffini corpuscle end-organs, and 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 fieldReceptive field
The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system....
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.
Others
Other mechanoreceptors than cutaneous ones include the hair cellHair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...
s, which are sensory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
s in the vestibular system
Vestibular system
The vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...
of the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
, where they contribute to the auditory system
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...
and equilibrioception
Equilibrioception
Equilibrioception or sense of balance is one of the physiological senses. It helps prevent humans and animals from falling over when walking or standing still. Balance is the result of a number of body systems working together: the eyes , ears and the body's sense of where it is in space ideally...
.
There are also Juxtacapillary (J) receptors
Juxtacapillary (J) receptors
J-receptors are nerves innervating into the body of the lung. They are present in the alveolar interstitium and are innervated by fibers of the vagus nerve. J-receptors respond to events such as pulmonary edema, pulmonary emboli, pneumonia, and barotrauma, which cause a decrease in oxygenation ...
, which respond to events such as pulmonary edema
Pulmonary edema
Pulmonary edema , or oedema , is fluid accumulation in the air spaces and parenchyma of the lungs. It leads to impaired gas exchange and may cause respiratory failure...
, pulmonary emboli
Pulmonary embolism
Pulmonary embolism is a blockage of the main artery of the lung or one of its branches by a substance that has travelled from elsewhere in the body through the bloodstream . Usually this is due to embolism of a thrombus from the deep veins in the legs, a process termed venous thromboembolism...
, pneumonia
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...
, and barotrauma
Barotrauma
Barotrauma is physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding fluid...
.
Pacinian Corpuscle
Pacinian corpuscles are 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 Pacinian 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 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 Pacinian corpuscle is 250 Hz, the frequency range generated upon finger tips by textures made of features smaller than 200 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s.
Muscle Spindles and the Stretch Reflex
The knee jerk is the popularly known stretch reflexStretch reflex
The stretch reflex is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length....
(involuntary kick of the lower leg) induced by a physician tapping the knee with a rubber-headed hammer. The hammer strikes a tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...
that inserts
Insertion (anatomy)
Insertion is the point at which a muscle attaches to the skin, a bone, or another muscle. The insertion attaches to the structure that will be moved by the contraction of the muscle. Insertions are usually connections of muscle via tendon to bone. The opposite end of the muscle is called the origin....
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 receptor
Stretch receptors are mechanoreceptors responsive to distention of various organs and muscles, and are neurologically linked to the medulla in the brain stem via afferent nerve fibers...
s within the muscle called muscle spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
s. Each muscle spindle consists of sensory nerve endings wrapped around special muscle fibers called spindle fibers (also called intrafusal fibers). Stretching a spindle fiber initiates a volley of impulses in the sensory neuron (a I-a
Type Ia sensory fiber
Type Ia Sensory Fiber also called Primary Afferent Fiber is a type of sensory fiber. It is a component of a muscle fiber's muscle spindle which keeps track of how fast a muscle stretch changes .-Function of muscle spindles:...
neuron) attached to it. The impulses travel along the sensory axon to the spinal cord where they form several kinds of synapses:
- Some of the branches of the I-a axons synapse directly with alpha motor neuronAlpha motor neuronAlpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
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.