A
mechanoreceptor is a
sensory receptorIn a sensory system, a sensory receptor is a sensory nerve ending that recognizes 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 corpusclePacinian corpuscles are one of the four major types of mechanoreceptor. They are nerve endings in the skin, responsible for sensitivity to pain and pressure.-Structure:...
s,
Meissner's corpuscleMeissner's corpuscles are a type of mechanoreceptor. They are a type of nerve endings in the skin that are responsible for sensitivity to light touch. In particular, they have highest sensitivity when sensing vibrations lower than 50 Hertz...
s,
Merkel's discsMerkel 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 the hairy skin, and the hair cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into sound.
Mechanism of sensation
Mechanoreceptors are primary neurons that respond to mechanical stimuli by firing
action potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
s. Peripheral transduction is believed to occur in the end-organs.
In somatosensory transduction, the afferent neurons transmit the message through synapses in the
dorsal column nucleiIn 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 the second order neurons send the signal to the
thalamusThe thalamus is a midline paired symmetrical structure within the brain of vertebrate animals, including humans. It is between the cerebral cortex and the midbrain, both in terms of its location and its neurological connections...
and synapse with the third order neurons in the
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
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 fibersA delta fibers, or Aδ fibers, are a type of sensory fiber. They are associated with cold and pressure, and as nociceptors they convey fast pain information....
. They can be categorized both by morphology, by what kind of sensation they perceive and by the rate of adaptation. Furthermore, they have different
receptive fieldThe 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 organ detects tension deep in the skin.
- Meissner's corpuscle
Meissner's corpuscles are a type of mechanoreceptor. They are a type of nerve endings in the skin that are responsible for sensitivity to light touch. In particular, they have highest sensitivity when sensing vibrations lower than 50 Hertz...
detects changes in texture (vibrations around 50 Hz); adapts rapidly.
- Pacinian corpuscle
Pacinian corpuscles are one of the four major types of mechanoreceptor. They are nerve endings in the skin, responsible for sensitivity to pain and pressure.-Structure:...
detects rapid vibrations (about 200-300 Hz).
- 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 position changes of hairs.
By sensation
Cutaneous mechanoreceptors provide the senses of touch, pressure, vibration,
proprioceptionProprioception is the sense of the relative position of neighbouring parts of the body...
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 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 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 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.
- The Rapidly Adapting (RA) mechanoreceptor underlies the perception of flutter, and slip on the skin. They have small 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....
s 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 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.
By rate of adaptation
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 potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, 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, vibrations, etc; whereas tonic receptors are useful for temperature and
proprioceptionProprioception is the sense of the relative position of neighbouring parts of the body...
among others.
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.
Some free nerve endings are intermediate 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 fieldThe 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 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 systemThe vestibular system, which contributes to our balance and our sense of spatial orientation, is the sensory system that provides the dominant input about movement and equilibrioception. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of the inner ear,...
in the
inner earThe inner ear is the bony labyrinth, a system of passages comprising two main functional parts:* The cochlea is the hearing part of the inner ear.* The semicircular canals , the utricle and the saccule are the balance part of the inner ear...
, where they contribute to the
auditory system- Ear :- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the direction from which the sounds came.The sound waves...
and
equilibrioceptionEquilibrioception or sense of balance is one of the physiological senses. It helps prevent humans and animals from falling over when walking or standing still.-Normal balance functioning:...
.
The Pacinian Corpuscle
Pacinian corpuscles are pressure receptors. They are 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 is applied to the corpuscle by the stylus. The electrical activity is 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 (also called 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 Pacinian Corpuscle is 250 Hz and this is the frequency range generated upon finger tips by textures made of features smaller than 200
µmA micrometer , sometimes known as a micrometer screw gauge, is a device used widely in mechanical engineering and machining for precision measurement, along with other metrological instruments such as dial calipers and vernier calipers...
s.
Muscle Spindles and the Stretch Reflex
The knee jerk is a stretch reflex. Your physician taps you just below the knee with a rubber-headed hammer. You respond with an involuntary kick of the lower leg.
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. This activates stretch receptors within the muscle called muscle spindles. 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 (called an "I-a" 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 neurons
(1). 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
(2). 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.
(3). Still other branches of the I-a axons synapse with interneurons leading to brain centers, e.g., the cerebellum, that coordinate body movements.