Mechanoreceptor

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For broader coverage of this topic, see Mechanosensation.

A mechanoreceptor, also called mechanoceptor, is a

sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system
.

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

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.

Tactile receptors.

By sensation

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 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 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.

  • Slowly adapting: Slowly adapting mechanoreceptors include
    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
    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
      .

By receptive field

Cutaneous mechanoreceptors with small, accurate

receptive fields
.

Lamellar corpuscles

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.

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 lamellar corpuscle is 250 Hz, the frequency range generated upon finger tips by textures made of features smaller than 200 micrometres.[9]

Ligamentous mechanoreceptors

There are four types of mechanoreceptors embedded in ligaments. As all these types of mechanoreceptors are myelinated, they can rapidly transmit sensory information regarding joint positions to the central 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 of proprioception.

Other mechanoreceptors

Other mechanoreceptors than cutaneous ones include the

juxtacapillary (J) receptors, which respond to events such as pulmonary edema, pulmonary emboli, pneumonia, and barotrauma
.

Muscle spindles and the stretch reflex

The

extensor muscle in the front of the thigh into the lower leg. Tapping the tendon stretches the thigh muscle, which activates stretch receptors within the muscle called muscle spindles. 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 synapses
:

  1. Some of the branches of the I-a axons synapse directly with alpha motor neurons. These carry impulses back to the same muscle causing it to contract. The leg straightens.
  2. 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.
  3. Still other branches of the I-a axons synapse with interneurons leading to brain centers, e.g., the cerebellum, that coordinate body movements.[11]

Mechanism of sensation

In

somatosensory cortex
.

More recent work has expanded the role of the cutaneous mechanoreceptors for feedback in

Merkel cell-neurite complex activation does not trigger muscle activity.[13]

Invertebrate mechanoreceptors

Insect and arthropod mechanoreceptors include:[14]

  • Campaniform sensilla: Small domes in the exoskeleton 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 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 have proprioceptive 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 both extero- and proprioceptive 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.[16]
  • Slit sensilla:Slits in the exoskeleton that detect physical deformation of the animal's exoskeleton, have proprioceptive function
  • 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]

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.[20] Mechanoreceptors aid the Venus flytrap (Dionaea muscipula Ellis) in capturing large[21] prey.[22]

Molecular biology

Mechanoreceptor proteins are ion channels whose ion flow is induced by touch. Early research showed that touch transduction in the nematode Caenorhabditis elegans was found to require a two transmembrane, amiloride-sensitive ion channel protein related to epithelial 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 in sensory neurons 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).[24]

See also

References

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External links

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