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How do afferent mechanoreceptors work on the finger pads?

How do afferent mechanoreceptors work on the finger pads?


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I'm having some difficulty understanding how the afferent signals are sensed in the finger pads. My understanding is that for mechanoreceptors, as the indenting force increases, their effective response also increases. Additionally, as contact area increases, so does the overall stimulus area and hence a larger response amplitude from the mechanoreceptor afferents should be found.

However, the results shown in this article in the Journal of Neuroscience that discusses this physiology shows contradicting results. In this article, the closer the dot spacing (1 mm or less) and the higher the contact area the lesser the receptors seem to fire, and stop firing altogether at about 1 mm dot spacing?

Why would this occur?


Short answer
Tactile grating stimuli with a higher line density (number of lines per surface area) and applied with an equal force result in a lower amount of pressure per unit of skin area under the grating, and hence smaller afferent responses.

Background
In the Discussion section of the linked article (Phillips et al, 1993) the authors explain on p. 838 (left column) that the reduction in the response of SA afferents (slowly adapting fibers) when grating density is increased is due to the higher proximity of neighboring areas of stimulation, i.e., the lines in the grating stimuli are more closely spaced. The authors reason that this leads to a reduction in the effective stimulus (i.e., compressive strain) at the receptor terminal due to the presence of adjacent stimulus elements that distributes, and hence reduces the skin loads per line.

Reference
- Phillips et al, J Neurosci; 12(3): 927-39


How do afferent mechanoreceptors work on the finger pads? - Biology

Clinical examination of joint position sense and vibration sense can provide important information concerning specific cutaneous sensory receptors, peripheral nerves, dorsal roots, and central nervous system pathways and should be included as a regular component of the neurological examination. Although these sensory modalities share a spinal cord and brainstem pathway, they arise in different receptors and terminate in separate distributions within the thalamus and cerebral cortex. Consequently, both modalities should be tested as part of the neurological examination. Clinical testing of these modalities requires simultaneous stimulation of tactile receptors hence this review will include information about the receptors and pathways responsible for tactile sensation.

  • joint position sense
  • vibration sense
  • peripheral receptors
  • central nervous system pathways
  • clinical testing

Introduction

One of the two classes of slowly adapting, low-threshold mechanoreceptors found in human glabrous skin are the slowly adapting type II afferents (SA-II), which are thought to innervate Ruffini-like nerve endings that respond to tension in collagenous fiber strands running in and between dermal and subdermal tissues (Vallbo and Johansson, 1984) (see also Chambers et al., 1972). While these afferents are distributed uniformly over the glabrous skin of the hand, there is an accumulation of slowly adapting afferents with similar discharge properties in the soft tissue surrounding the lateral borders of the nail (Knibestöl, 1975 Johansson, 1978 Johansson and Vallbo, 1979), i.e., in the paronychium where the fingertip skin is anchored to the distal phalanx via the nail bed (Zook, 2003 Schmidt and Lanz, 2004). Indeed, Ruffini-like structures in the human fingertips skin are most easily found around the nails (Paré et al., 2003). As typical SA-II afferents, these SA-IInail afferents show a modest dynamic sensitivity and a regular static discharge some also show an ongoing impulse activity in the absence of external stimuli (Knibestöl, 1975).

Relatively little is known about the function of the SA-IInail afferents, although they constitute some 17% of the tactile afferents that innervate the distal segment of a finger, which corresponds to ∼200 afferents/digit (Johansson and Vallbo, 1979). It has been proposed that they can signal forces applied to the nails (Johansson, 1978) and, since their firing rate can be modulated with the position of the distal interphalangeal joint, that they play a role in proprioception (Knibestöl, 1975). Given their abundance, in the present study we asked whether the SA-IInail afferents might contribute important information about mechanical events at volar skin areas of the fingertips that directly contact objects during natural use of the digits. Because widespread complex stresses and strains occur all over the fingertip when it deforms in response to forces applied on objects, tactile afferents not only in the skin area contacting an object, but also at the end and sides of the fingertip, can convey information about contact forces (Bisley et al., 2000 Birznieks et al., 2001 Jenmalm et al., 2003 Johansson and Birznieks, 2004). We hypothesized that also SA-IInail afferents might encode fingertip forces because of significant changes of the tension in collagenous fiber strands of the paronychium where their end organs are situated. Specifically, we asked whether SA-IInail afferents reliably respond to forces applied to the fingertip, and, in particular, whether they can transmit complex force information namely, direction of fingertip forces. Vectorial force information is not only important for planning and control of object manipulation tasks (for review, see Johansson and Flanagan, 2008, 2009) but also in haptics perception, where people actively explore an object to identify and locate shape features based on force cues (Robles-De-La-Torre and Hayward, 2001). Encoding fingertip forces in afferents that terminate dorsally in fingertip might be advantageous because fine-form features of the contacted surfaces would less influence the afferent signals than with afferents terminating in the finger pulp.


36.2 Somatosensation

By the end of this section, you will be able to do the following:

  • Describe four important mechanoreceptors in human skin
  • Describe the topographical distribution of somatosensory receptors between glabrous and hairy skin
  • Explain why the perception of pain is subjective

Somatosensation is a mixed sensory category and includes all sensation received from the skin and mucous membranes, as well from as the limbs and joints. Somatosensation is also known as tactile sense, or more familiarly, as the sense of touch. Somatosensation occurs all over the exterior of the body and at some interior locations as well. A variety of receptor types—embedded in the skin, mucous membranes, muscles, joints, internal organs, and cardiovascular system—play a role.

Recall that the epidermis is the outermost layer of skin in mammals. It is relatively thin, is composed of keratin-filled cells, and has no blood supply. The epidermis serves as a barrier to water and to invasion by pathogens. Below this, the much thicker dermis contains blood vessels, sweat glands, hair follicles, lymph vessels, and lipid-secreting sebaceous glands (Figure 36.4). Below the epidermis and dermis is the subcutaneous tissue, or hypodermis, the fatty layer that contains blood vessels, connective tissue, and the axons of sensory neurons. The hypodermis, which holds about 50 percent of the body’s fat, attaches the dermis to the bone and muscle, and supplies nerves and blood vessels to the dermis.

Somatosensory Receptors

Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, pain receptors, and chemoreceptors. These categories are based on the nature of stimuli each receptor class transduces. What is commonly referred to as “touch” involves more than one kind of stimulus and more than one kind of receptor. Mechanoreceptors in the skin are described as encapsulated (that is, surrounded by a capsule) or unencapsulated (a group that includes free nerve endings). A free nerve ending , as its name implies, is an unencapsulated dendrite of a sensory neuron. Free nerve endings are the most common nerve endings in skin, and they extend into the middle of the epidermis. Free nerve endings are sensitive to painful stimuli, to hot and cold, and to light touch. They are slow to adjust to a stimulus and so are less sensitive to abrupt changes in stimulation.

There are three classes of mechanoreceptors: tactile, proprioceptors, and baroreceptors. Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound.” There are four primary tactile mechanoreceptors in human skin: Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscles two are located toward the surface of the skin and two are located deeper. A fifth type of mechanoreceptor, Krause end bulbs, are found only in specialized regions. Merkel’s disks (shown in Figure 36.5) are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on glabrous skin, that is, the hairless skin found on the palms and fingers, the soles of the feet, and the lips of humans and other primates. Merkel’s disks are densely distributed in the fingertips and lips. They are slow-adapting, encapsulated nerve endings, and they respond to light touch. Light touch, also known as discriminative touch, is a light pressure that allows the location of a stimulus to be pinpointed. The receptive fields of Merkel’s disks are small with well-defined borders. That makes them finely sensitive to edges and they come into use in tasks such as typing on a keyboard.

Visual Connection

Which of the following statements about mechanoreceptors is false?

  1. Pacinian corpuscles are found in both glabrous and hairy skin.
  2. Merkel’s disks are abundant on the fingertips and lips.
  3. Ruffini endings are encapsulated mechanoreceptors.
  4. Meissner’s corpuscles extend into the lower dermis.

Meissner’s corpuscles , (shown in Figure 36.6) also known as tactile corpuscles, are found in the upper dermis, but they project into the epidermis. They, too, are found primarily in the glabrous skin on the fingertips and eyelids. They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter. They are rapidly adapting, fluid-filled, encapsulated neurons with small, well-defined borders and are responsive to fine details. Like Merkel’s disks, Meissner’s corpuscles are not as plentiful in the palms as they are in the fingertips.

Deeper in the epidermis, near the base, are Ruffini endings , which are also known as bulbous corpuscles. They are found in both glabrous and hairy skin. These are slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints, so they provide valuable feedback for gripping objects and controlling finger position and movement. Thus, they also contribute to proprioception and kinesthesia. Ruffini endings also detect warmth. Note that these warmth detectors are situated deeper in the skin than are the cold detectors. It is not surprising, then, that humans detect cold stimuli before they detect warm stimuli.

Pacinian corpuscles (seen in Figure 36.7) are located deep in the dermis of both glabrous and hairy skin and are structurally similar to Meissner’s corpuscles they are found in the bone periosteum, joint capsules, pancreas and other viscera, breast, and genitals. They are rapidly adapting mechanoreceptors that sense deep transient (but not prolonged) pressure and high-frequency vibration. Pacinian receptors detect pressure and vibration by being compressed, stimulating their internal dendrites. There are fewer Pacinian corpuscles and Ruffini endings in skin than there are Merkel’s disks and Meissner’s corpuscles.

In proprioception, proprioceptive and kinesthetic signals travel through myelinated afferent neurons running from the spinal cord to the medulla. Neurons are not physically connected, but communicate via neurotransmitters secreted into synapses or “gaps” between communicating neurons. Once in the medulla, the neurons continue carrying the signals to the thalamus.

Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. Related to these are Golgi tendon organs , which are tension receptors that detect the force of muscle contraction. Proprioceptive and kinesthetic signals come from limbs. Unconscious proprioceptive signals run from the spinal cord to the cerebellum, the brain region that coordinates muscle contraction, rather than to the thalamus, like most other sensory information.

Baroreceptors detect pressure changes in an organ. They are found in the walls of the carotid artery and the aorta where they monitor blood pressure, and in the lungs where they detect the degree of lung expansion. Stretch receptors are found at various sites in the digestive and urinary systems.

In addition to these two types of deeper receptors, there are also rapidly adapting hair receptors, which are found on nerve endings that wrap around the base of hair follicles. There are a few types of hair receptors that detect slow and rapid hair movement, and they differ in their sensitivity to movement. Some hair receptors also detect skin deflection, and certain rapidly adapting hair receptors allow detection of stimuli that have not yet touched the skin.

Integration of Signals from Mechanoreceptors

The configuration of the different types of receptors working in concert in human skin results in a very refined sense of touch. The nociceptive receptors—those that detect pain—are located near the surface. Small, finely calibrated mechanoreceptors—Merkel’s disks and Meissner’s corpuscles—are located in the upper layers and can precisely localize even gentle touch. The large mechanoreceptors—Pacinian corpuscles and Ruffini endings—are located in the lower layers and respond to deeper touch. (Consider that the deep pressure that reaches those deeper receptors would not need to be finely localized.) Both the upper and lower layers of the skin hold rapidly and slowly adapting receptors. Both primary somatosensory cortex and secondary cortical areas are responsible for processing the complex picture of stimuli transmitted from the interplay of mechanoreceptors.

Density of Mechanoreceptors

The distribution of touch receptors in human skin is not consistent over the body. In humans, touch receptors are less dense in skin covered with any type of hair, such as the arms, legs, torso, and face. Touch receptors are denser in glabrous skin (the type found on human fingertips and lips, for example), which is typically more sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm).

How is receptor density estimated in a human subject? The relative density of pressure receptors in different locations on the body can be demonstrated experimentally using a two-point discrimination test. In this demonstration, two sharp points, such as two thumbtacks, are brought into contact with the subject’s skin (though not hard enough to cause pain or break the skin). The subject reports if he or she feels one point or two points. If the two points are felt as one point, it can be inferred that the two points are both in the receptive field of a single sensory receptor. If two points are felt as two separate points, each is in the receptive field of two separate sensory receptors. The points could then be moved closer and retested until the subject reports feeling only one point, and the size of the receptive field of a single receptor could be estimated from that distance.

Thermoreception

In addition to Krause end bulbs that detect cold and Ruffini endings that detect warmth, there are different types of cold receptors on some free nerve endings: thermoreceptors, located in the dermis, skeletal muscles, liver, and hypothalamus, that are activated by different temperatures. Their pathways into the brain run from the spinal cord through the thalamus to the primary somatosensory cortex. Warmth and cold information from the face travels through one of the cranial nerves to the brain. You know from experience that a tolerably cold or hot stimulus can quickly progress to a much more intense stimulus that is no longer tolerable. Any stimulus that is too intense can be perceived as pain because temperature sensations are conducted along the same pathways that carry pain sensations.

Pain is the name given to nociception , which is the neural processing of injurious stimuli in response to tissue damage. Pain is caused by true sources of injury, such as contact with a heat source that causes a thermal burn or contact with a corrosive chemical. But pain also can be caused by harmless stimuli that mimic the action of damaging stimuli, such as contact with capsaicins, the compounds that cause peppers to taste hot and which are used in self-defense pepper sprays and certain topical medications. Peppers taste “hot” because the protein receptors that bind capsaicin open the same calcium channels that are activated by warm receptors.

Nociception starts at the sensory receptors, but pain, inasmuch as it is the perception of nociception, does not start until it is communicated to the brain. There are several nociceptive pathways to and through the brain. Most axons carrying nociceptive information into the brain from the spinal cord project to the thalamus (as do other sensory neurons) and the neural signal undergoes final processing in the primary somatosensory cortex. Interestingly, one nociceptive pathway projects not to the thalamus but directly to the hypothalamus in the forebrain, which modulates the cardiovascular and neuroendocrine functions of the autonomic nervous system. Recall that threatening—or painful—stimuli stimulate the sympathetic branch of the visceral sensory system, readying a fight-or-flight response.


Animal Physiology Lab Report: Mechanoreceptors of Cockroach

Figure 1: Mean action potential activity of tactile sensilla receptors in femural of cockroach.

In this experiment, it was found that there exists a spontaneous resting activity within the cockroach tibia even though tactile sensilla receptors have not yet been stimulated. From figure 1 it can be observed that the mean spontaneous resting activity of the cockroach leg is (42+/-9.7) action potentials/sec. After the tactile sensilla receptors were stimulated by moving the receptor with and against direction of growth there was an increase of action potentials. From figure 1 it can be seen that tactile sensilla receptors are least sensitive when stimulated with their direction of growth which resulted in mean of (94+/-9.9) action potentials/seconds. However, from figure 1 it can be observed that tactile sensilla receptors are most sensitive when stimulated against their direction of growth which resulted in mean of (197+/-12) action potentials/second.

After performing Mann-Whitney U statistical tests it was found that the results are statistically highly significant as seen on table 1. When tactile sensilla receptors are stimulated in any direction, there is a significant change in action potential activity with a statistical p value of less than 0.01. Also the results found about the sensitivity of tactile sensilla receptors are statistically highly significant (p<0.01).

Table 1: Neural discharge activity within femoral of cockroach and statistical significance:

Individual #

Spontaneous (spikes/sec)

with direction of growth (spikes/sec)

against direction of growth (spikes / sec)

Statistical Significance:

Between spontaneous and with direction of growth:

Between Spontaneous and against direction of growth:

Between with direction of growth and against direction of growth:

In this study we observed when cockroach tactile sensilla receptor detect signals such as air movement or touch initiates action potential activities higher than its spontaneous resting activity. This result was highly significant (p<0.1). With great significance, (p<0.01) we also observed that if tactile sensilla receptor are stimulated against their direction of growth, they will initiate more action potentials than if they are stimulates is received with their direction of growth. As seen in figure 1 above, there were more action potentials initiated when tactile sensilla receptors received signal against direction of growth than with direction of growth. Highly significant difference was found between the sensitivity of the direction of growth of tactile sensilla receptor.

The cockroach has many different mechanoreceptors in the leg which detects external mechanical stimuli, such as touch, air, and sound. Tactile sensilla receptors are one type of the sensory mechanoreceptors in the leg which are responsible in detecting signals from mechanical stimulation such as touch. One function of tactile sensilla receptor is to provide a predator response mechanism for the cockroach. These receptors detect signals initiated by the predators such as touch to detect their presence. After the signal is received by tactile sensilla receptors, increased neural discharge is initiated which initiate the organisms' escape response. The leg muscles detect these increases in action potential and contract moving the cockroach away from the source of signals.

Cockroach femoral tactile spine is a type I mechanoreceptors and it contains a single bipolar sensory neuron. At the base of the tactile spine, there are afferent neural sensory dendrites which are sensitive to signals. These dendrite in the wall of the spine leads through the spine lumen to a cell body, and then to an axon that proceeds along the femur. One interesting point is that the cell body is surrounded by glial cell however there is no glial cell wrapped around axon body or dendrites. The location of glial wrapping can suggest that they assist in passive conduction action potential from dendrites to the axon. (French 1993).

Originally the dendrites are surrounded by fluid called endolymph which is rich in K+. When there is no stimulation there are few movement of ions in and out of the dendrites, this is can be seen in figure 1 as resting spontaneous potential activity. When there is a movement in tactile receptors bristle, the K+ channels open. Due to different concentration of K+ ions across the cell membrane, K+ ions enter the cell, causing the cell to depolarize. These causes increase in neural discharge resulting in increase in number of action potential. (Moyes & Schulte 2006).Depending on which direction the bristle moves, more or less K+ channel opens and therefore result in different amounts of action potential discharge as seen in figure 1.

Sensory neurons associated with peripheral sensory structures, tactile spines, are activated by a threat. The sensory neurons converge by attaching to interganglionic interneurons called TIAs. TIAs are were the information are translated to escape response in cockroach determining the escape turn and direction.(Schaefer 2001) Therefore sensitivity of the tactile receptors bristles is important in identifying the direction in which the cockroach escapes.

This experiment can improve by trying to control the other stimulation caused by noise and air vibration to clearly see the effect of mechanical stress on the tactile sensilla receptor. Also in future experiment we should include a device that can detect action potential discharge at different bending angle of tactile bristle. There should be a clear observation that different bending angle of bristle results in different neural discharge intensity. Also longer duration of stimulation to tactile sensilla receptor should be done in order to see how adaptive the receptor is to constant stimulation and how this fact help organism to escape from predators that are following them for long period of time.

Study done by Pringle in 1937 clearly shows that when tactile receptor undergoes mechanical strain there is an increase in neural discharge of action potential. It was also seen that tactile sensilla is most sensitive when bending in certain direction. The sensitivity of the sensillum will be dependent on the absolute length of the long diameter of the cap of the tactile bristle. These results support our result that tactile receptors function when there is a mechanical stimulus and they are more sensitive bending in one direction (against their direction of growth), than bending in other directions (with their direction of growth). An experiment done by Moran et al in 1971 showed that, when there is a mechanical strain on leg, the leg cuticle is strained and the thin cap of the tactile sensillum is displaced. The mechanical depression of the cap of a tactile sensillum stimulates the sensillum to produce a train of nerve impulses detectable in the afferent neuron. The strain increases the cell conductance thus depolarizing the cell and therefore creating action potential in the leg. This clearly supports our result that when tactile sensilla are stimulated by mechanical strain there is an increase in action potential. One important experiment performed to find the function of tactile receptors was done by Librastat in 2003. In this experiment it was observed that wasp uses venom cocktail to manipulate the behavior of its cockroach prey. It was seen that, before wasp approaches cockroach, it placed venom in the cockroach leg to block all sensory inputs including tactile sensilla receptors. The wasp then approaches the cockroach and even bites off its antennae without initiating any escape behavior in cockroach. In so doing, the wasp had applied tactile stimuli that would, in a normal cockroach, immediately trigger an escape and run. Therefore, it showed that tactile sensilla are important in initiating escape response mechanism in the cockroach. Some experiments are done to illustrate the escape movements evoked by tactile stimulation in the cockroach. One such experiment was performed by Schaefer et al in 1994) which studied the affect of tactile stimuli on different mechanoreceptors. It was observed that 85.7% resulted in vicious escape movement but in different direction. Tactile stimuli to the head and thoracic regions resulted in different escape direction than tactile stimuli to the abdomen resulted. It was concluded that the direction of escape of cockroach depended on the direction that the tactile sensilla receptor is bent. Another experiment was done by (Schaefer et al., 2001). It was observed that when there is a tactile stimulation to the body or to the antennae, the escape movement of cockroach is composed of an initial directional turn that turn the animal away from the threat followed by a run of more random direction. This depended on which receptor is stimulated and in which direction. This finding support our result that the mechanoreceptors are more sensitive depending in which direction they are bent due to their different sensitivity.

Mechanoreceptors are one of the main ways that organisms interact with the environment. Mechanoreceptor helps detect changes in external environment to initiate change in internal system of the species in order for species to be able to avoid any danger and survive. One of the main importances of mechanoreceptors is to identify the location of predator and to determine if there is an attack initiated by the predator. Therefore, escape response can be initiated by the prey in order to avoid predation and to survive. Throughout history, mechanoreceptors have been under effect of evolution changing from being simple to having highly complex mechanism as seen in tactile sensilla in cockroach. Simple mechanoreceptor could not transfer and translate signal as fast as recent mechanoreceptors. However throughout history organisms that were able to detect and translate signals faster were more able to avoid predation and survive. Therefore this led to evolution of more complex mechanoreceptors. Cockroaches have been living for million of year and showing little sign of evolution. This is partly due to their complexity mechanoreceptors which allowed them to adapt and interact with their environment and avoid extinction. Therefore, by studying mechanoreceptors we can get more information on how species try to adapt to their environment and use the information to help human adapt to the current changing environment due to global warming and many other factors.

�PAGE � �PAGE �7� The Sensitivity of Tactile Sensilla Mechanoreceptors of Cockroach


Density of Mechanoreceptors

The distribution of touch receptors in human skin is not consistent over the body. In humans, touch receptors are less dense in skin covered with any type of hair, such as the arms, legs, torso, and face. Touch receptors are denser in glabrous skin (the type found on human fingertips and lips, for example), which is typically more sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm).

How is receptor density estimated in a human subject? The relative density of pressure receptors in different locations on the body can be demonstrated experimentally using a two-point discrimination test. In this demonstration, two sharp points, such as two thumbtacks, are brought into contact with the subject’s skin (though not hard enough to cause pain or break the skin). The subject reports if he or she feels one point or two points. If the two points are felt as one point, it can be inferred that the two points are both in the receptive field of a single sensory receptor. If two points are felt as two separate points, each is in the receptive field of two separate sensory receptors. The points could then be moved closer and re-tested until the subject reports feeling only one point, and the size of the receptive field of a single receptor could be estimated from that distance.


Contents

Tactile corpuscles are encapsulated myelinated nerve endings, [3] surrounded by Schwann cells. [3] The encapsulation consists of flattened supportive cells arranged as horizontal lamellae surrounded by a connective tissue capsule. The corpuscle is 30–140 μm in length and 40–60 μm in diameter. A single nerve fiber meanders between the lamellae and throughout the corpuscle.

Location Edit

They are distributed on various areas of the skin, but concentrated in areas especially sensitive to light touch, such as the fingers and lips. [4] [5] [6] [7] [8] More specifically, they are primarily located in glabrous skin just beneath the epidermis within the dermal papillae. [9]

Comparison with other receptors Edit

Feelings of deep pressure (from a poke, for instance) are generated from lamellar corpuscles (the only other type of phasic tactile mechanoreceptor), which are located deeper in the dermis, and some free nerve endings.

Also, tactile corpuscles do not detect noxious stimuli this is signaled exclusively by free nerve endings.

The number of tactile corpuscles per square millimeter of human skin on the fingertips drops fourfold [ clarification needed ] between the ages of 12 and 50. The rate at which they are lost correlates well with the age-related loss in touch sensitivity for small probes. [10] [ clarification needed ]

Tactile corpuscles are rapidly adapting mechanoreceptors. They are sensitive to shape and textural changes in exploratory and discriminatory touch. Their acute sensitivity provides the neural basis for reading Braille text. Because of their superficial location in the dermis, these corpuscles are particularly sensitive to touch and vibrations, but for the same reasons, they are limited in their detection because they can only signal that something is touching the skin. [11]

Any physical deformation of the corpuscle will cause sodium ions to enter it, creating an action potential in the corpuscle's nerve fiber. Since they are rapidly adapting or phasic, the action potentials generated quickly decrease and eventually cease (this is the reason one stops "feeling" one's clothes). [11]


Mechanoreceptors

A mechanoreceptor is a sensory receptor that detects the mechanical stimuli of stretch and distortion. It is also known as the tactile receptor. In human beings, four types of mechanoreceptors are present in hairless skin: Pacinian corpuscles, Meissner’s corpuscles, Merkel’s cells, and Ruffini endings. They are discussed in detail below.

The afferent neurons carry the signals from the sensory receptors to the dorsal column nuclei. From there, the second order neurons carry the message forward to the thalamus. The second order neurons synapse with the third order neurons in the ventrobasal complex. The third order neurons deliver the signals to the somatosensory cortex, which sends appropriate signals via the motor neurons.

Image: “Mechanoreceptors in the Skin.” by Blausen.com Staff. “Blausen gallery 2014”. Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. – Own Work. License: CC BY 3.0

The afferents from the Pacinian corpuscle, Meissner’s corpuscle, and Ruffini ending are thought to link directly the muscle activation.

There are receptors in the hair follicle which detect a change in the hair’s position. Mechanoreceptors are also present in the hair of the cochlea, which transmits vibration signals to the brain.

Baroreceptors are a type of mechanoreceptor in large blood vessels. They detect a rise in blood pressure and send the signal to the vasomotor center in the brain.

Pacinian corpuscles

Image: “Pacinian Corpuscle” by Henry Vandyke Carter, Henry Gray (1918) Anatomy of the Human Body. Bartleby.com: Gray’s Anatomy, Plate 935. License: Public Domain

Pacinian corpuscles, also known as lamellar corpuscles, are type II fibers which detect vibration and pressure. They can detect rapid vibrations of 200-300 Hz.

They are the largest of the major types of corpuscles, measuring around 1mm in length, with an oval-cylindrical shape. They are also the fewest in number. There is an outer capsule present, comprised of fibroblasts and fibrous connective tissue, which is mainly a type II and type IV collagen network.

They are rapidly adapting receptors with a large receptive field. Any disruption in their lamellae causes the sodium channels to open, creating an action potential. The magnitude of the disruption dictates the response. The greater the pressure change, the larger the impulse.
However, as mentioned earlier, they adapt very rapidly, and impulses are not generated after some time. Pacinian corpuscles are present in the skin and fascia. The pancreas also has these corpuscles, which detect vibration changes.

Meissner’s corpuscles

Image: “A Meissner’s corpuscle. a. Cortical layer. b. Nerve fiber. c. Outer layer of the tactile body, with nuclei. d. Clear interior substance.” by Henry Vandyke Carter, Henry Gray (1918) Anatomy of the Human Body. Bartleby.com: Gray’s Anatomy, Plate 936. License: Public Domain

Meissner’s corpuscles, or tactile corpuscles, are type II fibers and respond to a light touch. They have the lowest threshold and are sensitive to vibrations of 10-50 Hz. The length of the corpuscle is twice its diameter. It is an encapsulated, unmyelinated nerve ending, where the cells are arranged in the form of horizontal lamellae, surrounded by a capsule of connective tissue.

Any physical deformation of the corpuscle generates an action potential however, they are also very rapidly adapting nerve endings. These nerve endings are concentrated in the thick, hairless skin, such as the finger pads and lips. Their number declines with increasing age.

Merkel’s cells

Image: “Arrangement of Mechanoreceptors in the Skin.” by Thomas.haslwanter – Own Work. License: CC BY-SA 3.0

Merkel’s cells consist of type II fibers. They detect sustained pressure and deep static touch. They are slowly adapting, unencapsulated, and myelinated nerve endings.

Merkel’s disc, or Merkel’s neurite complex, refers to a group of Merkel’s cells linked to a single afferent nerve fiber. They have a small receptive field and, therefore, are capable of two-point discrimination. They are found in the basal epidermis of the glabrous, as well as the hairy skin, and are usually lost in case of skin burn.

Ruffini endings

Ruffini endings, or bulbous corpuscles, are type II fibers that detect pressure tension deep in the skin and fascia. They are slow adapting, enlarged dendritic endings with elongated capsules. They are responsible for detecting angle changes of the joints, up to 3 degrees. They also detect slippage of an object or sustained holding. They are most abundant around the fingernails.

Overview of encapsulated mechanoreceptor afferents

  • Sense vibrations (40-500 Hz range)
  • Adapt rapidly
  • Wide receptive field
  • Touch and fluttering sensations (2-40 Hz range)
  • Adapt rapidly
  • Narrow receptive field
  • Stretch sensation (100-500 Hz range)
  • Adapt slowly
  • Wide reception field
  • Free nerve endings that plexus around the hair follicle
  • Detects hair deflection
  • More fully developed in some comparative species
  • Sense light pressure (< 0.2-2.0 Hz range)
  • Adapt slowly
  • Narrow receptive field

Density of Mechanoreceptors

The distribution of touch receptors in human skin is not consistent over the body. In humans, touch receptors are less dense in skin covered with any type of hair, such as the arms, legs, torso, and face. Touch receptors are denser in glabrous skin (the type found on human fingertips and lips, for example), which is typically more sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm).

How is receptor density estimated in a human subject? The relative density of pressure receptors in different locations on the body can be demonstrated experimentally using a two-point discrimination test. In this demonstration, two sharp points, such as two thumbtacks, are brought into contact with the subject’s skin (though not hard enough to cause pain or break the skin). The subject reports if he or she feels one point or two points. If the two points are felt as one point, it can be inferred that the two points are both in the receptive field of a single sensory receptor. If two points are felt as two separate points, each is in the receptive field of two separate sensory receptors. The points could then be moved closer and re-tested until the subject reports feeling only one point, and the size of the receptive field of a single receptor could be estimated from that distance.


Methods

Eight subjects, 24.3 ± 5.7 years of age (mean ± SD) participated in this study. The study was approved by the local ethics committee, at Western Sydney University (Ethics Approval H9967). The procedures followed were in accordance with the ethical standards of the local ethics committee on human experimentation and with the Helsinki Declaration of 1975, as revised in 2000. Each participant had no known neurological disorders. All subjects provided written informed consent before taking part in the study. Normal force was applied to the finger-pad of an immobilised finger of the right hand using a six axis robotic manipulator AGILUS R900 (KUKA Roboter GmbH, Germany). A force transducer (Nano F/T, ATI Industrial Automation, Garner, USA) was attached at the tip of the robotic manipulator. The robot was programmed to safely deliver the force stimulus at the human finger-tip. Upon touching the finger-tip, the robotic manipulator switched from position to force control mode. A device to immobilize the finger was used. The device was adjustable and could accommodate different finger sizes.

Tactile data were recorded from SA-I afferent fibers of the right hand. The needle electrode was percutaneously inserted into the median nerve and positioned in such a way as to obtain action potentials (AP) waveforms [37, 38]. Force profiles and the corresponding tactile afferent signals were recorded simultaneously, using a 16-bit data acquisition system (PowerLab, ADInstruments Dunedin New Zealand). Force data were sampled at 1kHz and afferent data were sampled at 20kHz. The acquisition system was set up with a monitor to provide visual feedback, and speakers to provide audio feedback. The feedback from the monitor and speakers was used to ensure that the quality of the data recorded is suitable for analysis. Spike sorting techniques—where the occurrences of AP waveforms that pertain to an individual cell are grouped—were applied to the afferent data based on methods described in [39�]. In cases where AP waveforms overlapped, as result of recording from more than one afferent fiber, we used a combination of automated and visual methods to identify which afferent fibers contributed to that AP waveform.

Fig 1 shows an example of a force profile and the corresponding (spike sorted) neural spikes that were recorded from an SA-I afferent (Panels A and B respectively). We apply the methods described below, to an ensemble of 28 SA-I afferents.

Panel A shows the stimulus used to elicit slowly adapting type I tactile response shown in B. Panel C shows a representation of a series of action potentials along with their corresponding times of occurrence (in ms). A zoomed in version of the left most action potential is represented in panel D. We fit a nonhomogeneous Poisson model to the first portion of the data (100� ms). The inverse problem�oding—was done using data in the range 451� ms.

Statistical methods

SA-I afferents are associated with Merkel discs that encode information about some properties of the object in the hand into neural spike patterns. We devise a model (encoding) to capture the mapping between the force stimulus and the corresponding SA-I afferent spike response. The data were split into two disjoint subsets. A subset was used to fit a model (encoding) and another was used to assess how well the decoding algorithms generalize. The encoding subset was defined as the data recorded during the first portion of recording (between 100� ms, see Fig 1 ). This subset was used to fit the nonhomogeneous Poisson process model for each SA-I afferent. The second subset was defined as the data recorded during the rest of the recording period (between 451� ms) and was used to reconstruct the force stimulus using a recursive Bayesian filter.

Encoding model

We define the model for SA-I afferents using a nonhomogeneous Poisson process. A nonhomogeneous Poisson process is a Poisson process where the rate parameter varies as a function of time and/or some other physical quantity but it retains the memoryless property [42]. In this study, the rate parameter of the nonhomogeneous Poisson process is modeled as a function of the force stimulus and the derivative of the force stimulus. This is because among three candidate models𠅊 first where we consider force only, a second where we take a combination of force and its derivative, and a third where force as well as its first and second derivatives are considered. We used the model which considers the force and its first derivative because this model yielded the lowest Akaike’s Information Criteria (AIC) value [43]𠅏or each of the afferents under the current model. The encoding model is defined as follows:

where β0 corresponds to the baseline firing rate, β is the vector of parameters corresponding to covariates that modulate firing rate, and S(t) is a matrix of covariates that modulate the firing activity. We assume that individual SA-I afferents form a population of conditionally independent Poisson processes (the SA-I afferents are independent given their model parameters). We fit the nonhomogeneous Poisson model defined in Eq 1 to each SA-I afferent. We estimated the model parameters based on the maximum likelihood method [44, 45]. The relative importance of the first and second derivatives of the components were assessed using Akaike Information Criterion (AIC) [27, 46].

Assessment of goodness-of-fit

After fitting the model to data, we assessed its validity in describing the observed SA-I afferent spike data. In order to use already established statistical methods, such as the Kolmogorov-Smirnov (K-S) test, we transformed the data into a simpler form. The Time rescaling theorem, in addition to simulation of point process data, can be used to transform the data [47�]. Using the rate (conditional intensity function), estimated from the data, we transformed the data using time rescaling to obtain a homogeneous Poisson process with rate equal to one, and further transformed the data into uniform random variables:

where tj is the spike time, uj is a uniform random variable. We then use the K-S test to assess how close the empirical distribution of rescaled spike times are to a reference uniform distribution on the interval (0, 1). If the nonhomogeneous model described fit the data correctly, the transformed data should lie on a 45° line on the K-S plot.

Decoding model

The state transition function for the force is defined as follows:

Following the encoding stage, the decoding stage aims to find the best estimate of s(tl) for each tl using a probability density given the A afferents, force and force derivative parameters. To facilitate the description of the decoding procedure, we start by defining a set of times in (tϱ, T], tϱt0 < t1 <, ⋯, < tl < tl+1, ⋯, tL ≤ T, and let ΔNa(tl) be an indicator function. The indicator function is equal to one if there is a spike at time tl and zero if there is no spike at time tl. We let Δ N ( t l ) = [ Δ N 1 ( t l ) , ⋯ , Δ N A ( t l ) ] ⊺ be a vector of all A afferents at time tl. The probability density of s(tl), given the spikes in (tϱ, T]) and parameters estimated during the encoding stage, is computed sequentially using Bayes’ rule from probability densities of previous force and force derivatives and that of the new afferent data recorded since the previous state prediction was made [27], [50]. The formulation of the recursive algorithm is based on two steps: the prediction and the update. The prediction stage is based on the relationship between the posterior, at the previous time step, and the state evolution model. The one-step prediction probability density is defined below.

The equations for tracking the mean and variance of the one-step prediction are defined below:


Watch the video: Mechanoreceptive pain and receptor based therapy (July 2022).


Comments:

  1. Shakajar

    This - is improbable!

  2. Dorisar

    that we would do without your brilliant idea

  3. Gardagis

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  4. Carlaisa

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  5. Pontus

    What the right words ... super, brilliant thought



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