22 Touch: The Skin

Touch can come in many forms: pressure, vibration, stretch, motion, edges, points, etc. Receptors in the skin allow for perception of these different characteristics, and when this information is combined in the central nervous system, we are able to determine the location, strength, duration, movement, shape, and texture of the object interacting with the skin.

Receptors

We can feel different modalities of touch because of the presence of specialized sensory receptors, called mechanoreceptors, located in the skin.

The Pacinian corpuscles are located deep in the dermis of the skin and are responsible for perception of vibration.

Ruffini endings detect skin stretch and are also located within the dermis layer of the skin.

The Meissner corpuscles are stimulated by skin motion and are located in the epidermis layer.

The Merkel cells are located at the border between the dermis and epidermis and are specialized to detect edges and points.

Illustration of a cross-section of skin showing location of touch receptors. Details in caption.
Figure 22.1. The different mechanoreceptor types are located in different regions of the skin and are responsible for perception of different characteristics of a touch timulus. Pacinian corpuscles and Ruffini endings are located deep in the dermis. Meissner corpuscles are located in the dermis near the epidermis, and Merkel cells are located in the epidermis, near the surface of the skin. ‘Mechanoreceptors’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Receptive Fields

Each mechanoreceptor responds to a touch stimulus in a specific area of the skin, a region called the receptive field of the receptor. When the receptive field is touched, the mechanoreceptor will be activated.

Illustration of mechanoreceptor receptive fields with and without stimulation. Details in caption.
Figure 22.2. Each mechanoreceptor will be activated by a specific region of skin, the receptive field. When no stimulation of the receptive field occurs on the surface of the skin, the mechanoreceptor will show a baseline firing rate. When stimulation of the receptive field occurs, the firing rate of the mechanoreceptor will increase. ‘Receptive Field Activation’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Receptive Field Size

Merkel cells and Meissner corpuscles, both of which are located near the skin surface, have small receptive fields. Ruffini endings and Pacinian corpuscles, located deeper in the skin layers, have larger receptive fields than the Merkel cells and Meissner corpuscles.

Illustration of mechanoreceptors and relative receptive field sizes. Details in caption.
Figure 22.3. Receptive field sizes vary depending on the underlying mechanoreceptor type and location. Merkel cells and Meissner corpuscles have small receptive fields, whereas Pacinian corpuscles and Ruffini endings have large receptive fields. ‘Mechanoreceptor Receptive Fields’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Receptive field sizes are different among the different mechanoreceptors, but they also vary among different body regions. Even within one receptor type (e.g. Meissner corpuscles), receptive fields in regions like the fingers or lips are smaller than in regions like the back or leg. This allows us to have finer spatial resolution with locating and identifying objects using our fingers. The smaller receptive fields in these regions are a result of a higher density of receptors in the skin.

Illustration of mechanoreceptors with small and large receptive fields. Details in caption.
Figure 22.4. Density of mechanoreceptors can affect the size of the receptive field for each receptor. High density leads to smaller receptive fields. Density and receptive field size varies by location on the body. Regions like the hands and face have smaller receptive fields than regions like the back. ‘Receptive Field Location’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Two-Point Discrimination

Receptive field sizes are important because they allow us to locate a stimulus on our bodies. Larger receptive fields are not as precise as smaller receptive fields. One measure of receptive field size is two-point discrimination (try it at home!), which determines the minimum distance needed between two stimuli to perceive two separate points on the skin and not one. The hand has a smaller threshold for discerning between two points than does the back, a result of the different sized in receptive fields.

Illustration of calipers interacting with receptive fields of different sizes. Details in caption.
Figure 22.5. The size of the receptive fields affect the sensitivity of the skin, which can be measured by the two-point discrimination test. Tools like calipers or even a paperclip can be used to measure two-point discrimination. If the two points of the caliper feel like one point, they are both activating the same receptive field, indicating the receptive field is large. If, however, it is possible to perceive two separate points on the skin, then the calipers are activating two different receptive fields. ‘Two-Point Discrimination’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Adaptation Rate

Another important characteristic of the somatic sensory receptors is that of adaptation rate. Fibers that are slowly adapting show action potential firing throughout the entire time a stimuli is present. Merkel cells and Ruffini endings are both slowly adapting fibers. Slowly adapting fibers are most useful for determining the pressure and shape of a stimulus.

Animation 22.1. Slowly adapting mechanoreceptors continuing firing action potentials throughout the duration of a stimulus. As the stimulus moves from not present, to weak, to strong, the action potential firing of the Ruffini ending fires throughout the entire stimulus. ‘Slowly Adapting Receptor’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License. View static image of animation.

Rapidly adapting fibers fire action potentials when a stimulus changes (e.g., starts, stops, gets stronger or weaker) but not when a stimulus is constant. This firing makes rapidly adapting fibers specialized for detecting movement and vibration. Meissner and Pacinian corpuscles are rapidly adapting.

Animation 22.2. Rapidly adapting mechanoreceptors firing action potentials when the strength of the stimulus changes. As the stimulus moves from not present, to weak, to strong, the action potential firing of the Pacinian corpuscle only fires when the stimulus changes strength. ‘Rapidly Adapting Receptor’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License. View static image of animation.

Sensory Transduction

In previous chapters we discussed ion channels that are gated by voltage changes in the neuron and channels that are gated by neurotransmitters. In the somatosensory system, we find ion channels that are gated by physical distortion or stretch of the membrane. These channels can open by stretch of the membrane itself or indirectly through movement of intra- or extracellular proteins that are linked to the channels. Sodium and calcium flow into the cell, causing both a depolarization and the initiation of second messenger cascades. If enough stimulus is applied, the depolarization reaches threshold of the axon and an action potential is sent toward the spinal cord.

Animation 22.3. Mechanoreceptors respond to touch stimuli via stretch-gated non-selective cation channels. The channels can either open due to stretch of the membrane itself which stretches open the channel or due to proteins associated with the channels that pull the channel open. ‘Stretch-Gated Ion Channels’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License. View static image of animation.

Key Takeaways

  • There are multiple types of mechanoreceptors in the skin that are activated by different types of touch stimuli
  • The receptive field size differs among the types of mechanoreceptors
  • The adaptation rate differs among the types of mechanoreceptors
  • Receptive field is a region of skin that activate a given mechanoreceptor
  • Receptive field size for a specific type of mechanoreceptor can vary in size across the body
  • Mechanoreceptors express stretch-gated non-selective ion channels that depolarize the cell during sensory transduction

Test Yourself!

Additional Review

Describe the relationship between density of receptors, receptive fields, and two-point discrimination.

Answers

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