Touch can come in many forms: pressure, vibration, stretch, motion, edges, points, and more. 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 an 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 upper dermis, just beneath the epidermis.
• The Merkel cells are located in the epidermis, near the skin surface, and are specialized to detect edges and points.

Where a mechanoreceptor sits in the skin, deep in the dermis or near the surface, matches the touch quality it is built to detect

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.

Smaller receptive fields mean finer spatial resolution, which is why receptor density, and two-point discrimination, is much higher on the fingers than on the back

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.

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.

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 difference in receptive field size.

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 stimulus 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 25.1. Slowly adapting mechanoreceptors fire action potentials continuously for the entire duration of a touch stimulus. As pressure steps from weak to strong, the Ruffini ending maintains continuous firing, increasing its spike frequency to reflect higher stimulus intensity. ‘Slowly Adapting Receptor’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

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 24.2. Rapidly adapting mechanoreceptors fire action potentials only during changes in stimulus intensity. When pressure increases from weak to strong, the Pacinian corpuscle generates brief bursts of action potentials during each transition, remaining silent while pressure is held constant. ‘Rapidly Adapting Receptor’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

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 the threshold of the axon and an action potential is sent toward the spinal cord.

Animation 24.3. Mechanoreceptors initiate sensory transduction using stretch-gated cation channels. Mechanical pressure opens these channels either directly through membrane tension or indirectly via attached structural proteins that pull the channel open to allow positive sodium and calcium ions to enter the cell. ‘Stretch-Gated Ion Channels’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Conclusion

Mechanoreceptors provide the foundation for our sense of touch through their specialized structures, locations, and response properties. Differences in receptor type, receptive field size, and adaptation rate allow the somatosensory system to distinguish pressure, vibration, stretch, and fine spatial detail.

Video Lecture