Mechanoreceptors of Skin

When discussing touch or tactile perception, it is essential to begin with the skin, the largest sensory organ in the human body (the average person has approximately 10-20 square feet of skin), which plays a crucial role in the interactions between our internal environment and external environments.  Mammalian skin is typically classified into two distinct types: hairy skin (which has hair) and glabrous skin (which is hairless). Each type can be further divided into three layers: the epidermis (outermost, thinnest layer), dermis (middle layer), and the hypodermis (subcutaneous tissue; innermost layer). In these layers, various distinct sensory receptors are located, enabling and contributing to our sense of touch. For this chapter, we will focus on mechanoreceptors.

Slow and Fast Adapting Mechanoreceptors

Mechanoreceptors convert skin deformations into neural signals that are transmitted to the brain through the dorsal column-medial lemniscal pathway (DCML). These receptors are classified into four main types based on their response time: slow-adapting (SA; I & II) and fast-adapting (FA; I & II).

Slow-adapting mechanoreceptors (types I and II) generate action potentials when the skin is initially deformed and then maintain a reduced, sustained response until the stimulus is removed. Fast-adapting mechanoreceptors (types I and II) only produce action potentials at the moment of skin deformation and again when the stimulus is removed.

The four mechanoreceptors are distinguished by the size of their receptive field and relative locations within the skin’s layers. A receptive field is the area in which a stimulus can alter the firing rate of a neuron. SAI and FAI receptors have smaller receptive fields and are situated closer to the skin’s surface within the upper dermis, where they are densely packed. This allows them to respond effectively to finer details. In contrast, SAII and FAII mechanoreceptors have larger receptive fields and are more spread out within the deeper levels within the dermis and hypodermis of the skin.

Specialized Endings

The mechanoreceptors involved in tactile perception feature specialized nerve endings that enable the detection of particular tactile qualities associated with objects or surfaces we touch. The primary receptor endings responsible for fine touch in human skin include Meissner corpuscles, Pacinian corpuscles (also known as Lamellar corpuscles), Merkel cells (or Merkel disks), and Ruffini endings (referred to as Bulbous corpuscles; see Table 1).

Table 1

Mechanoreceptors and their Endings

Merkel Cells

As indicated in Table 1, Merkel cells respond significantly to indentations and finer touch sensations, which allows us to perceive details such as scratches or intricate patterns, such as in Braille, for example. This ability is partly due to their high spatial resolution, enhanced by an inhibitory surround that improves sensitivity to spatial details. To determine the spatial resolution of tactile perception of different areas of the skin, the two-point threshold is utilized as a measure. This involves touching the skin with either one or two closely spaced points and asking the individual to identify whether they felt one point or two. A two-point threshold for a specific area of the skin is established when a person’s correct responses reach 75%. Spatial resolution is generally highest, meaning the threshold is smallest, in areas sensitive to fine touch, such as the lips and fingertips (where Merkel discs and Meissner’s corpuscles are densely distributed), compared to areas like our arms or legs, where spatial resolution is less pronounced. Our ability to detect the forms of different objects can be attributed largely to Merkel discs and Meissner corpuscles.

Meissner  Corpuscles

We observe a greater number of Meissner corpuscles than Merkel discs in areas sensitive to fine touch. However, Meissner corpuscles offer a smaller spatial resolution. As specialized endings of fast-adapting I receptors, Meissner corpuscles are particularly adept at detecting localized movements across the skin and low-frequency vibrations. Due to these qualities, they make Meissner corpuscles essential for “slip and grip control,” which allows us to maintain our hold on an object we are handling. This process can be divided into four phases: loading, lifting, holding, and unloading. Essentially, it involves picking up an object, exerting enough force to grip and lift it off a surface, and then releasing it and placing it back down. FA1 receptors are particularly active during the load and unload phases. This reflects the grip force needed to prevent the object from slipping and the response when releasing the grip from an object.

Ruffini Endings

Ruffini endings possess a receptive field approximately five times larger than that of Merkel cells. They are primarily activated by the stretching of the skin. When you manipulate your hand into various configurations, the skin stretches in different ways, providing distinct information about hand positioning and configuration. This mechanism also enhances our perception of movement across the skin, particularly during object grasping. When we grip an object, the skin stretches downward, helping us discern whether or not the object is slipping.

Pacinian Corpuscles

Pacinian corpuscles are fascinating structures characterized by their onion-like morphology, consisting of multiple layers of tissue separated by fluid, with the nerve ending positioned within the innermost capsule. These receptors are highly sensitive to vibrations, detecting changes as slight as 10 nanometers. Their extensive receptive field stems from their ability to sense these vibrations, which can travel across significant distances within the dermis. This sensitivity allows us to perceive objects and surfaces through vibrations transmitted via tools. For instance, when writing with a pen or pencil, we can sense the texture of the paper through the tip of the writing instrument, allowing us to become aware of the paper’s texture. This mechanism also helps us judge the textures of various surfaces.

Transduction

The mechanical forces cause ion channels to open in the mechanoreceptor cell membrane, wherein ions flow across the cell membrane to initiate a cascade of biochemical reactions that result in action potentials being sent through the nerve fibers. All the aforementioned mechanoreceptor fibers are myelinated, which allows for tactile information to be sent rapidly to the CNS. They utilize a relatively large, myelinated fiber called Aβ (A beta). Mechanoreceptors also use a protein referred to as Piezo, which depolarizes the cell when the membrane is stretched, particularly Piezo2.

Earlier, we discussed that these tactile receptors utilize the dorsal column-medial lemniscal pathway (DCML). The mechanoreceptor cell bodies reside in the dorsal root ganglia (DRGs). From these receptors in the skin, their axons travel through the dorsal roots and ascend directly to the brainstem via the spinal cord’s dorsal column of white matter, also known as the DCML. Here, the axons connect to the medulla and synapse with neurons in the dorsal column nuclei. After this, the axons cross the midline to the opposite side and continue ascending to nuclei in the thalamus, which then relay this information to the primary somatosensory cortex.