Tactile cells (Merkel cells) are specialized sensory receptor cells found within the skin. They function as mechanoreceptors, converting mechanical pressure into electrical signals the nervous system can interpret. These cells are essential for detecting light touch, sensing sustained pressure, and perceiving detailed information about shapes and edges. Their ability to respond to gentle, static contact provides the foundation for fine tactile discrimination in humans.
Primary Location and Cell Structure
Tactile cells are the only type of sensory receptor cell situated entirely within the epidermis, the outermost layer of the skin. They reside in the deepest layer of the epidermis, the stratum basale, sitting directly on the basement membrane that separates the epidermis from the dermis. Their positioning allows them to be the first responders to gentle pressure applied to the skin’s surface.
These oval-shaped cells measure approximately 10 micrometers in diameter and account for less than five percent of the epidermal cell population. Structurally, the cell features a lobulated nucleus and a pale, mitochondria-rich cytoplasm. A defining characteristic is the presence of small, electron-dense core granules concentrated on the side facing the underlying dermis and the associated nerve ending.
Tactile cells maintain their position by forming specialized junctions called desmosomes with the neighboring keratinocytes. They often appear in small clusters along the epidermal invaginations, particularly in areas responsible for high sensitivity. This physical arrangement within the basal layer ensures that even minimal deformation of the skin is effectively registered by the cell structure.
The Tactile Disc Functional Unit
A tactile cell does not function in isolation but forms a highly specialized structure known as the tactile disc (Merkel cell-neurite complex). This complex is formed by the Merkel cell and an enlarged, disc-shaped terminal of a myelinated afferent nerve fiber (Type I slowly adapting mechanoreceptor, or SAI-LTMR). The nerve fiber terminal closely apposes the base of the tactile cell, creating a synapse-like connection.
When mechanical pressure deforms the tactile cell, the specialized Piezo2 ion channel is activated. This activation is the initial step in mechanotransduction, converting the physical stimulus into an electrical signal within the cell. The resulting electrical changes trigger the release of neurotransmitters, such as serotonin, from the dense-core granules toward the nerve terminal.
The released neurotransmitters excite the associated A\(\beta\) afferent nerve ending, generating a signal sent rapidly to the brain. Because this complex is a slowly adapting receptor, it continues to fire impulses as long as the pressure is maintained. This sustained signaling enables the brain to perceive the continuous presence, shape, and texture of an object in contact with the skin.
Body Distribution and Sensitivity
The distribution of tactile cells is not uniform but is concentrated in areas requiring the highest degree of fine touch and texture discrimination. These highly sensitive areas, known as glabrous (hairless) skin, include the fingertips, palms, and soles of the feet. High concentrations are also found on the lips and in certain mucosal regions.
The density of tactile discs in a given area directly correlates with its tactile acuity. For instance, the fingertips have a high density of these complexes, making them adept at exploring and recognizing objects. This relationship between receptor density and sensory precision is physiologically demonstrated using the two-point discrimination test.
This test measures the minimum distance required between two simultaneous stimuli for a person to perceive them as two separate points rather than a single touch. Areas with a high density of Merkel cell-neurite complexes, such as the fingertips, have a small two-point discrimination threshold. This small threshold is a direct consequence of the tactile cells’ small receptive fields, allowing for the exquisite spatial acuity necessary for tasks like reading Braille or handling small tools.