Merkel cells

Meaning: Merkel cells are mechanoreceptor cells in the stratum basale, the deepest layer of the epidermis, concentrated in fingertips and lips. Each cell contacts a sensory neuron ending (a Merkel disc). Pressure deforms the cell and increases firing in the neuron, which carries the signal to the somatosensory cortex. They keep firing while a stimulus is maintained, so they are slowly adapting, and their small receptive fields give fine spatial detail such as edges and texture.

On the MCAT: Expect a skin receptor comparison. Merkel: sustained light touch and texture, slowly adapting. Meissner corpuscles: light touch and low-frequency flutter, rapidly adapting, in the dermis. Pacinian corpuscles: deep pressure and vibration, rapidly adapting. Ruffini endings: skin stretch, slowly adapting. Free nerve endings: pain and temperature. A firing-rate graph may show one receptor that fires throughout a steady press and another that fires only at onset. Dense, small-field receptors also lower the two-point discrimination threshold.

Common confusion: Merkel and Meissner receptors both signal light touch, so students swap them. The fact that settles it is adaptation: Merkel cells respond to a steady stimulus, while Meissner corpuscles respond mainly to changes. Merkel cells are also not melanocytes or Langerhans cells, which are other epidermal cell types.