Meissner's corpuscle

Meaning: Meissner's corpuscles sit in the dermal papillae just below the epidermis and are densest in glabrous skin: fingertips, palms, soles and lips. Light deformation of the skin opens mechanically gated ion channels in the sensory nerve ending. This produces a graded receptor potential, and if it reaches threshold, action potentials travel along myelinated afferent neurons to the spinal cord and on to the somatosensory cortex. They are rapidly adapting: they fire when a stimulus starts and stops, then quiet down during steady contact. Their small receptive fields give fine spatial resolution.

On the MCAT: Passages often list the skin's receptors and ask which one fits a stimulus. Fine discriminative touch, such as reading Braille or the two-point threshold on a fingertip, points to Meissner's corpuscles (and Merkel disks). Related receptors: Pacinian corpuscles detect deep pressure and high-frequency vibration and adapt rapidly. Merkel disks detect sustained light pressure and texture and adapt slowly. Ruffini endings detect skin stretch and adapt slowly. Free nerve endings detect pain and temperature.

Common confusion: Meissner and Pacinian corpuscles are both rapidly adapting, so adaptation rate cannot separate them. Use location (superficial versus deep), preferred vibration (low versus high frequency) and receptive field size (small versus large). Neither one reports sustained pressure.