Merkel disc · Lobeworks/17

The Merkel disc is a sensory receptor that detects shape and held pressure, a mechanoreceptor made of a nerve ending pressed against Merkel cells at the base of the epidermis, which keeps firing for as long as a pressure lasts and resolves the edges, corners and dots under the fingertip.

Merkel disc. The Merkel disc is a sensory receptor that detects shape and held pressure, a mechanoreceptor made of a nerve ending pressed against Merkel cells at the base of the epidermis, which keeps firing for as long as a pressure lasts and resolves the edges, corners and dots under the fingertip.

It is slowly adapting, so a finger resting on a coin keeps reporting its rim. Its fields are small and its response follows the local curvature of the skin, which makes the Merkel system the one that carries form and fine texture: the dots of Braille, the ridge of a key, the letters embossed on a card. The deformation is sensed through the stretch-gated channel Piezo2, in the Merkel cell and in the ending; without it in the Merkel cells the sustained part of the response weakens (Woo and colleagues, 2014).

It reports form. It answers to a still indentation that the Meissner and Pacinian endings ignore once it stops moving.

The fingertip's resolution comes largely from it. Reading Braille rests on the spatial detail of the Merkel population.

Two cells, one sensor. The Merkel cell is a skin cell of the epidermis, and with the ending it forms a complex that works as a unit.

Merkel holds the picture still.

While the motion detectors fall silent, it keeps drawing the edge under the finger.

Questions: Merkel and Ruffini both keep firing under a steady stimulus: what does each one measure? Merkel measures indentation in fine detail: small fields near the surface that resolve edges, dots and texture, the system that reads Braille. Ruffini measures stretch of the skin: large fields deeper in the dermis that report how the skin is pulled, which tells the hand how its fingers are bent and which way an object pulls. Both are tonic; one draws the shape, the other feels the strain.