Ruffini endings

Meaning: Ruffini endings are encapsulated mechanoreceptors in the deep dermis and underlying tissue. They respond to stretching of the skin and to sustained pressure, and they keep firing for as long as the stimulus lasts, so they are slowly adapting. Their receptive fields are large, so they localize a stimulus poorly. Some older texts also list them as warmth receptors, but current texts treat them mainly as stretch mechanoreceptors.

On the MCAT: Questions ask you to match a receptor to a stimulus. The slowly adapting receptors are Merkel disks (light touch, texture, edges, shallow, small field) and Ruffini endings (stretch, deep, large field). The rapidly adapting receptors are Meissner corpuscles (light touch, shallow) and Pacinian corpuscles (vibration, deep pressure). A passage may show firing rate against time during a steady indentation of the skin. A receptor that keeps firing is slowly adapting. One that fires mainly at the start and end of the stimulus is rapidly adapting. Free nerve endings handle pain and temperature.

Common confusion: Students mix up Ruffini endings and Pacinian corpuscles because both lie deep in the skin. Depth does not separate them. Adaptation rate does: Pacinian corpuscles adapt rapidly and detect vibration, while Ruffini endings adapt slowly and detect sustained stretch.