Merkel nerve endings

Merkel nerve endings are mechanoreceptors found in the skin and mucosa of vertebrates that provide touch information to the brain. Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal. This is sometimes referred to as a Merkel cell-neurite complex, or a Merkel disk receptor. A single afferent nerve fibre branches to innervate up to 90 such endings. They are classified as slowly adapting type I mechanoreceptors.

Location
In mammals, Merkel nerve endings have a wide distribution. Merkel nerve endings are found in the basal layer of glabrous and hairy skin, in hair follicles, and in oral and anal mucosa. In humans, Merkel cells (along with Meissner's corpuscles) occur in the superficial skin layers, and are found clustered beneath the ridges of the fingertips that make up fingerprints. (Some other types of mechanoreceptors, such as Pacinian corpuscles and Ruffini endings, are found primarily in subcutaneous tissue.) In birds, Merkel receptors are located in the dermis. Wherever they are found, the epithelium is arranged to optimize the transfer of pressure to the ending.

Functions
Their somewhat rigid structure, and the fact that they are not encapsulated, causes them to have a sustained response (in the form of action potentials or spikes) to mechanical deflection of the tissue. They are the most sensitive of the four main types of mechanoreceptors to vibrations at low frequencies, around 5 to 15 Hz.

Because of their sustained response to pressure, Merkel nerve endings are classified as slowly adapting. This is in contrast to Pacinian corpuscles (rapidly adapting receptors which respond only to the onset and offset of mechanical deflection, and to higher frequency vibrations.

In mammals, electrical recordings from single afferent nerve fibres have shown that the responses Merkel nerve endings are characterized by a vigorous response to the onset of a mechanical ramp stimulus (dynamic), and then continued firing during the plateau phase (static). Firing during the static phase can continue for more than 30 minutes. The inter-spike intervals during sustained firing are irregular, in contrast to the highly regular pattern of inter-spike intervals obtained from slowly adapting type II mechanoreceptors.

They fire fastest when small points indent the skin and fire at a low rate on slow curves or flat surfaces. Convexities reduce their rate of firing further still.

Merkel nerve endings are extremely sensitive to tissue displacement, and may respond to displacements of less than 1 &mu;m. Type I afferent fibres have smaller receptive fields than type II fibres. Several studies indicate that type I fibres mediate high resolution tactile discrimination, and are responsible for the ability of our finger tips to feel fine detailed surface patterns (e.g. for reading Braille).

Receptive fields
A mechanoreceptor's receptive field is the area within which a stimulus can excite the cell. If the skin is touched in two separate points within a single receptive field, the person will be unable to feel the two separate points. If the two points touched span more than a single receptive field then both will be felt. The size of mechanoreceptors' receptive fields in a given area determines the degree to which detailed stimuli can be resolved: the smaller and more densely clustered the receptive fields, the higher the resolution. For this reason, Merkel nerve endings and Meissner's corpuscles are most densely clustered in the highly sensitive finger tips, and less so in the palms.