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 and Meissner's 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 of 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 μm. 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).
Read more about this topic: Merkel Nerve Ending
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