Sensory neuron. Sensory neurons are nerve cells within the nervous system responsible for converting external stimuli from the organism’s environment into internal electrical impulses. For example, some sensory neurons respond to tactile stimuli and can activate motor neurons in order to achieve muscle contraction.
Thus, in humans, the nail of the index finger grows faster than that of the little finger; and fingernails grow up to four times faster than toenails. In humans, nails grow at an average rate of 3 mm (0.12 in) a month. Fingernails require three to six months to regrow completely, and toenails require 12 to 18 months.