The mystacial macrovibrissae are shared by a large group of land and marine mammals (see images), and it is this group that has received by far the most scientific study. The arrangement of these whiskers is not random: they form an ordered grid of arcs (columns) and rows, with shorter whiskers at the front and longer whiskers at the rear (see images). In the mouse, gerbil, hamster, rat, guinea pig, rabbit, and cat, each individual follicle is innervated by 100–200 primary afferent nerve cells. These cells serve an even larger number of mechanoreceptors of at least eight distinct types. Accordingly, even small deflections of the vibrissal hair can evoke a sensory response in the animal. Rats and mice typically have approximately 30 macrovibrissae on each side of the face, with whisker lengths up to around 50 mm in (laboratory) rats, 30 mm in (laboratory) mice, and a slightly larger number of microvibrissae. Thus, an estimate for the total number of sensory nerve cells serving the mystacial vibrissal array on the face of a rat or mouse might be 25,000.
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