Vibrissae are usually thicker and stiffer than other types of hair but, like other hairs, consist of inert material and contain no nerves. However, vibrissae are different from other hairs because they are implanted in a special Hair follicle incorporating a capsule of blood called a blood sinus and heavily innervated by sensory nerves.
A wide range of species have a similar arrangement of mystacial vibrissae. The arrangement of 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 In 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. Seal whiskers, which are similarly arrayed across the mystacial region, are served by as many as 1,500 nerve cells each.
Rats and mice typically sport around 30 whiskers on each side of the face, with whisker lengths up to around 50 mm in (laboratory) rats and 30 mm in (laboratory) mice. Thus, a rough estimate for the total number of sensory nerve cells serving the vibrissal array on the face of a rat or mouse might be over 9000. Manatees, remarkably, have around 600 vibrissae on or around their lips - indeed, it seems that all of their hairs, all over their body, are vibrissae rather than fur (pelagic hairs).
Whiskers can be very long in some species; the length of a chinchilla's whiskers can be more than a third of its body length (see image). Even in species with shorter whiskers, they can be very prominent appendages
In some mammals, some vibrissa follicles are motile. Typically, these are the large vibrissae (macrovibrissae) towards the rear of the mystacial area, whilst the supraorbital (above the eye) vibrissae and the much shorter vibrissae arrayed around the mouth or on the lips (microvibrissae) are immotile. A small muscle 'sling' is attached to each macrovibrissa and can move it more-or-less independently of the others, whilst larger muscles in the surrounding tissue move many or all of the whiskers together.
Amongst those species with motile whiskers, some (rats, mice, flying squirrels, gerbils, chincillas, hamsters, shrews, porcupines, opossums) palpate their vibrissae, a movement known as whisking (Video of rat whisking), while other species (cats, dogs, racoons, pandas) do not appear to. The distribution of mechanoreceptor types in the whisker follicle differs between rats and cats, which may correspond to this difference in the way they are used. Whisking movements are amongst the fastest produced by mammals. In all whisking animals in which it has so far been measured, these whisking movements are precisely and rapidly controlled in response to behavioural and environmental conditions.
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