Whiskers, or facial vibrissae, function primarily as a replacement or supplement to short-distance vision by detecting forces and vibrations around them (Williams and Kramer 2010). Each whisker acts as a transducer, converting mechanical information about the environment into an electrical signal by means of a mechanoreceptor (Williams and Kramer 2010). Striated muscles near the follicle-sinus complex, the highly-innervated part where the whisker meets the face, allows the cat to move and change the angle of the whiskers projecting out of its face (Williams and Kramer 2010). Blood vessels near the follicles constrict upon whisker stimulation, causing the skin to cool slightly, and this does not seem to alter the sensory process (Nilsson 1971). The nerve terminals of cat whisker follicles are adrenergic, meaning the synapses between nerves communicate using neurotransmitters called epinephrine and/or norepinephrine, which is not the case for other kinds of hair follicles (Nilsson 1971). Parts of the brain called the trigeminal nucleus and somatosensory cortex receive that information and are able to accurately determine the distance, direction, and texture of the object sensed by the whisker (Williams and Kramer 2010). The specific route of information is from the whisker, through the trigeminal ganglion to the primary somatosensory cortex, and lastly to the thalamus (Williams and Kramer 2010). The primary somatosensory cortex is made up of groups of neurons in columns called barrels, each of which corresponds to a specific whisker (Williams and Kramer 2010).
One study found patchy activity patterns of an enzyme in parts of the brain that match up with a spatial map of the vibrissae on the cats’ faces (Nomura and Mizuno 1986). The patterns were the same in adult cats and kittens, indicating that the sensory abilities of the vibrissae are of full strength even at young ages (Nomura and Mizuno 1986). An intermediate group of neurons in the map was in 4 horizontal rows, lining up rather exactly with the 4 rows of whiskers on the mystacial pad (moustache-area) of a cat’s face (Nomura and Mizuno 1986).
The length of whiskers varies so that they can be used to determine the distance of an object from the face, and in the case of flexible whiskers, the amount of stress on the follicle-sinus complex can indicate how close an object is as well (Williams and Kramer 2010). Interestingly, whiskers respond best to a narrow range of vibrational frequencies with neurons geared toward those specific frequencies, making them better suited for some movements and sounds than others (Williams and Kramer 2010). The frequency of nerve impulses transmitted by the whiskers is related to the velocity of the movement that causes stimulation (Nilsson 1971). After repeated stimulation, the threshold of movement required to make the nerve cells fire increases (Nilsson 1971).
Whiskers of terrestrial mammals have circular cross sections and taper in diameter near the ends, which is thought to have been the case throughout mammalian evolutionary history, though none have been found in the fossil record (Williams and Kramer 2010). Aquatic mammals with whiskers have a different shape that is better adapted for underwater sensing (Williams and Kramer 2010). The small diameter near the end is thought to be so that the whiskers can serve as a more fine-tuned probe (Williams and Kramer 2010). Whiskers fall off and get replaced at regular intervals likely so that the tips of the whiskers are always damage-free for precise sensory abilities (Williams and Kramer 2010).
Summary: ~In cats, whiskers supplement close-range vision by perceiving vibrations and motion around them and converting that into electrical signals. ~Unlike other types of hair follicles, the whisker follicle-sinus complex has many adrenergic nerves that use epinephrine and norepinephrine as neurotransmitters. ~The trigeminal nucleus of a cat’s brain contains a spatial map of the whiskers, or vibrissae. ~The sensory cortex has barrels of neurons which correspond to specific facial whiskers. ~Whiskers of cats and other terrestrial mammals have a tapered shape that is adapted to be able to finely and accurately discriminate motion on land and in air.
~Nilsson BY. 1971. Effects of sympathetic stimulation on mechanoreceptors of cat vibrissae. Acta Physiol Scand. 1972(85): 390-397.
~Nomura S, Mizuno N. 1986. Histochemical demonstration of the vibrissae-representing patchy patterns of cytochrome oxidase activity within the trigeminal sensory nuclei in the cat. 380(1986): 167-171.
~Williams CM, Kramer CM. 2010. The advantages of a tapered whisker. Plos One. 5(1): e8806.
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