Welcome back! This week’s blog post will focus on vibrissae in rabbits. The brains of rabbits contain two complete maps of the entire body surface, both of which are in the somatosensory cortex of the parietal cortex (Gould 1986). The first map, called SI, has the vibrissae represented near the back and in the center, or caudomedially, within the head region of the map (Gould 1986). In SI, the vibrissae region takes up 17.9% of SI, even though the vibrissae are only 0.6% of the body surface, indicating the vibrissae transduce a lot of information to be processed in the brain (Gould 1986). Six rows of 3-5 vibrissae each, labelled A-F from top to bottom, can be identified (Gould 1986). This brain region containing SI is a kind of koniocortex, or glandular cortex region, called the medial parietal area, or Pm, and the different portions of the body surface map make up subregions of this Pm (Gould 1986). In the second body surface map, called SII, the area around the mouth is represented in the center and takes of 53.9% of the map, while these parts are only 2.7% of the body surface, meaning that overall, the sensation of the face is very sensitive (Gould 1986). In mammals, parts of the body with larger cortical representations are thought to be more important for survival of the species as a whole, so vibrissae are hypothesized to be rather essential to rabbit fitness (Gould 1986). Other important parts include the nose, lips, and pinna of the ear (Gould 1986). It is also important to note that there was some individual variation in size of the different parts of the map from one rabbit to another and there are no duplicate representations of any parts of the body surface in either SI or SII (Gould 1986).
The rabbit body surface maps in the somatosensory cortex are similar to that of other nonprimate mammals, though it is simpler than that of rodents (Gould 1986). One major difference between rabbits and some other mammals with whiskers is that rabbits lack barrel structures that correspond to whiskers (McMullen et al. 1994). The exact reason for the existence of barrel cytoarchitecture is unknown in other species (McMullen et al. 1994). Rabbits do, however, have distinct whisker representations made of a protein called parvalbumin in SI, which is commonly found in the central nervous system (McMullen et al. 1994). Rabbits have dense patches of parvalbumin-like immunoreactivity (PV-LIR) in a vibrissae-like array with no septa in between, while rodents have been found to have a uniform distribution of PV-LIR in barrels and septa between barrels (McMullen et al. 1994).
Whisker deflection has been used as a neutral conditioned stimulus for trace eyeblink conditioning, and the barrel-like cortex in rabbits has been shown to increase in size due to this training (Galvez et al. 2007). One study lesioned the barrel areas of rabbits’ brains and found that this conditioning could not be trained in animals that were lesioned prior to conditioning (Galvez et al. 2007). Interestingly, rabbits lesioned after training showed a great reduction but not complete loss in conditioned response (Galvez et al. 2007). Overall, this study indicated that the barrel vibrissae representation in the brain is essential for acquisition of this kind of training, important for retention, and is a long-term storage area for associations having to do with whiskers (Galvez et al. 2007).
Summary: ~Rabbits have two complete body surface maps in the somatosensory cortex of their brains called SI and SII, in which the representations of the vibrissae are disproportionately large, indicating that they are very sensitive and important for survival.
~Unlike some other mammalian species, rabbits have representations of their six rows of whiskers made out of PV-LIR instead of the regular barrel cortex representation separated by septa in between.
~The region of the brain in which whiskers are represented is used as a site for storing memories of conditioning training involving the whiskers.
~Galvez R, Weible AP, Disterhoft JF. 2007. Cortical barrel lesions impair whisker-CS trace eyeblink conditioning. Learn Mem. 2007(14): 84-100.
~Gould HJ. 1986. Body surface maps in the somatosensory cortex of rabbit. J Comp Neurol. 243(1886): 207-233.
~McMullen NT, Smelser CB, Rice FL. 1994. Parvalbumin expression reveals a vibrissa-related pattern in rabbit SI cortex. Brain Res. 660(1994): 225-231.
Thanks for reading, and see you next week when we will talk about vibrissae in rodents!