Hello, fellow neuroethologists! This week, I will be discussing the whisker sensorimotor system in rodents, which has been studied quite well. Rodents collect the majority of the information they perceive from the environment through their whiskers by determining where and what about an object: its location in space, size, shape, and texture (Diamond et al. 2008). In fact, rats will only use vision to detect the edge of a floor if their whiskers are cut (Diamond et al. 2008). Rodents sense by “whisking” their whiskers in forward and backward sweeping motions until they find an object and then palpating it while moving their head (Diamond et al. 2008). Golden hamsters can do 16 sweeps per second of either contact, non-contact, or double pumping, which combines contact and non-contact whisking (Wineski 1983). In rats, some neurons fire during whisking, some fire during touch, and others fire during both only (Diamond et al. 2008). In rodents, each follicle has about 200 cells of the trigeminal ganglion where mechanical information is converted into action potentials fired from one neuron to another (Diamond et al. 2008). There are four different pathways to the cortex, and the pathways contain cells called barrelettes and barreloids (Diamond et al. 2008). Each follicle contains one primary vibrissa and occasionally a secondary smaller vibrissa that is shorter and thinner and may serve as a future replacement for the primary whisker (Wineski 1983). As in cats, the layout of the macrovibrissae (large whiskers on the snout) is replicated in the barrels of the somatosensory cortex (Diamond et al. 2008). Rats sense in three dimensions: radial, horizontal, and vertical; multiple whiskers work together to determine radial distance, but only 1 whisker along with head stabilization is needed to determine horizontal distance (Diamond et al. 2008). It has been hypothesized that rodent whiskers are as sensitive as primate fingertips, which as you might guess, are quite good for fine sensing (Diamond et al. 2008). The rate at which neurons in the barrel cortex fire is important in how rodents perceive texture; a higher firing rate is associated with a smooth surface, and a slower firing rate is associated with rougher surfaces (Diamond et al. 2008). Whiskers can be at rest, such as while the animal is sleeping, alert, such as when they are eating, or in active investigation, and the whiskers are moved to different angles away from the face depending on the state (Wineski 1983). This likely is to conserve energy and protect the rather important whisker structures while at rest, which would be a mechanism that increases survival and therefore fitness. The positioning of the mystacial pad just above the lip changes shape during whisking as well, and it moves even when the whiskers have been cut off (Wineski 1983).
The cortical barrels are not present in rodents at birth, and they develop improperly if the whiskers are damaged in the first five days of life (Durham and Woolsey 1984). The cortical barrels are unaffected by damaging whiskers after only postnatal day six, making six days the “critical period.” (Durham and Woolsey 1984). Studies suggest that the whisker sensorimotor system develops from the peripheral to the central, with the critical period ending first in the brainstem, followed by the thalamus, and lastly the cortex (Durham and Woolsey 1984). However, trimming the whiskers before postnatal day 20 impairs development at a smaller level, interfering with development of extensions off of neurons called dendritic spines in certain layers of the brain (Briner et al. 2010). Rodents that had their whiskers trimmed before postnatal day 20 had fewer and smaller dendritic spine heads, but in adulthood, no differences were noticeable, showing the incredible plasticity of the brain (Briner et al. 2010).
Summary: ~Rodents use a “whisking” motion of their whiskers and mystacial pads to determine where and what when actively investigating their environments.
~The rate at which neurons fire is important in how rodents detect texture.
~The cortical barrels in the brain develop about six days after birth, beginning from the periphery of the brain and moving in toward the cortex.
~Briner A, De Roo M, Dayer A, Muller D, Kiss JZ, Vutskits L. 2010. Bilateral whisker trimming during early postnatal life impairs dendritic spine development in the mouse somatosensory barrel cortex. J Comp Neurol. 518(2010): 1711-1723.
~Diamond ME, von Heimendahl M, Knutsen PM, Kleinfeld D, Ahissar E. 2008. ‘Where’ and ‘what’ in the whisker sensorimotor system. Nat Rev Neurosci. 9(2008): 601-611.
~Durham D, Woolsey TA. 1984. Effects of neonatal whisker lesions on mouse central trigeminal pathways. J Comp Neurol. 223(1984): 424-447.
~Wineski LE. 1983. Movements of the cranial vibrissae in the Golden hamster (Mesocricetus auratus). J Zool Lond. 1983(200): 261-280.
Check in next week to learn about vibrissae in manatees from a psychologist who studies them himself!
I like how this post hints at a few of Tinbergen’s “Four Questions” – comparisons between species addresses phylogeny; the whisker-trimming experiment addresses ontogeny, neural coding of whisking addressing mechanism.