Barrel cortex
The 'barrels' of the barrel field are regions within cortical layer IV that are visibly darker when stained to reveal the presence of cytochrome c oxidase and are separated from each other by lighter areas called septa.
Due to this distinctive cellular structure, organisation, and functional significance, the barrel cortex is a useful tool to understand cortical processing and has played an important role in neuroscience.
In the case of the barrel field, the map is somatotopic - based on the arrangement of body parts.
Areas corresponding to the nose and mouth are more rostral and lateral in the map, the forelimb, hindlimb and trunk are more medial, with the forelimb rostral of the hindlimb, and the whisker barrel subfields - the posteromedial barrel subfield, which corresponds to the major facial whiskers (the mystacial vibrissae), and the anteriolateral barrel subfield, which corresponds to the smaller whiskers of the face - are caudal and lateral.
Although the whiskers make up a relatively small portion of the animal, they dominate the somatotopic map.
[6] The organisation of the mystacial vibrissae and corresponding barrels is so consistent that there is a naming convention to identify each whisker in rats and mice.
[6] Sensory information flows from whisker follicles to barrel cortex via the trigeminal nerve nuclei and the thalamus.
Projections from the trigeminal nuclei to the thalamus are split into pathways designated lemniscal, extralemniscal, and paralemniscal.
The simplest response, seen in neurons within the layer IV barrel cortex, directly code for whisker displacement.
Because the barrel cortex has a well-organised structure that relates clearly to the whisker pad, it has been used extensively as a tool to study sensory processing and development, and the phenomenon of experience-dependent plasticity - changes in the activity, connectivity, and structure of neural circuits in response to experience.
Neurons in the barrel cortex exhibit the property of synaptic plasticity that allows them to alter the vibrissae to which they respond depending on the rodent's history of tactile experience.
[12] Experience-dependent plasticity is commonly studied in the barrel cortex by partially depriving it of sensory input, either by lesioning elements of the afferent pathway (e.g. the trigeminal nerve) or by ablating, plucking, or trimming some of the facial whiskers.
[15][16] Two cortical processes run alongside each other when barrel cortex is deprived of sensory input from some whiskers to produce representational plasticity.
