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Barrel cortex
The barrel cortex is a region of the somatosensory cortex that is identifiable in some species of rodents and species of at least two other orders and contains the barrel field. 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. These dark-staining regions are a major target for somatosensory inputs from the thalamus, and each barrel corresponds to a region of the body. 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. The majority of what is known about corticothalamic processing comes from studying the barrel cortex, and researchers have intensively studied the barrel cortex as a model of neocortical column.
The most distinctive aspect of the barrel field are the whisker barrels. These structures were first discovered by Woolsey and Van der Loos in 1970. Staining in the whisker barrels is more distinct than that in other areas of the somatosensory cortex. Recognizing that the array was similar to that of the vibrissae (whiskers) on the mystacial pad (region where whiskers grow from) of certain mammals, they hypothesized that the barrels were the "cortical correlates of the mystacial vibrissae" and that "one barrel represents one vibrissa". Whereas small non-whisker areas of barrel cortex correspond to large and sometimes overlapping areas of the body, each much larger whisker barrel corresponds to a single whisker. As a result, the whisker barrels are the focus of the majority of barrel cortex research, and 'barrel cortex' is often used to refer primarily to the whisker barrels. Consequently, much of this article focuses on rodent whisker barrel cortex.
The barrel field, like many regions of cortex, is organised into a topographic map. 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.
The barrels that correspond to the major facial whiskers (mystacial vibrissae) are contained within the posteromedial barrel subfield (PMBSF). The barrels here are the largest and most elliptical in shape and have a striking topographical organization that is identical to that of the whiskers; they are organized into 5 rows of 4-7 large whiskers that run close to parallel with the bridge of the nose. 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. Rows are designated A to E from top to bottom, and columns of whiskers within each row are numbered from back to front. The first four rows also have an additional whisker behind column 1, which is designated with a lower case letter or a Greek letter (α, β, γ, or δ). These four whiskers are also called straddlers.
The barrels of the barrel cortex were named because the densities of cells resembled barrels. That is, they are collected into cylindrical shapes that are narrowed at the top and bottom. The centre of the barrel is designated the hollow, and the spaces between the barrels are the septa (singular: septum).
Sensory information flows from whisker follicles to barrel cortex via the trigeminal nerve nuclei and the thalamus. Barrel-like divisions can be seen in some, but not all parts of the trigeminal nuclei (where they are called barrelets) and the thalamus (where they are called barreloids). The trigeminal nerve carries afferent fibres from the follicles into the brainstem where they connect to neurons in four different trigeminal nerve nuclei: principal, interpolar, oral, and caudal.
Projections from the trigeminal nuclei to the thalamus are split into pathways designated lemniscal, extralemniscal, and paralemniscal. In the lemniscal pathway, axons from the principal trigeminal nucleus cross over the midline and project to “barreloids” in the thalamus, specifically in the dorsomedial section of the ventroposterior medial nucleus (VPMdm). Neurons in VPMdm project mainly to barrels in layer 4 of primary somatosensory cortex (S1). In the extralemniscal pathway, neurons of the interpolar nucleus project to the ventrolateral section of the ventroposterior medial nucleus (VPMvl). Neurons in VPMvl project to septa between the barrels and to secondary somatosensory cortex (S2). The paralemniscal pathway runs from the interpolar trigeminal nucleus via the posterior nucleus (POm) of the thalamus to S2 and to diffuse targets in barrel cortex, especially layer 5.
Each pathway also has secondary projections to other layers within barrel cortex and other regions of cortex, including motor cortex. These different pathways are thought to transmit different modalities of sensory information from the whisker.
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Barrel cortex
The barrel cortex is a region of the somatosensory cortex that is identifiable in some species of rodents and species of at least two other orders and contains the barrel field. 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. These dark-staining regions are a major target for somatosensory inputs from the thalamus, and each barrel corresponds to a region of the body. 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. The majority of what is known about corticothalamic processing comes from studying the barrel cortex, and researchers have intensively studied the barrel cortex as a model of neocortical column.
The most distinctive aspect of the barrel field are the whisker barrels. These structures were first discovered by Woolsey and Van der Loos in 1970. Staining in the whisker barrels is more distinct than that in other areas of the somatosensory cortex. Recognizing that the array was similar to that of the vibrissae (whiskers) on the mystacial pad (region where whiskers grow from) of certain mammals, they hypothesized that the barrels were the "cortical correlates of the mystacial vibrissae" and that "one barrel represents one vibrissa". Whereas small non-whisker areas of barrel cortex correspond to large and sometimes overlapping areas of the body, each much larger whisker barrel corresponds to a single whisker. As a result, the whisker barrels are the focus of the majority of barrel cortex research, and 'barrel cortex' is often used to refer primarily to the whisker barrels. Consequently, much of this article focuses on rodent whisker barrel cortex.
The barrel field, like many regions of cortex, is organised into a topographic map. 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.
The barrels that correspond to the major facial whiskers (mystacial vibrissae) are contained within the posteromedial barrel subfield (PMBSF). The barrels here are the largest and most elliptical in shape and have a striking topographical organization that is identical to that of the whiskers; they are organized into 5 rows of 4-7 large whiskers that run close to parallel with the bridge of the nose. 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. Rows are designated A to E from top to bottom, and columns of whiskers within each row are numbered from back to front. The first four rows also have an additional whisker behind column 1, which is designated with a lower case letter or a Greek letter (α, β, γ, or δ). These four whiskers are also called straddlers.
The barrels of the barrel cortex were named because the densities of cells resembled barrels. That is, they are collected into cylindrical shapes that are narrowed at the top and bottom. The centre of the barrel is designated the hollow, and the spaces between the barrels are the septa (singular: septum).
Sensory information flows from whisker follicles to barrel cortex via the trigeminal nerve nuclei and the thalamus. Barrel-like divisions can be seen in some, but not all parts of the trigeminal nuclei (where they are called barrelets) and the thalamus (where they are called barreloids). The trigeminal nerve carries afferent fibres from the follicles into the brainstem where they connect to neurons in four different trigeminal nerve nuclei: principal, interpolar, oral, and caudal.
Projections from the trigeminal nuclei to the thalamus are split into pathways designated lemniscal, extralemniscal, and paralemniscal. In the lemniscal pathway, axons from the principal trigeminal nucleus cross over the midline and project to “barreloids” in the thalamus, specifically in the dorsomedial section of the ventroposterior medial nucleus (VPMdm). Neurons in VPMdm project mainly to barrels in layer 4 of primary somatosensory cortex (S1). In the extralemniscal pathway, neurons of the interpolar nucleus project to the ventrolateral section of the ventroposterior medial nucleus (VPMvl). Neurons in VPMvl project to septa between the barrels and to secondary somatosensory cortex (S2). The paralemniscal pathway runs from the interpolar trigeminal nucleus via the posterior nucleus (POm) of the thalamus to S2 and to diffuse targets in barrel cortex, especially layer 5.
Each pathway also has secondary projections to other layers within barrel cortex and other regions of cortex, including motor cortex. These different pathways are thought to transmit different modalities of sensory information from the whisker.
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