Barrel Cortex - Afferent Pathways

Afferent Pathways

Sensory information flows from whisker follicles to somatosensory cortex through at least three parallel pathways. The trigeminal nerve carries afferent fibres from the follicles into the brainstem where they synapse with neurons in four different nuclei: principal, interpolar, oral, and caudal. The most important of these pathways for our purposes is the lemniscal pathway, in which axons from the principal nucleus cross over and project to “barreloids” in the dorsomedial section of the ventroposterior medial nucleus (VPMdm) of the thalamus. 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 VPM (VPMvl). Neurons in VPMvl project to septa between the barrels and to secondary somatosensory cortex (S2). The paralemniscal pathway runs via the interpolar nucleus and the posterior nucleus (POm) of the thalamus to S2 and to diffuse targets in barrel cortex especially layer 5. These different pathways are thought to transmit different modalities of sensory information from the whisker.