Spatial organization of thalamocortical and corticothalamic projection systems in the rat SmI barrel cortex

Abstract

Axonal tracing techniques were used to examine the distribution of corticothalamic projection neurons in relation to the organization of the thalamocortical recipient zones in the whisker representation of the rat first somatic sensory cortex. Following injection of horseradish peroxidase into the physiologically defined vibrissa area in the ventrobasal complex of the thalamus, labeling in the cortex had a columnar appearance. Dense patches of anterograde labeling were located within the centers of the layer IV barrels and extended superficially through lamina III; the septa between barrels contained considerably less reaction product. Retrogradely labeled neurons were observed in lower layer V and layer VI where they were concentrated preferentially deep to the barrel centers. Regions deep to the septa displayed less overall labeling and a lower relative number of thalamic projecting neurons. Zones having the larger numbers of retrogradely labeled cells also contained terminallike labeling of either corticothalamic or thalamocortical origin. Following an injection that included the posterior group medial to the ventrobasal complex, anterograde labeling in layer IV was located largely in the septa. In conjunction with previous findings concerning the origin and termination of other projection systems in the barrel cortex, these results suggest that a vibrissal column contains a central core zone intimately linked with the ventrobasal thalamus that is bounded by narrower regions of more diverse inputs and outputs that form an interface between adjacent cortical columns.

Laminar and Columnar Organization of Ascending Excitatory Projections to Layer 2/3 Pyramidal Neurons in Rat Barrel Cortex

“…Based on receptive-field properties of barrel cortical neurons, lemniscal and paralemniscal pathways appear to segregate by column in L2-L4 (lemniscal3barrel related; paralemniscal3septum related) but by layer in L5 (lemniscal3 L5B; paralemniscal3 L5A) (Ahissar et al, 2000;Brecht and Sakmann, 2002;Brecht et al, 2003;Manns et al, 2004). Lemniscal3 L4 and paralemniscal3 L5A projections have also been identified anatomically, based on anterograde tracer injections into the VPM and POm (Koralek et al, 1988;Chmielowska et al, 1989;Lu and Lin, 1992). We speculate that this L5A32 septum projection represents an intracortical continuation of the paralemniscal pathway, providing a mechanism for the transfer of paralemniscal excitation to supragranular layers (Fig.…”

“…Cells in septa and septum-related columns are associated with distinct thalamocortical (Koralek et al, 1988;Chmielowska et al, 1989;Lu and Lin, 1992) and intracortical (Kim and Ebner, 1999;Shepherd et al, 2003) circuits and display different receptive-field properties compared with barrel-related cells (Brecht and Sakmann, 2002;Brecht et al, 2003).…”

Efficient Recruitment of Layer 2/3 Interneurons by Layer 4 Input in Single Columns of Rat Somatosensory Cortex

“…Spiny neurons in layer 4 of a cortical column are a main target of excitatory thalamic afferents from ventroposterior medial nucleus of the thalamus (Hubel and Wiesel, 1962;Killackey, 1973;Chmielowska et al, 1989;Lu and Lin, 1993;Bruno and Sakmann, 2006). In parallel, thalamic afferents excite interneurons in layer 4, which in turn can release GABA onto spiny neurons in layer 4 (Simons, 1978;White and Rock, 1981;Agmon and Connors, 1991;Kim et al, 1995;Staiger et al, 1996;Castro-Alamancos and Connors, 1997;Gibson et al, 1999;Porter et al, 2001;Swadlow, 2002;Beierlein et al, 2003;Gabernet et al, 2005).…”

Monosynaptic Connections between Pairs of Spiny Stellate Cells in Layer 4 and Pyramidal Cells in Layer 5A Indicate That Lemniscal and Paralemniscal Afferent Pathways Converge in the Infragranular Somatosensory Cortex

“…L5A pyramidal cells are considered to be part of the paralemniscal sensory pathway: anterograde tracer injections into the posterior medial nucleus of the thalamus (POm) indicate that layer 5A is targeted by paralemniscal afferents from this nucleus (Koralek et al, 1988;Chmielowska et al, 1989;Lu and Lin, 1993). Functional evidence indicates that the temporal response characteristics are similar for POm thalamic neurons and L5A pyramidal cells.…”