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

Abstract

Monosynaptic interlaminar connections between spiny stellate cells in layer 4 (L4), the main cortical recipient layer for thalamic projections, and pyramidal cells in layer 5A (L5A), one of the main cortical output layers, were examined anatomically and functionally by paired recordings in acute brain slices. The somata of pairs forming interlaminar L4-to-L5A connections were located predominantly close to or directly under the barrel-septum wall in layer 4. Superposition of spiny stellate axon arbors and L5A pyramidal cell dendritic arbors suggested an innervation domain underneath an L4 barrel wall. Functionally, the L4-to-L5A connections were of high reliability and relatively low efficacy, with a unitary EPSP amplitude of 0.6 mV, and the connectivity was moderately high (one in seven pairs tested was connected). The EPSP amplitude was weakly depressing (paired-pulse ratio of Ϸ0.8) during repetitive presynaptic action potentials at 10 Hz. The existence of monosynaptic L4-to-L5A connections indicates that the specific "lemniscal" thalamic input from the ventrobasal nucleus of the thalamus to the cortex and the more unspecific "paralemniscal" afferent thalamic projections from the posterior medial nucleus of the thalamus merge already at an initial stage of cortical signal processing. These monosynaptic connections establish a monosynaptic coupling of the input to the cortex and its output, thereby effectively bypassing the supragranular layers.

Layer- and Cell-Type-Specific Subthreshold and Suprathreshold Effects of Long-Term Monocular Deprivation in Rat Visual Cortex

“…In line with this, slice work indicates that 5NPs are only rarely contacted by 2/3Ps (Thomson and Bannister, 1998). Moreover, their latency of subthreshold activation is shorter compared with 2/3Ps (my unpublished data); finally, in S1, they receive monosynaptic excitation inputs from layer 4 spiny neurons (Feldmeyer et al, 2005).…”

Recurrent Connection Patterns of Corticostriatal Pyramidal Cells in Frontal Cortex

“…Synaptic connection selectivity between pyramidal cell subtypes Pyramidal cells are functionally connected with each other by local collaterals (Thomson and Deuchars, 1994;Markram et al, 1997;Mercer et al, 2005), and pyramidal cells in different layers are selectively connected (Thomson and Bannister, 2003;Feldmeyer et al, 2005). Pyramidal cells in the same layer appear to be connected in nonrandom ways Song et al, 2005).…”

Developmental and comparative aspects of posterior medial thalamocortical innervation of the barrel cortex in mice and rats

“…Thus, the uniform innervation of layer Va across barrel and septal columns may indicate that in mice, POm TCA inputs have more direct access to, and thus can rapidly modulate, infragranular motor output circuits distributed across barrel and septal columns (Wise and Jones, 1977; Porter and White, 1983; Crandall et al, 1986; Hoffer et al, 2005; Mitchell and Macklis, 2005). In rats, such integration might occur more indirectly through POm TCA innervation of layers Va and IV within septal columns that is then relayed via intrinsic circuitry that couples layers IV and Va (Feldmeyer et al, 2005) or that couples infragranular motor output neurons occupying septal columns with those occupying barrel columns (Frick et al, 2007). The POm TCA innervation of layer IV appeared very sparse in mice in comparison with the denser innervation through the full extent of layer IV septa in rats (Lu and Lin, 1993).…”