Synaptic excitation of neurones in area 17 of the cat by intracortical axon collaterals of cortico‐geniculate cells.

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Abstract
1. Antidromic activation of layer 6 cortico-geniculate cells from the lateral geniculate nucleus (l.g.n.) was used to study synaptic effects mediated by their intracortical axon collaterals. A specific stimulation procedure, earlier shown from layer suppress geniculo-cotical synaptic effects and to enhance synaptic effects from layer 6 cell collaterals was employed to differentiate between inputs from the two pathways. Single cells in different cortical layers were recorded both extra- and intracellularly with glass micro-electrodes. 2. Antidromic activation of cortico-geniculate cells at 10-16 Hz induced massive, usually repetitive, spike discharges in almost all cells in layers 2-5. The shortest latency for this synaptic activation (3.4-7.5 ms) was found for simple cells in layer 4. Cells in layers 2, 3 and 5 responded with progressively longer latencies. Corticogeniculate cells in layer 6 were atypical in that hardly any responded wiht synaptic discharges to the stimulation. 3. Intracellular recordings from layer 4 cells revealed, besides a monosynaptic excitatroy post-synaptic potential (e.p.s.p.) from gneiculo-cortical fibres a late e.p.s.p. with a latency of 3.0-4.2 ms. This e.p.s.p. could only be evoked from stimulation sites within the A laminae of the l.g.n. in retinotopic register with the recording site in the cortex. No corresponding potential was obtained by stimulation of the optic tract or the superior colliculus. 4. The threshold intensity for the lte e.p.s.p. in layer 4 cellswa was much higher than for the geniculo-cortical e.p.s.p.s from the same stimulation sites, indicating that the effect was mediated by thin fibres. The late e.s.p.s. increased dramatically in size with repetitive stimualtion of the l.g.n. at frequencies above 7 Hz, while the geniculo-cortical e.p.s.p.s remained unchanged. In all these properties, the late e.p.s.p.s resemble similar e.p.s.p.s evoked monosynaptically in principal cells of the l.g.n. by orthodromic activation of cortico-geniculate fibres. 5. Large e.p.s.s were evoked also in complex of layers 2,3 and 5 and in simple cells of layer 6 upon antidromic stimulation of cortico-geniculate fibres. The sample included many efferent neurones, identified as projection cells by antidromic activation from extracortical stimulation sites. The latency and behaviour of the e.p.s.ps in these cells indicate included many efferent neurones, identified as projection cells by antidromic activation from extracortical stimulation sites. The latency and behaviour of the e.p.s.p.s. in these cells indicate that the effect was mediated indirectly via layer 4 cells. Presumably, the excitation travelled along an earlier identified chain of neurones going from layer 4 to layers 2 and 3, from there to layer 5 and then to layer 6. 6. Late inhibitory post-synaptic potentials (i.p.s.p.s) were evoked in many cells of different cortical layers after antidromic activation of layer 6 cells. Thus the system has access also to cortical inhibitory neurones. No early i.p.s.p.s were observed in layer 6 cells, however as would be expected if the cells received inhibition through a simple recurrent inhibitory pathway. 7. It is concluded that the collaterals of layer 6 cortico-geniculate neurones terminate with excitary synapses on simple cells in layer 4. The excitation is subsequently spread to practicary all neurones throughout the cortex via pre-existing pathways. The possible function of this collateral system is discussed.