Optic nerve innervation of so-called interneurons of the rat lateral geniculate body

In urethane-anesthetized rats, neurons responding to electrical stimulation of the optic nerve (ON) with bursts of spikes were searched for in the region of the thalamic reticular nucleus close to the dorsal nucleus of the lateral geniculate body (LGBd). These neurons were identical with those which had been presumed to be inhibitory internerons (I-cells) of LGBd. Conduction velocityes of ON fibers innervating I-cells were determined by measuring differences in response latency between stimulations of two separate sites along the contralateral ON. The velocity ranged from 1.9 to 7.3 m/sec with an average of 4.6 m/sec, indicating that among the three groups of ON fibers with different velocities, only the slowest group is involved in activation of I-Cells. Calculation of synaptic delays revealed that one group of I-cells was excited monosynaptically and another, disynaptically. Experiments on rats with the visual cortex chronically ablated provided evidence that the disynaptically excited I-cells received ON impulses via axon collaterals of principal cells of LGBd. The region of the thalamic reticular nucleus containing I-cells was found to receive inputs not only from the contralateral but also from ipsilateral ON.