[Various aspects of the physiology and pathophysiology of spindles in the cat]

Spindles are a thalamocortical rhythm entirely dependent upon a thalamic mechanism. Generators of cortical spindle waves are the pyramidal neurons of the cortex. A positive statistical relationship exists between postsynaptic excitatory events of these neurons and spindle waves, but there is no significant association with inhibitory postsynaptic events. The excitatory synaptic coupling between thalamocortical fibers and cortical neurons implicated in the genesis of spindles is weak, but increases considerably after i.m. injection of penicillin which induces a transformation of spindles into generalized spike and wave discharges (feline generalized penicillin epilepsy). The bursts of increased discharges of pyramidal neurons after penicillin recruit the recurrent intracortical inhibitory pathway. There results from this an oscillation between phases of increased excitability and phases of diminished excitability which correspond respectively to the spike and to the slow wave component of the spike and wave complex. In its turn the cortex imposes this pattern of discharge upon the thalamus which brings about a close coupling of cortical and thalamic neuronal discharges during spike and wave activity.