Analysis of extrathalamic synaptic influences on reactions of sensorimotor cortical neurons during conditioning

The effects of iontophoretic application of acetylcholine, noradrenaline, serotonin and their blockers on neuronal activity were studied in the cat before and during fulfillment of conditioned instrumental placing reflex. It was found that acetylcholine increased the background neuronal activity through muscarinic cholinergic receptors and noradrenaline decreased it through beta-adrenoceptors in a considerable proportion of the cortical neurons. Serotonin had no reliable effect on the background activity. At the same time, it facilitated an initial component of the impulse reaction to conditioned stimulus and part of the impulse reaction preceding the start of the conditioned movement. Acetylcholine applied iontophoretically also facilitated the evoked responses in some cortical neurons via nicotinic cholinergic receptors. On the contrary, iontophoretic application of noradrenaline or ephedrine decreased the evoked activity of some neurons. Application of beta-adrenergic receptor blocker, propranolol, led to an increase of neuronal responses to conditioned stimuli. Evidently, noradrenergic projections exert a steady inhibitory influence on the cortical neurons during natural functioning of the cortex. It is concluded that cortical reactions evoked by activation of thalamic projections and intracortical connections are modulated and regulated by extrathalamic projections to the cortex.