Contribution of superficial layer neurons to premotor bursts in the superior colliculus

In vitro whole-cell patch-clamp methods were used to examine the contributi on of one component of intracollicular circuitry, the superficial gray laye r, to the generation of bursts of action potentials that occur in the inter mediate layer and that command head and eye movements in vivo. Applying a s ingle brief (0.5 ms) pulse of current to the superficial layer of rat colli cular slices evoked prolonged bursts of excitatory postsynaptic currents (E PSCs) in the cells of the intermediate layer. The EPSCs were sufficient to elicit bursts of action potentials that lasted as long as 300 ms and resemb led presaccadic command bursts. To examine the contribution of neurons with in the superficial layer to the production of these bursts, we determined h ow superficial neurons respond to the same current pulses that evoke bursts in the intermediate layer. Recordings from 61 superficial layer cells reve aled 19 neurons that produced multiple action potentials following stimulat ion. Nine of these 19 neurons were wide- and narrow-field vertical cells, w hich are known to project to the intermediate layer and could contribute to producing the EPSC bursts. The remaining cells (n = 42) did not generate t rains of action potentials and 21 of these showed only subthreshold potenti al changes in response to the stimulus. Our results indicate that most supe rficial cells do not directly contribute to production of the EPSC bursts, but a small number do have the properties necessary to provide a prolonged excitatory drive to the premotor neurons.