Firing patterns and morphological features of neurons in the pigeon nucleus rotundus

In birds, the thalamic nucleus rotundus relays visual information from the midbrain optic tectum to the forebrain ectostriatum. Using brain slices, the present study investigates the firing patterns and morphological features of 41 neurons in various divisions of the pigeon nucleus rotundus. The results indicate that these rotundal cells could be physiologically categorized into two types according to their firing patterns in response to depolarizing current pulses. Type I cells (76%) are characterized by a multi-phase firing pattern producing a single spike, late-spiking and fast-spiking as current intensity increases. Cells of type II (24%) are characterized by a rapid spiking-inactivation, discharging only one or two small spikes at the onset of current injections. However, intracellular staining shows no significant morphological differences between the two physiological types of cells in terms of somatic and dendritic field sizes, or an average density of dendritic varicosities, although each cell type includes both small- and large-sized dendritic fields. It is likely that the firing patterns and morphological features of rotundal neurons might not be correlated with each other or with rotundal divisions.