Intracellular recording and staining of neurons in the pigeon nucleus lentiformis mesencephali

The pretectal nucleus lentiformis mesencephali in pigeons is involved in optokinetic nystagmus and consists of lateral (nLMl) and medial (nLMm) subnuclei. The present study using intracellular recordings and brain slices shows that pretectal cells respond to depolarizing current injection in different ways. Type I cells (32%) fire spontaneously and have regular spikes. Type II cells (20%) discharge regular spikes, whose frequency increases as current intensity increases. Type III cells (8%) produce a series of bursts, each of which consists of 2-5 spikes depending on current intensities. Type IV cells (39%) fire several spikes in a cluster at the onset of current injection and are then rapidly adapted. One cell of type V (1%) shows spontaneous firing and is inactivated by depolarizing currents. Cells of types III and V are only found in nLMm, and other types of cells exist in both subnuclei. This physiological difference might be a bias due to the small sampling of cells. Twenty-six cells are labeled with dye and they could be categorized into fusiform (23.1%), piriform (7.7%), or multipolar (69.2%) cells. Some correlation seems to exist between the physiological and morphological properties of pretectal neurons. Statistically, the somatic size of nLMm cells is significantly larger than that of nLMl cells, indicating that the nucleus could be divided cytoarchitecturally into magnocellular and parvocellular components as suggested previously.