Tectal neurons of the frog: intracellular recording and labeling with cobalt electrodes

With the aid of cobalt-filled intracellular electrodes, responses evoked by electrical and visual stimulation were recorded from tectal neurons. Diffuse light and "edge" and "spot" stimuli were used for visual stimulation; electrical stimuli were delivered by means of bipolar electrodes to the optic tract. The cells were subsequently filled with cobalt for a correlative histological study. The following types of neurons were distinguished: (1) Pear-shaped neurons with restricted and broad dendritic fields occurring in tectal layers 8, 6, 4, and 2. In one group, the dendrites bore beads and small protrusions and the short axon arborized within, or close to, the dendritic arbor. They responded mostly with inhibitory postsynaptic potentials to the various stimulations. This type of neuron was interpreted to be a tectal interneuron. In the second group the dendrites were smooth, the dendritic arbor was large, and the axon could be followed to layer 7 in a few cases. Excitatory activities with frequent spike discharges prevailed in their responses. They were regarded as efferent neurons of the tectum. (2) Large ganglionic neurons showed a greater variation in form than has been described in previous Golgi studies. Their responses to the different stimulations were also very variable. The axons of five neurons could be followed to the dorsal part of the thalamus, to the postoptic commissural system, and to the direct and crossed tectobulbar tract. These axons emitted several collaterals to tectal, pretectal, and tegmental structures. (3) A single stellate neuron was impaled in layer 9. It responded only to electrical stimulation with excitation. (4) Deep bipolar neurons were not mentioned in earlier works. Their somata were found in layer 6; the vertically oriented dendrites arborized in plexiform layers 7, 5, and 3, over a distance of more than 1 mm. These cells were sensitive only to electrical stimulation.