Kevan A. C. Martin

Evidence for the connections between a clutch cell and a corticotectal neuron in area 17 of the cat visual cortex

Evidence is presented for the synaptic connectivity between a physiologically characterized and intracellularly filled GABAergic interneuron and a corticotectal pyramidal neuron in area 17 of the cat visual cortex. The interneuron was located in layer 4 and had the morphological characteristics of a clutch cell. The physiological data demonstrated that the clutch cell received direct X-type innervation from the dorsal lateral geniculate nucleus. These results indicate that a GABAergic neuron is directly involved during the first cortical stages of geniculocorticotectal interactions. Furthermore, the proximal location of the clutch-cell inputs to the labelled dendrite suggests a strategic siting of intracortical feedforward inhibition.

The Neurons in Layer 1 of Cat Visual Cortex

We have examined the morphology of neurons in layer 1 by injecting them intracellularly with lucifer yellow in lightly fixed brain slices (250 µm thick) taken from the medial bank of area 17 in adult cats. Of 22 neurons with well- filled dendrites, 16 had smooth dendrites, two had sparsely spiny dendrites ( < 200 spines) and, unexpectedly, four had spiny dendrites typical of pyramidal cells. The axon was generally not well filled. Computer reconstructions showed that parts of the dendritic tree had been lost in the sectioning. Nevertheless, measurements of the length of intact dendrites suggested an average diameter of the dendritic tree of 220 µm. The density of the neurons was such that the dendritic trees of about six neurons cover each point in layer 1. Thus, despite the very low density of neurons that characterizes layer 1 there are more than sufficient neurons to sample from the entire representation of the visual field in area 17.

The connection from cortical area V2 to V5 in macaque monkey

Cortical area V2 of two rhesus monkeys was microinjected with Phaseolus vulgaris Leucoaggluttinin (Pha-L). Injection sites were from 1 2 mm in diameter. After a survival period of 14 days the animals were anaesthetised and perfused with a solution of para-formaldehyde (4%), gluteraldehyde (0.3%) and picric acid (15%) in saline. Tissue from areas V1 and V5 was sunk in sucrose (10-30%) and vibratome sectioned. The PhaL was visualised by a Vector ABC kit and DAB histology. Portions of labeled axon and boutons were selected in the light microscope (LM) for serial ultrathin sectioning and subsequent electron microscopic (EM) examination. Complete boutons, their synapses and targets were reconstructed from electron micrographs. We used the physical disector method to estimate the number of labeled boutons in area V5. Light micrograph of cortical area V5 showing PhaL labeled axons and boutons. A, Patchy distribution of labeled terminals in layer 4. B, Adjacent section to A, higher magnification. Profuse axon branching in layer 4 and numerous axons projecting through layer 3 (arrow). Lamina boundaries indicated to right. Scale A, 0.5mm; B, 100μm. Electron micrographs of PhaL labeled boutons in layer 4 of area V5, asymmetric synapses are indicated (arrowheads). A; Two boutons form synapses with spine heads (sp, green). B; Bouton forms synapse and puncta (small arrow) with a dendritic shaft (d, yellow) that also forms symmetric synapse (open arrow) with unidentified bouton. Scale; 0.5μm. B Histogram of the synaptic targets of labeled V2 boutons in V5.