Meynert cells in the primate visual cortex.

The solitary cells of Meynert are distinguished by their specific location in layer V of the striate cortex, very large size, argyrophilia, and the profusion of neurofilaments in their dendrites and perikarya. They occur with greater frequency in the macular region of the cortex, spaced a minimal distance of 110 mum apart, at a maximum density of about 8000/cm-2. In the perifoveal cortex, Meynert cells are spaced about 400 mum apart and packed at a density of approximately 625/cm-2. Each Meynert cell has an apical dendrite and many large basal dendrites. The perikaryon and primary segments of all dendrites are spine-free; however, more distally a total of 36 000 spines are present, differentially disposed upon the dendritic surfaces. The basal dendrites bear over 77% of the spines on the Meynert cell, although they account for only 66% of the total length of the dendritic arborization. The first part of the apical dendrite is the most densely decorated with appendages, accounting for almost 10% of the spines on the whole dendritic tree. The apical dendrite becomes progressively less spiny as it passes through the superficial part of layers IV and III; less than 2.5% of the total number of spines of the Meynert cell project from this part of the apical dendrite. When the dendrite reaches layer II it bursts into an umbel of rapidly tapering branches. These are highly spinose, accounting for 8-13% of the cell's total, dispersed over only 23% of the linear dendritic length. It is suggested that this differential distribution of thorns can be correlated with the axonal inputs in the various cortical layers, and that the Meynert cell is designed to receive maximal information from layers I and II, and from layers V and VI, which are sources mainly of intracortical inputs. Thus the Meynert cell may be principally concerned with integrative information. In the perifoveal cortex, the basal dendrites of adjacent Meynert cells overlap considerably, and the apical terminal bouquet dendrites do not. In the macular cortex, because of the increased frequency of these neurons, both basaal and apical terminal dendritic fields overlap. A model is developed to illustrate these hypotheses.