Magnopyramidal neurons in the anterior motor speech region - Dendritic features and interhemispheric comparisons

Objective: To test the hypothesis that the larger somal size of layer III magnopyramidal neurons in left than in right Brodmann area 45 is associated with greater measures of dendritic arborization in the left hemisphere neurons. Design: A case series involving postmortem human brain specimens was used to compare dendritic parameters of Golgi-impregnated layer III pyramidal neurons in Brodmann area 45 in the left and right hemispheres. Subjects: A convenience sample consisting of 9 subjects with no known neurologic or psychiatric disorders was obtained at autopsy. Main Outcome Measures: Dendritic parameters of the 10 largest Golgi-impregnated layer III pyramidal neurons were measured in each hemisphere of each brain using a eutectic neuron tracing system. The measures examined were somal size, neuron depth, mean segment length, number of branch points, maximal branch order, combined dendritic diameter, total dendritic length, horizontal extent of the dendritic field, and spine density. The Golgi-impregnated slides were also compared with Nissl-stained slides from adjacent blocks to determine the laminar location of the sampled neurons. Results: Although the mean somal size was greater in the left than in the right hemisphere, none of the dendritic parameters examined was larger for the left hemisphere neurons. As expected, total dendritic length was positively correlated to somal size (r=0.43, P.<001) in the left hemisphere. However, there was no correlation between these parameters in the right hemisphere (r=0.02, P=.76). Conclusions: The difference between left and right Brodmann area 45 magnopyramidal neurons in the correlation of somal size and dendritic length provides additional evidence of anatomical differences between these 2 populations of neurons. However, the lack of interhemispheric differences in measures of dendritic arborization suggests that additional factors contribute significantly to the marked difference in somal size between the large magnopyramidal neurons in the left and right hemispheres.