Number and size of neurons and synapses in the motor cortex of cats raised in different environmental complexities

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Abstract
In a previous study we have shown that the richness of the environment affects the number of neurons, the size of their nuclei, the number of round‐asymmetrical synapses per neuron, the numerical density (number per unit volume; Nv) of flat‐symmetrical synaptic contacts, their number per neuron and their size in the visual cortex of cats. Of these, the number of flat‐symmetrical synapses per unit volume is particularly affected (there are nearly twice as many per mm3 in the impoverished cortex). Several studies in the rat have shown that environmentally induced changes in cortical thickness occur in the occipital regions but are much smaller or absent in the frontal regions. In order to determine if the cat motor cortex is also resistant to environmental changes, we have estimated the number and size of neurons and of synapses in individual laminae of motor cortex, area 4γ, in six pairs of cats raised either in a colony (EC: enriched condition) or in isolation (IC: impoverished condition).For the neurons, we have found that the numerical density (28,900 neurons per mm3 of EC and 29,500 neurons/mm3 of IC motor cortex), the number under 1 mm2 of cortical surface (49,400 and 49,200 in EC and IC cats), and the size of the neuronal nuclei (82 vs 80 μm2 in EC and IC animals) were not significantly affected.The number of flat‐symmetrical synapses per neuron (1,470 in EC vs 1,400 in IC cortex), their size (0.33 μm in both groups) and even their number per unit volume, which was so greatly affected in the visual cortex, remains unchanged (43 million/mm3 and 41 million/mm3 in EC and IC motor cortex). We did find however, a significant difference (p < 0.05) in the numerical density of round‐asymmetrical synapses which is 13% greater in the impoverished motor cortex (216 million/mm3 in EC vs 247 million/mm3 in IC cortex).Our results confirm that the motor cortex is much less affected by the richness of the environment than the visual cortex: In fact, the cat motor cortex is hardly affected at all. Furthermore our results represent the most complete data presently available on the number and size of neurons and synapses in individual laminae of the cat motor cortex.