Size, laminar and columnar distribution of efferent cells in the sensory‐motor cortex of monkeys

Abstract The cells of origin of cortico‐cortical and subcortical projections from the subfields of the somatic sensory area and from the motor cortex have been identified in cynomolgus and squirrel monkeys by the retrograde axonal transport method. The somata of the cells of origin of a particular fiber system have a specific laminar or sublaminar distribution. The somata of the majority of cortico‐cortical cells lie in the supragranular layers. Those projecting to the opposite cortex are confined to the deeper half of layer III (layer IIIB). Ipsilateral cortico‐cortical neurons lie mainly superficial to them in layers IIIA and II, but in the second somatic sensory area (SII) and in area 2 of the first (SI), small numbers are also found in layer V. Corticospinal cells lie in the deeper part of layer V and corticostriatal cells in the superficial part. Corticopontine, corticobulbar and corticorubral cells lie in between. The majority of corticothalamic cells lies in layer VI but a second, smaller population is found in the deep part of layer V. The cells giving rise to a particular set of efferent connections can be distinguished in terms of size and, with the exception of the corticospinal cells, their size does not vary greatly from area to area. In many cases, the size and laminar specificity indicates that cells sending axons to one site cannot have collateral branches projecting to another. In most of the fiber systems studied, labeled cells form single or multiple strips, 0.5–1 mm wide and oriented mediolaterally across the cortex. The strips appear in all of the subfields of the somatic sensory and motor areas and may form the basis of the clustering of like groups of efferent neurons demonstrable in physiological studies.