The organization of pyramidal and non-pyramidal cell dendrites in relation to thalamic afferent terminations in the monkey somatic sensory cortex

Golgi-stained neurons of the monkey first somatic sensory cortex (areas 3b and 1-2) and the adjacent area 5 were examined in thionin-counterstained preparations and their dendritic fields related to the position of the major thalamic afferent plexuses (in layers IV and IIIB in area 3b, in layer IIIB only in the other areas and in a subsidiary plexus at the border of layers V and VI). Many types of pyramidal cells of layers IIIB, V and VI have dendrites that consistently branch within the laminae of the thalamic axon terminations: 1. The largest, most deeply situated layer IIIB pyramidal cells have basal dendrites which descend into layer IV of area 3b but become progressively more horizontally oriented in layer IIIB of areas 1-2 and 5. 2. Some layer V pyramidal cells, particularly medium-to-large cells of layer VA, have apical dendrites which give off many branches and spines in both layer IV and layer IIIB of area 3b but only in layer IIIB of areas 1-2 and 5. 3. Some of the modified pyramidal cells of layer VI give rise to apical dendrites that branch profusely in layer IV of area 3b and in layer IIIB of areas 1-2. Many of the non-pyramidal cell types have somata and dendrites among the thalamic afferent plexus. The small spiny (type 7) cells have their somata confined to layer IV of all areas but have an ascending dendritic tuft which is relatively short in area 3b, often barely reaching the upper border of layer IV, but is considerably longer in areas 1-2 and 5, reaching the layer IIIB/IIIA border. Several classes of small aspiny non-pyramidal cells have their somata in layers IIIB and IV of areas 3b and 1-2 but only in layer IIIB of area 5. Large, multipolar neurons (basket or type 1 cells) with somata in layers IIIB to VI of each area have dendrites that branch extensively either in layers IIIB and IV or at the layer V/VI border. It is concluded that virtually all pyramidal cells in layers IIIB, V and VI have substantial dendritic ramifications among the terminations of thalamic afferents. The dendrites of many pyramidal and non-pyramidal cells change their dendritic branching patterns and in some cases the positions of their somata, in a manner that conforms to the change in position of the thalamic afferent plexus from area to area. These observations suggest a close relationship between thalamic terminations and cell form, and may imply that all cells with dendrites among the thalamic terminations in monkey somatic sensory and parietal cortex receive significant numbers of thalamic synapses.