MORPHOLOGICAL CHARACTERIZATION OF A CORTICOCORTICAL RELAY IN THE CAT SENSORIMOTOR CORTEX

One feature of the cerebral cortex circuitry is the complex network of fibers which links its different functional regions. Our knowledge of the specific relationships between neurons which form these pathways is limited. The cortico-cortical connections between primary somatosen sory cortex (SI) and primary motor cortex (MI) were the focus of the s tudy. The aims were twofold: first, to identify characteristics of int er-areal cortico-cortical connections; and second, to determine if pat hways exist which support the notion that peripheral signals are integ rated in the somatosensory cortex before being relayed to the motor co rtex. Neurons in area 2 of SI, which projected to the motor cortex wer e identified. The morphological characteristics of these neurons and t he pattern of input that they received from area 3a were determined. T he fluorescent retrograde tracer, fast blue, was injected into the ele ctrophysiologically defined forepaw representation of motor cortex and the anterograde tracer, dextran-tetramethylrhodamine (DR), was inject ed into the somatotopically matched region of area 3a. Labeled neurons in area 2 which were located in a field of labeled axons arising from area 3a were identified in fixed tissue sections. Some of these label ed cells were impaled with a Lucifer yellow (LY)-filled micropipette a nd were intracellulary labeled by iontophoretic injection of LY. Cells in area 2 that projected to the motor cortex were located primarily i n layers II-III. They were all classified as pyramidal neurons and wer e morphologically similar. Their apical dendrites for the most part di d not extend beyond layer II. Their apical tufts exhibited 2-4 branche s within layers II-III, while basal dendrites exhibited more numerous tertiary basal dendritic branches. Light microscopic (LM) examination revealed the presence of appositions between LY filled profiles and DR -labeled axons. Appositions were observed between swellings along DR-l abeled axons and dendritic shafts or spines of 1 degrees, 2 degrees an d 3 degrees branches of apical and 1 degrees and 2 degrees branches of basal dendrites. The appositions were primarily on proximal segments of labeled dendritic shafts. Fewer appositions with distal dendrites w ere observed and some of these were with dendritic spines. No appositi ons with the somata were observed. Only one or two appositions were ob served for individual cells. The pattern of cortico-cortical synaptic input arising from area 3a onto this population of cells was predicted from these LM findings. An ultrastructural analysis was performed to confirm the existence of contacts and the predicted pattern of connect ivity. Neurons in area 2 which projected to the motor cortex, and area 3a axons which projected to area 2, were identified with electron den se retrograde and anterograde tracers respectively. Labeled neurons lo cated in a field of labeled axons were examined throughout a sequentia l series of ultrathin sections. Electron microscopic analysis revealed a similar pattern, but with a slightly higher density of synaptic inp ut (1-8 contacts per target cell) than that predicted from the LM stud ies. These results revealed a specific density and pattern of cortico- cortical input onto an identified population of cortico-cortical proje ction neurons. Individual target cells received only sparse input from a functionally different but somatotopically related region of the co rtex. The pattern of input onto cells was unexpected in that most axon s contacted the shafts of proximal dendrites. This aspect of the conne ction may exemplify a unique feature of the cortical circuit which hel ps to define its functional role. The significance of these results in defining cortical function is that the particular cortical circuit de scribed may provide an anatomical substrate for the modulation of moto r cortex activity by integrated signals from the sensory cortex. The s ynaptic relationships of neurons in this pathway may be characteristic of inter areal cortico-cortical connections in other functional regio ns of the cortex.