Afferent connections of the lateral agranular field of the rat motor cortex.

Retrograde axonal transport techniques were used to identify the afferent connections of the lateral agranular field (AG1) of the rat frontal cortex. This cytoarchitectonically distinct cortical field forms the bulk of the primary motor cortex (MI) as defined by intracortical microstimulation studies (Donoghue and Wise, '82). Following injections of horseradish peroxidase (HRP) or wheat germ agglutinin conjugated to HRP into AG1, retrogradely labeled cells are found in the forebrain, thalamus, and brainstem. Within the cerebral cortex labeled neurons are mainly present in the first somatic sensory area (SI), the second somatic sensory area (SII), and the medial agranular field, which lies medial and rostral to AG1. In SI, labeled cells are found primarily in a cytoarchitecturally distinct region called the dysgranular field of SI. Labeled neurons are present in layers II and III, Va, and the deepest part of layer VI in this field and in SII. Labeled cells are also present in layers Va and VI of the densely granular field of SI, which is the part of SI that is strongly activated by cutaneous inputs. Commissural inputs to AG1 arise from layers II-VI of the contralateral AG1 and thalamic inputs arise from the ventrolateral, ventromedial, posterior medial, and central lateral nuclei. Additional afferent fibers originate from neurons in the basal forebrain, the ventral thalamus, the midbrain raphé nuclei, and the locus coeruleus. This combination of inputs to AG1 from somatic sensory and frontal cortical fields, thalamic motor centers, and several other subcortical areas implies that AG1 forms a subdivision of sensorimotor cortex that is important in centrally directed movements as well as those that are guided by somatic sensory feedback.