Somatotopic organization of vibrissae afferents in the trigeminal sensory nuclei of the rat studied by transganglionic transport of HRP.

Transganglionic transport of horseradish peroxidase (HRP) has been used to study the cell bodies and central projections of neurons innervating the vibrissae in the rat. These can be grouped into five horizontal rows and one posterior vertical row. Twenty-four to 48 hours after the nerves innervating different vibrissae were exposed to HRP, the trigeminal ganglia, brainstem, and upper cervical spinal cord were fixed by perfusion and serial sections were processed according to the tetramethylbenzidine technique. The results revealed a tendency for somatotopic organization in the trigeminal ganglion of cell bodies innervating the different vibrissae. Corresponding termination areas in the trigeminal sensory nuclei showed a detailed pattern of organization replicating the peripheral organization of the vibrissae. In all trigeminal sensory nuclei the horizontal rows are represented in an inverted fashion from dorsal to ventral, i.e., the most dorsal row is represented most ventrally. In addition, the more anterior a vibrissa is located, the deeper is it represented in the rostral nonlaminated nuclei. The situation is reversed in the laminated nucleus caudalis. The posterior vertical row is represented most superficially in the rostral nonlaminated nuclei, but most deeply in the laminated nucleus caudalis. In nucleus caudalis there are also rostrocaudal differences in the representation of different vibrissae. Thus, the posterior vibrissae in a horizontal row have their main representations more caudally than the anterior vibrissae. The posterior vertical row has its main representation most caudally, in the C1 segment.