Axon morphology of mesencephalic trigeminal neurons in a snake, Thamnophis sirtalis.

The morphology of single axons of mesencephalic trigeminal neurons (Mes V) was studied in the eastern garter snake (Thamnophis sirtalis) by solid filling them with an extracellular horseradish peroxidase technique. Each Mes V axon can be divided into central, peripheral, and descending branches. The central branch descends from its soma of origin in the mid-brain to the dorsal aspect of the motor nucleus of the trigeminal (Motor V) and the motor root, where it splits into peripheral and descending branches. The descending branch travels caudally from Motor V to the brainstem-spinal cord junction. The peripheral branch passes dorsal to motor V and joins the motor root of V to exit the brainstem. All three branches issue a massive collateral system that distributes terminal swelling within the nuclear boundaries of Motor V. Single Mes V axons diverge to sparsely contact a large number of motoneurons throughout the nucleus, suggesting that single motoneurons receive a convergent input from many Mes V neurons. Since Motor V contains multiple, highly overlapping motor pools, single afferents are positioned to contact different motor pools. The descending branch is situated medial and adjacent to the spinal sensory nucleus of the trigeminal (Sensory V). It issues a collateral field to the entire length of Sensory V. The terminal swellings of these collaterals form rostrocaudally aligned sheets, flattened in the horizontal plane. Single terminal sheets have a divergent projection to a large field of sensory cells and single, fusiform sensory cells are positioned to receive a convergent projection from many terminal sheets. The results provide the first detailed description of Mes V axon morphology. The overall pattern of these axons closely resembles that recently described for spinal Ia afferent fibers in cat. There is evidence in both cases for divergence of single afferent terminal fields to set of spatially overlapping motor pools and a convergence of input to single motoneurons from a large population of afferents. This anatomical pattern is consistent with the recently proposed role of sensory feedback in the activity of single motoneurons.