The Journal of comparative neurology
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Double retrograde axonal tracing experiments were carried out in order to reveal potential patterns of divergence in axonal projections from the two major sensory nuclei of the mouse brainstem trigeminal complex: the principal sensory and spinal trigeminal nuclei (oralis, interpolaris, and caudalis divisions). The tracers wheat germ agglutinin, N-[acetyl-3H] and horseradish peroxidase were used in paired injection strategies within portions of the cerebellum, superior colliculus, and thalamic ventrobasal complex and/or posterior group of adult ICR white mice. Trigeminal neurons with projections to tactile areas of the cerebellar cortex or underlying deep cerebellar nuclei were found scattered throughout the principal sensory nucleus and interpolaris division, and mainly in dorsal regions of the oralis division of the spinal trigeminal nucleus. ⋯ Double-labeled neurons were observed only after paired injections of the tracers in the thalamus and ipsilateral superior colliculus, and they were found within the caudoventral portion of the principal sensory nucleus near the oralis border, throughout the interpolaris division, within the magnocellular layer of caudalis, and only a few double-labeled neurons were also found within the marginal layer. After such injections, 50% of the labeled tectum-projecting neurons in the principal sensory nucleus, 64% in the interpolaris division, and 57% in the caudalis division are branched neurons which have collateralized projections to both the superior colliculus and thalamus. These projections, which have not been described before, appear to arise from more than one class of projection neuron which is differentially distributed within different regions of the trigeminus.
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Horseradish peroxidase was intra-axonally injected into functionally identified primary afferent fibers within the rat spinal trigeminal tract in order to study the morphology of their central terminations. They were physiologically determined to be large, myelinated, cutaneous primary afferents by means of electrical and mechanical stimulation of their receptive fields. Ninety-three axons that innervated vibrissa follicles, guard hair follicles, and slowly adapting receptors were stained for distances of 4-12 mm at the levels of the main sensory nucleus, spinal trigeminal nucleus, and rostral cervical spinal cord. ⋯ The terminal arbors of the rostral type of collaterals formed an interrupted, rostrocaudally oriented column like those seen in the lumbar dorsal horn, but the column shifted down to lamina V near the obex, and more caudally, gradually shifted upward to lamina III. Major morphological differences were not observed among the three different functional types of collaterals with respect to the rostrocaudal distribution of collaterals, and the shape and location of collaterals. The differential laminar distribution of collateral arbors of single axons along the rostrocaudal axis distinguishes the spinal trigeminal nucleus from the spinal dorsal horn where functional types of mechanoreceptive afferents form continuous or interrupted sagittal columns of terminal arbors that do not shift dorsoventrally within segments.