Retrograde hrp identification of neurons in the rhombencephalon and spinal cord of the rat that project to the dorsal mesencephalon

Abstract

Neurons in the rhombencephalon and spinal cord of the rat that project to the dorsal midbrain were identified using the HRP retrograde neuroanatomical tracing method. Retrogradely labeled neurons were most numerous in the reticular formation, specifically in nucleus pontis oralis, caudalis, and gigantocellularis and in the sensory trigeminal complex. Neurons that project were also found in certain raphe nuclei, the parabrachial nuclei, the deep cerebellar nuclei, the vestibular cochlear complex, and the spinal cord. The diverse distribution of neurons that project to the midbrain is understandable in the context of the known diverse functions of the region, which include a role in sexual and other behaviors, nociception, and various autonomic functions.

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

“…Lamina VIII projections to the contralateral cerebellum (Matsushita et al, 1979; cat), medulla (Villanueva et al, 1991; rat), and thalamus (Craig et al, 1989; rat). Further, a contralateral, reciprocal projection to the superior colliculus from lamina VIII has also been identified in the rat (Morrell & Pfaff, 1983), suggesting a communication pathway where visual, auditory, and somatosensory information from the superior colliculus goes on to influence motor responses (and vice‐versa; Ito & Feldheim, 2018). Spinal commissural neurons are found in lamina VIII and project more locally; to contralateral lamina VII, VIII, and IX (Harrison et al, 1986; Matsushita, 1970; Scheibel & Scheibel, 1968).…”

“…Projection neurons are predominately found in this in the medial zone (Molenaar & Kuypers, 1978) and project to a host of supraspinal regions such as the cerebellum (cat; as part of the cuneocerebellar tract; Edgley & Gallimore, 1988), the thalamus (rat; as part of the spinothalamic tract; Burstein et al, 1990; Kayalioglu et al, 1996), the periaqueductal gray (rat; as part of the spinomesencephalic tract; Liu, 1983), and the superior colliculus (rat; as part of the spinomesencephalic tract; Morrell & Pfaff, 1983). The lateral zone on the other hand occupies the remaining third of these layers and neurons in this sub‐region are comparatively larger (30–45 μm; Rexed, 1952b) and adopt a multipolar morphology.…”

“…In addition, lamina VII contributes to the formation of the spinocuneocerebellar tract (Sengul & Watson, 2015). Other projection targets include the superior colliculus (Morrell & Pfaff, 1983) [indicating contributions to eye movement and coordinating the bodily response to visual stimuli (Dean et al, 1989)], the periaqueductal gray (Liu, 1983; suggesting involvement in autonomic body function; projections to the superior colliculus and periaqueductal gray ascend via the spinomesencephalic tract), and the pontomedullary reticular formation [Krutki et al, 1998; which is involved in the regulation and maintenance of posture (Miller et al, 2017)].…”

Cytoarchitecture of the Spinal Cord

“…Lamina 7 has also been shown to have supraspinal projections, including projections to the amygdala and hypothalamus (Menétrey and De Pommery, 1991), cerebellum (Rivero-Melián and Grant, 1990;Matsushita, 1999;), the contralateral superior colliculus (Morrell and Pfaff, 1983), parabrachial nucleus (Kitamura et al, 1993), periaqueductal gray (Liu, 1983;Keay and Bandler, 1993), and the pontomedulllary reticular formation (Chaouch et al, 1983;Krutki et al, 1999). Lamina 7 has also been shown to have supraspinal projections, including projections to the amygdala and hypothalamus (Menétrey and De Pommery, 1991), cerebellum (Rivero-Melián and Grant, 1990;Matsushita, 1999;), the contralateral superior colliculus (Morrell and Pfaff, 1983), parabrachial nucleus (Kitamura et al, 1993), periaqueductal gray (Liu, 1983;Keay and Bandler, 1993), and the pontomedulllary reticular formation (Chaouch et al, 1983;Krutki et al, 1999).…”