Spinal ascending pathways in amphibians: Cells of origin and main targets

Abstract

As part of a research program on the evolution of somatosensory systems in vertebrates, the various components of ascending spinal projections were studied with in vivo and in vitro tract-tracing techniques in representative species of amphibians (the large green frog, Rana perezi, the clawed toad, Xenopus laevis and the ribbed newt, Pleurodeles waltl). Three main ascending sensory channels, each with largely separate targets, were demonstrated: 1. Ascending projections via the dorsal funiculus include primary and nonprimary projections that ascend to terminate mainly in the dorsal column nucleus at obex levels. A small component ascends farther rostralwards to terminate in the reticular formation, the octavolateral area, the trigeminal nuclear complex, and in the granular layer of the cerebellum. 2. Projections ascending via the dorsolateral funiculus reach other spinal and supraspinal targets than the dorsal funicular fibers, mainly ipsilaterally. At upper cervical cord and obex levels, many fibers innervate a region considered the amphibian homologue of the lateral cervical nucleus of mammals. In the medulla, these fibers ascend ventral to the descending trigeminal tract to terminate in the dorsal column and the solitary tract nuclei, and more rostrally, in the reticular formation, the descending trigeminal nucleus and the medial aspect of the ventral octaval nucleus. Major projections reach the area between the facial motor nucleus and the ventral octaval nucleus, and a mediolateral subcerebellar band. These projections arise in neurons located mainly in the ipsilateral deep dorsal and lateral fields throughout the spinal cord. 3. Ascending spinal projections via the ventral quadrant of the spinal cord (the ventral and ventrolateral funiculi) ascend throughout the brainstem up to the diencephalon. Along its course, this component innervates various parts of the reticular formation, the octavolateral area, the granular layer of the cerebellum, the region ventromedial and ventrolateral to the isthmic nucleus, and the subcerebellar region. In the mesencephalon, the torus semicircularis, the midbrain tegmentum and, sparsely, the tectum mesencephali are innervated. Beyond the midbrain, various dorsal and particularly ventral thalamic nuclei and the posterior tubercle are innervated by this ascending sensory channel. The cells of origin of some of these projections were observed in the dorsal, and to a lesser extent, in the lateral and ventral spinal fields of the spinal cord. Evidence for the presence of these three main ascending sensory channels throughout vertebrates will be discussed. The presence of such channels appears to be a shared character in the brain of both amniotes and anamniotes.

Descending supraspinal pathways in amphibians. II. Distribution and origin of the catecholaminergic innervation of the spinal cord

“…Moreover, in mammals dopamine is involved in the sensory transmission by means of cells in the dorsal horn that project to the dorsal column nucleus (DCN), but not by means of cells that give rise to the spinocervical tract (Doyle and Maxwell, 1993;Doyle, 1994). A similar condition may exist in amphibians, although double-labeling studies are needed to distinguish between the CA innervation of cells that project to the DCN and those constituting the origin of the spinocervical tract (Muñ oz et al, 1995(Muñ oz et al, , 1996(Muñ oz et al, , 1997.…”

Anurans

“…Munoz et al 1991, A. Munoz et al 1995 studies demonstrated rather extensive, somatotopically arranged, spinal afferents to the dorsal column nucleus. The dorsal funicular projection continues rostrally to innervate the vestibular nuclear complex as well as the granular layer of the cerebellum Szekely et al 1980;A. 19.7.4) consists of medial ('gracile') and lateral ('cuneate') compartments.…”

“…The dorsal funicular projection continues rostrally to innervate the vestibular nuclear complex as well as the granular layer of the cerebellum Szekely et al 1980;A. Munoz et al 1997). Fibres terminating in the vestibular region and in the cerebellum (Ebbesson's dorsal spinocerebellar pathway) arise from limbinnervating spinal ganglia Gonzalez et al 1984).…”

“…the spinal lemniscus, are grouped into two relatively discrete fascicles, each issuing fibres to deeper structures (Ebbesson 1969(Ebbesson , 1976A. the spinal lemniscus, are grouped into two relatively discrete fascicles, each issuing fibres to deeper structures (Ebbesson 1969(Ebbesson , 1976A.…”

Analgesia in Amphibians: Preclinical Studies and Clinical Applications

“…A single study found that electrical stimulation of the sciatic nerve produces evoked potentials in posterior thalamic nuclei and primordial hippocampal structures in frogs 35. However, the general sensory tracts from spinal neurons make direct connections with neuron groups farther towards the front of the brain as the phylogeny of vertebrates is ascended 36. Throughout phylogeny, all the target sites of spinal tracts in the brain increase in complexity, specialization, and number of neurons, suggesting that even nociceptive messages to the thalamus in an amphibian and mammal may not be similar.…”