Reptilian Somatosensory Midbrain: Identification Based on Input from the Spinal Cord and Dorsal Column Nucleus

Abstract

A midbrain somatosensory area was identified in reptiles Caiman crocodilus, by orthograde degeneration techniques after midcervical spinal cord transections or after dorsal column lesions. Terminations of these somesthetic inputs were located in a caudal lateral part of the central nucleus of the torus semicircularis and contiguous dorsal mesencephalon. Dorsal column projections were contralateral while spinal connections were mainly ipsilateral. In order to confirm the results of the anterograde degeneration experiments independently, injections of horseradish peroxidase were placed in this midbrain somatosensory recipient area. Retrogradely labeled neurons were identified in the contralateral dorsal column nucleus. These findings indicated that the pattern of degeneration seen after dorsal column lesions originated from the dorsal column nucleus itself. The results of the present experiments when compared with similar studies in other amniotes emphasize the following two points. First, dorsal column and spinal cord projections to the midbrain are features common to reptiles, birds, and mammals. Second, since these pathways and midbrain somatosensory terminations are common to amniotes, dorsal column nucleus and spinal circuits to the midbrain and their mesencephalic projection areas are phylogenetically ancient.

Thalamic Projections from a Midbrain Somatosensory Area in a Reptile, <i>Caiman crocodilus</i>

“…To confirm these assumptions, HRP injections into the medialis complex resulted in retrogradely labeled cells primarily restricted to the caudolateral portion of the central nucleus of the torus semicircularis and adja cent lateral mesencephalic gray. This area was coinci dent with the locus of terminals from the spinal cord and dorsal column nucleus identified in a previous study [Pritz and Stritzel, 1989]. The retrograde HRP experiments identified labeled cells in the somatosen sory midbrain that were heaviest ipsilaterally but unequivocally identified neurons contralaterally as well.…”

“…A craniectomy was performed over the optic tectum which was then removed to expose the underlying torus semicircularis. A detailed account of the injection types, perfusion of animals and tissue processing for HRP ncurohistochcmistry is available elsewhere [Pritz and Stritzel, 1989].…”

“…Brains were then removed and blocked in a standard plane [Pritz, 1974]. Brains were then postfixed, embedded in albumin-ge latin, sectioned transversely on a freezing microtome at 40 pm, and then processed for HRP neurohistochemistry using tetramethylbcnzidinc as the chromogen [Mesulam, 1979], according to proce dures described in detail previously (Pritz and Stritzel, 1989]. Sec tions were mounted on gelatinized slides, dehydrated and coverslipped.…”

“…This somesthetic region is located in the caudolateral portion of the torus semicircularis and contiguous dorsal mesencephalon -an area anatomi cally separate from the midbrain auditory region [Pritz and Stritzel, 1989]. The present study deter mined the thalamic projections of this mesencephalic somatosensory area utilizing horseradish peroxidase (HRP) neurohistochemical methodology.…”

Afferent Connections of the Optic Tectum in Lampreys: An Experimental Study

“…Following tectal injections, retrograde labeled cells were found in the spinal cord of several vertebrate species, including fish [Hayle, 1973;Smeets, 1982;Becker et al, 1997;Prasada Rao and Sharma, 1999], amphibians [Wilczynski and Northcutt, 1977;Gruberg and Solish, 1978;Rettig, 1988], and reptiles [Ebbesson, 1967;Ulinski, 1977;Stein and Gaither, 1981;Welker et al, 1983;Pritz and Stritzel, 1989]. However, they could not be demonstrated in other species such as fish [Luiten, 1981;Northcutt, 1982;Fiebig et al, 1983;Oka et al, 1986;Schlussman et al, 1990] or reptiles [Ebbesson, 1969;Hay-le, 1973;Gruberg et al, 1979;Kishida et al, 1980].…”