Ascending projections of the dorsal column in a garter snake (Thamnophis siritalis): a degeneration study

Projections ascending from the spinal cord to the brain in petromyzontid and myxinoid agnathans

The course of projections ascending through the rostral spinal cord to nuclei in the brains of petromyzontid and myxinoid agnathans was examined with silver staining of anterograde degeneration and horseradish peroxidase histochemistry. As in jawed vertebrates, the ascending spinal projections of lampreys and hagfishes appear to be organized into two major systems, the spinal lemniscal and dorsal column pathways. The spinal lemniscal pathway, extending rostrally along the ventrolateral margin of the spinal and medullary central gray, consists of a spinoreticular and possibly a spinovestibular projection in both aganthan groups. In Pacific hagfish, spinal lemniscal fibers reach the ipsilateral mesencephalic tectum, but no spinal projection to the thalamus was evident. The spinal lemniscus of lampreys ascends to the region of the isthmus and may extend into the mesencephalic tegmentum. Anterograde and retrograde tracing methods indicate that a very small population of cells in the far rostral cord of lampreys may project to the optic tectum and diencephalon; however, spinotectal and spinothalamic projections, if present, are limited in extent. The dorsal column pathway in agnathans, consisting in part of primary spinal afferents, ascends in the dorsal funiculus of the cord. The dorsal column fibers of agnathans, like those of some other anamniotes, continue beyond the spinomedullary junction through the length of the hindbrain, possibly conveying ascending somatosensory input to the sensory nuclei of the alar medulla.

Dorsal funicular projections to the dorsal column nucleus in a reptile, Caiman crocodilus

Projections to the dorsal column nucleus were investigated in the reptile Caiman crocodilus following application of horseradish peroxidase (HRP) crystals to the cervical spinal cord after dorsal myelotomy. Brains were processed for HRP histochemistry by use of tetramethylbenzidine as the chromogen. Injection of the dorsal funiculus alone labelled axons that terminated solely in the ipsilateral dorsal column nucleus. Application of HRP that extended beyond the dorsal funiculus labelled axons that terminated in brainstem areas in addition to the dorsal column nucleus depending upon the injection site. Retrogradely labelled neurons in the brainstem were seen only after HRP injections that extended beyond the dorsal funiculus and varied depending upon the injection site. Dorsal funiculus fibers travel rostrally and turn laterally to reach more anterior portions of the dorsal column nucleus. Fibers peel off in an oblique ventrolateral fashion to terminate in the posterior part of the nucleus where boutons are intermixed with fibers. The anterior portion of the nucleus contains a greater proportion of axon terminals as compared to fibers than does the posterior part. A graphic reconstruction of the dorsal column nucleus was made by superimposing serial sections to view the nucleus in transverse, sagittal, and horizontal planes.

The nucleus dorsalis myelencephali of snakes: relay nucleus between the spinal cord and the posterior colliculus (paratorus)

Nucleus dorsalis myelencephali is in the dorsolateral area of the caudal medulla in snakes. The parvocellular area projects bilaterally to the paratorus and receives ipsilateral projections from the spinal cord. The magnocellular area projects bilaterally to the spinal cord. This nucleus has been only briefly described in snakes but not in any other reptilian group.

Primary afferent projections to the spinal cord and the dorsal column nuclear complex in the turtle Pseudemys

Primary afferent projections from cervical and lumbar levels were studied in the turtle Pseudemys scripta elegans. Injections of radioactive amino acids, wheat germ agglutinin and horseradish peroxidase were made into the dorsal root ganglia or the spinal cord. Previous reports on the terminal distribution of primary afferents within the ipsilateral segment of entry were confirmed (Kusuma and ten Donkelaar 1979, 1980) and additional dorsal root projections were demonstrated to the contralateral side and to several neighboring spinal segments. The primary afferent projections to the brainstem were essentially restricted to a dorsolateral area that appears to be homologous to the main dorsal column nuclei (n. gracilis and n. cuneatus medialis) in mammals. While exhibiting a similarly extensive rostro-caudal span, the projections originating from lumbar injections terminated more medially, those from cervical injections more laterally. The labeling pattern suggested that terminations are mainly on dorsally extending dendrites.

Afferent connections of the cerebellum in various types of reptiles

The origin of cerebellar afferents was studied in various types of reptiles, viz., the turtles Pseudemys scripta elegans and Testudo hermanni, the lizard Varanus exanthematicus, and the snake Python regius, with retrograde tracers (the enzyme horseradish peroxidase and the fluorescent tracer "Fast Blue"). Projections to the cerebellum were demonstrated from the nucleus of the basal optic root, the interstitial nucleus of the fasciculus longitudinalis medialis, the vestibular ganglion, and the vestibular nuclear complex, two somatosensory nuclei, viz., the descending nucleus of the trigeminal nerve and the nucleus of the dorsal funiculus, the nucleus of the solitary tract, the reticular formation, and throughout the spinal cord. A distinct bilateral projection to the cerebellum was found to arise in a nucleus previously called nucleus parvocellularis medialis (Ebbesson, '67). In the present study this cell mass is termed the perihypoglossal nuclear complex, considering its comparable position and fiber connections to the perihypoglossal nuclei in mammals. In all reptilian species studied a contralateral cerebellar projection of a cell mass located in the caudal brainstem adjacent to the nucleus raphes inferior was observed. It seems likely that this cell mass represents the reptilian homologue of the mammalian inferior olive. Most of the spinocerebellar fibers appeared to arise in neurons located in area VII-VIII of the gray matter. In this respect the origin of the spinocerebellar projection in reptiles resembles the origin of the rostral and ventral spinocerebellar tracts in mammals. No indications for the existence of a column of Clarke or a central cervical nucleus in the reptilian spinal cord were obtained. On comparison of the cerebellum afferents in reptiles with the known connections of the cerebellum in amphibians, birds, and mammals, a basic pattern of cerebellar afferent projections appears to exist in these vertebrate classes, including retinal, vestibular, precerebellar, somatosensory, and spinal afferents.