Noemi Corvaja

The Origin, Course and Termination of Vestibulospinal Fibers in the Toad

The course and distribution of the vestibulospinal tract in the toad have been studied in silver-impregnated sections following electrolytic lesions of the terminal nuclei of the VIIIth cranial nerve. The tract originates from the ventral, vestibular nucleus of the VIIIth nerve and descends bilaterally in the ventral funiculi throughout the whole length of the spinal cord. Contralateral fibers are less numerous than ipsilateral and cross the midline in the caudal medulla oblongata. Some fibers cross in the spinal gray matter ventral to the central canal.The vestibulospinal fibers terminate in the ventromedial region of the ventral horn which harbors the medial column of motoneurons, innervating the axial musculature. Silver-impregnated sections suggest that some fibers might terminate on the cell bodies and proximal dendrites of these motoneurons. Preliminary electron microscopical observations, however, confirm only the presence of degenerating boutons in the neuropile of this region.Short comment has been devoted to other descending fiber systems which were observed following hemisection of the high cervical cord.

Spinal Projections from the Mesencephalon in the Toad

Mesencephalic cell groups projecting to the spinal cord have been identified by means of the retrograde axonal transport of the enzyme horseradish peroxidase (HRP). The injections were made either in the cervical or lumbar enlargements of the toad spinal cord. Following injections in the cervical cord, labeled cells are located in the isthmus region, in the ipsilateral laminated nucleus posteroventralis tegmenti mesencephali (Potter). At more rostral levels the labeled cells are in the nucleus of the fasciculus longitudinalis medialis, in the nucleus interstitialis of the fasciculus longitudinalis medialis, in the contralateral red nucleus, in lamina six of the contralateral optic tectum, bilaterally in the nucleus of the posterior commissure and in the mesencephalic nucleus in the Vth nerve. Injections in the lumbar cord label neurons of the nucleus posteroventralis tegmenti mesencephali (Potter) and nucleus interstitialis of the fasciculus longitudinalis medialis. Nuclei that had not been previously identified in anurans but which were labeled after HRP spinal injections (i.e., the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus of the posterior commissure and the red nucleus) have been delimited in normal material in Nissl-stained transverse sections. The spinal pathways from the mesencephalon can be classified into four projections: reticulospinal, rubrospinal, tectospinal and trigeminospinal. A comparison of these descending fiber systems with homologous pathways in other vertebrate species has been made.

Identification of different size motoneurons labeled by the retrograde axonal transport of horseradish peroxidase

SummaryThree main groups of motoneurons of different size have been labeled in adult cats by using the method for retrograde axonal transport following injection of horseradish peroxidase in the medial gastrocnemius and soleus muscles. In particular small, medium-size and large neurons which probably correspond respectively to gamma, small alpha and large alpha motoneurons innervating the calf muscles, have been identified and the corresponding area measured.

Ultrastructure of supraspinal dorsal root projections in the toad. II. The cerebellar granular layer

SummaryFollowing section of the left dorsal roots, degenerating fibres and boutons were observed in the granular layer of the ipsilateral cerebellum. The degenerating terminals were identified as largeen passant varicosities of mossy fibres contacting the dendrites of presumptive granule cells. They contained round synaptic vesicles and neurofilaments and established Gray type I contacts. The terminals initially underwent filamentous degeneration with neurofilamentous hypertrophy, swollen mitochondria and loss of synaptic vesicles. At later survival times (6–30 days) they acquired an electron-dense appearance due to an increase and clumping of the filamentous component.After injection of horseradish peroxidase into the left cerebellum, all ipsilateral spinal ganglia showed a few (2–3%) labelled cells, indicating that a primary afferent contribution to this pathway originated from each segment of the spinal cord.