Axons in the peripheral nervous system (PNS) and central nervous system (CNS) form sprouts after injury. Elongation of regenerating axonal sprouts has been observed as the exception within the adult mammalian CNS but is the rule in the PNS of mammals as well as in the CNS of some fish and amphibians. The relative importance of intrinsic neuronal properties and axonal environment in determining the extent of axonal regrowth is unknown. Neuroglial cells, nerve growth factor and target tissues such as smooth muscle are known to influence neuronal responses to injury. Here we have examined the capacity of transected axons originating in the CNS to regrow into nerve grafts containing Schwann cells.

Axons from CNS neurons regenerate into PNS grafts.

The cytoarchitecture of the motoneuron pool of the male rat was studied at the lumbo-sacral transition area, particularly in L6. In the latter segment a dorso-medial (DM), ventral (V), dorso-lateral (DL), and retrodorso-lateral group (RDL) could be defined. The DL group was associated with a prominent longitudinal dendrite bundle and the DM group with smaller transverse bundles. Morever, close soma-somatic apposition was found between neurons in these columns.



Because L6 gives rise to n. pudendus and contributes to n. ischiadicus, horseradish peroxidase (HRP) was applied to the cut n. ischiadicus and in other experiments injected into the pelvic muscles. Neurons in RDL were labeled following exposure of n. ischiadicus to HRP. Injections in m. levator ani resulted in labeled neurons in the V group, mainly below L6. Injections in m. sphincter urethrae resulted in labeled neurons in the DL group as well as neurons immediately cranial to this column. Musculus ischiocavernosus injections resulted in transport of HRP to neurons in the DL group, primarily in its medial part, and to more cranially located neurons. In addition, some neurons in the V group in L6 were labeled. Following injections in m. bulbocavernosus and m. sphincter ani, labeled neurons were found primarily in the DM group, and to a lesser extent in the V group.



Histochemical investigations with staining methods for the localization of acetyl cholinesterase (AChE) and heavy metals (the Timm method) demonstrated that part of the neuropil of DL and of DM were different from the rest of the motoneuron neuropil. In the DL group the area with the diverging staining patterns corresponded to the region of the dendrite bundle. The experimental data indicated and the ultrastructural studies demonstrated that the histochemical differences could be correlated with differences in the composition of the populations of boutons.



The comparison of the cytoarchitectural and histochemical data with the results obtained by the aid of the retrograde HRP tracing technique established that mm. sphincter urethrae, ischiocavernosus, bulbocavernosus, and sphincter ani were each innervated by two populations of neurons that were situated in separate areas which had different histochemical properties, and which thus probably have different compositions of their afferent inputs.



The daulity in the motoneuron pool that innervates the pelvic muscles might be a reflection of the dual influence on these muscles. As all other striated muscles the pelvic muscles are under voluntary control. However, they are also tightly linked to the function of the pelvic viscera and thus under influence of the autonomic nervous system.

Organization of the motoneurons innervating the pelvic muscles of the male rat.

This study used the retrograde transport of a protein-gold complex to examine the distribution of spinal cord and trigeminal nucleus caudalis neurons that project to the nucleus of the solitary tract (NST) in the rat.

In the spinal grey matter, retrogradely labeled cells were common in the marginal zone (lamina I), in the lateral spinal nucleus of the dorsolateral funiculus, in the reticular part of the neck of the dorsal horn (lamina V), around the central canal (lamina X), and in the region of the thoracic and sacral autonomic cell columns. The pattern of labeling closely resembled that seen for the cells at the origin of the spinomesencephalic tract and shared some features with that of the spinoreticular and spinothalamic tracts. Labeled cells in lamina IV of the dorsal horn were only observed when injections spread dorsally, into the dorsal column nuclei, and are thus not considered to be at the origin of the spinosolitary tract. They are probably neurons of the postsynaptic fibers of the dorsal column.

Retrogradely labeled cells were also numerous in the superficial laminae of the trigeminal nucleus caudalis, through its rostrocaudal extent. The pattern of marginal cell labeling appeared to be continuous with that of labeled neurons in the paratrigeminal nucleus, located in the descending tract of trigeminal nerve.

Since the NST is an important relay for visceral afferents from both the glossopharyngeal and vagus nerves, we suggest that the spinal and trigeminal neurons that project to the NST may be part of a larger system that integrates somatic and visceral afferent inputs from wide areas of the body. The projections may underlie somatovisceral and/or viscerovisceral reflexes, perhaps with a significant afferent nociceptive component.

Spinal and trigeminal projections to the nucleus of the solitary tract: a possible substrate for somatovisceral and viscerovisceral reflex activation.

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Central Control of the Cardiovascular and Respiratory Systems and Their Interactions in Vertebrates

Spinal origin of sympathetic preganglionic neurons in the rat.

The distribution in the spinal cord of the rat of preganglionic neurons sending fibers into the hypogastric nerve was determined with the retrograde transport of horseradish peroxidase (HRP). Labeled cells were present in the intermediate gray matter of spinal segments L1-L2. The majority (81%) of HRP-filled cells formed a continuous column along the midline in the dorsal gray commissure. This cell column was termed the dorsal commissural nucleus (DCN). The remainder of HRP-labeled cells were present bilaterally in the middle and lateral regions of the intermediate gray; the majority of the latter cells were located along the lateral border of the intermediate gray.



The present finding of a midline preganglionic autonomic cell column in the spinal cord of a mammal is contrary to previous reports, in which sympathetic preganglionic neurons have been localized primarily in the lateral intermediate gray. The DCN may be species-specific and related to the system of short adrenergic neurons present in the pelvis.

Cynthia A. Peveto

Papers

A preganglionic autonomic nucleus in the dorsal gray commissure of the lumbar spinal cord of the rat