Crossed receptive field components and crossed dendrites in cat sacrocaudal dorsal horn

Sensory-induced activation of pattern generators in the absence of supraspinal control

Sacrocaudal afferent (SCA) stimulation is used in this work to study neural pathways involved in sensory-activation of central pattern generators (CPGs) in the isolated spinal cord of the neonatal rat. Surgical manipulations of the white matter funiculi and confocal imaging of back-labeled funicular pathways suggest that the CPGs are activated during SCA stimulation by crossed and uncrossed multifunicular projections of sacral neurons and that activation of short projecting proprioneurons is sufficient for the generation of the rhythm by SCA stimulation. The versatile organization of the pathways involved in the SCA-induced rhythm makes it a potent and durable activator of the CPGs in the absence of descending control from the brain. The significance of our findings and their potential clinical use are discussed.

Alteration in axial motoneuronal morphology in the spinal cord injured spastic rat

Following spinal cord injury (SCI), exaggerated reflexes and muscle tone emerge that contribute to a general spastic syndrome in humans. At present, the underlying mechanisms involved with the development of spasticity following traumatic spinal cord injury, especially with regard to axial musculature, remains unclear. The purpose of the present study was to examine the temporal changes in sacrocaudal motoneuronal morphology following complete transection of the sacral spinal cord and to correlate these changes with the onset and progression of spasticity within the tail musculature. The spinal cords of rats were transected at the upper sacral (S(2)) level. Animals were behaviorally tested for the onset and progression of spasticity in the tail and at 1, 2, 4, or 12 weeks postinjury were sacrificed. At these time points, the animals demonstrated stage 1, 2, 3, or 4 spastic behavior, respectively. Sacrocaudal motoneurons innervating selected flexor muscles within the tail were retrogradely labeled with cholera toxin beta-subunit and neuronal morphology was analyzed using a combination of immunocytochemistry and standard microscopy. Initially over the first 2 weeks postinjury, a transient increase in the lengths of primary and secondary dendrites occurred. However, a progressive decrease in the overall number of dendritic branches was observed between 2 and 12 weeks postinjury, which parallels the time frame for the progressive increase in spastic behavior in the tail musculature. Following spinal cord injury, there is an alteration in the morphology of tail flexor motoneurons, which may be relevant to the development of spasticity within the tail.

Crossed and uncrossed projections to the cat sacrocaudal spinal cord: III. Axons expressing calcitonin gene-related peptide immunoreactivity

We have investigated the projection patterns of peptidergic small-diameter primary afferent fibers to the cat sacrocaudal spinal cord, a region associated with midline structures of the lower urogenital system and of the tail. Calcitonin gene-related peptide (CGRP)-immunoreactive (CGRP-IR) primary afferent fibers were observed within the superficial laminae, rostrally as the typical inverted U-shaped band that capped the separate dorsal horns (S1 to rostral S2) and caudally as a broad band that spanned the entire mediolateral extent of the fused dorsal horns (caudal S2 and caudal). Within the dorsal gray commissure, labeling was seen as a periodic vertical, midline band. CGRP-IR labeling was prevalent in an extensive mediolateral distribution at the base of the dorsal horn, originating from both lateral and medial collateral bundles that extend from the superficial dorsal horn. Some bundles, in part traveling within the dorsal commissure, conspicuously crossed the midline. In addition to the robust projection to the superficial dorsal horn, there was a more extensive distribution of CGRP-IR fibers within the deeper portions of the cat sacrocaudal dorsal horn than has been reported for other regions of the cat spinal cord. Presumably, these deep projections convey visceral information to projection or segmental neurons at the neck of the dorsal horn and in the region of the central canal. This deep distribution overlaps the reported projections of the pelvic and pudendal nerves. In addition, the contralateral projections of CGRP-IR fibers may form an anatomical substrate of the bilateral receptive fields for selective dorsal horn neurons. The density and variety of CGRP-IR projection patterns is a reflection of the functional attributes of the innervated structures.