We have investigated the organizational and morphological features of motoneurons from cat sacrocaudal spinal cord, the portion of the neuraxis that innervates the tail. This information is pertinent for development of a new model of spinal cord injury. An understanding of sacrocaudal circuitry is essential for physiological and behavioral assessment of the effects of sacrocaudal lesions. Observations from Nissl-stained sections corroborated Rexed's cytoarchitectural scheme. Putative motoneurons were located within two regions of the ventral horn: the ventromedial nucleus (lamina IX) and the nucleus commissuralis. To map motoneuron pools, cholera toxin-horseradish peroxidase conjugate was injected into each dorsal tail muscle. The dorsomedial muscle was innervated by ipsilateral nucleus commissuralis motoneurons. The dorsolateral and intertransversarius muscles were innervated by ipsilateral lamina IX and nucleus commissuralis motoneurons. Cell bodies of retrogradely labeled sacrocaudal motoneurons ranged from 22 to 82 microns in diameter; the unimodal distributions peaked between 45 and 50 microns. Dendritic trees of motoneurons, revealed by retrograde labeling or by intracellular injection with horseradish peroxidase, were extensive. Five to eight primary dendrites originated from the cell body. Dendritic branches extended throughout the ipsilateral ventral gray matter, with processes spreading into the surrounding white matter and the base of the dorsal horn. Dendrites from motoneurons with their soma in the lateral portion of lamina IX formed a longitudinal plexus at the gray/white border. Medial dendrites from motoneurons in the nucleus commissuralis formed bundles in the ventral gray commissure and spread throughout the contralateral ventral horn. It is speculated that contralateral dendrites subserve synchronized co-contraction of medial muscles from both sides of the tail.
Intra-axonal recording and horseradish peroxidase staining techniques were used to map terminal fields of primary afferent fibers from cutaneous receptors within the cat sacrocaudal spinal cord. It was hypothesized that projection patterns of cutaneous afferent fibers mirror the known somatotopic organization of sacrocaudal dorsal horn cells. Forty-three primary afferent fibers, innervating either slowly adapting type I receptors, hair follicles, or slowly adapting type II receptors, all on the tail, were recovered. All collaterals (N = 372) branched from parent axons in the dorsal columns. Most collaterals coursed rostromedially to the ipsilateral gray matter, penetrated the medial dorsal horn, and arborized within laminae III, IV, and to a lesser extent, V. Ipsilateral projections to dorsal horn were as follows: axons with dorsal or dorsolateral receptive fields (RFs; n = 20) to the lateral portion, axons with lateral RFs (n = 4) to the central portion, and axons with ventral or ventro-lateral RFs (n = 19) to the medial portion. Most axons (16 of 20) with dorsal or dorsolateral RFs also had contralateral projections to lateral dorsal horn and most axons (15 of 19) with ventral or ventrolateral RFs also had contralateral projections to medial dorsal horn. No axons with lateral RFs had crossed projections. These data represent the first complete mapping of the somatotopic organization of primary afferent fiber projection patterns to a spinal cord level. The findings demonstrate that ipsilateral projection patterns of sacrocaudal primary afferent fibers are in register with the somatotopic organization of the dorsal horn. Our earlier suggestion that crossed projections of primary afferent fibers give rise to crossed components of dorsal horn RFs spanning the midline is supported by these results.
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