Organization of motor pools supplying the cervical musculature in a cryptodyran turtle, Pseudemys scripta elegans. I. Dorsal and ventral motor nuclei of the cervical spinal cord and muscles supplied by a single motor nucleus

Abstract

In this and the accompanying paper (Yeow and Peterson, '86) we characterize motor nuclei of the cervical spinal cord in Pseudemys scripta and the motor pools of eight cervical muscles. We have identified three motor nuclei that supply the cervical musculature by using serial reconstructions of Nissl-stained spinal cords cut in three cardinal planes, and in experimental cases in which horseradish peroxidase (HRP) was applied to individual neck muscles. These nuclei are named according to their position as visualized in the transverse plane: dorsal, ventral, and medial. A fourth (ventrolateral) motor nucleus was never labelled following application of HRP to the cervical musculature and presumably innervates the forelimbs. The dorsal motor nucleus occupies the mid-dorsal to dorsolateral ventral horn of C1 and C2. It is composed of at least two morphological groups of motor neurons; one of these is a population of very large, fusiform profiles with transversely oriented dendrites that is found primarily in C1. The ventral motor nucleus occupies the tip of the ventral horn from C1 to C8. Its cells are significantly smaller and more numerous in rostral than in caudal cervical segments. In Nissl material, ventral nuclear profiles show little tendency to cluster into subgroups, but experimental cases suggest that there is some spatial dissociation of different motor pools within the ventral nucleus. The medial motor nucleus is described in the accompanying paper together with the motor pools of three cervical muscles that it supplies. Having identified the cervical motor nuclei we then used retrograde transport of HRP to characterize the motor pools of individual cervical muscles. Two superficial ventral muscles (mm. coracohyoideus and plastro-squamosus) are supplied by dorsal nuclear cells. M. coracohyoideus motor neurons are significantly larger than those of m. plastrosquamosus and the very large, fusiform cell type is relatively more numerous in the m. coracohyoideus motor pool. Dorsal and lateral muscles (mm. cervicocapitis, testocapitis, and transversalis cervicis) are supplied by ventral nuclear motor neurons. These cells are smaller, on average, than motor neurons supplying ventral musculature. The m. cervicocapitis motor pool lies dorsomedially in the tip of the ventral horn of C1 and C2; motor neurons supplying the more laterally placed mm. testocapitis and transversalis cervicis occur more ventrolaterally, in C2-C3 and C3-C5, respectively. Thus each of the five cervical muscles is supplied by a single motor nucleus, and their motor pools are organized into a musculotopic pattern.

Properties of spinal motoneurons and interneurons in the adult turtle: Provisional classification by cluster analysis

“…11B-D). In summary on this point, although there is now some morphology to support R N measurements showing turtle MNs to be larger than INs, there is no morphology indicating a bimodal distribution of hindlimb MN sizes to support the supposition (based on R N values) that the Overlap MNs were much smaller than the Main MNs (for a bimodal distribution of MNs supplying a turtle neck muscle, see Yeow and Peterson, 1986).…”

Homology of the enigmatic nuchal bone reveals novel reorganization of the shoulder girdle in the evolution of the turtle shell

“…Rectus capitis cervico-plastralis Shah (1963) testocapitis Rectus capitis cervico-plastralis Schumacher (1973) n.a. M. plastrosquamosus Yeow and Peterson (1986) Testo-capitis Plastrosquamosus Herrel et al (2008) m. testocapitis n.a.…”

Distribution of motoneurons supplying dorsal and ventral suboccipital muscles in the feline neck

“…The motoneurons of these muscles were among the first to have been studied using retrograde and intracellular tracers (Richmond et al, 1978;Rose, 1981;Keirstead and Rose, 1983). Their distribution in the cervical spinal cord has since been identified in cats and a number of other vertebrate species (Richmond et al, 1978;Robards et al, 1980;Abrahams and Keane, 1984;Pfister and Zenker, 1984;Yeow and Peterson, 1986;Callister et al, 1987). However, much less is known about shorter, deeper neck muscles, at least in part because of the technical difficulties encountered when trying to carry out anatomical and electrophysiological studies on muscles whose nerves are short and often hidden.…”