Identification of beta fibers at spindle entry

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

The purpose of the present study was to compare, in motoneurons (MNs) vs. interneurons (INs), selected passive, transitional, and active (firing) properties, as recorded in slices of lumbosacral spinal cord (SC) taken from the adult turtle. The cells were provisionally classified on the basis of (1) the presence (in selected INs) or absence (MNs and other INs) of spontaneous discharge, (2) a cluster analysis of selected properties of the nonspontaneously firing cells, (3) a comparison to previous data on turtle MNs and INs, and (4) a qualitative comparison of the results with those reported for other vertebrate species (lamprey, cat). The provisional nomenclature accommodated properties appropriate for solely MNs (Main MN group) vs. nonspontaneously firing INs (Main IN-N) vs. spontaneously firing INs (IN-S) and for neurons with two degrees of intermediacy between the Main MN and the Main IN-N groups (Overlap MN, Overlap MN/IN). Morphological reconstructions of additional cells, which had been injected with biocytin during the electrophysiological tests, were shown to provide clear-cut support for the provisional classification procedure. The values for the measured parameters in the 96 tested cells covered the spectrum reported previously across adult vertebrate species and were robust in measurements made on different SC slices up to 5 days after their removal from the host animal. The interspecies comparisons permitted the predictions that (1) our Main MN and Overlap MN cells would be analogous to two MN types that innervate fast-twitch and slow-twitch skeletomotor muscle fibers, respectively, in the cat, and (2) the MNs in our Overlap MN/IN group probably innervate slow (nontwitch, tonic) muscle fibers whose presence has recently been established in the turtle hindlimb. In summary, the results bring out the utility of the SC slice preparation of the turtle for study of spinal motor mechanisms in adult tetrapod vertebrates, particularly as an adjunct to the in vivo cat, because of the ease with which robust measurements can be made of the active properties of both MNs and INs.

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

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.