Embryonic development and survival of brachial motoneurons projecting to muscleless chick wings

The role of muscle cells in the survival of embryonic motoneurons projecting to the developing wing was directly examined. Embryos lacking muscle in one of their wings were produced by surgically removing the embryonic precursors of muscle cells, the somites. The resulting limb lacked only muscle cells, with the derivatives of the other limb contributor, the lateral plate mesoderm, left intact. Counts of apparently healthy lateral motor column (LMC) motoneurons supplying normal wings between stages 28 and 36 showed little decline in motoneuron number until stage 34; approximately 24% of the motoneurons died between stages 34 and 36. In contrast, the number of LMC motoneurons supplying muscleless wings declined progressively from stages 28 to 36. This decline resulted in the loss of about 77% of the motoneurons present at stage 28. In addition, the LMCs supplying muscleless wings had fewer motoneurons at all stages examined than similarly staged controls; this difference ranged from 27% in the youngest cases to 75% in the older embryos. Motoneurons were lost equivalently from all rostrocaudal levels of the brachial LMC. From these studies we conclude that motoneurons survival depends on the presence of muscle cells in the developing wing. In the absence of muscle cells, motoneuron death was increased compared to normal embryos at stages prior to the onset of naturally occurring cell death.     

Leukemia inhibitory factor, glial cell line-derived neurotrophic factor, and their receptor expressions following muscle crush injury.

Using in situ hybridization histochemistry, we characterized the spatiotemporal gene expression patterns of leukemia inhibitory factor (LIF) and glial cell line-derived neurotrophic factor (GDNF), and their receptor components (LIFR, GFR-alpha1, RET) induced in muscle cells, intramuscular nerves, and motoneurons in the regeneration processes of both muscle cells and nerves following muscle contusion. Muscle contusion induced upregulation of GDNF and GFR-alpha1 mRNAs in Schwann cell-like cells in the intramuscular nerves and of LIFR mRNA in damaged muscle cells. LIFR, GFR-alpha1, and RET mRNA expressions in motoneurons were upregulated following muscle contusion. Muscle contusion also induced more rapid, prominent transactivations of GFR-alpha1 and RET genes in motoneurons than did sciatic nerve axotomy. These findings suggest that rapid and prominent upregulation of the receptor components for LIF and GDNF in motoneurons is important for the regeneration of intramuscular motor nerves damaged by muscle contusion.     

Location of motor pools innervating chick wing.

Intramuscular injections of HRP were used to map the spinal cord location of motoneurons innervating wing and shoulder girdle muscles in newly hatched chicks. Motor pools are grouped in the lateral motor column in relationship to embryonic origin of the muscles: muscles derived from the ventral muscle mass are innervated by medially lying motor pools, while muscles derived from the dorsal mass are innervated by lateral pools. Motor pool position is also well correlated with nerve supply. Muscles innervated by nerves diverging from common nerve trunks are innervated by neighboring motor pools. The rostro-caudal organization of the motor pools reflects both proximo-distal and antero-posterior axes of the limb with proximal and anterior muscles innervated from rostral motor pools.     

Chick wing innervation. I. Time course of innervation and early differentiation of the peripheral nerve pattern

Anterograde transport of horseradish peroxidase was used to map the initial projection patterns of motor and sensory axons innervating the wing of the chick embryo. Injections which resulted in labeling large numbers of motor and sensory axons, separately or in combination, were used to define the time course of innervation and to visualize the progressive morphogenesis of the peripheral nerve pattern. Motor axons emerged from the spinal cord and accumulated near the ventromedial border of the myotome where they remained for up to 16 hours before growing into the plexus region and limb bud. Despite the known later time of sensory neuron production, the first sensory axons projected to the wing at the same time as motor axons. When axons first entered the wing bud, they were distributed in two loosely organized sheets of axon fascicles, one projecting to dorsal muscle mass, the other to ventral muscle mass. The width of the sheets was between one-third to one-half the width of the wing bud, and this distance was more than twice the diameter of the proximal nerve trunks measured at stage 28. In the proximal limb the basic pattern of peripheral nerves emerged gradually from stages 26 to 28. During these stages, the loosely organized sheets of axonal fascicles seen at younger stages were progressively transformed into several coherent nerve trunks and muscle nerves extended from common nerve trunks. The implication of these observations is that many outgrowing axons appear not to follow preformed pathways corresponding to the mature peripheral nerve branching pattern. This pattern may instead result from axonal recognition of cues within a largely undifferentiated limb bud, and from the subsequent bundling together of loosely organized axon fascicles. These events occur concurrently with limb growth and differentiation. © 1995 Wiley-Liss, Inc.     

Specificity of initial axonal projections to embryonic chick wing.

Anterograde transport of HRP was used to map initial segmental projection patterns of motor and sensory axons to the chick embryo wing. At the earliest stages of innervation, before the peripheral nerves had formed, motor axons were distributed as two loosely organized sheets, one directed towards the dorsal muscle mass and the other to the ventral. Labeled axons emerging from a given spinal cord segment were distributed within the sheets in a pattern that reflected the origin of the axons along the rostro-caudal axis of the lateral motor column. The segmental specificity of the motor projections was analyzed again after the axonal sheets had formed recognizable nerve trunks and muscle nerves. As was observed at the younger stages, axons were distributed in a pattern that was correlated with both the relative antero-posterior (A/P) and proximo-distal (P/D) position of the forming nerve, with cranial spinal nerves projecting to anterior and proximal limb regions. Whereas most axons emerging from a given ventral root projected in accord with what would be expected from the known adult motor pool maps, segmentally unexpected projections were frequently observed. The proportion of aberrantly projecting axons appears to be quite small, and in most embryos, it was impossible to determine whether the erroneous projections originated from unbranched axons or were collateral branches of others. The findings indicate that the initial projections of motor axons to the developing wing are patterned along the A/P axis of the wing and are largely accurate; however, the guidance processes are not sufficiently precise to exclude fibers entirely from inappropriate nerves. It is likely that later developmental processes act to fine-tune the initial projection pattern.     

Rules of motor innervation in chick embryos with supernumerary limbs

The positions of motoneurons supplying individual muscles in chick embryos with grafted supernumerary limbs have been identified using retrograde transport of horseradish peroxidase. For a given muscle, motor pool location varied depending on the embryonic origin of the muscle, the position of the limb along the rostrocaudal axis of the body, and the limb's orientation with respect to the body wall. Limb muscles derived from the ventral part of the embryonic premuscle mass were always innervated by medially located motoneurons. Muscles derived from the dorsal portion were consistently innervated by motoneurons in either intermediate or far lateral positions. These relationships were invariant with changes in limb position and orientation. These findings indicate that motor axons can recognize and selectively innervate muscles derived from either dorsal or ventral muscle mass. In addition, the spinal nerves innervating each limb were identified. The type of limb plexus (e.g., crural, sciatic, or wing) and the peripheral branching patterns of the nerves within the limbs were also studied and found to be controlled by the limbs. The rostrocaudal variation in motor pool position similarly depended on the position and orientation of the limb. This rostrocaudal variation in motor pool position can be explained by the limb's ability to determine axonal outgrowth pathways and hence to constrain the possible target choices of outgrowing axons. The process of limb innervation involves interactions between motoneuron axons having intrinsic differences or specificities, and the character of the local environment of the limb into which they grow.     

Development of the caudal nerve cord, motoneurons, and muscle innervation in the appendicularian urochordate Oikopleura dioica

The development of the caudal nerve cord and muscle innervation in the appendicularian Oikopleura dioica was assessed using differential interference contrast and confocal microscopy, phalloidin staining of actin, and in situ hybridization for the neuronal markers tubulin and choline acetyltransferase (ChAT). The caudal nerve cord first appears as a stream of tubulin mRNA-positive neurons that extends into the tail from the caudal ganglion. By this stage a few actin-rich nerve fibers course longitudinally along the cord. As the tail lengthens, the caudal nerve cord extends and becomes more fasciculated and the neurons cluster at stereotyped longitudinal positions. The number of neurons in the nerve cord reaches a relatively stable maximum of about 29. A subset of neurons in the caudal ganglion and caudal nerve cord expresses ChAT mRNA. These putative motoneurons are distributed along nearly the full extent of the tail in numbers consistent with an independent innervation of each tail muscle cell. The longitudinal series of putative motoneurons is not aligned with the muscle cells, but peripheral nerve fibers extending to the muscle cells are, indicating that motor axons grow along the cord before exiting adjacent to their peripheral target. Muscle innervation occurs roughly coincident with the onset of ChAT mRNA expression. Our results provide the first molecular identification of motoneurons and the first developmental characterization of the motor system in an appendicularian and help set the stage for gene expression studies aimed at understanding the evolution of developmental patterning in this part of the chordate central nervous system.     

Organization of hindlimb muscle afferent projections to lumbosacral motoneurons in the chick embryo.

We have examined the organization of muscle afferent projections to motoneurons in the lumbosacral spinal cord of chick embryos between stage 37, when muscle afferents first reach the motor nucleus, and stage 44, which is just before hatching. Connectivity between afferents and motoneurons was assessed by stimulating individual muscle nerves and recording the resulting motoneuron synaptic potentials intracellularly or electrotonically from other muscle nerves. Most of the recordings were made in the presence of DL-2-amino-5-phosphonovaleric acid (APV), picrotoxin, and strychnine to block long-latency excitatory and inhibitory pathways. Activation of muscle afferents evoked slow, positive potentials in muscle nerves but not in cutaneous nerves. These potentials were abolished in 0 mM Ca2+, 2mM Mn2+ solutions, indicating that they were generated by the action of chemical synapses. The muscle nerve recordings revealed a wide-spread pattern of excitatory connections between afferents and motoneurons innervating six different thigh muscles, which were not organized according to synergist-antagonist relationships. This pattern of connectivity was confirmed using intracellular recording from identified motoneurons, which allowed the latency of the responses to be determined. Short-latency potentials in motoneurons were produced by activation of homonymous afferents and the heteronymous afferents innervating the hip flexors sartorius and anterior iliotibialis. Stimulation of anterior iliotibialis afferents also resulted in some short-latency excitatory postsynaptic potentials (EPSPs) in motoneurons innervating the knee extensor femorotibialis, though other connections were of longer latency. Afferents from the adductor, a hip extensor, did not evoke short-latency EPSPs in any of these three types of motoneurons. Short-latency, but not long-latency EPSPs, persisted during repetitive stimulation at 5 Hz, suggesting that they were mediated monosynaptically. Long-latency, fatigue-sensitive potentials were maintained in the presence of APV, picrotoxin, and strychnine, suggesting that polysynaptic pathways utilize non-NMDA receptors as well as NMDA receptors. We found no difference in the pattern of inputs to femorotibialis motoneurons between stage 37-39 and near hatching at stage 44, suggesting muscle afferent projections to these motoneurons are correct at stage 37, when the afferents first reach the lateral motor column in substantial numbers.     

Specificity of early motoneuron growth cone outgrowth in the chick embryo

During development, chick lumbosacral motoneurons have been reported to form precise topographic projections within the limb from the time of initial outgrowth. This observation implies, first, that motoneurons select the appropriate muscle nerve pathway and, second, that they restrict their ramification within the primary uncleaved muscle masses to appropriate regions. Several reports based on electrophysiology and orthograde horseradish peroxidase (HRP) labeling have shown muscle nerve pathway selection to be fairly precise. However, studies based on retrograde labeling with HRP have produced conflicting reports on the extent to which vertebrate motoneurons make projection errors. Since it is difficult to distinguish between true projection errors and HRP leakage when using retrograde labeling, we decided to assess the distribution of labeled growth cones in 25-micron serial plastic sections, following orthograde labeling of identifiable subpopulations of motoneurons during the period of initial axon outgrowth. Examination of a large number of muscle nerves revealed no segmentally inappropriate axons, confirming earlier reports that muscle nerve pathway selection is very accurate. In addition, we observed that growth cones take widely divergent trajectories into the same muscle nerve, suggesting that growth cones are responding independently to some specific environmental cue rather than being passively channeled at this point. The distribution of labeled growth cones within the muscle masses provided direct evidence that motoneurons did not at any time project to obviously inappropriate muscle regions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myokymia, muscle hypertrophy and percussion "myotonia" in chronic recurrent polyneuropathy

Three unusual features were observed in a patient with chronic relapsing polyneuropathy: myokymia, muscle hypertrophy, and prolonged contraction in response to muscle percussion. Low nerve conduction velocity and conduction block were demonstrated in all motor nerves tested, indicating a demyelinating peripheral neuropathy. Myokymia was caused by spontaneous motor unit activity which was shown to originate in peripheral nerves, since it persisted after nerve block and was abolished by regional curarization. Muscle hypertrophy was attributed to increased peripheral nerve activity, and the prolonged contraction of muscle in response to direct percussion was attributed to irritability of intramuscular nerve terminals.     

Projection pattern and target selection of Clione limacina motoneurons sprouting within an intact environment

In the pteropod mollusc Clione limacina, two groups of locomotor motoneurons, located in the pedal ganglion, innervate the dorsal and ventral muscle layers of the ipsilateral wing through the wing nerve. Separate branches of this nerve go either only to the dorsal muscle layer or only to the ventral one. In the present study, growth of novel neurites of the wing motoneurons was induced by cutting the wing nerve. In addition, all other peripheral nerves and connectives of the pedal ganglion were cut, except for the pedal commissure to the contralateral pedal ganglion. Thus, the neurites were allowed to grow only towards the contralateral pedal ganglion. We have found that the novel neurites, entering the contralateral pedal ganglion, were capable of growing everywhere inside the central nervous system (CNS) and into any peripheral nerve. However, they preferred the wing nerve. This finding suggests that the preference is caused by the guiding cues in the wing nerve or the attractive influence of the wing muscles. Because the contralateral pedal ganglion and nerves were left intact, the growth direction of the new neurites could be determined only by factors permanently existing in the CNS, rather than induced by nerve injury or muscle denervation. Within the wing nerve, the neurites could not discriminate between the nerve branches going to the dorsal and ventral muscle layers. They formed synapses on muscles of both layers, despite the fact that the muscles were innervated by their own motoneurons.     

Complex expression pattern of tenascin during innervation of the posterior limb buds of the developing chicken.

The histological localization of the extracellular matrix glycoprotein tenascin was studied during the formation of peripheral nerves in the developing chick hindlimb (embryonic stages 21.5 to 30) by light and electron microscopic immunological methods to obtain insights into the molecule's functional role in the pathway formation by motor and sensory nerves. At stages 21.5 and 23, nerve roots and plexus were surrounded by high tenascin-immunoreactivity, whereas the not yet innervated limb bud was not immunoreactive. During innervation of the limb bud at stages 24.5 and 25, tenascin was detectable at the limb bud base and restricted in its expression to the proximal nerve regions. The nerve tips did not contact areas of elevated tenascin-immunoreactivity. At stages 26 to 28 the dorsal and ventral trunks of the crural and sciatic nerves were surrounded by tenascin-immunoreactivity, which was localized between Schwann and mesenchymal cells. The tips of the growing nerve had now reached the tenascin-positive interface between bone and muscle anlagen. This interface was contacted tangentially rather than penetrated by the nerve tips. The medial and lateral femoral cutaneous nerves were surrounded by high and weak tenascin-immunoreactivity, respectively. In both nerves, tenascin-immunoreactivity was absent where the nerves branched extensively to innervate the skin. The cutaneous nerves diverging from the sciatic nerve were of very low tenascin-immunoreactivity or tenascin-negative at all developmental stages tested. At stages 29 and 30, muscle nerves, having just entered the tenascin-negative muscles, exhibited strong immunoreactivity, whereas the more proximally situated trunks of the sciatic nerve were weakly and discontinuously labeled, particularly at sites where smaller nerves were branching off. Since the cutaneous branches of the sciatic nerve were always of low tenascin-immunoreactivity, the question was raised whether tenascin expression in the sciatic nerve depended on the presence of motor axons. Spinal cords of stage 19 or 20 embryos were therefore removed and tenascin expression was investigated at stages 26 and 27. Some of the residual nerves were weakly tenascin-immunoreactive, whereas others were tenascin-negative. Our observations suggest that tenascin is not involved in the initial guidance of peripheral nerves to their targets. Rather, neuron-induced tenascin appears to stabilize the proximal nerve trunks during a transient time period, possibly by preventing axons and Schwann cells from intermingling with the surrounding mesenchyme, thus contributing to nerve fiber compaction. Conversely, nerve branching may be elicited by reduced levels of tenascin. Furthermore, tenascin may divert growth cones from the developing bone tissue and direct muscle afferents to their appropriate targets.     

Developmental approaches to the analysis of vertebrate central pattern generators

The isolated spinal cord of the chick embryo is a new preparation for analyzing the neural mechanisms and development of vertebrate motor activity. The embryonic cord can be isolated in vitro during the period of development when antagonist alternation of hindlimb motoneurons matures. The preparation is spontaneously active in vitro generating episodes of motor activity that can be recorded from muscle nerves and the ventral roots. The neural mechanisms responsible for the development and genesis of motor activity are being investigated using intra- and extracellular recording from motoneurons and electrotonic recordings of motoneuron synaptic activity from muscle nerves. The results suggest that alternating motor activity in the isolated chick cord may be generated by a mechanism in which a synaptically induced motoneuronal shunt conductance regulates the time of discharge of flexor and extensor motoneurons.     

Accuracy of reinnervation of rat internal intercostal muscles by their own segmental nerves

The positions of internal intercostal motoneurons within their motor pool were studied, following reinnervation of the intercostal muscles by their original nerves. Six to 9 weeks after proximal nerve section in 10-d-old and adult rats, 0.1 microliter injections of wheat germ agglutinin (WGA)-HRP were made in the distal part of the reinnervated internal intercostal muscle. The corresponding region of the contralateral control muscle was also injected. The positions of the retrogradely labeled motoneurons were mapped in 100 microns transverse sections of thoracic spinal cord that had been stained for HRP according to the method of Mesulam (1982). In normal rats, motoneurons innervating distal muscle fibers are found largely in the more dorsal part of the internal intercostal motoneuron pool (Hardman and Brown, 1985). In adult rats, regenerated motor axons did not show any selectivity; distal muscle fibers were innervated by motoneurons whose cell bodies were distributed throughout the internal intercostal pool. However, in rats operated on at 10 d of age, distal intercostal muscle fibers were reinnervated by motoneurons that were distributed mainly in the dorsal part of the motor pool. These results support the view that positional signals may be of importance in organizing the distribution of axon terminals within muscles during development.     

Peripheral nerve regeneration through a long detergent-denatured muscle autografts in rabbits.

Muscle segments excised from rabbit biceps femoris muscles were treated with detergent sodium dodecyl sulphate to denature cellular constituents, and each was autografted in a 5 cm gap of the sciatic nerve in the same rabbit. Axonal regrowth through the grafts and reinnervation into the host sciatic nerves and muscles were studied morphologically, and electrophysiologically, 4 months after grafting. Regenerating axons accompanied by Schwann cells extended through basal lamina tubes of the grafts into the distal host nerves. Reinnervation of the tibialis anterior muscles by motor nerves was confirmed by recovery of the compound muscle action potentials (CMAP) and the reinnervation of the muscle spindles was demonstrated by electron microscopy. These findings indicated that the basal lamina tubes of denatured muscles were effective scaffolds through which the regenerating nerve fibers grew across as large a gap as 5 cm.     

Distribution of innervating neurons of masticatory muscle spindles in the rat: An HRP study

Neurons innervating representatives of supramandibular and suprahyoidal muscles of the rat were identified in the mesencephalic and motor nuclei of the trigeminal nerve and in the accessory nuclei of the trigeminal and facial nerves after intramuscular injections of horseradish peroxidase. Labeling was always ipsilateral with respect to the injection site. The supramandibular motoneurons showed a bimodal size distribution, whereas suprahyoidal motoneurons were unimodally distributed. Mesencephalic neurons were labeled only after supramandibular injections. These results indicate a strict ipsilateral organization of muscle spindles supplying sensory and motoneurons.     

Retractor bulbi muscle responses to oculomotor nerve and nucleus stimulation in the cat

This study demonstratesthe presence of retractor bulbi motoneurons within the oculomotor nucleus which activate muscle units within all 4 slips of the cat retractor bulbi muscle. These muscle units are mechanically different and physiologically separate from retractor bulbi muscle units innervated by the abducens nerve. The retractor bulbi muscle, then, is innervated by two separate pools of motoneurons whose axons are carried in two different cranial nerves. These observations of mechanical properties of retractor bulbi muscle suggest that the oculomotor retractor bulbi motor units may be activated during patterned eye movements.     

Chick wing innervation. II. Morphology of motor and sensory axons and their growth cones during early development

The development and distribution of neuronal projections to the developing chick wing was studied using anterograde transport of horseradish peroxidase (HRP). Small injections of HRP were made into motor or sensory neuronal populations in order to visualize individual axons and their associated growth cones. Motor growth cones were observed in different regions of the embryo at different stages, in a proximal-to-distal pattern of distribution which paralleled the process of axon outgrowth and nerve formation. Different growth cone morphologies were associated with differing regions of the developing projection. In the spinal nerves, axons destined for the limb were unbranched and terminated in simply shaped growth cones. As axons approached the developing limb and entered the plexus region, their growth cones became more complex and larger primarily because of widening, and they sometimes branched, producing processes which could extend tens of microns from a tricorne branch point on the parent axon. Both motor and sensory fibers showed similar morphological changes in the plexus region. A distinctively shaped growth cone expanded on its leading edge was observed, sequentially apparent in the distal spinal nerves, in the plexus region, in the loosely organized axonal sheets projecting to the uncleaved dorsal or ventral muscle masses, and where muscle nerves diverged from nerve trunks and within muscle nerves. It is likely that some of these are transitional growth cones preparing to branch, because complex and branched growth cones were also observed in these regions. Branched axons oriented along the anteroposterior axis were similarly observed in the plexus region and distal to the plexus when axons first projected to the limb bud. At somewhat older stages when the basic peripheral nerve branching pattern had formed, motor growth cones were observed in common nerve trunks and in individual muscle nerves, but they were no longer found in the plexus region. Branched axons were likewise restricted to these peripheral Imations. Taken together, these observations suggest that one of the ways in which axons navigate is by exploration in the form of growth cone widening, and in some cases terminal bifurcation which may produce axon branches. Selection of the most appropriately directed growth cone process and/or precocious axonal branches may be one of the ways in which axons respond to specific growth cues which guide axons into the limb bud. Alternatively, this precocious branching may be an early neurotrophic response to developing muscle and play no significant role in axon navigation. © 1995 Wiley-Liss, Inc.     

Regeneration of neurons of the pedal ganglion of the pteropodal mollusk Clione limacina

In pedal ganglia of mollusc Clione limacina the growth of axons was studied in motoneurons and interneurons after transections of the wing nerve or of the pedal comissure. Neurons were stained by Lucifer Yellow. In motoneurons, neurites grown both from the transected end of the axon and from the neuron soma spread to all nerve trunks of ipsi- and contralateral ganglia. After nerve transection in the whole mollusc, wing movements restored 10 days later. In interneurons, neurites branched within the pedal ganglion or spread into cerebral ganglia but they did not extend out peripheral nerve trunks. Thus, the patterns of neurite sprouting in moto- and interneurons are different.     

The organization of the motoneurons innervating the axial musculature of vertebrates. I. Goldfish (Carassius auratus) and mudpuppies (Necturus maculosus)

The motoneurons innervating different regions of the myomeres in goldfish and mudpuppies were examined by applying HRP to the musculature or to branches of spinal nerves. In goldfish, the populations of motoneurons innervating epaxial or hypaxial muscle occupied similar positions in the motor column and had similar size distributions. There was no relationship between the size or location of a motoneuron in the motor column and the dorsoventral location of the muscle it innervated in the myomeres. Instead, different populations of motoneurons innervated the functionally different red and white musculature. The red muscle was innervated only by small motoneurons that occupied the ventral portion of the motor column. Their small axons passed lateral to the Mauthner axon in the cord, and most of them traveled in a separate branch of each spinal nerve that ran in the horizontal septum to the red muscle. The white muscle was innervated by a population of motoneurons that did not innervate red. They were large and they occupied a characteristic position in the extreme dorsal part of the motor column. Their large axons traveled medial to the Mauthner axon in the cord and entered branches of spinal nerves running deep in the epaxial or hypaxial muscle. The white muscle was probably also innervated by some smaller motoneurons similar to those innervating red; however, these may have been motoneurons whose axons ran through white muscle to reach other muscle. The large motoneurons innervating only white muscle are similar to the primary motoneurons identified in developmental studies in teleosts (Myers: Soc. Neurosci. Abstr. 9:848, '83); the smaller ones, innervating both red and white, are like secondary motoneurons. Therefore, in goldfish, motoneurons having different morphology and developmental history also innervate different regions in the myomeres. The motor column in mudpuppies was, in general respects, similar to the column in goldfish. There were large primary motoneurons and small secondary ones. Though there were slight differences in the locations of motoneurons filled from nerves entering epaxial and hypaxial muscle, their distributions in the cord overlapped substantially. The motor columns in these two anamniotes differ substantially from the motor columns in those amniotes that have been studied. In amniotes, the motoneurons innervating epaxial and hypaxial muscles are spatially segregated in the cord (Smith and Hollyday: J. Comp. Neurol. 220:16-28, '83; Fetcho: J. Comp. Neurol. 249:551-563, '86).(ABSTRACT TRUNCATED AT 400 WORDS)