Somatotopic relations between the motor nucleus and its innervated muscle fibers in the cat tibialis anterior

The normal development of the anatomic relationships between the motoneurons of the tibialis anterior (TA) muscle and their innervated muscle fibers was studied in 1-, 6-, and 12-week-old and adult cats. The motoneurons of the anterior branch and the contralateral posterior branch of the TA nerve were retrogradely labeled with horseradish peroxidase. Within the TA motor nucleus, anterior branch motoneurons (63% of total) were located rostrally and posterior branch motoneurons (37% of total) were located more caudally. The distributions of soma diameters of labeled motoneurons were bimodal in all age groups, allowing a presumptive division into gamma (small) and alpha (large) motoneurons. The posterior branch contained 52% of the total gamma motoneurons but only 28% of the total alpha motoneurons. Within the TA muscle, the regions innervated by the anterior and posterior branches were clearly segregated as determined by glycogen depletion. Myofibrillar ATPase staining at pH 4.4 demonstrated that the posterior branch innervated a higher proportion (56%) of types I and IIA fibers than the anterior. Our results support the hypothesis that a topographic relationship exists between the locus of a motoneuron within its motor nucleus and the position of its innervated muscle fibers within the muscle. Since these topographic relationships apply to all age groups studied, the muscle volume innervated by each muscle nerve branch appears to represent a reproducible developmental unit with distinct anatomic, physiologic and possibly functional properties. This unit may be termed a muscle “compartment.”     

Central connections of the IXth and Xth cranial nerves in the clearnose skate,Raja eglanteria

The anterograde and retrograde transport of horseradish peroxidase was utilized to identify the motor nuclei and sensory connections of the IXth and Xth cranial nerves in the clearnose skate, Raja eglanteria. The majority of VIIth, IXth, and Xth nerve motoneurons form an ipsilateral dorsal visceromotor column, extending from the level of the posterior lateral line nerve root to the 3rd-4th ventral spinal roots. Within this column, the motor nucleus of IX (IXm) occurs rostral to vagal motoneurons (dorsal motor nucleus of X, Xmd). Vagal motoneurons are also located ventrolaterally in a ventral motor nucleus (Xmv), which extends from the level of the middle vagal rootlets to the 5th-6th ventral spinal roots. IXth and Xth nerve afferents terminate predominantly in the ipsilateral visceral sensory column (vS). Many vagal but few glossopharyngeal afferents form a solitary tract. A distinct nucleus of the solitary tract could not be identified; rather fibers terminate among cells scattered in the tract, adjacent vS, and nucleus of the commissura infima, where some vagal fibers decussate. Vagal and glossopharyngeal somatic sensory fibers were not found.     

Plantar motoneuron columns in the rat

In the rat, the numbers and locations of motoneurons innervating the short plantar muscles of the hindlimb (supplied by the medial and lateral plantar nerves, as well as a branch of the sural nerve) were determined by using both horseradish peroxidase (HRP) and fluorochromes as retrograde labels. Topographical organization within the plantar motor nucleus was examined by exposing individually the cut ends (encapsulated in low melting-point paraffin) of medial plantar, lateral plantar, and sural nerves to HRP. In addition, double-labeling experiments were conducted in which the medial plantar nerve was labeled with one fluorochrome (either true blue or diamidino yellow) and the lateral plantar nerve with another. The plantar motor pool is located in the extreme dorsolateral portion of the ventral horn, usually concentrated in the fifth lumbar (L5) spinal segment. Labeled motoneurons extended caudally into the sixth lumbar (L6) segment and rostrally into portions of the fourth lumber (L4) segment. Motoneurons of the medial plantar, lateral plantar, and sural nerve have overlapping territories. Sural motoneurons (about 70 cells per side) are generally confined to L5, medial plantar motoneurons (about 180 cells per side) tend to be concentrated in caudal L5 and rostral L6, whereas the lateral plantar motoneurons (about 310 cells per side) extend throughout the entire length of the plantar motor pool. The distribution of motoneuronal cell size is unimodal (mean cross-sectional area = 610 +/- 150 microns2). Cell bodies of plantar motoneurons tend to have similar geometries in all three major planes of sectioning. In all, the combined plantar plus sural nerve population amounts to about 560 motoneurons on each side of the spinal cord. On the basis of these data, and those published by others, the innervation of the small muscles of the foot accounts for about 25% of the motor axons carried by the entire sciatic nerve.     

Changes in the expression of parvalbumin immunoreactivity in the lumbar spinal cord of the rat following neonatal nerve injury

Nerve injury in newborn animals results in the loss of motoneurons and dorsal root ganglion neurons and long-term changes in reflex activation of surviving motoneurons. Parvalbumin has been previously shown to be found in large-diameter primary afferent axons and interneurons in the spinal cord, and was used here to study the changes in parvalbumin-immunoreactive appositions onto identified tibialis anterior/extensor digitorum longus (TA/EDL) motoneurons, during both normal development and following neonatal nerve injury in the rat spinal cord. During normal development, there was a decrease in the number of parvalbumin-immunoreactive appositions onto TA/EDL motoneurons. Thus, at postnatal day 7 (P7), there were 72.8 +/- 17.5 (mean +/- SD) appositions per motoneuron and by P14, it had decreased to 38.8 +/- 13.2 (mean +/- SD; p > 0.05). Following neonatal nerve injury at P2, there were fewer parvalbumin-positive afferent appositions close to the TA/EDL motoneurons than normal, so that at P7, there were 53.5 +/- 17.1 (mean +/- SD), and at P14, it further decreased to 25.8 +/- 8.6 (mean +/- SD; p > 0.05). This injury-induced reduction in the number of parvalbumin-immunoreactive boutons apposing TA/EDL motoneurons may result, at least in part, from the death of dorsal root ganglion cells with the consequent loss of their central projections. The alterations in the number of parvalbumin-positive appositions close to motoneurons observed in this study may contribute to the changes in the pattern of reflex activity observed in the developing spinal cord both during normal development and following neonatal injury.     

The organization of the pudendal nerve in the male and female rat

Mature male and female Sprague-Dawley rats were used in this study to compare the organization of the pudendal nerve in the two sexes. Experiments included (1) incubating the cut pudendal nerve in diamidino-2-phenylindole HCl (DAPI), fast blue (FB), or horseradish peroxidase (HRP) and (2) injecting individual perineal muscles with these substances. In both sexes, incubation of the pudendal nerve labelled two motoneuron nuclei in the L5-L6 segments of the spinal cord. These nuclei are the dorsomedial (DM) and dorsolateral (DL) cell columns described by Schr?der (J. Comp. Neurol. 192:567–587, 1980). In agreement with previous studies, there were significantly more neurons in both nuclei in the male than in the female and the neurons were larger in the male. In both sexes, the DL and DM nuclei were characterized by a longitudinal dendritic structure. The DM nucleus also had numerous dendritic bundles extending across the midline, linking the DM nuclei bilaterally. Pudendal nerve afferent neurons were located in the L6 and S1 dorsal root ganglia. In the male, the afferent neurons were larger and more numerous. In both sexes, labelled pudendal afferent fibers in the spinal cord were located in the dorsal columns, the medial half of Lissauer's tract, the extreme medial edge of the dorsal horn, both ipsilaterally and contralaterally, and in a large terminal field in the dorsal gray commissure. No labelled afferents were seen in the intermediate or ventral gray. Perineal muscle injections established that there was no difference between males and females in the number of motoneurons innervating the external anal or urethral sphincters. In the female, urethral sphincter motoneurons accounted for almost all the DL motoneurons, and anal sphincter motoneurons accounted for almost all the DM motoneurons. The ischiocavernosus and bulbospongiosus muscles are vestigial in the female rat. In the male, neurons innervating the anal sphincter and bulbospongiosus muscles were intermingled in the DM nucleus. In contrast, in the DL nucleus, the urethral sphincter neurons were located in the lateral portion of the nucleus and the ischiocavernosus neurons were located in the medial portion.     

Is there morphological separation between the spinal cord motor nuclei which innervate the heads of a multiheaded muscle?

Horseradish peroxidase was injected into the individual heads of the pectoralis muscles of the dog or applied to the nerve which supplies each of these heads. The location and numbers of labeled motoneurons in the spinal cord were studied using light microscopy. There was longitudinal overlap of the pectoral nuclei, but no separation in their mediolateral or dorsoventral positions. The cutaneous trunci muscle motor nucleus is distinctly separate from the motor nuclei of the pectoral muscles, even though they share a common nerve supply. The methods of horseradish peroxidase application to the cut nerve or injection into the muscle are compared.     

Motor nuclei of peroneal muscles in the cat spinal cord

The cat peroneal muscles have been used in numerous investigations dealing with the physiological properties of motor units, muscle spindles, and Golgi tendon organs. This report presents a study of the organization of peroneal motor pools in the cat spinal cord by means of retrograde axonal transport of horseradish peroxidase from individual muscles to the corresponding motoneurons. The motor nuclei of peroneus longus (PL), peroneus brevis (PB), and peroneus tertius (PT) muscles formed thin columns in the lateral part of the ventral horn in spinal segments L6-S1. In the transverse plane, the PT and PL nuclei occupied, respectively, dorsolateral and ventromedial positions, with PB nucleus in an intermediate position overlapping with the other two nuclei. Measurements of cell body diameters allowed identification of alpha and gamma subgroups in peroneal motoneuron populations. The average numbers of motoneurons were about 96 alpha and 60 gamma in PL, 75 alpha and 54 gamma in PB, and 34 alpha and 23 gamma in PT. Comparison with data from electrophysiological studies indicated that whole populations of motoneurons were labeled in each motor nucleus. The proportions of gamma motoneurons were the same, and cell bodies of gamma motoneurons had similar sizes in the three peroneal populations. In contrast, alpha motoneurons were significantly smaller in PB than in the two other pools, in keeping with the fact that PB contains a proportion of slow motor units larger than the two other muscles. In large samples of homonymous motoneurons, the numbers of first-order dendrites correlated linearly with motoneuron sizes.     

Distribution of motoneurons supplying feline neck muscles taking origin from the shoulder girdle

A combination of fluorescent retrograde tracers and horseradish peroxidase (HRP) was used to compare the spinal distributions of motoneurons supplying shoulder muscles with attachments to the skull and cervical spinal cord that suggest a significant role in head movement. Two muscles, the rhomboideus and the levator scapulae, were innervated by multiple segmental nerve bundles that entered the muscles at different rostrocaudal locations. Motoneurons that were labelled retrogradely from rhomboideus nerve bundles formed a single, long column in the ventral horn from C4 to C6, lateral to previously studied motor nuclei supplying deep neck muscles. When different tracers were used to differentiate motoneurons supplying specific nerve bundles, discrete subnuclei could be identified that were organized in a rostrocaudal sequence corresponding to the rostrocaudal order of the nerve bundles. Levator scapulae motoneurons formed a second elongate column immediately lateral to the rhomboideus motor nucleus. Three other muscles, the trapezius, sternomastoideus, and cleidomastoideus, were supplied by cranial nerve XI. Labelled motoneurons from these muscles formed a single column from the spinomedullary junction to middle C6. Within this column, the three motor nuclei supplying the sternomastoideus, cleidomastoideus, and trapezius were laminated mediolaterally. Sternomastoideus and cleidomastoideus motoneurons were confined to upper cervical segments, whereas trapezius motoneurons were found from C1 to C6. In C1 and C6, the motoneuron column was located centrally in the gray matter, but, between C2 and C5, the column lay on the lateral wall of the ventral horn in a position dorsolateral to motor nuclei supplying the rhomboideus and the deeper neck muscles. The findings in this study suggest that descending and propriospinal systems responsible for coordinating head movement may have to descend as far caudally as C6 if they are to project onto muscles controlling the mobility of the lower neck. J. Comp. Neurol. 377:298–312, 1997. © 1997 Wiley-Liss, Inc.     

Distribution and connections of inspiratory premotor neurons in the brainstem of the pigeon (Columba livia)

We have recorded extracellular, inspiratory-related (IR) unit activity in the medulla at locations corresponding to those of neurons retrogradely labeled by injections of retrograde tracers in the lower brachial and upper thoracic spinal cord, injections that covered cell bodies and dendrites of motoneurons innervating inspiratory muscles. Bulbospinal neurons were distributed throughout the dorsomedial and ventrolateral medulla, from the spinomedullary junction through about 0.8 mm rostral to the obex. Almost all of the 104 IR units recorded were located in corresponding parts of the ventrolateral medulla, rostral to nucleus retroambigualis, where expiratory related units are found. Injections of biotinylated dextran amine at the recording sites labeled projections both to the spinal cord and to the brainstem. In the lower brachial and upper thoracic spinal cord, bulbospinal axons traveled predominantly in the contralateral dorsolateral funiculus and terminated in close relation to the dendrites of inspiratory motoneurons retrogradely labeled with cholera toxin B-chain. In the brainstem, there were predominantly ipsilateral projections to the nucleus retroambigualis, tracheosyringeal motor nucleus (XIIts), ventrolateral nucleus of the rostral medulla, infraolivary superior nucleus, ventrolateral parabrachial nucleus, and dorsomedial nucleus of the intercollicular complex. In all these nuclei, except XIIts, retrogradely labeled neurons were also found, indicating reciprocity of the connections. These results suggest the possibility of monosynaptic connections between inspiratory premotor neurons and inspiratory motoneurons, which, together with connections of IR neurons with other brainstem respiratory-vocal nuclei, seem likely to mediate the close coordination that exists in birds between the vocal and respiratory systems. The distribution of IR neurons in birds is similar to that of the rostral ventral respiratory group (rVRG) in mammals. J. Comp. Neurol. 379:347–362, 1997. © 1997 Wiley-Liss, Inc.     

Maintenance of specificity by sprouting and regenerating peripheral nerves. I. Normal variability

Inter-animal variability in the spinal representation of a single hindlimb muscle, tibialis anterior (TA), in the cat, was examined by retrograde transport of intramuscularly injected HRP, dissection of the lumbosacral plexus and reflex testing after acute section of spinal nerves L5, L6, S1 and S2 sparing L7. No variability between the two sides of the same animal was seen. The transverse position of the TA motor nucleus and the number of labeled cells was constant between the two sides in each animal. Inter-animal variability was considerable, however, in that the number of motor neurons and rostrocaudal extent of the motor neuron column supplying TA varied considerably from animal to animal. According to the relationship between the position of the lumbosacral plexus and the distribution of spinal nerves, 3 classes of representation of the plexus were found: prefixed, postfixed and intermediate. In animals in which the lumbosacral plexus was prefixed, more than one half of labeled cells were rostral to the L7 segment; in those with postfixed plexus more than half the cells were caudal to L7. Section of L5, L6, S1 and S2 spinal nerves weakened the tibialis anterior tendon reflex in 'prefixed plexus' animals but abolished that reflex in 'postfixed' plexus animals, in spite of the presence of labeled motor neurons projecting through the spared L7 nerve. This suggests that some of the afferents and efferents comprising the TA tendon reflex may travel in different spinal roots or that a particular distribution of motor axons within a muscle is required for the maintenance of this particular reflex activity.     

Brainstem origin of preganglionic cardiac motoneurons in the muskrat

The muskrat, an aquatic rodent with a brisk and reliable diving response, shows a remarkable bradycardia after nasal stimulation. However, the medullary origin of cardiac preganglionic motoneurons is unknown in this species. We injected fat pads near the base of the heart of muskrats with a WGA-HRP solution to label retrogradely preganglionic parasympathetic neurons that project to the cardiac plexi. Results showed that the preponderance of labeled neurons was in ventrolateral parts of the medulla from 1.5 mm caudal to the obex to 2.0 mm rostral. Eighty-nine percent of the labeled neurons were located bilaterally in the external formation of the nucleus ambiguus, 5.6% were in the lateral extreme of the dorsal motor nucleus of the vagus nerve and 5.3% were found in the intermediate area in between these two nuclei. Although controversy still exists concerning the medullary origin of preganglionic cardiac motoneurons, our results from muskrats agree with those from most other species where preganglionic cardiac motoneurons were located just ventral to the nucleus ambiguus.     

Organization of choline acetyltransferase-containing structures in the cranial nerve motor nuclei and spinal cord of the monkey

Cholinergic structures in the cranial nerve motor nuclei and ventral and lateral horns of the spinal cord of the monkey, Macaca fuscata, were investigated immunohistochemically with a monoclonal antibody against monkey choline acetyltransferase (ChAT). ChAT-immunoreactive perikarya and dendrites were present in the oculomotor, trochlear, abducent, trigeminal motor, facial and hypoglossal nuclei, nucleus of Edinger–Westphal, nucleus ambiguus, dorsal nucleus of the vagus, lamina IX of the cervical, thoracic and lumbar spinal cords, and intermediolateral nucleus of the thoracic spinal cord. The neuropil of the trigeminal motor, facial and hypoglossal nuclei, nucleus ambiguus and lamina IX of the cervical, thoracic and lumbar spinal cords contained many ChAT-positive bouton-like structures and they were seemingly in contact with perikarya and dendrites of motoneurons, suggesting that motoneurons in these nuclei are cholinoceptive as well as cholinergic. The oculomotor, trochlear and abducent nuclei, nucleus of Edinger–Westphal, dorsal nucleus of the vagus and intermediolateral nucleus of the thoracic spinal cord contained a small number of ChAT-immunoreactive bouton-like structures, but they did not contact with perikarya and dendrites of ChAT-positive neurons. These observations suggest that the organization of the motor nuclei is complex, at least regarding the cholinoceptivity.     

On the location and size of laryngeal motoneurons in the cat and rabbit

Motoneurons supplying the posterior crico-arytenoid (PCA), thyro-arytenoid (TA), lateral crico-arytenoid (LCA), and crico-thyroid (CT) laryngeal muscles were localized in the cat, the rabbit, and the 6-week-old kitten by using the technique of intramuscular injection of horseradish peroxidase. Each muscle was found to be innervated by a single, ipsilateral pool of motoneurons, a result which was reliably established only after controlling adventitious spread of the label to nontarget muscles by prior denervation of adjacent musculature. The laryngeal motoneuron column extended in the nucleus ambiguus for a distance of 5-6 mm caudally from the facial nucleus. CT motoneurons were located in the rostral third of this column while the PCA, TA, and LCA motoneurons were located more caudally. These results are in general agreement with earlier degeneration studies (Lawn, '66a; Szentágothai, '43). Although labelled cells were widely dispersed in the nucleus, particularly in the adult cat, a limited amount of topographical structure could still be discerned in the arrangement of recurrent laryngeal nerve motoneurons. In the cat, the PCA pool was located in the ventral part of the recurrent laryngeal nerve representation and did not extend as far caudally as the TA or LCA pools; the LCA pool was located in the caudal and dorsomedial part of the recurrent laryngeal nerve pool; TA motoneurons appeared to overlap the PCA and LCA pools on all three anatomical planes. TA motoneurons were more numerous than PCA or LCA motoneurons, the numbers of cells in the three pools being estimated at 170, 111, and 112, respectively. In the cat bilateral labelling of different pools pointed to certain differences in morphology between cells from these pools and also suggested a functional basis for such differences. The mean soma diameter for the PCA and CT motoneurons was each significantly smaller than that for the TA and LCA motoneurons. The rabbit data were similar. The findings on motoneuron morphology are considered in relation to anatomical and physiological characteristics known to have been established for individual laryngeal muscles and with which they appear to be consistent.     

Distribution of motoneurons supplying cat sartorius and tensor fasciae latae, demonstrated by retrograde multiple-labelling methods.

Sartorius (SART) and tensor fasciae latae (TFL) in the cat hindlimb are functionally heterogeneous muscles with regions that differ in their skeletal actions and electromyographic recruitment during normal activity. The topographical organization of motoneurons supplying different regions of SART or TFL has been investigated by exposing cut nerve branches supplying different peripheral territories to a combination of retrograde tracers, including Fast Blue (FB), Fluorogold (FG), and horseradish peroxidase (HRP). Motoneurons supplying medial, central, and anterior regions of SART were intermixed extensively throughout a single columnar nucleus located in the ventrolateral part of segments L4 and L5. With this column, motoneurons supplying medial SART tended to lie more rostrally than those supplying anterior regions, but the gradient was modest and showed some cat-to-cat variation. Two major branches entered anterior SART at different proximodistal levels. When these two branches were exposed to different tracers, most motoneurons contained a single tracer; only a few double-labelled cells were apparent. The labelling suggests that anterior SART may contain two separate, in-series divisions of motor units. In TFL, motoneurons supplying nerve branches to posterior, central, and anterior parts of the muscle were intermingled indiscriminately in a single ventrolateral cell column in L6 and rostral L7. These results suggest that topographical organization in lumbar motor nuclei does not always reflect the highly ordered biomechanical and functional specialization evident in the peripheral organization of the muscles themselves.     

Localization of the spinal accessory motoneurons in the cervical cord in connection with the phrenic nucleus: an HRP study in cats

The localization of the spinal accessory motoneurons (SAMNs) that innervate the accessory respiratory muscles, the sternocleidomastoid (SCM) and trapezius (TP) muscles, was identified in the cat using the horseradish peroxidase (HRP) method. In the cases of HRP bathing of the transected spinal accessory nerve (SAN), HRP-labeled motoneurons were observed ipsilaterally from the C1 to the rostral C6 segments of the spinal cord. Labeled neurons were located principally in the medial and central regions of the dorsomedial cell column of the ventral horn in the C1 segment, in the lateral region of the ventrolateral cell column in the C2-C4 segments, between the ventrolateral and ventromedial cell columns in the C5 segment and in the lateral region of the ventromedial cell column in the C6 segment. In the cases of HRP injection into either SCM or TP muscles, labeled SCM motoneurons were found in the C1-C3 segments of the spinal cord and labeled TP motoneurons were chiefly localized more caudally within the spinal accessory nucleus. The present study revealed that, in the C5 and C6 segments, the SAMNs have a very similar topographic localization to the phrenic nucleus in the ventral horn. This finding implicated the functional linkage of the SAMNs with the phrenic motoneurons in particular types of respiration.     

Innervation and Properties of the Rat FDSBQ Muscle: An Animal Model to Evaluate Voluntary Muscle Strength after Incomplete Spinal Cord Injury

Muscles innervated from spinal segments close to the site of a human spinal cord injury are often under voluntary control but are weak because they are partially paralyzed and partially denervated. Our objective was to develop an animal model of this clinical condition to evaluate strategies to improve voluntary muscle strength. To do so, we examined the spinal and peripheral innervation of the flexor digitorum superficialis brevis quinti (FDSBQ) muscle of the rat foot, characterized the muscle and motor unit properties, and located the FDSBQ motoneurons. Retrograde labeled motoneurons were in L4 to L6 spinal cord. Unilateral stimulation of L4 to S1 ventral roots and recording of evoked force showed that FDSBQ motor axons exited via two ventral roots (L5 and L6 or L6 and S1) in 38% of rats and via one ventral root in 62% of rats. FDSBQ motor axons traveled via two peripheral nerves, the lateral plantar (76% of axons) and sural nerves (24%). Each ventral root contributed motor axons to each nerve branch. Thus, by combining conduction block of one peripheral nerve to induce partial muscle paralysis and ventral root section to induce partial denervation, it is possible to produce in one rat muscle the consequences of many human cervical spinal cord injuries. FDSBQ muscles and motor units were mainly fast-twitch, fatigable, and composed of fast-type muscle fibers. The narrow range of motor unit forces (1-13 mN), the low mean twitch force (5.1 +/- 0.3 mN), and the large number of motoneurons (31 +/- 4) suggest that rat FDSBQ muscle is a good model of distal human musculature which is frequently influenced by spinal cord injury. We conclude that the FDSBQ muscle and its innervation provide a useful animal model in which to study the consequences of many spinal cord injuries which spare some descending inputs but also induce substantial motoneuron death near the lesion.     

Differences in the connectivity of rat pudendal motor nuclei as revealed by retrograde transneuronal transport of wheat germ agglutinin

Bilateral coordinated activation of pudendal motoneurons is an essential component of penile reflexes in male rats. However, little is known about the intraspinal organization of these reflexes. In the present study, retrograde transneuronal transport of wheat germ agglutinin (WGA) was used to examine the organization of spinal motoneurons and putative interneurons mediating penile reflexes in adult male rats. Injection of WGA into the ventral bulbospongiosus muscle resulted in direct retrograde labeling of motoneurons in the ipsilateral dorsomedial (DM) nucleus and transneuronal labeling of ipsilateral and contralateral DM motoneurons. Motoneurons in the ipsilateral and contralateral dorsolateral (DL) nuclei were not labeled. WGA-labeled putative interneurons were observed bilaterally, primarily in the ventromedial spinal gray matter extending dorsally to the central canal and the dorsal gray commissure. The number of transneuronally labeled putative interneurons increased with longer survival times. Injection of WGA into the ischiocavernosus muscle resulted in direct retrograde labeling of motoneurons in the medial subdivision of the ipsilateral DL nucleus. However, no WGA labeling was detected in motoneurons in the lateral subdivision of the ipsilateral DL nucleus, the contralateral DL nucleus, or the DM nuclei at any of the survival times studied (1–7 days). Only a small number of transneuronally labeled putative interneurons was observed in the ventrolateral gray matter at longer survival times (3–7 days). Thus, marked differences were observed between the DM and DL nuclei with respect to the transneuronal transport of WGA. These results are discussed with respect to the organization of the spinal circuits that mediate pudendal motor reflexes. © 1995 Wiley-Liss, Inc.     

Absence of Nerve - muscle Interaction Influences the Survival of Developing Motoneurons

Following sciatic nerve crush at birth, approximately 70% of motoneurons to the soleus and 60% of motoneurons to the tibialis anterior (TA) and to the extensor digitorum longus (EDL) die. However, following nerve injury at 5 days, there is negligible motoneuron death. We investigated whether the interaction between the nerve and its target during these 5 days is an important factor for the ability of the motoneuron to survive injury. Nerve - muscle interaction was blocked shortly after birth by alpha-bungarotoxin (BTX) and the effect on motoneuron survival after subsequent injury was examined. It was confirmed that sciatic nerve crush at 5 days produced no significant reduction in motoneuron numbers. However, if nerve crush was preceded by paralysis with alpha-bungarotoxin, the number of surviving motoneurons after nerve injury at 5 days was substantially reduced. On the operated side only 43 +/- 6.68% of the motoneurons of the soleus pool survived and even fewer, 14 +/- 5.0%, in the TA and EDL pool. In a control group of animals paralysed with alpha-bungarotoxin at birth but receiving no nerve crush, there was no appreciable reduction in the motoneuron numbers at 28 days in either motor pool. It is concluded that blocking of nerve - muscle interaction by paralysis in early postnatal life reduces the motoneurons' ability to survive nerve injury later in life, and that this effect is more severe for the motoneurons to the TA and EDL than to the soleus.     

Central location of the motoneurons that supply the cucullaris (trapezius) of the clearnose skate, Raja eglanteria.

A complex of three muscles (one lateral, one intermediate and one medial in position) in the clearnose skate, Raja eglanteria, is believed to be wholly, or in part, homologous to the cucullaris (trapezius). The retrograde transport of horseradish peroxidase was used to discover the central location of the motoneurons that supply each of these muscles. Motoneurons that project to the lateral muscle occupy the caudal part of the ventral nucleus of X. This nucleus is situated ventrolateral to the dorsal vagal motor column at caudal medullary levels, and lateral to the main ventral motor column of the rostral spinal cord. The axons of these motoneurons exit the medulla within the caudal vagal rootlets and course peripherally within the intestinal (visceral) ramus of the vagus nerve. Motoneurons that innervate the intermediate and medial muscles are located along the ventral border of the ventral column of gray at spinal cord segments 10-15. Their axons course peripherally within the ventral roots of spinal nerves. The caudal ventral nucleus of X, the nerve that supplies the lateral muscle, and the lateral muscle are likely homologues of the accessory nucleus, accessory nerve, and cucullaris (trapezius), respectively, among other fishes and tetrapods. Intermediate and medial muscles, based on the central location of motoneurons that supply them, are part of the longitudinal epaxial musculature and are not part of a trapezius complex.