Maturation and recycling of trigeminal motoneurons in anuran larvae

Development of the trigeminal motor system was analyzed in Rana pipiens larvae and adults. The aim of this investigation was to determine the postmetamorphic fate of the primary motoneurons that innervate the larval jaw muscles. Specifically, we wanted to ascertain whether these neurons were deleted in conjunction with their muscular targets during metamorphosis or reused to innervate the adult jaw muscles. Cell counts and horseradish peroxidase tracer were used to distinguish between these two possibilities. The number of trigeminal motoneurons was relatively constant in premetamorphic and prometamorphic larvae. A small reduction in the cellular complement of the motor nucleus occurred during metamorphic climax, but the majority (approximately equal to 90%) of the primary motoneurons were retained from the larval to the adult nervous system. The cell loss may represent motoneurons that innervated specific larval muscles that have no adult successors and thus the entire myoneural unit degenerates. Retrograde tracers indicated that all trigeminal motoneurons extended axons into the jaw muscles of both premetamorphic larvae and adult frogs. These observations provide further support for the recycling of the trigeminal motoneurons.     

Postembryonic development of the dorsal longitudinal flight muscle and its innervation in Manduca sexta

The neuromuscular systems of holometabolous insects must be remodeled during metamorphosis to allow striking behavioral changes, such as the acquisition of flight. The fast contracting dorsal longitudinal flight muscle (DLM) of Manduca arises from an anlage containing both remnants of specific larval dorsal body wall muscles and extrinsic myoblasts. In the mesothorax, the DLM is innervated by five persisting larval motoneurons: one in the mesothoracic and four in the prothoracic ganglion. These motoneurons innervate two slowly contracting body wall muscles in the larva. 2 days before pupation, the DLM template fibers begin to degenerate, whereas other muscles remain intact until pupation. Correspondingly, the motor terminals retract from the template fibers while they remain on other muscle fibers until pupation. Accumulation and proliferation of putative myoblasts also starts 2 days before pupation in close spatial relationship to the retracted motor tufts around the degenerating larval template fibers. Proliferation increases through the early pupal stages, and is detected within the anlage until the ninth day after pupation. 2 days after pupation, the anlage splits into five bundles, each innervated by one motoneuron. Striations occur on the seventh day after pupation when the growing motor axons reach the attachment sites. Subsequently, the muscle grows in volume and higher-order motor branches are formed. Within the central nervous system, there is dramatic regression of larval dendrites followed by growth of new dendrites as the persistent motoneurons assume their new role in flight behavior. Both central and peripheral remodeling follow similar time courses.     

Reutilization of trigeminal motoneurons during amphibian metamorphosis

Indirect evidence suggests that trigeminal motoneurons (Vmns) in Rana pipiens innervate distinct myofiber populations in tadpoles and adult frogs. Redeployment occurs when the larval myofibers die and are replaced during metamorphosis. To directly test this hypothesis, DiI was injected into the larval muscle and Fast Blue into the replacement myofiber population. Over 95% of the Vmns contained both tracers, providing support for the innervation of sequential targets by the same motoneurons.     

Cell death of motoneurons in the chick embryo spinal cord. II. A quantitative and qualitative analysis of degeneration in the ventral root, including evidence for axon outgrowth and limb innervation prior to cell death.

Development of the ventral roots in the caudal half of the chick lumbar spinal cord (segments 26-29) was studied by electron microscopy. The ventral root fibers at 4, 5, 6, 7, 8, 9, and 13 days of incubation and at 1, 10 days and 5 weeks post-hatching were counted either from photomontages or directly in the electron microscope. In the chick, most, if not all, the fibers in the caudal ventral roots studied here probably arise from axons of motoneurons in the lateral and medial motor columns. At four days of incubation, there is an average of 800 axons per segment. The number increases very rapidly reaching a peak of 5,500 axons at day 5.5 In other words, by day 5.5 all the axons of motoneurons hav ealready reached the ventral root region. Between days 6 and 9, the number dreastically declines to 2,200 axons poer segment, a 57% reduction in ventral root fibers. After day 9, there is only aminor and rather slow additional loss of axons, reaching 1, 700 in the 13-day embryo and 1,500 in the 1-day post-hatching chick. In brief, during embryonic development about 71% of the axons are depleted in the ventral roots. Quantitatuve comparisons of Motoneurons in the lateral motor column (LMC) of segments 26-29 with the axon counts from the same segments have demonstrated: (a) that there is a massive natural cell loss in this region between days 5.5 and 9 amounting to 53%; (b) that axons are lost to the same extent as the motoneurons during this period, Resulting in a close to 1:1 relationship between the two by day 9. When horseradish peroxidase (HRP) was injected into the limb-buds of 5-day embryos, prior to the onset of massive cell death, virtually all motoneurons in the LMC were found to contain the HRP reaction product. Since approximately 50% of the cells present on day 5 typically degenerate by day 9, this finding, coupled with the observed close correspondence between axon and cell counts, strongly indicates that all motoneurons, even those destined to die, normally innervate the leg. Ultrastructural changes of motoneuron axons undergoing spontaneous degeneration in the ventral root were also described. The degenerating axons are found in the ventral root as early as the fourth day of incubation, although the number at this time is very low. More massive degeneration occurs between 5.5 and 9 days of incubation. The increrased number of degenerating axons in the ventral root during this period is in agreement with the increased number of degenerating cell bodies in the spinal lateral motor column at these same stages. Between day 4 and day 9, the degenerating axons are characterized by the presence of numerous vesiculated structures, membrane-bounded autophagic vacuoles, membranous lamellar figures and electron dense bodies in focal, swollen portions of the axon, as well as the disruption of the axolemma. The degeneration process seems to be due to progressive autolysis with the final axonal remnants being phagocytozed by the surrounding Schwann cells and some mononuclear leukocytes. Approximately 60% of the ventral root fibers completely disappear within three to four days, leaving very little evidence of axonal debris. We have found no differences in the details of axonal degeneration of ventral roots from limbbud removal embryos. The spontaneous degeneration of axons continues even after hatching but on a much reduced scale. The post-hatching degeneration is evidenced by the loosening of myelin sheath, shrinking of th axoplasm, an increase in both multivesicular bodies and lamellated dense bodies, and the disintergration of neurofilaments and neurotubules.     

Birth dates of trigeminal motoneurons and metamorphic reorganization of the jaw myoneural system in frogs

Drastic alterations in oral behavior characterize metamorphosis of anuran amphibians. Changes cascade through all components of the jaw apparatus from bone to muscle to nerve. In this investigation, tritiated thymidine autoradiography was used to determine the production schedule of the trigeminal motoneurons in the leopard frog, Rana pipiens. The time of origin of these neurons and their subsequent fate are of special interest given the breakdown of the larval jaw muscles and the de novo generation of adult muscle fibers during metamorphosis. Specifically, we wanted to learn whether trigeminal motoneurons are added, deleted, or reused during metamorphic climax. The entire complement of trigeminal motoneurons was produced over a 4-day span commencing at embryonic stage 13 and terminating at stage 20. Newly formed neurons are added to the primordial trigeminal nucleus in an orderly pattern. Firstborn neurons settle in the ventrorostral region of the nucleus; cells with progressively later birth dates were added in a posterodorsal direction. No additional trigeminal motoneurons are generated during larval maturation or at metamorphosis, thus indicating that the same population of neurons is present throughout the lifespan of the animal. From these observations we suggest that, during metamorphosis, the trigeminal motoneurons that supply the larval muscles switch their allegiance to the newly formed adult jaw muscles. This change of peripheral targets can be viewed as a respecification of the trigeminal motoneurons.     

Synaptic connections between primary trigeminal afferents and accessory abducens motoneurons in the monitor lizard, Varanus exanthematicus

We studied the anatomical pathway underlying the nictitating reflex in the monitor lizard Varanus exanthematicus by the anterograde degeneration technique combined with retrograde transport of horseradish peroxidase (HRP) and electron microscopy. After application of HRP to the abducens nerve, retrogradely labeled neurons were observed in the ipsilateral principal and accessory abducens motor nuclei. The transection, in the same experiments, of the root of the trigeminal nerve resulted in massive degeneration of myelinated fibers in the descending trigeminal tract. In the ipsilateral accessory abducens nucleus, we observed electron-dense degenerating axon terminals that formed asymmetric synaptic contacts with the primary and secondary dendrites of large neurons retrogradely labeled with HRP. A few of the degenerating terminals could be traced in serial sections to myelinated axons. No terminal degeneration was found in the contralateral accessory abducens nucleus or in the ipsilateral and contralateral principal abducens nuclei. The present results are complementary with the findings of previous light microscopic experimental tracing studies (Barbas-Henry, H.A., and A.H.M. Lohman, J. Comp. Neurol. 1986, 254:314-329; see also J. Comp. Neurol. 1988, 267:370-386), and strongly suggest the existence in Varanus of a monosynaptic, unilateral reflex pathway in which trigeminal fibers, presumably originating from the cornea, synapse with motoneurons of the bursalis and retractor bulbi muscles, which are located in the accessory abducens nucleus. This monosynaptic pathway may mediate a rapid unilateral eyeball retraction and nictitating membrane extension.     

Amygdaloid pathway to the trigeminal motor nucleus via the pontine reticular formation in the rat

The connections of the amygdala with the trigeminal motor nucleus were studied by light and electron microscopy. Horseradish peroxidase (HRP) experiments showed that the pontine reticular formation, ventromedial to the spinal trigeminal nucleus at the level rostral to the genu of the facial nerve, receives fibers from the central nucleus of the amygdala ipsilaterally and sends fibers to the trigeminal motor nucleus contralaterally. Electron microscopic observations were carried out on the pontine reticular formation after electrolytic lesions in the central nucleus of the amygdala and HRP injections into the contralateral trigeminal motor nucleus were made on the same animal. These experiments using the combined degeneration and HRP technique clearly demonstrated that degenerating amygdaloid fibers made synaptic contacts with retrogradely labeled neurons.     

Sex-specific neuronal respecification during the metamorphosis of the genital segments of the tobacco hornworm moth Manduca sexta

At metamorphosis, the terminal abdominal segments of larvae of the moth Manduca sexta transform into either male or female genitalia. At the start of this transformation, the larval muscles degenerate but their remains may persist to form the scaffolding on which the new adult muscles differentiate. The survival and subsequent orientation of larval muscle remnants is determined by the sex of the individual and is independent of motor innervation at the start of metamorphosis. Many of the larval motoneurons persist through metamorphosis and innervate the skeletal muscle of the adult. The survival of particular motoneurons is also sex-dependent and correlated with the survival of its respective muscle remnant. No new skeletal motoneurons arise postembryonically, so all of the adult skeletal muscle motoneurons are derived from preexisting larval skeletal muscle motoneurons. The fates during metamorphosis are more complex for the visceral muscle motoneurons. Those innervating the adult hindgut of both sexes are identical and are derived from the larval hindgut motoneurons. Other hindgut motoneurons in the larva switch targets during metamorphosis and come to innervate the oviduct in adult females or perish in adult males. Other regions of the reproductive tract become innervated by adult-specific cells that differentiate during metamorphosis. These cells come from distinct lineages in males and females.     

Localization of the calcium/calmodulin-dependent protein phosphatase,calcineurin, in the hindbrain and spinal cord of the rat

The calcium/calmodulin-dependent protein phosphatase calcineurin was localized at the light microscopic level in the rat hindbrain and spinal cord by using an antibody against the α-isoform of the catalytic subunit. Calcineurin was highly concentrated in axons, dendrites, and cell bodies of a subpopulation of α-motoneurons in hindbrain motor nuclei and the lateral motor column along the length of the spinal cord. These calcineurin-positive α-motoneurons appeared to be randomly distributed and represented approximately 25% of the total α-motoneuron pool in the motor trigeminal nucleus and the spinal cord lateral motor column. Within the facial nucleus, calcineurin-containing motoneurons were present in the medial and dorsal subdivision but not in the lateral and intermediate subdivision. In addition to the enrichment in motoneurons, calcineurin was enriched in cells of the superficial laminae of the spinal cord dorsal horn and its extension into the medulla, the caudal spinal trigeminal nucleus. Axonal staining in the white matter of the spinal cord was generally weak, except in the dorsolateral funiculus, where strongly calcineurin-positive axons formed a putative ascending tract that appeared to terminate uncrossed in the caudal lateral reticular nucleus of the medulla. This tract may originate from calcineurin-positive cells in the dorsolateral funiculus. We also compared the distribution of calcineurin with calcium/calmodulin-dependent kinase II in the spinal cord and found that the kinase is more widely expressed. Thus, calcineurin is highly restricted to a few locations in the hindbrain and spinal cord. Selective staining in facial subnuclei that innervate phasically active muscles suggests that calcineurin-positive motoneurons represent a subset of α-motoneurons innervating a metabolic subtype of muscle fibers, possibly fast-twitch fibers. © 1996 Wiley-Liss, Inc.     

Accurate reinnervation of motor end plates after disruption of sheath cells and muscle fibers

After injury, regenerating motor axons grow back to form neuromuscular junctions at the original synaptic sites on muscle fibers. The pathways they grow along consist of basement membrane, Schwann cells, and perineurium that remained after degeneration of the original axons. All the factors necessary for directing axons to the original synaptic sites persist in muscles even after disruption of myofibers. The aim of the present experiments was to determine whether structural integrity of nerve sheath cells is required for precise reinnervation in the presence and absence of muscle fiber targets. The region of innervation of the cutaneous pectoris muscle of the frog was briefly frozen to eliminate all living cells from neuromuscular junctions, intramuscular nerve bundles, and from a 1-3-mm length of the nerve trunk. Only extracellular matrices persisted within the frozen region of muscle and nerve. These consisted of the basement membrane sheaths of myofibers, of Schwann cells, and of perineurial cells and the small fragments of disrupted cells that were bound to them. In some preparations new muscle fibers developed within the basement membrane sheaths. Regenerating axons grew through the naked basement membrane sheaths of original Schwann cells, formed numerous branches, and contacted the myofibers precisely at the original synaptic sites. By 5 weeks 75% of the original synaptic sites became reinnervated; the terminals were indistinguishable from those at normal neuromuscular junctions. In contrast, preparations in which all muscle fibers were prevented from regenerating far fewer synaptic sites became reinnervated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Repeated in vivo visualization of neuromuscular junctions in adult mouse lateral gastrocnemius

The structure of individually identified neuromuscular junctions (NMJs) in mouse lateral gastrocnemius (LG) muscles was studied on 2 or more occasions over 3-6 months. Presynaptic motor nerve terminals and their underlying acetylcholine receptors were stained in living animals with the fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodide) and tetramethylrhodamine isothiocyanate-conjugated alpha-bungarotoxin (R alpha BTX), respectively, and visualized by video-enhanced fluorescence microscopy. The overall shape of most NMJs changed very little over this time except for enlargement of some junctions attributable to growth of the animals. A few junctions did, however, change appreciably: over 3-6 months about 15% underwent modifications such as additions to, or losses from, their original configuration. The frequency and extent of changes in LG NMJs were substantially less than in a similar study of NMJs from mouse soleus (Wigston, 1989). These findings, together with those from other laboratories, indicate a correlation between the extent of NMJ remodeling and the fiber-type composition of a muscle: NMJs in muscles consisting of predominantly fast-twitch myofibers appear to undergo less remodeling than NMJs in muscles containing a substantial fraction of slow-twitch fibers. Since fast- and slow-twitch muscles and their motoneurons exhibit strikingly different patterns of electrical activity, these findings suggest that structural remodeling at mammalian NMJs may depend on the amount of impulse activity experienced by motoneurons, their target muscle, or individual synaptic terminals.     

Remodeling of the peripheral processes and presynaptic terminals of leg motoneurons during metamorphosis of the hawkmoth,Manduca sexta

During metamorphosis of the hawkmoth, Manduca sexta, the muscles, cuticular structures, and most sensory neurons of the larval thoracic legs are replaced by new elements in the adult legs. The thoracic leg motoneurons, however, survive the loss of the larval muscles and persist to innervate new targets in the imaginal legs. Here we have used biocytin staining, immunocytochemistry, and confocal microscopy to follow the fates of the peripheral processes and presynaptic terminals of the leg motoneurons. Although the most distal processes of the motor nerves retract following the degeneration of larval leg muscles, the axon terminals always retain close association with the muscle remnants and the anlagen of the new adult muscles. As the imaginal muscles differentiate and enlarge, the motor terminals expand to form adult presynaptic terminals. An antibody to the presynaptic protein, synaptotagmin, revealed its localization to the terminal varicosities in both larval and adult stages but distribution within pre-terminal branches during adult development. Electrophysiological methods revealed that functional neuromuscular transmission first occurs quite early during metamorphosis, before the differentiation of contractile elements in the muscle fibers. © 1996 Wiley-Liss, Inc.     

Motoneurons of twitch and nontwitch extraocular muscle fibers in the abducens, trochlear, and oculomotor nuclei of monkeys

Eye muscle fibers can be divided into two categories: nontwitch, multiply innervated muscle fibers (MIFs), and twitch, singly innervated muscle fibers (SIFs). We investigated the location of motoneurons supplying SIFs and MIFs in the six extraocular muscles of monkeys. Injections of retrograde tracers into eye muscles were placed either centrally, within the central SIF endplate zone; in an intermediate zone, outside the SIF endplate zone, targeting MIF endplates along the length of muscle fiber; or distally, into the myotendinous junction containing palisade endings. Central injections labeled large motoneurons within the abducens, trochlear or oculomotor nucleus, and smaller motoneurons lying mainly around the periphery of the motor nuclei. Intermediate injections labeled some large motoneurons within the motor nuclei but also labeled many peripheral motoneurons. Distal injections labeled small and medium-large peripheral neurons strongly and almost exclusively. The peripheral neurons labeled from the lateral rectus muscle surround the medial half of the abducens nucleus: from superior oblique, they form a cap over the dorsal trochlear nucleus; from inferior oblique and superior rectus, they are scattered bilaterally around the midline, between the oculomotor nucleus; from both medial and inferior rectus, they lie mainly in the C-group, on the dorsomedial border of oculomotor nucleus. In the medial rectus distal injections, a "C-group extension" extended up to the Edinger-Westphal nucleus and labeled dendrites within the supraoculomotor area. We conclude that large motoneurons within the motor nuclei innervate twitch fibers, whereas smaller motoneurons around the periphery innervate nontwitch, MIF fibers. The peripheral subgroups also contain medium-large neurons which may be associated with the palisade endings of global MIFs. The role of MIFs in eye movements is unclear, but the concept of a final common pathway must now be reconsidered.     

Fibers supplying the laryngeal musculature in the cranial root of the rabbit accessory nerve: Nucleus of origin, peripheral course, and innervated muscles

The location of the rabbit laryngeal motoneurons whose axons traverse the cranial root of the accessory nerve was studied with injection of HRP or nuclear yellow into the laryngeal muscles in combination with the intracranial severing of either the rootlets of the vagus nerve or those of the cranial root. The motoneurons were located in the diffuse cell group that forms a subnucleus occupying the caudal two-thirds of the nucleus ambiguus and sending fibers to the inferior laryngeal nerve. The caudal one-third of the diffuse cell group supplying the lateral cricoarytenoid muscle, was occupied only by these motoneurons, whereas in its rostral two-thirds, they were intermingled with motoneurons having axons that traversed the vagal rootlets. The thyroarytenoid and posterior cricoarytenoid motoneurons are present in the rostral two-thirds of the diffuse cell group; axons of the former traversed the rootlets of the cranial root, and of the latter traversed the vagal rootlets. On the other hand, the medial scattered cell group, located in the rostral one-third of the nucleus ambiguus and sending fibers to the cricothyroid muscle via the superior laryngeal nerve, contained only motoneurons with axons traversing the vagal rootlets. The above findings clarified that fibers of the cranial root enter the inferior laryngeal nerve after joining the vagus, and then reach the adductor muscles for the vocal fold, with their neurons of origin in a caudal portion of the nucleus ambiguus. The vagal rootlet fibers, with their neurons of origin situated more rostrally in the nucleus, innervate the tensor and abductor muscles via the superior and inferior laryngeal nerve, respectively.     

Regeneration of mouse peripheral nerves in degenerating skeletal muscle: guidance by residual muscle fibre basement membrane

The part played by basement membrane in the guidance of peripheral nerve growth in vivo has been assessed by examining the capacity of degenerating mouse muscle to support the regeneration of the cut sciatic and saphenous nerves. Ethanol and formaldehydefixed gluteus maximus muscles were implanted around the contralateral cut nerves. The subsequent nerve growth into the degenerating muscle was assessed by silver staining after 3, 4 and 10 days. By 4 days, linear axonal growth was seen, parallel to the length of the muscle fibers, and coinciding with the onset of degeneration of the sarcoplasm. Transverse sections of the 10 day preparations showed that over 90% of linearly growing axons were located inside the remaining sheaths of muscle fibre basement membrane. This relationship was confirmed by electron microscopy of ruthenium red-stained preparations. Both motor and sensory axons were able to grow in this manner, for electrophysiological testing revealed the presence of motor axons from the sciatic nerve, while the saphenous nerve contains only sensory axons. Identical growth was seen at 10 days in muscles caused to degenerate by incubation in distilled water. However, linear growth did not occur in live-innervated and glutaraldehyde-fixed muscles, in which muscle fibre architecture was preserved.It is concluded that basement membrane derived from muscle can promote peripheral nerve regeneration. Furthermore, both motor and sensory axons show a strong preference for growth along its inner surface, the basal lamina.     

Action potential shape of rabbit masseter motor units and jaw angle.

Action potentials of rabbit masseter motor units (n = 42) were registered at different jaw angles to examine whether the shape of the action potential is related to length of muscle fibers in motor units and depends on the intramuscular location of the motor unit. Twitches were elicited by stimulating motoneurons in the trigeminal motor nucleus. During jaw opening (0-21 degrees), the duration of the action potentials increased by about 10%. Anteriorly located motor units showed an increase in duration larger than that of more posteriorly located units, which was probably due to a larger stretching of the more anteriorly located units.     

A theory on the lability and stability of spinal motoneuron soma size and induction of synaptogenesis in the adult spinal cord

There exists a dynamic relationship between the soma size of a motoneuron and its motor unit size. Adult motoneuron soma size can be experimentally increased if the neuron is allowed to innervate more muscle fibers than it normally innervates. In postnatal mammals a transition from polyneuronal to mononeuronal innervation of limb muscle fibers occurs which is temporally related to a plastic change in the perikaryal size. This lability of postnatal motoneuron size is temporally related to growth of synaptic connections on the motoneuron. In adult mammal, regenerating motor axons polyneuronally innervate the muscle fibers for a transient period. This hypothesis proposes that a plastic change in soma size occurs in these adult motoneurons. This short-lived labile state may revive the embryonic properties and evoke growth of synaptic boutons. Experimentally induced labile state in motoneuron pools and spinal ganglion neurons in the adult mammal should offer a basis for the study of mechanisms of synaptogenesis in the spinal cord.     

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)     

Amygdaloid projections to commissural interneurons for masticatory motoneurons

Injections of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) into the trigeminal motor nucleus resulted in retrograde labeling of neurons, called commissural interneurons for masticatory motoneurons, in the contralateral supratrigeminal region. Further HRP studies, in which WGA-HRP injections were made into the amygdala and supratrigeminal region, indicated that the supratrigeminal region receives fibers from the central nucleus of the amygdala ipsilaterally. These findings raised the possibility of direct connections between the amygdala and commissural interneurons. In order to confirm the connections, electrolytic lesions in the central nucleus of the amygdala and WGA-HRP injections into the contralateral trigeminal motor nucleus were made on the same animal and electron microscopic observation was carried out on the supratrigeminal region. Of particular interest was that degenerating amygdalotegmental fibers synapsed upon HRP-labeled neurons.     

Innervation pattern of muscles of one-legged Xenopus laevis supplied by motoneurons from both sides of the spinal cord

In Xenopus tadpoles one limb bud was removed before innervation and motoneurons from both sides of the spinal cord were induced to innervate the remaining limb. When examined after metamorphosis the motor innervation of the limb had the following characteristics. In agreement with previous findings a large proportion of contralateral motoneurons survived (51-82% of the ipsilateral numbers) and sent axons to the limb. By acetylcholinesterase staining and intracellular recording from muscle fibers of the response to electrical stimulation of the two limb innervations, the neuromuscular junctions from contralateral motoneurons were indistinguishable from those from the ipsilateral side in their morphology, spacing along the fiber, and physiological properties. Many single muscle fibers shared innervation from both sides of the cord by symmetrically placed spinal nerves. By the same techniques junctions in one-legged frogs were morphologically indistinguishable from those in normal frogs, but the quantal content of transmitter release was increased by up to 63%. Recording twitch and tetanic tensions from individual motor units from the gastrocnemius muscle showed that the one-legged animals had many more and smaller motor units than do normal frogs. We confirm that the hind-limb musculature has the ability, normally unexpressed, to sustain, through the period of normal developmental cell death, up to twice the usual number of motoneurons. In maturity, motoneurons accommodate themselves to the limb muscles by making fewer than the normal number of synapses. The above suggests that developmental motoneuron death is not primarily a mechanism for adjusting the number of motoneurons to the size of the peripheral musculature and is likely to be related to mechanisms for securing specific neuromuscular connections.