Influence of grafting a smaller target muscle on the magnitude of naturally occurring trochlear motor neuron death during development

About half of the motor neurons produced by some neural centers die during the course of normal development. It is thought that the size of the target muscle determines the number of surviving motor neurons. Previously, we tested the role of target size in limiting the number of survivors by forcing neurons to innervate a larger target (Sohal et al., '86). Results did not support the size-matching hypothesis because quail trochlear motor neurons innervating duck superior oblique muscle were not rescued. We have now performed the opposite experiment, i.e., forcing neurons to innervate a smaller target. By substituting the embryonic forebrain region of the duck with the same region of the quail before cell death begins, chimera embryos were produced that had a smaller quail superior oblique muscle successfully innervated by the trochlear motor neurons of the duck. The number of surviving trochlear motor neurons in chimeras was significantly higher than in the normal quail but less than in the normal duck. The smaller target resulted in some additional loss of neurons, suggesting that the target size may regulate neuron survival to a limited extent. Failure to achieve neuron loss corresponding to the reduction in target size suggests that there must be other factors that regulate neuron numbers during development.     

Afferent and efferent innervation patterns of the superior olivary nucleus of the leopard frog

An HRP study of the frog's superior olivary nucleus (SON) revealed that it (1) receives reciprocal tonotopic projections from dorsal medullary nuclei and principal nuclei of the torus semicircularis (TS) bilaterally; (2) receives bilateral projections from caudalis nuclei and brainstem reticular nuclei, and unilateral projections from the ipsilateral ventral tegmental nuclei and the contralateral SON; (3) is reciprocally connected with the ipsilateral laminar and magnocellular nuclei of the TS, dorsal tegmental nuclei and the posterior thalamic nucleus; (4) projects directly to the ipsilateral central thalamic nucleus.     

Location of motoneurons in the oculomotor nucleus and the course of their axons in the oculomotor nerve

Subdivisions of the oculomotor nucleus, and the course of axons in the brainstem and more peripherally in the oculomotor nerve of the cat, were studied by directly applying horseradish peroxidase solution to the transected nerve-branch stump in the orbit. The medial rectus subdivision consisted of two subgroups, and intermingling between subdivisions was found. About 20% of the motoneurons controlling the medial rectus muscle were scattered in the medial longitudinal fasciculus or a more ventrolateral area. A few motoneurons controlling the inferior rectus or inferior oblique muscle were also located in the medial longitudinal fasciculus. Axons to the superior branch that supplied the superior rectus and levator muscle coursed in the dorsolateral half of the oculomotor nerve. In contrast, those to the medial rectus, inferior rectus, and inferior oblique muscles were scattered diffusely in the oculomotor nerve.     

Afferent and efferent innervation patterns of the cochlear nucleus (dorsal medullary nucleus) of the leopard frog

The afferent and efferent innervation patterns of the frog dorsal medullary nucleus (DMN; anuran homolog of the cochlear nucleus) were examined by studying the anterograde and retrograde transport patterns of horseradish peroxidase injected focally into the nucleus. It was found that this structure projected bilaterally to the superior olivary nuclei (SON) and dorsal midbrain tegmental nuclei, and contralaterally to the opposite DMN, the lateral lemniscus nucleus (LLN) and the torus semicircularis (TS). The termination sites in the TS were restricted to the laminar and principal nuclei. The DMN in turn received projections from these structures with the exception of the TS and dorsal tegmental nuclei. The projection to the ipsilateral LLN and TS was not pronounced. In addition to the above findings, the ascending projection to the DMN, SON and TS, as well as the centrifugal projection from the SON, were found to be organized tonotopically.     

A direct hypothalamic projection to the superior salivatory nucleus neurons in the rat. A study using anterograde autoradiographic and retrograde HRP methods

The location of the superior salivatory nucleus and terminal labelings of the hypothalamic descending fibers were demonstrated in the nucleus reticularis parvocellularis using HRP and the autoradiographic techniques, respectively. When both techniques were used in the same animals, some HRP-labeled neurons were seen among the accumulations of silver grains, suggesting pericellular terminations. The present study demonstrates that the hypothalamic efferents project directly to the superior salivatory nucleus innervating salivary and lacrimal glands.     

Excitatory projection from the interpeduncular nucleas to central superior raphe neurons

The projection from the interpeduncular nucleus (IP) to the central superior raphe nucleus (CS) was studied using intracellular electrophysiologic methods. IP stimulation generates monosynaptic EPSPs in a large number of CS neurons studied with latency of 1–2 ms. Intracellular peroxidase injections into CS neurons responding to IP shock confirmed the location and somatic origin of intracellular potentials. These findings document the existence of a direct excitatory projection from IP onto CS neurons.     

The location of the preganglionic neurons that innervate the submandibular gland of the cat. A horseradish peroxidase study

Horseradish peroxidase was injected under pressure in the main excretory duct of the submandibular gland of cats, to locate the perikarya of the preganglionic parasympathetic salivatory neurons in the brainstem. Labeled multipolar neurons were found ipsilaterally in the lateral reticular formation, where they extended from the rostral plane that contains the genu of the facial nerve to the caudal plane that contains the rostral pole of the dorsal motor nucleus of the vagus nerve.     

Location of the cutaneous trunci motor nucleus in the dog

Using the methods of muscle injection of horseradish peroxidase (HRP) and application of HRP to the proximal stump of the cut lateral thoracic nerve (LTN), the motor nucleus of the cutaneous trunci muscle (CTM) has been located predominantly in C8 and T1 spinal cord segments in the ventral and ventrolateral nuclei of Rexed's Lamina IX. Cells of the nucleus are somatotopically arranged rostro-caudally to correspond to the rostro-caudal segmentation. There was a higher average number of HRP-filled cells per section and greater rostro-caudal extent of the nucleus for the cut nerve technique than for the muscle injection technique. Evidence is presented that the CTM is an appendicular muscle in spite of its obvious axial position on the trunk.     

Morphological and functional evaluation of the superior salivatory nucleus in rabbits

Horseradish peroxidase (HRP) was applied to the submandibular ganglion of the rabbit to determine the locus of the parasympathetic preganglionic neurons in the medulla. Labeled cells were recognized in the ipsilateral bulbar reticular formation at the level between the caudal end of the facial genu and the caudal part of the root of the facial nerve. The size of the labeled cells were distributed from 150 to 1150 μm² with a variety of forms such as spindle shape, triangular, and polygonal. Moreover, these cells of different sizes and forms were evenly scattered in the superior salivatory nucleus. To evaluate the physiologic functions of the bulbar salivatory nucleus, electrical stimulation was applied to the medulla, and salivary secretion and thermal change of the submandibular gland were measured simultaneously after bilateral cervical sympathectomy. A very good parallelism between the volume of salivary secretion and the magnitude of temperature change at each stimulating point suggests that secretory and vasodilator cells are intermingled with each other in the salivatory nucleus.     

Development of the trochlear nucleus in quail and comparative study of the trochlear nucleus, nerve, and innervation of the superior oblique muscle in quail, chick, and duck

The present study was undertaken to examine the development of the trochlear nucleus in quail and to compare the mature trochlear nucleus, nerve, and their sole target of innervation, the superior oblique muscle, in quail, chick, and duck. Study of the trochlear nucleus in quail from embryonic day 5 through hatching shows a maximum of 1,248 neurons on embryonic day 10 followed by spontaneous degeneration of 40% of the neurons between days 10 and 16. Previous studies have shown that although the initial and final number of neurons is different in chick and duck, the magnitude of trochlear cell loss in both species is about 40%. This study shows the average number of neurons in the nucleus of quail, chick, and duck, 2 weeks post-hatching, to be 658, 743 and 1,459, respectively. Fiber counts in the trochlear nerve from electron micrograph montages at the same period indicated a ratio of about 1:1 between neurons and axons. While a majority of the fibers in these nerves are myelinated, an average of 3-6% of the fibers are unmyelinated. The nucleus in the quail not only contains the smallest number of neurons but it also innervates the smallest muscle in terms of total number of muscle cells and endplates. However, the opposite relationship does not hold true. The nucleus in duck contains the largest number of neurons, yet the largest number of muscle cells and endplates were found in the chick. The ratios between the neurons and muscle cells as well as between neurons and endplates are about the same in quail and duck. These ratios are much higher in the chick, reflecting the relatively small neuron pool destined for a relatively large target. In spite of variations in the number of neurons, muscle fibers, and endplates the average number of endplates per muscle fiber is relatively constant among the three species.     

Evidence for a sympathetic component in motor branches of the facial nerve: a horseradish peroxidase study in the cat

The retrograde transport of horseradish peroxidase (HRP) was used to examine the location of sympathetic ganglion cells with axons in facial motor branches of the cat. Large numbers of HRP-labeled neurons were observed in the rostro-anterior part of the superior cervical ganglion. In addition, some labeled neurons were found in the cervical sympathetic trunk, the accessory cervical, middle cervical and stellate ganglia.     

Interpeduncular nucleus afferents in the rat

Afferents to the interpeduncular nucleus (IPN) of the rat were studied with the horseradish peroxidase (HRP) retrograde transport method. HRP was deposited microelectrophoretically in the IPN of adult rats. Major projections to the IPN originate in the medial habenular nucleus, the region surrounding the dorsal tegmental nucleus (accessory dorsal tegmental nucleus and the so-called dorsal tegmental nucleus pars lateralis), and the midbrain raphe (nucleus centralis superior and nucleus raphe dorsalis). Also, minor projections originate in the central gray and nucleus locus coeruleus. Our results indicate that the habenulointerpeduncular projection originates solely from the medial habenular nuclei and is topographically organized; medial regions of the medial habenular nuclei project to ventral portions of IPN and lateral regions project to the dorsal IPN.     

Interpeducular nucleus afferents in the rat

Afferents to the interpeduncular nucleus (IPN) of the rat were studied with the horseradish peroxide (HRP) retrograde transport method. HRP was deposited microelectrophoretically in the IPN of adult rats. Major projections to the IPN originate in the medial habenular nucleus, the region surrounding the dorsal tegmental nucleus (accessory dorsal tegmental nucleus and the so-called dorsal tegmental nucleus pars lateralis), and the midbrain raphe (nucleus centralis superior and nucleus raphe dorsalis). Also, minor projections originate in the central gray and nucleus locus coeruleus. Our results indicate that the habenulointerpeduncular projection originates solely from the medial habenular nuclei and is topographically organized; medial regions of the medial habenular nuclei project to ventral portions of IPN and lateral regions project to the dorsal IPN.     

Efferent acoustic neurons within the lateral superior olivary nucleus of the guinea pig

The relative potencies of the stereoisomers of ketamine, N-allylnormetazocine and cyclazocine were determined at mu opiate and sigma opiate/phencyclidine (PCP) receptors in vitro in rat brain homogenates, as well as in a discriminative stimulus behavioral paradigm in rats trained to discriminate PCP from saline. In all cases the in vivo and in vitro data were in agreement. The (+)-isomers of N-allylnormetazocine and cyclazocine are highly specific sigma opiate/PCP ligands.     

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.     

The organization of the gastric efferent projections in the brainstem of monkey: An HRP study

Topographic localization of neurons in the dorsal motor nucleus (DMN) of the vagus innervating the stomach and the extent of central decussation of its fibers were investigated in the monkeys after chronic unilateral cervical vagotomy. This study presents findings in regard to the non-topographic representation of the stomach in both DMN and lack of a well-defined decussation of its fibers in the brainstem of monkeys. Reported findings in the rat and cat vis-a-vis our own results in the monkey indicate species difference in the organization of efferent vagal gastric fibers. The evidence of a very small number of HRP-labeled cells in DMN on the vagotomized side after intervals of 2-36 weeks is being reported here, for what we believe to be the first time, and its significance has been discussed.     

Anatomical localization of the cells of origin of efferent fibers in the superior laryngeal and recurrent laryngeal nerves of the dogs

Using horseradish peroxidase, we identified the cells of origin of motor fibers in the superior laryngeal nerves of dogs. Cells giving rise to fibers in the superior laryngeal nerve were found in the dorsal motor nucleus and the nucleus ambiggus, whereas cells giving rise to fibers in the recurrent laryngeal nerve were found in the nucleus ambiguus and nucleus retroambigualis, but usually not in the dorsal motor nucleus.