Somatic Motor Nerve Research Articles

Somatic motor nerve and function as conceived by Alcmaeon of Croton.

Somatic motor nerve and function as conceived by Alcmaeon of Croton.

Patterning the rostrocaudal axis of the hindbrain

During early development the hindbrain is segmentally organised along the rostrocaudal axis into units termed rhombomeres. This segmental organisation is underlain by patterns of cellular differentiation. A relationship between the organisation of the branchiomotor and somatic motor nerves and the segmented ground plan has been found. It has also been shown that neural crest cells have a segmental origin. The expression patterns of a number of putative regulatory genes relate to the rhombomere pattern. Foremost amongst these are the Hox gene family. This family of genes are particularly interesting as they are thought to be involved in rostrocaudal patterning. A number of recent experiments, both embryological and genetic, support such a role for these genes in the rhombencephalon.

Vagal stimulation and exogenous acetylcholine in isolated rat stomach: calcitonin effect on contractile activity

Calcitonin induces contraction in the vascular and extravascular smooth muscle and facilitates the transmission of the excitation in somatic motor nerve endings. These actions are Ca 2+-dependent. The calcitonin effect on autonomic nerve endings has been studied here by testing influence of calcitonin on the contractile responses of the isolated rat stomach. The organ was submitted to electrical vagal stimulation or, after denervation, to exogenous acetylcholine. Calcitonin invariably increased the muscular tone and reduced the contractile responses to vagal stimulation. Opposite effects were noted after a serotoninergic block with nicergoline. Calcitonin also increased the contractile response evoked by exogenous acetylcholine and the Ca 2+-antagonist nicardipine counteracted the facilitatory effects. We suggest that the inhibitory action of calcitonin is serotonine-dependent while the facilitatory one is Ca 2+-dependent.

The Physiology of the Nose

The nose, as an organ initiating reflexes affecting itself and the rest of the body, and as a target organ of control, is highly complex. Its innervation includes parasympathetic, sympathetic, sensory/afferent, and somatic motor nerves, which combine in a variety of morphologic pathways. The vasculature of the nose contains capacitance vessels such as sinusoids and distensible venules, as well as arteriovenous anastomoses, arterioles, capillaries, and venules. The secretory tissue of the nose includes epithelial cells, submucosal glands, and relatively large anterior or lateral serous glands; in addition, some species have specialized secretory glands. The nose is the source of many powerful reflexes, including the diving response, sneeze and sniff reflexes, and reflexes affecting autonomic nervous function to the cardiovascular system, airways in the lungs, the larynx, and other organs. Axon reflex control of the nasal vasculature is also important. The nasal vasculature can be shown to be under parasympathetic and sympathetic control, but there is little precise information concerning the effect of nerves on different types of blood vessels. Pharmacologic experiments show that vascular resistance and vascular volume can be separately influenced by nerves and mediators, and that vascular resistance and airway resistance are not necessarily inversely related. Nasal secretion is also under the influence of both parasympathetic and sympathetic nerves, and can be induced by a wide range of neurotransmitters and mediators. In general, the concentrations needed to promote secretion are considerably higher than those that affect vascular resistance, at least with regard to the lateral nasal gland of the dog. In humans, nasal patency is affected by several nervous inputs, presumably acting via vascular beds. The nasal cycle is the alternation of resistances between the two sides of the nose, on which other changes are superimposed. In exercise and hyperpnea, nasal airflow resistance decreases, presumably with vascular decongestion. Recent studies of the crutch reflex (the ipsilateral nasal congestion caused by stimulation of the axilla) show that some of the reflex inputs to the nose can be unilateral.

Are cat lingual muscles dually innervated by the hypoglossal nerve and facial nerve?

The origins of the somatic motor nerve innervating the cat lingual muscles were studied using the horseradish peroxidase (HRP) method. Following HRP injection into the lingual muscles, labeled neurons were found not only in the hypoglossal nucleus but also in the facial nucleus, ipsilateral to the injection side. HRP-labeled neurons in the facial nucleus were principally observed in the ventromedial and ventrolateral divisions of the nucleus. This study suggests that cat lingual muscles are innervated by both hypoglossal motoneurons and some of the motoneurons in the ventromedial and ventrolateral divisions of the facial nucleus.

Distribution of cholinergic neurons in rat brain: demonstrated by the immunocytochemical localization of choline acetyltransferase.

The neuroanatomical location and cytological features of cholinergic neurons in the rat brain were determined by the immunocytochemical localization of the biosynthetic enzyme, choline acetyltransferase (ChAT). Perikarya labeled with ChAT were detected in four major cell groups: (1) the striatum, (2) the magnocellular basal nucleus, (3) the pontine tegmentum, and (4) the cranial nerve motor nuclei. Labeled neurons in the striatum were observed scattered throughout the neostriatum (caudate, putamen) and associated areas (nucleus accumbens, olfactory tubercle). Larger ChAT-labeled neurons were seen in an extensive cell system which comprises the magnocellular basal nucleus. This more or less continuous set of neuronal clusters consists of labeled neurons in the nucleus of the diagonal band (horizontal and vertical limbs), the magnocellular preoptic nucleus, the substantia innominata, and the globus pallidus. Labeled neurons in the pontine tegmentum were seen as a group of large neurons in the caudal midbrain, dorsolateral to the most caudal part of the substantia nigra, and extended in a caudodorsal direction through the midbrain reticular formation into the area surrounding the superior cerebellar peduncle. The neurons in this latter group constitute the pedunculopontine tegmental nucleus (PPT). An additional cluster of cells was observed medially adjacent to the PPT, in the lateral part of the central gray matter at the rostral end of the fourth ventricle. This group corresponds to the laterodorsal tegmental nucleus. Large ChAT-labeled neurons were also observed in all somatic and visceral motor nerve nuclei. The correspondence of the distribution of ChAT-labeled neurons identified by our methods to earlier immunocytochemical and acetylcholinesterase histochemical studies and to connectional studies of these groups argues for the specificity of the ChAT antibody used.

Noradrenergic-mediated potentiation of acetylcholine release from the phrenic nerve: Evidence for presynaptic α1-adrenoceptor involvement

This study, conducted in the rat phrenic nerve-diaphragm preparation, was designed to establish more direct evidence that norepinephrine enhances acetylcholine (ACh) release from motor neurons and characterize the α-adrenoceptor type mediating this action. Norepinephrine (50 μM, α 1 + α 2 agonist) increased nerve-stimulated release by 183%, as determined by radioenzymatic assay. This effect was completely abolished by pretreatment with the α-adrenoceptor antagonists phentolamine ( α 1 + α 2) and by WB 4101 ( α 1) but only modestly reduced by yohimbine ( α 2). Clonidine ( α 2 agonist) did not enhance ACh release or nerve-stimulated muscle contractions, while phenylephrine ( α 1 agonist) and norepinephrine increased muscle contractions up to 19.5–22.4%. These results support the hypothesis that norepinephrine increases ACh release from somatic motor nerves via a presynaptic α 1 interaction.

Somatic motor axons can innervate autonomic neurones in the frog heart.

1. The effects of sympathetic and parasympathetic stimulation on the heart rate in frogs were tested after hearts were reinnervated with a somatic motor nerve. When frogs were vagotomized and hypoglossal axons were redirected to the heart for 8 or more weeks, stimulating the redirected hypoglossus nerve produced a parasympathetic-like inhibition of the heart. Stimulating sympathetic rami of the anastomosed hypoglossus nerve produced cardiac acceleration.2. Individual parasympathetic neurones received synaptic input from hypoglossal terminals. The excitatory post-synaptic potentials evoked by hypoglossal stimulation were much smaller than those evoked by vagal stimulation in control or vagal-reinnervated ganglia. However, hypoglossal axons innervated most (71%) of the ganglion cells and this level of innervation persisted for at least 60 weeks.3. Hypoglossal axons formed networks of varicose terminals within cardiac ganglia and established axo-axonic synapses with parasympathetic neurones. Hypoglossal terminals did not reinnervate the neuronal perikarya, in contrast to vagal axons in control or vagal-reinnervated ganglia.4. Axo-axonic synapses from redirected hypoglossal axons were identified in cardiac ganglia by bathing isolated hearts in horseradish peroxidase (HRP) and stimulating the redirected nerve. Electron micrographs showed that axo-axonic synapses contained HRP-labelled presynaptic vesicles.5. The source of foreign innervation in experimental cardiac ganglia was confirmed to be hypoglossal motoneurones (a), by comparing the conduction velocity of the redirected presynaptic axons (1.32 m/sec) with regenerating vagal preganglionic fibres (< 0.3 m/sec), and (b), by retrograde HRP-labelling of large motoneurones in the hypoglossal nucleus after applying peroxidase to the axons which had grown into the heart.

Multiple Molecular Forms of Acetylcholinesterase in Rat Vagus Nerve, Smooth Muscle, and Heart

Extracts of rat tissues were subjected to ultracentrifugation on linear density gradients of sucrose (5-20%) and fractions of these gradients were analyzed for acetylcholinesterase (AChE) activity. Most of the AChE activity in extracts of vagus nerve was found to sediment at 10S. However, 16S AChE, previously thought to be confined to somatic motor nerves and skeletal muscle, was observed to accumulate rapidly at a nerve ligation. A search for this rapidly sedimenting form of AChE was carried out in tissues receiving vagal innervation. Significant amounts of 16S AChE were detected in the heart, especially in the right atrium, and in several regions of the gut. It was concluded that 16S AChE is distributed more widely than has previously been recognized.

Observations on neuromuscular connection between the vagus nerve and skeletal muscle

The reinnervation of rat skeletal muscle fibres by the vagus nerve was studied. Vagal fibres in the rat can establish connections with regenerating muscle fibres and those muscle fibres supplied by the vagus become multiply innervated. The vagus nerve made connections with a normally innervated sternohyoid muscle when this muscle was rendered chemosensitive. Several endplates were formed by the vagus nerve on the same muscle fibre, and the structure of these differed from the endplates formed by the muscle's own nerve. If the muscle's own nerve and the vagus were cut and resutured into the sternohyoid muscle of the rat, many muscle fibres become innervated by their own nerve and by the vagus. Evidence is presented to show that each nerve established contacts at a separate site along the same muscle fibre. These connections were permanent, for even after 2 years following the operation the muscle was supplied by both nerves. Thus, in these experiments the muscle's own nerve did not ‘suppress’ the contacts made by an alien nerve. In rabbits the vagus and the accessory nerve were sutured into the sternomastoid muscle. Only a small proportion of the muscle became reinnervated by the vagus, whereas the accessory nerve reinnervated most of the muscle. Very few muscle fibres had become innervated by both nerves. The sternohyoid and sternomastoid are fast contracting muscles. When their fibres were reinnervated by the vagus nerve, their speed of contraction became significantly slower than those reinnervated by the original somatic motor nerve. The vagus nerve therefore determines the properties of the muscle fibres that it supplies. It is suggested that it is the slow tonic pattern of activity of the vagus nerves that determines the contractile speed of the muscles.

Organization of brainstem noradrenaline hyperinnervation following neonatal 6-hydroxydopamine treatment in rat.

The organization and origin of NA axonal sprouting in individual brainstem nuclei was examined by fluorescence histochemistry and a radioenzymatic assay for noradrenaline (NA) in adult rats which were administered 6-hydroxydopamine (6-OHDA) as neonates. Significant 2–6 fold increases in NA content were measured in primary sensory nuclei, associational nuclei, somatic motor and visceral cranial nerve nuclei, reticular formation, raphe nuclei, and the inferior olivary complex. Fluorescence histochemical analysis reveals a major increase in the number of fluorescent fibers in most areas of the brainstem after 6-OHDA treatment. The normal pattern of innervation and axon morphology of each nucleus is retained after drug treatment. Bilateral locus coeruleus (LC) lesions in treated animals results in the denervation of only those nuclei which normally receive LC innervation (Levitt and Moore, 1979). The increased number of NA-containing axons in nuclei innervated by the lateral tegmental cell groups (LT) remains intact after LC ablation. The present results indicate that the brainstem NA sprouting following neonatal 6-OHDA administration occurs in a highly specified manner. The brainstem projections of both the LC and LT neuron groups exhibit marked axonal growth which is restricted to their specific brainstem terminal fields and probably occurs in response to the denervation of their respective, more rostral target areas.

The physiology, pharmacology, and trophic effectiveness of synapses formed by autonomic preganglionic nerves on frog skeletal muscle.

1. Frog sartorius muscles, newly denervated and transplanted to the lymph sac of the back, are reinnervated by implanted cholinergic nerves (spinal somatic motor nerves or the preganglionic sympathetic splanchnic nerve), but not by nerves). 2. Foreign somatic motor nerves (s.m.n.s) form synapses that resemble normal sartorius neuromuscular junctions electrophysiologically. 3. Axons of the sympathetic preganglionic splanchnic nerve (s.p.n.) grow throughout the muscle, but only a small percentage of fibres form synapses. Most e.p.p.s are of low quantal content, generally subthreshold. Long onset latencies and multiple post-synaptic responses indicate that innervation is multiple, multi-terminal, and by unmyelinated axons. 4. Spontaneous miniature e.p.p.s at splanchnic junctions occur at an average rate under 0.1/sec. Their average amplitude and time course are about the same as for control muscles, but the variability of amplitudes is greater than for control muscles. 5. The amount of facilitation shown by s.p.n.-evoked e.p.p.s is the same as by s.m.n. e.p.p.s, but the time course is almost twice as long. 6. S.p.n.-reinnervated fibres show dramatic post-tetanic potentiation preceded by depression, following as few as 20--50 stimuli. 7. As judged by standard physiological and histochemical criteria, AChEsterase is absent at s.p.n. junctions. 8. The pharmacological responses of the s.p.n. junctions are similar to those of normal or foreign s.m.n. innervated neuromuscular junctions in their sensitivity to the cholinergic blocking agents D-tubocurarine and hexamethonium. 9 The s.p.n. is capable of restricting ACh sensitivity to the sites of nerve contacts, although this restriction occurs more slowly and less completely than with s.m.n. reinnervation. The loss of extrajunctional ACh sensitivity can be correlated with effectiveness of innervation; but significant restriction occurs even in s.p.n. reinnervated fibres that probably never contract to nerve stimulation.

Competitive interaction between foreign nerves innervating frog skeletal muscle.

1. Competition between two foreign nerves innervating frog skeletal muscle has been studied by using pairs of somatic motor nerves (s.m.n.s) or one s.m.n. and the preganglionic splanchnic nerve (s.p.n.) implanted into a denervated sartorius muscle that has been transplanted to the lymph sac of the back. 2. A single s.m.n. implanted into the muscle succeeded in innervating essentially every fibre within 2--3 months; tetanic stimulation of the nerve elicited 9--100% of the maximal direct tetanus tension. Most of the e.p.p.s were suprathreshold, since a single indirect stimulus evoked a twitch 60--100% as large as that to a direct stimulus. 3. If two s.m.n.s were implanted simultaneously, tetanic stimulation of either elicited 80--100% of the maximal tension to direct stimulation. If one nerve was implanted 2--3 months before the other, the second, although usually less effective than the first, normally innervated 50--100% of the fibres, with approximately the same time course of innervation as a single s.m.n. 4. Mutual synaptic repression was seen on examination of twitch tensions. With either simultaneous or staggered innervation, stimulation of each s.m.n. resulted in a twitch of 30--50% of the total direct twitch tension, with little overlap between the fields driven by the two nerves. Intracellular recordings showed that the distribution of subthreshold and spike-producing e.p.p.s reflected the existence of separate twitch fields. Even if one s.m.n. was implanted several months before the other and had time to establish suprathreshold junctions on most muscle fibres, an s.m.n. implanted later was able to reduce sharply the effectiveness of many junctions from the earlier nerve while itself innervating most muscle fibres. 5. The subthreshold e.p.p.s had low quantal content, typically ten or fewer quanta/e.p.p. The min e.p.p. frequency was very low, while min e.p.p. amplitude appeared to be normal. 6. In the vast majority of muscle fibres, junctions from the two nerves were not within recording distance of each other. Hence, we infer that the competitive interaction was mediated somehow via the muscle fibre. 7. The preganglionic splanchnic nerve, which also successfully reinnervated frog skeletal muscle, competed with a foreign s.m.n. in ways which differ qualitatively from the competition by a second s.m.n. In the presence of a s.m.n., synapses of the s.p.n. were almost universally subthreshold. However, if the s.p.n. was implanted 2--3 months before the s.m.n., the s.m.n. was prevented for several months from innervating fibres driven by the s.p.n. This delay in s.m.n. reinnervation was greater than if the first nerve implanted was also an s.m.n. 8. After 6--8 months of dual innervation by s.m.n. and s.p.n., the s.m.n. became almost totally dominant. However, if the s.m.n. was then sectioned, the s.p.n. became as effective, within approximately 1 week, as it would have been in the absence of the s.m.n.

Hemifacial spasm and the facial nucleus.

Spontaneous and associated hyperkinetic facial movements and contracture which follow injury to the seventh cranial nerve (postparalytic hemifacial spasm) or arise without known previous injury (cryptogenic hemifacial spasm) are pathological motor phenomena not found in the distribution of other cranial or somatic motor nerves. The commonly expressed hypotheses of pathogenesis--aberrant regeneration and fiber excitation by false synapse formation (ephapses) at the site of injury--cannot account for all aspects of these phenomena or for the uniqueness of such movements to the distribution of the seventh nerve. The suggestion is made that the existing diversity of facial motor behavior, which encompasses voluntary, emotional, and especially automatic, associated, and reflexive movements, is based on a unique central organization that sets it apart from other motor groups. I hypothesize that because of this organization, the changes following axonal injury--which include selective deafferentation, glial response, axonal sprouting, functional reconnection, and hyperexcitability from dendritic spike generation--can unmask and augment automatic, associated, and reflexive movements already present in the facial neuronal network to result in facial hyperkinesia.

Electron-microscopic analysis of the mouse facial nerve near the geniculate ganglion.

An electron-microscopic analysis of the mouse facial nerve near the geniculate ganglion shows that there are, on the everage, 603 more nerve fibers in the portion of the nerve distal to the geniculate ganglion than there are in the part proximal to the ganglion. The average distal increase in the number of unmyelinated fibers is 444 and that in the myelinated fibers is 165. The somatic motor nerve fibers and the parasympathetic fibers in the mouse facial nerve may not contribute to the distal excess. It is possible that the increase in the number of unmyelinated fibers distal to the geniculate ganglion is mainly due to the presence of postganglionic sympathetic fibers in the facial trunk distal to the geniculate ganglion and the greater petrosal nerve. The distal increase in the number of myelinated fibers may be mainly contributed by the sensory fibers.

Interaction between foreign and original nerves innervating gill muscles in fish.

Foreign somatic motor nerves were made to innervate denervated gill muscles of the perch. The foreign innervation remained functional after reinnervation by the original nerve. Intracellular recordings showed that individual muscle fibers could be activated by both foreign and original neurons. Extensive foreign innervation appeared to inhibit the ability of the original nerve to reinnervate its own muscle. These studies provide evidence for a functional coexistence of appropriate and inappropriate innervation and show that foreign synapses need not be repressed by the "correct" nerve.

Slow rhythms of sympathetic discharge induced by convulsant drugs.

We have previously reported (Can. J. Physiol. Pharmacol. 1969. 47, 815–826) that sustained oscillations of blood pressure, similar to Mayer waves, can be produced by the drug picrotoxin in anesthetized cats. We have investigated further the properties of these oscillations by recording the electrical activity of sympathetic preganglionic, somatic motor, and phrenic nerves. We found that the sympathetic preganglionic and somatic motor nerves show a periodic increase of electrical activity in synchrony with the oscillations of blood pressure. The phrenic nerve shows changes in repetition rate and duration of its bursting activity, which are synchronous with the blood pressure waves (i.e. phrenic bursts are more frequent and longer lasting on the ascending than on the descending limb of the blood pressure waves). The synchronous oscillation of these three output systems is not due to proprioceptive feedback because it is present when the animals are paralyzed, ventilated artificially, and even when the blood pressure is prevented from oscillating by ganglionic block. We conclude that the observed rhythmic pattern of neural activity is generated in the central nervous system.

Nervus tenuis, a Hitherto Unknown Cranial Nerve of the Fourth Metamere

AbstractIn hynobiid urodele embryos there are three serially arranged muscles associated with the otic and occipital regions of the endocranium. These muscles are continuous posteriorly with the succession of trunk myomeres and most likely represent myotomic derivatives attributable to the third, fourth, and fifth metameres of the body. From front to back these muscles are designated: anterior basiotic, posterior basiotic, and occipital. A hitherto unknown cranial nerve is directed towards the posterior basiotic muscle in embryos of Hynobius retardatus. For this new somatic motor nerve the term nervus tenuis is proposed. Together with the glossopharyngeal nerve it constitutes the fourth cranionerval segment.

Differentiation of the actions of tetrodotoxin and saxitoxin

While tetrodotoxin and saxitoxin are very similar in their actions, they can be differentiated biologically by the observation that newts of the genus Taricha are resistant to tetrodotoxin but not to saxitoxin. The resistance of these newts is due to a remarkable insensitivity of the somatic motor nerves to tetrodotoxin. The isolated desheathed sciatic nerve of Taricha torosa is at least 30,000 times more resistant than frog nerve. As seen in electronmicroscopic sections, there is no special enveloping structure around taricha nerves that can be considered as a particularly effective diffusion barrier. The isolated, desheathed brachial nerve of the Atlantic puffer fish, Spheroides maculatus, is about 1000 times more resistant to tetrodotoxin than frog nerve. Both taricha and tetrodon nerves are readily blocked by saxitoxin. Both nerves require external sodium ions for activity since replacement of Na + with choline, tris (hydroxymethyl) aminomethane or dimethyl-diethanol ammonium ion reduced the spike amplitudes. The resistance of these nerves to tetrodotoxin is interpreted as being a result of peculiarities in membrane structures that prevent an effective association with tetrodotoxin.

The diameters of the fibres of the peripheral nerves of Octopus

The diameters and numbers of fibres have been measured throughout the peripheral nervous system. The nerves of the muscles that act upon the outside world contain few fibres, having very large and medium-sized fibres but no very small ones. Thus the muscles of the head receive 6000 fibres, the largest of 30 μ m diameter. The eye muscles receive 3300 fibres, reaching 24 μ m. The stellar nerve fibres are more numerous (150 000), but smaller (&lt; 20 μ m). The preganglionic fibres joining the c. n. s. to the stellate ganglia are fewer than the postganglionic ones (4000, &lt; 16 μ m). In some of the somatic motor nerves the longest bundles contain the largest fibres. However these are accompanied by a distinct group of smaller fibres, whose significance is uncertain. It is not clear that there is a distinct proprioceptor group. The fibres to the chromatophores are numerous (30000) and of medium diameter (&lt; 12 μ m). The visceral motor and vasomotor nerves, however, contain hundreds of thousands of minute fibres (&lt; 3 μ m). The significance of these numerous small fibres can hardly be to obtain great resolution of movement in such acts as secretion of saliva. Presumably the great number has a special significance. The fibres to the muscles of the buccal mass are more numerous and smaller than the somatic motor fibres, but fewer and larger than those for the viscera. The muscles of the arms and suckers have 3.0 x 10 6 fibres, all &lt; 6 μ m, originating from neurons within the arms. They are controlled from the brain by relatively few but large fibres (32000, &lt; 26 μ m). There is also a reduction of 100 times on the afferent pathway of the arms, from some 18 x 10 6 receptors at the periphery of the suckers to 140000 fibres entering the brain. The brain thus serves for major decisions, whose detailed execution is left to peripheral reflex centres in the arms. The optic nerve fibres are very numerous, and all small (20 x 10 6 , &lt; 1.3 μ m) presumably for economy of space and material. By contrast the static nerves, although they are short, contain a small number of large fibres (1400 between 6 and 22 μ m) as well as several thousand smaller ones. The presence of sets of fibres with their distinctive diameters, conduction velocities and other properties is evidently a fundamental feature of the design of the nervous system of cephalopods as it is of vertebrates, although the significance of many features remains to be explored.