Neuronal analysis of pharyngeal peristalsis in the gastropod Navanax in terms of identified motoneurons innervating identified muscle bands. II. Radial and circumferential motor fields

The neuronal basis of pharyngeal ingestion and peristalsis was studied in the gastropod Navanax inermis. Radially and circumferentially oriented muscles produce expansion and constriction of the pharynx. Motor fields of 11 identified radial motoneurons and 13 identified circumferential motoneurons were determined with respect to circumferential and longitudinal muscle band coordinates by muscle movements, electromyography, antidromic stimulation and axonal anatomy. Activation of these identified motoneurons can account for all the elemental pharyngeal movements observed during feeding. Four motoneurons, each innervating most of radial muscle, can mediate ingestion. Three radial motoneurons with anterior motor fields can mediate anterior expansion during sealing of the pharyngeal lips around prey and during regurgitation. Ten circumferential motoneurons have small arciform motor fields, the distributions of which correspond to the regional specializations in circumferential band organization. Arciform constriction can center eccentric ingested prey within the pharyngeal lumen during peristalsis. Arciform constrictions could combine to form an annular constriction in peristalsis. Small, non-overlapping, circumferential motor fields maximize the number of independent annular units available to produce a fine peristaltic wave. Sphincters have more circumferential motoneurons with smaller motor fields; this innervation permits finer motor control. Radial motoneurons with posterior motor fields can produce expansion caudal to a circumferential constriction during peristalsis. Motor fields of regional radial motoneurons show greater interanimal variability than circumferential motor fields, which is correlated with a less essential role of radial motoneurons in peristalsis. Two circumferential motoneurons with giant posterior pharyngeal motor fields can mediate pharyngeal emptying either in swallowing or in regurgitation.     

Neuronal analysis of pharyngeal peristalsis in the gastropod Navanax in terms of identified motoneurons innervating identified muscle bands. I. Muscle band identifiability

The neuronal basis of pharyngeal ingestion and swallowing can be conveniently studied in the marine mollusc Navanax inermis because of the small number of neurons involved in the behavior, reproducible identifiability of many individual neurons and simple muscle arrangement. The Navanax pharynx contains 3 approximately orthogonal muscle groups, circumferential, longitudinal and radial, arranged in well defined layers. Pharyngeal peristalsis involves sequential circumferential constriction, apparently with coordinated local pharyngeal expansion produced by radial muscle. Circumferential and longitudinal muscles consist of discrete, well defined bands. The number, position and arrangement of circumferential and longitudinal bands show little interanimal variability; these bands are individually identified by sequential number. Regional morphologic specializations of circumferential bands presumably facilitate peristalsis. Circumferential and longitudinal bands form a two dimensional reference system defining position on the pharyngeal surface. In the accompanying paper circumferential motor fields are described in terms of identified motoneurons innervating identified bands, and radial motor fields are located with respect to the coordinate system of overlying longitudinal and circumferential bands.     

Synaptic organization of expansion motoneurons ofNavanax inermis

The opisthobranch mollusc, Navanax, feeds by rapid pharyngeal expansion that sucks in prey followed by peristaltic swallowing that moves prey into the esophagus. Several identifiable neurons on the ventral surface of the buccal ganglia control radial musculature within the pharyngeal wall, contraction of which leads to pharyngeal expansion. These are considered expansion motoneurons because their axons run into the muscle and twitches and EMGs occur one for one with action potentials. The motoneurons are electrotonically coupled. Electrotonic PSPs, the components of spread associated with impulses, can summate with subthreshold DC depolarizations to yield synchronous impulses in coupled cells. During a train of responses the later electrotonic PSPs can be facilitated because of increase in amplitude and duration of the presynaptic impulses. Expansion motoneurons are synaptically connected by two apparently interneuronal pathways: a low threshold pathway activated by subthreshold depolarization of the two largest expansion motoneurons (the G-cells) that inhibits the entire population, and a high threshold pathway that is activated by a train of G-cell impulses and produces largely excitatory PSPs in the smaller expansion motoneurons and an EPSP--IPSP sequence in the G-cells. Coupling among expansion motoneurons can be abolished by chemical inhibitory synaptic inputs that are activated by electrical stimulation of the pharyngeal nerve or tactile stimulation of the pharyngeal wall. This uncoupling phenomenon can be explained by a simple equivalent circuit in which inhibitory synapses along the coupling pathway short circuit electrotonic spread. Uncoupling can outlast the evoking stimulus by several seconds. During uncoupling the smaller expansion motoneurones can fire independently while the G-cell is inhibited, and impulses still propagate from somata to the periphery. The expansion motoneuron population receives excitatory input from the mechanoreceptors in protractor muscles. Mechanical stimulation of the pharyngeal wall activates primary sensory neurons in the buccal ganglia that fire during excitation and during inhibition and uncoupling of expansion motoneurons.     

Classically conditioned alterations in single motor unit activity in the spinal cat

Muscle tension and single motor unit EMG recordings from a flexor muscle of acute spinal cats were obtained during presentation of classical conditioning and control paradigms. Conditioned increases in muscle tension were similar to previously obtained results. Motor unit recordings suggested that this conditioned reflex facilitation is brought about by an increased probability firing of motoneurons initially responsive to the conditioned stimulus, as well as orderly, size-dependent recruitment of initially non-responsive motoneurons. These results indicate that the same physiological mechanisms for grading motor output may apply for conditioned responses as have been demonstrated for reflexive and voluntary movements.     

Synaptic connections of buccal mechanosensory neurons in the opisthobranch mollusc, Navanax inermis

Mechanical stimulation of various areas of the pharyngeal wall and lips can produce EPSPs and IPSPs, as well as abruptly rising impulses, in primary sensory cells. IPSP fields are generally larger than EPSP fields and these fields are distributed without obvious order around fields from which afferent spikes are evoked. Apparently monosynaptic excitatory and inhibitory contacts are formed between primary sensory neurons. These synapses are blocked by high Mg2+ indicating chemical transmission. IPSPs are inverted by Cl- injection. Excitatory inputs can be electrically far from the soma. Sensory cells form apparently monosynsptic excitatory or inhibitory contacts on motoneurons mediating pharyngeal expansion. Brief sensory excitation can initiate sustained firing within this neuronal population and sustained synaptic activity in motoneurons. Interactions of sensory neurons may be important in information processing and in generating motor paterns. These neurons serve both primary sensory and interneuronal functions.     

Extraocular muscle motor units characterized by spike-triggered averaging in alert monkey

Single-unit recording in macaque monkeys has been widely used to study extraocular motoneuron behavior during eye movements. However, primate extraocular motor units have only been studied using electrical stimulation in anesthetized animals. To study motor units in alert, behaving macaques, we combined chronic muscle force transducer (MFT) and single-unit extracellular motoneuron recordings. During steady fixation with low motoneuron firing rates, we used motoneuron spike-triggered averaging of MFT signals (STA-MFT) to extract individual motor unit twitches, thereby characterizing each motor unit in terms of twitch force and dynamics. It is then possible, as in conventional studies, to determine motoneuron activity during eye movements, but now with knowledge of underlying motor unit characteristics.We demonstrate the STA-MFT technique for medial rectus motor units. Recordings from 33 medial rectus motoneurons in three animals identified 20 motor units, which had peak twitch tensions of 0.5-5.25 mg, initial twitch delays averaging 2.4 ms, and time to peak contraction averaging 9.3 ms. These twitch tensions are consistent with those reported in unanesthetized rabbits, and with estimates of the total number of medial rectus motoneurons and twitch tension generated by whole-nerve stimulation in monkey, but are substantially lower than those reported for lateral rectus motor units in anesthetized squirrel monkey. Motor units were recruited in order of twitch tension magnitude with stronger motor units reaching threshold further in the muscle's ON-direction, showing that, as in other skeletal muscles, medial rectus motor units are recruited according to the “size principle”.     

Intracellular recording in trigeminal motoneurons of the anesthetized guinea pig during rhythmic jaw movements

Intracellular recordings were obtained from guinea pig trigeminal motoneurons during rhythmic jaw movements. The cells were identified by stimulation of the trigeminal mesencephalic nucleus which evokes excitatory postsynaptic potentials and spikes in jaw-closer motoneurons. The anesthetized guinea pig in the stereotaxic apparatus demonstrated spontaneous rhythmic jaw movements which were characterized by hyperpolarization of jaw-closer motoneurons, occurring concurrently with digastric muscle excitation during the jaw-opening phase of the cycle. The anesthetized guinea pig could also be induced to rhythmically clench and release a stick placed between the molar teeth. During this behavior intracellular recordings in jaw-closer motoneurons revealed a rapid depolarization leading to bursts of action potentials following the hyperpolarization which occurred during the jaw-opening phase of the cycle. The results demonstrate the feasibility of intracellular recording in guinea pig trigeminal motoneurons during rhythmic jaw movements. It was also shown that during the opening phase of the rhythmic jaw movement cycle there is a pronounced hyperpolarization present in the membrane potential of jaw-closer motoneurons. Resolution of the problem of central vs. peripheral origin of this hyperpolarization is significant for our understanding of the motor control of the jaw.     

The spinal motor system in early vertebrates and some of its evolutionary changes.

Recent studies of the spinal motor systems of vertebrates allow us to begin to infer the organization of the motor apparatus of primitive vertebrates. This paper attempts to define some of the features of the motor system of early vertebrates based on studies of the motor systems in anamniotes and in Branchiostoma. It also deals with some changes in the primitive motor system during evolution. The primitive motor system consisted of myomeric axial muscles, with a functional subdivision of the musculature into non-spiking slow muscle fibers segregated in the myomeres from spiking fast ones. These fibers were innervated by two major classes of motoneurons in the cord-large motoneurons innervating faster fibers and small motoneurons innervating slow fibers. There was not a simple isomorphic mapping of the position of motoneurons in the motor column onto the location of the muscle fibers they innervated in the myomeres. Early vertebrates used these axial muscles to bend the body, and the different types of muscle fibers and motoneurons reflect the ability to produce slow swimming movements as well as very rapid bending associated with fast swimming or escapes. The premotor network producing bending was most likely a circuit composed of a class of descending interneurons (DIs) that provided excitation of ipsilateral motoneurons and other interneurons, and inhibitory commissural interneurons (CIs) that blocked contralateral activity and played an important role in generating the rhythmic alternating bending during swimming. This DI/CI network was retained in living anamniotes. At least two major descending systems linked the sensory systems in the head to these premotor networks in the spinal cord. The ability to turn on swimming by activation of DI/CI premotor networks in the cord resided at least in part in a midbrain locomotor region (MLR) that influenced spinal networks via projections to the reticular formation. Reticulospinal neurons were important not only for initiation of rhythmic swimming but also in the production of turning movements. The reticulospinal cells involved in turns produced their effects in part via monosynaptic connections with motor neurons and premotor interneurons, including some involved in rhythmic swimming. A prominent and powerful Mauthner cell was most likely present and important for rapid escape or startle movements. Some features of this primitive motor apparatus were conserved during the evolution of vertebrate motor systems, and others changed substantially. Many features of the early motor system were retained in living anamniotes; major changes occur among amniotes.(ABSTRACT TRUNCATED AT 400 WORDS)     

Development of a polar morphology by identified embryonic motoneurons

Motoneuron morphology arises through the coordinated growth of the motor axon and dendrites. In the Drosophila embryo the RP motoneurons have a contralaterally-extended motor axon, ipsilateral dendrites that extend a short distance in the ipsilateral connective, and a tuft of short dendrites in the contralateral connective. In the present study mechanical and genetic manipulations were utilized to test if (i) the ipsilateral dendrites can develop an axon morphology, (ii) the presence of the contralateral motor axon suppresses the development of an axon-like morphology by the ipsilateral dendrites and (iii) whether establishment of a contralateral motor axon can be genetically suppressed. It was found that an ipsilateral motor axon could develop-but only at the expense of the contralateral motor axon. Axotomy could overturn the normal polarity of the RP motoneurons in favor of the development of an ipsilateral motor axon, and this reversed morphology was also observed when the motor axon could not extend across the midline in the commissureless mutant. These findings show that the RP motoneurons have the plasticity for an alternative polarity, but that the extension of an ipsilateral axon is normally suppressed by the presence of the contralateral axon. The RP motoneurons now represent a genetically amenable in vivo system for analyzing the basis of polarity formation in neurons.     

Location of motor pools innervating chick wing.

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

Physiological properties of isolated motor units in normal and bilaterally innervated Xenopus gastrocnemius muscles.

Physiological properties of isolated gastrocnemius motor units were measured in normal juvenile postmetamorphic Xenopus frogs and in a group of juvenile animals with a single bilaterally innervated hindlimb. The aim of this study was to evaluate changes to the organisation of the motor unit when the muscle is hyperinnervated. Animals with a single bilaterally innervated hindlimb have previously been shown to support up to twice the normal number of motoneurons projecting into a single hindlimb. Under these circumstances there was a lowering of average neuromuscular efficacy (as judged by motor unit twitch/tetanus ratio) in comparison with normal age-matched siblings. Motor units with neurons in the lateral motor column contralateral to the remaining hindlimb were indistinguishable from those originating ipsilaterally. There is a wide range of safety margins for neuromuscular transmission at the various terminals of individual frog motor units, and comparison of motor unit contraction times with twitch/tetanus ratios showed that under pressure of hyperinnervation, motoneurons tend to retain their safest terminals on muscle fibres with fast contraction times.     

Identification of stapedius muscle motoneurons in squirrel monkey and bush baby

The location of brainstem neurons which mediate the stapedius reflex was identified by injecting horseradish peroxidase into the stapedius muscle of squirrel monkeys and bush babies. Retrogradely labeled neurons, arranged in a one- to three-cell column, were found medial to the main facial motor nucleus in squirrel monkeys and ventral to it in bush babies. Nissl, protargol, and acetylcholinesterase stains were subsequently used to identify and describe this unique column of cells. It was found that staining characteristics, as well as shape, size, and location, distinguish stapedius muscle motoneurons from closely associated cell groups. Furthermore, stapedius muscle motoneurons are morphologically similar to periolivary cells and morphologically dissimilar to cells within the facial motor nucleus.     

The organization of the motoneurons innervating the axial musculature of vertebrates. II. Florida water snakes (Nerodia fasciata pictiventris)

The motor pools of axial muscles in Florida water snakes (Nerodia fasciata pictiventris) were studied by applying horseradish peroxidase (HRP) to branches of spinal nerves innervating individual muscles or groups of muscles. Motor pools of different muscles or muscle groups were located in characteristic positions in both the transverse and the longitudinal extent of the motor column. Epaxial pools were located ventromedially in the column, segregated from most hypaxial ones, which were dorsolateral. The only exception to this general rule was the motoneurons innervating the levator costae muscle. Some of the motoneurons innervating this hypaxial muscle were located in the ventral part of the motor column, like epaxial motoneurons, but they were segregated longitudinally from epaxial ones. The arrangement of the motor pools was strikingly similar to the motor pools of presumptive homologous muscles in rats (Smith and Hollyday: J. Comp. Neurol. 220:29-43, '83), even though the locomotor mechanics in the two animals are very different. The similarities may reflect a comparable relationship between the location of motoneurons in the motor column and the location, in embryonic life, of the muscles they innervate. They also suggest that differences in the locomotor mechanics in the two species are accomplished without any dramatic reorganization of the medial motor column, in marked contrast to the substantial reorganization necessary to account for differences in the motor columns of amniotes and anamniotes.     

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

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

Time course of preferential motor unit loss in the SOD1 G93A mouse model of amyotrophic lateral sclerosis

Electromyographical analyses of pre-symptomatic motor unit loss in the SOD1 G93A transgenic mouse model of amyotrophic lateral sclerosis (ALS) have yielded contradictory findings as to the onset and time course. We recorded hindlimb muscle and motor unit isometric forces to determine motor unit number and size throughout the life span of the mice. Motor unit numbers in fast-twitch tibialis anterior, extensor digitorum longus and medial gastrocnemius muscles declined from 40 days of age, 50 days before reported overt symptoms and motoneuron loss. Motor unit numbers fell after overt symptoms in the slow-twitch soleus muscle. Muscle forces declined in parallel with motor unit numbers, indicating little or no functional compensation by sprouting. Early muscle-specific decline was due to selective preferential vulnerability of large, fast motor units, innervated by large motoneurons. Large motoneurons are hence the most vulnerable in ALS with die-back occurring prior to overt symptoms. We conclude that size of motoneurons, their axons, and their motor unit size are important determinants of motoneuron susceptibility in ALS.     

An identified motoneuron with variable fates in embryonic zebrafish

Every trunk hemisegment of the zebrafish is innervated by 3 identified primary motoneurons whose development can be observed directly in living embryos. In this paper, we describe another identified neuron that is part of this system. Unlike the other primary motoneurons which are present in all trunk hemisegments, this cell is present in slightly less than half of the trunk hemisegments. Additionally, this cell has at least 2 different fates: it may become a primary motoneuron and arborize in an exclusive muscle territory, or it may die during embryonic development. We have named this cell VaP, for variable primary. We show that the presence of VaP does not affect the early development of the other primary motoneurons in the same hemisegment. Moreover, we show that ablation of both VaP and caudal primary does not alter pathfinding by another identified primary motoneuron.     

The control of eye movements by the cerebellar nuclei: polysynaptic projections from the fastigial,interpositus posterior and dentate nuclei to lateral rectus motoneurons in primates

Premotor circuits driving extraocular motoneurons and downstream motor outputs of cerebellar nuclei are well known. However, there is, as yet, no unequivocal account of cerebellar output pathways controlling eye movements in primates. Using retrograde transneuronal transfer of rabies virus from the lateral rectus (LR) eye muscle, we studied polysynaptic pathways to LR motoneurons in primates. Injections were placed either into the central or distal muscle portion, to identify innervation differences of LR motoneurons supplying singly innervated (SIFs) or multiply innervated muscle fibers (MIFs). We found that SIF motoneurons receive major cerebellar ‘output channels’ bilaterally, while oligosynaptic cerebellar innervation of MIF motoneurons is negligible and/or more indirect. Inputs originate from the fastigial nuclei di‐ and trisynaptically, and from a circumscribed rostral portion of the ventrolateral interpositus posterior and from the caudal pole of the dentate nuclei trisynaptically. While disynaptic cerebellar inputs to LR motoneurons stem exclusively from the caudal fastigial region involved in saccades, pursuit and convergence (via its projections to brainstem oculomotor populations), minor trisynaptic inputs from the rostral fastigial nucleus, which contributes to gaze shifts, may reflect access to vestibular and reticular eye‐head control pathways. Trisynaptic inputs to LR motoneurons from the rostral ventrolateral interpositus posterior, involved in divergence (far‐response), is likely mediated by projections to the supraoculomotor area, contributing to LR motoneuron activation during divergence. Trisynaptic inputs to LR motoneurons from the caudal dentate, which also innervates disynaptically the frontal and parietal eye fields, can be explained by its superior colliculus projections, and likely target saccade‐related burst neurons.     

Serotonergic modulation of the feeding behavior of the medicinal leech

Hungry medicinal leeches, Hirudo medicinalis, bite warm surfaces and ingest blood meals averaging 890% of their weight. Satiation lasts 12–18 months during which leeches avoid warm surfaces and will not bite. The segmental nervous system of the leech is distinguished by a population of neurons which contain serotonin (5-Hydroxytryptamine, 5-HT) at high concentrations. Some of these identified 5-HT neurons directly activate the effectors responsible for three physiological components of feeding: salivary secretion, bite-like movements and pharyngeal peristalsis. A localized warming of the lip is sufficient to initiate ingestion and synaptically excites anterior 5-HT cells into high frequency impulses or bursts. Distension of the body wall terminates ingestion and also hyperpolarizes these 5-HT neurons. Serotonin treatment produces hyperphagic behavior by the leech, while a specific pharmacological lesion of its 5-HT cells produces the anorexic behavior of satiation. This anorexia is transiently reversed by 5-HT treatment. Serotonin plays an obligatory role in the initiation and expression of leech feeding behavior by its differential modulation of central neuronal networks and peripheral glands and muscles.     

Motor excitability in a patient with a somatosensory cortex lesion.

OBJECTIVE: We report a patient with an ischemic lesion in right somatosensory cortex who developed dystonic posturing and pseudo-athetotic involuntary left-sided finger movements during voluntary muscle contractions. METHODS: Motor excitability was assessed using transcranial magnetic stimulation techniques and electrical peripheral nerve stimulation. Results obtained from abductor digiti minimi muscles of both hands were compared. RESULTS: On the affected side, silent period duration and intracortical inhibition were reduced, indicating a loss of inhibitory properties. Intracortical facilitation was enhanced. Stimulus-response curves showed a smaller increase of motor evoked potential amplitudes when recorded during muscle relaxation, but not during voluntary muscle activation. CONCLUSIONS: The results suggest that, under normal conditions, somatosensory cortex modifies inhibitory as well as excitatory properties in the motor system.