Showing papers on "Motor neuron published in 1984"

Motoneuron death after sciatic nerve section in newborn rats.

Abstract: When the sciatic nerve was cut in newborn rats almost all of the motoneurons were subsequently lost. The adult sciatic nerve contained 7,640 myelinated fibers of which 1,650 were motor axons. The ventral roots of the sciatic nerve (L4-L6) contained in all 4,080 myelinated fibres; only 2,380 were present 5–30 weeks after the sciatic nerve had been cut at birth. The deficit of 1,700 fibres corresponded to the number of motor axons in the sciatic nerve. The motoneuron pools were depleted and practically no labelled or unlabelled perikarya were after found after HRP had been applied to the nerve proximal to the former lesion. The deficit of ventral root fibres was 1,030 when the nerve had been cut at age 1 week; nerve section 4 weeks after birth was not followed by retrograde loss of motor axons. Nerve crush at birth caused a deficit of 1,230 ventral root fibres. The incomplete loss of motor axons after crushing was due to the fact that not all axons had been severed. The continuity of the nerves cut at birth was always restored and myelinated axons which probably were dorsal root ganglion afferents grew into the atrophic muscles. Roughly two-thirds of the myelinated sensory fibres degenerated after axotomy at birth. The denervated muscle fibres were not reinnervated but vanished eventually and were replaced by fat cells.

Mutations altering synaptic connectivity between identified neurons in Drosophila

Abstract: By studying the effects of mutations on a simple circuit of identified neurons in Drosophila, we have found genes whose proper functioning is necessary to produce normal synaptic connections between the neurons. These neurons comprise the giant fiber (GF) system; the GFs are command neurons activated by a light-off stimulus and evoke a stereotyped pattern of activity in the thoracic muscles producing an escape jump. Each GF monosynaptically drives a motor neuron innervating the tergotrochanteral muscle (jump muscle, TTM). Each GF also disynaptically drives the motor neurons innervating the dorsal longitudinal flight muscle (DLM) via the peripherally synapsing interneuron (PSI) (King, D. G., and R. J. Wyman (1980) J. Neurocytol. 9: 753–770; M. A. Tanouye and R. J. Wyman (1980) J. Neurophysiol. 44: 405–421). A search was made for mutations affecting these identified synapses. Fifty thousand mutagenized flies were screened for nonjumping behavior to the light-off stimulus. Fifty-seven nonjumping mutant lines were established from individuals selected in the screen. Members of the lines were then tested for abnormal GF motor output to the TTM and DLM. From these lines, four X-linked mutations (representing three complementation groups) were isolated which affect the circuit. The mutations differentially disrupt specific synapses within the GF system. One mutation, bendless, disrupts synaptic transmission between the GF and the TTM motor neuron. Another, gfA, disrupts the synaptic connections of the PSI, and a third mutation, passover, disrupts transmission in both pathways.

Recruitment order of motoneurons in stretch reflexes is highly correlated with their axonal conduction velocity

Abstract: Motor units of soleus and medial gastrocnemius (MG) muscles were studied in pairs during stretch reflexes in the decerebrate cat to determine the relation between their recruitment orders and axonal conduction velocities. In 97% of soleus pairs, the motor unit with the lower axonal conduction velocity was recruited first. Since the soleus is a homogeneous muscle in the cat, differences in motor-unit type are, therefore, not a sine qua non for orderly recruitment nor is recruitment random within homogeneous populations of motor units, as recently proposed (28). In the medial gastrocnemius, a heterogeneous muscle, the same high correlation (97%) between recruitment sequence and conduction velocity was observed. Thus, the factors that determine recruitment order in heterogeneous muscles are as closely correlated with axonal diameter as they are in homogeneous muscles. Comparison of axonal conduction velocities in our sample of MG units with those in three samples of type-identified MG units studied by other investigators also suggests that motor-unit type is not the critical factor controlling the sequence of activation in heterogeneous muscles. It is concluded that the combined effects of all presynaptic and postsynaptic factors that determine susceptibility to discharge in motoneurons during stretch reflexes are strictly correlated with their axonal conduction velocities, as predicted by the size principle.

Transmitter identification of pyloric neurons: electrically coupled neurons use different transmitters

Abstract: The neurotransmitters mediating the synaptic interactions among the neurons of the pyloric system of the stomatogastric ganglion (STG) of the lobster, Panulirus interruptus, were examined using a combination of electrophysiological, pharmacological, and biochemical techniques. Iontophoretically applied L-glutamate inhibited all motor neurons of the pyloric system. This inhibitory response was blocked by low concentrations of picrotoxin but unaffected by atropine. The anterior burster (AB) interneuron, pyloric dilator (PD) motor neurons, and ventricular dilator (VD) motor neuron were depolarized and excited by iontophoretically applied acetylcholine (ACh). The lateral pyloric (LP) and pyloric (PY) constrictor motor neurons were inhibited by ACh and by the cholinergic agonist, carbachol. These inhibitory cholinergic responses were blocked by atropine but not by picrotoxin. The inhibitory postsynaptic potentials (IPSPs) evoked by the constrictor motor neurons were blocked by picrotoxin but not by atropine. Taken together with previously published data (15, 18), this suggests that the constrictor motor neurons release glutamate at both their excitatory neuromuscular junctions and their inhibitory intraganglionic junctions. The lucifer yellow photoinactivation technique (27) was used to study separately the neurotransmitters released by the electrically coupled PD and AB neurons. The AB-evoked IPSPs were blocked by picrotoxin but not by atropine. The PD-evoked IPSPs were blocked by atropine and other muscarinic antagonists but not by picrotoxin. Somata of PD neurons contained choline acetyltransferase (CAT) activity, but somata of AB neurons contained no detectable CAT activity. On the basis of the data in this paper and previously published data (17, 18), we conclude that the PD neurons release ACh at both their excitatory neuromuscular junctions and their inhibitory intraganglionic connections. Although the AB neuron is electrically coupled to the PD neurons, the AB neuron is not cholinergic. Glutamate is a likely transmitter candidate for the AB neuron. These data show that electrically coupled neurons can release different transmitters. Furthermore, these data show that an IPSP can be the result of the combined actions of two different neurotransmitters, each released from a different neuron. The functional consequences of these conclusions are explored in the following papers (9, 22).

Steroid regulation of neuronal death in the moth nervous system.

Abstract: The emergence of the adult Manduca sexta moth is followed by the programmed degeneration of about 50% of the insect's abdominal interneurons and motoneurons. Neurons in implanted ganglia undergo degeneration in concert with neurons of the host, thereby indicating that a hormonal cue is important. The neuronal death follows the normal decline in the levels of the steroid hormones, ecdysteroids, that occurs at the end of metamorphosis. Manipulations that cause a precocious ecdysteroid decline result in an early death of the neurons. Also, prolongation of the ecdysteroid titer extend the life of the neurons. By properly timed steroid application it was possible to dissociate the neuronal degeneration from the behavioral events and muscle degeneration that also occur at emergence. Also the stereotyped sequence of death among identified motoneurons could be stopped at various points in the sequence by the carefully timed application of steroid. It was concluded that the signal for neuronal death was the withdrawal of ecdysteroids at the end of metamorphosis and that the death was probably a direct response of the individual neurons to the hormone withdrawal.

The Pattern of Monosynaptic Ia-Connections to Hindlimb Motor Nuclei in the Baboon: A Comparison with the Cat

Abstract: The pattern of Ia-connections to motor nuclei of 17 hindlimb muscles (or groups of muscles) has been investigated in baboons by intracellular recording of Ia-e.ps.p.s evoked in motoneurons from different muscle nerves. The amplitudes are normalized to 70 m V resting potential and compared with similarly normalized Ia-e.ps.p.s in cats. As in the cat, Ia-excitation is drawn from a restricted number of muscles and the homonymous effect is usually dominating. Heteronymous connections to many motor nuclei are different in the two species. For example, hip extensors are generally more Ia-isolated from each other in baboons than in cats, and also knee flexors have fewer Ia-interconnections than in cats. A unidirectional Ia-synergism between some hip extensors and knee flexors in cats has changed to a bidirectional one in baboons, with a tendency to lateralization of the connections. Among ankle extensors, soleus has smaller heteronymous Ia-connections from its synergic ankle extensors than in cats. In baboons, plantaris is heteronymously Ia-excited from gastrocnemius-soleus but not from the intrinsic plantar muscles; whereas in casts there exists a considerable Ia-projection from the intrinsic plantar muscles but not from gastrocnemius-soleus. There is a corresponding difference in the insertion of the plantaris tendon, which shows that this muscle acts as toe extensor in cats but as ankle extensor in baboons. For most of the motor nuclei, the homonymous as well as the total aggregate of Ia-e.ps.p.s is smaller in the baboon than in the cat; but the amplitude range between different motor nuclei is larger in the baboon. Reciprocal Ia-i.ps.p.s are evoked only after spinal transection or when brain function is depressed. It is postulated that baboons, contrary to cats, have descending tonic inhibition of transmission in the reciprocal Ia-inhibitory pathway. The phylogenetic flexibility of Ia-connections is discussed and contrasted with their ontogenetic stability.

Monosynaptic inhibition of phrenic motoneurons: a long descending projection from Bötzinger neurons.

Abstract: Synaptic connections between medullary expiratory neurons and phrenic motoneurons were studied in anesthetized cats using spike-triggered averaging of synaptic noise in phrenic motoneurons. Monosynaptic inhibition of phrenic motoneurons originating from the rostral medullary expiratory neurons of the Botzinger complex was revealed. Neither mono- nor disynaptic inhibition from expiratory neurons of the nucleus retroambigualis was found. It was concluded that Botzinger complex expiratory neurons make long descending inhibitory connections with phrenic motoneurons: the expiratory neurons of the nucleus retroambigualis do not appear to make synaptic connections with phrenic motoneurons.

A mechanism for production of phase shifts in a pattern generator.

Abstract: During motor activity of the pyloric system of the lobster stomatogastric ganglion, there are rhythmic alternations between activity in the pyloric dilator (PD) and pyloric (PY) motor neurons. We studied the phase relations between PD motor neuron activity and PY motor neuron activity in preparations cycling at a wide range of frequencies and after altering the activity of the PD neurons. The PY neurons fall into two classes, early (PE) and late (PL) (21), distinguished by the different phases in the pyloric cycle at which they fire. The phase at which PE neurons fired and the phase at which PL neurons fired was independent of pyloric cycle frequency over a range of frequencies from 0.5 to 2.25 Hz. The anterior burster (AB) interneuron is electrically coupled to the PD motor neurons. Together the AB and PD neurons form the pacemaker for the pyloric system. Synchronous depolarization of the AB and PD neurons evokes a complex inhibitory post-synaptic potential (IPSP) in PY neurons. This IPSP has two components: an early, AB neuron-derived component and a late, PD neuron-derived component. Deletion of the PD neurons from the pyloric circuit by photoinactivation removed the PD-evoked component of the pacemaker-evoked IPSP. This resulted in a decrease in the duration of the IPSP evoked by pacemaker depolarization and a significant advance in the firing phase of PY neurons. Bath application of dopamine was used to hyperpolarize and inhibit the PD neurons (30), causing them to release less neurotransmitter. As a consequence, the duration of the IPSP evoked by pacemaker depolarization was decreased and the firing phase of the PY neurons was significantly advanced. Stimulation of the inferior ventricular nerve (IVN) produces a slow excitation of the PD neurons (30), causing them to release more neurotransmitter. Consequently, the duration of the IPSP evoked by pacemaker depolarization was increased and the firing phase of the PY neurons was significantly retarded for several cycles of pyloric activity following IVN stimulation. Thus, modulation of the strength of PD-evoked inhibition in PY neurons is responsible for altering the firing phase of the PY neurons with respect to the pyloric pacemaker. We suggest that frequency of the pyloric output and the phase relations of the elements within the pyloric cycle can be regulated independently. The potential implications of these data for modulation of synaptic efficacy in other preparations are discussed.

Motoneuron death and motor unit size during embryonic development of the rat

Abstract: Chronic paralysis of rat embryos during the last 4 to 6 prenatal days causes a diminution in skeletal muscle fiber numbers but inhibits motoneuron death. Consequently, as paralyzed muscles develop, an increased number of motoneurons attempts to form synapses at a reduced number of synaptic sites. Paralyzed muscle fibers receive their synapses at a single endplate, as in control muscles, but these endplates are hyperinnervated, with about twice the normal number of inputs. Counts of axons, synaptic inputs, and muscle units showed that motoneurons normally contact a maximum number of muscle fibers shortly before birth, and this number remains stable for several days postnatal until it finally is reduced to the adult number. The average motor unit size in paralyzed embryos at the time of birth was the same as in controls. We suggest that it is not necessary to postulate the existence of competition between embryonic nerve terminals in order to explain regulation of the number of muscle fibers initially contacted by a motoneuron. Motoneuron death was not immediately affected by paralysis, but paralysis “rescued” all motoneurons whose death normally would have occurred 24 hr or more after the time when paralysis was initiated, regardless of when this was. This implies that the peak period for determination to die is during embryonic day 14, when myotube formation is just beginning and no recognizable endplate structures are present in muscles. When paralyzed, motoneurons normally destined to die are capable of forming a normal number of functional nerve-muscle contacts.

Anatomical basis of specific connections between sensory axons and motor neurons in the brachial spinal cord of the bullfrog.

Abstract: The anatomical basis for the specificity of the monosynaptic stretch reflex has been studied in the brachial spinal cord of bullfrogs. Sensory axons from the triceps brachii muscle innervate the corresponding triceps motoneurons but do not innervate two types of unrelated motoneurons (subscapularis and pectoralis) (Lichtman, J.W., and E. Frank (1984) J. Neurosci. 4: 1745-1753). Retrograde labeling of these three types of motoneurons with horseradish peroxidase (HRP) demonstrated that their cell bodies had overlapping distributions in the lateral motor column, and their dendrites all occupied the same region of the dorsal horn. In addition, triceps sensory axons aborized extensively in the dorsal horn throughout the brachial spinal cord, with no obvious predilection for the region of the triceps motoneurons. Thus, the physiological specificity of these sensory-motor connections was not apparent from the anatomical location of the sensory or motor neurons. However, by injecting single pairs of related or unrelated sensory and motor cells with HRP, we found that related pairs formed anatomical contacts with each other more frequently than unrelated sensory-motor pairs did. These observations suggest that the specificity of these connections is most likely the result of local interactions between sensory and motor processes.

Dementia and motor neuron disease: Morphometric, biochemical, and Golgi studies

Abstract: In three patients dementia without neurofibrillary tangles or Pick bodies antedated amyotrophy by several years. The motor neuron disorder in two patients was characterized by terminal bulbar symptoms; in one it was similar to classic amyotrophic lateral sclerosis. In two patients, quantitative studies of selected regions of the cortex using a computerized image analyzer disclosed, as in patients with senile dementia of Alzheimer type, a marked reduction in the number of neurons, especially those larger than 90 mu 2. The findings differed from those in Alzheimer dementia, however, in that the cells in the substantia innominata were not reduced and the levels of choline acetyltransferase and somatostatin-like immunoreactivity, determined in one patient, were within normal limits. A variable degree of sponginess of the upper layers of the cortex was attributed to attrition of pyramidal cell dendrites, observed in the one patient in whom Golgi study was successful. Because of severe degeneration of the substantia nigra in all three, the disease in these patients may represent a subset of motor neuron disease or a multisystem atrophy.

Absence of somatotopic projection of muscle afferents onto motoneurons of same muscle.

Abstract: During weak voluntary contractions, muscle afferents from a restricted region of the tibialis anterior of humans were activated with mechanical or electrical stimuli while electromyographic recordings (EMG) were made from pairs of motor units. One motor unit of a pair was located in the region of muscle from which the afferents arose; the other was at least 10 cm distal. The territories of the motor units of each pair did not overlap. All motor units were of low threshold, recruited at less than 5% of maximal voluntary strength. Direct recordings of muscle afferent activity, using a microelectrode in the nerve fascicle innervating the tibialis anterior, showed that taps delivered to the muscle belly activated submaximally a discrete population of receptors, restricted to the site of the taps. The spread of the disturbance set up by the taps or by vibration at 100 Hz applied to the muscle belly was measured with an accelerometer and was also found to be restricted to the site of application of the vibrator tip. With each of 12 pairs of motor units, vibration at 100 Hz was applied to the muscle near the motor unit of higher threshold for recruitment in a voluntary contraction. The vibration produced detectable reflex effects but did not alter recruitment order within any pair of motor units. During voluntary contractions involving 10 pairs of motor units, taps were applied to the muscle belly near each motor unit of a pair. The taps produced, at short latency, an increase in the probability of discharge of each motor unit. This change in probability is related to a composite excitatory postsynaptic potential (EPSP) caused by dynamically responding mechanoreceptors near the tap site and is generated through monosynaptic/oligosynaptic pathways. Taps near one motor unit of a pair did not selectively or preferentially affect the discharge of that motoneuron. During a voluntary contraction of tibialis anterior, electrical stimuli just below threshold for efferent axons, and so probably above threshold for afferent axons, were delivered singly and in brief trains through a microelectrode in a nerve fascicle innervating that muscle. Such stimulation had no detectable effect on the discharge pattern of four pairs of motor units. These findings suggest that the feedback from a few receptors in the muscle has a negligible effect on the motoneuron pool, in comparison with the total excitatory drive present during a voluntary contraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of individual Ia-afferent EPSPs in a homonymous motoneuron pool with respect to muscle topography

Abstract: The spike-triggered averaging technique (26) was used to determine whether the synaptic input from medial gastrocnemius (MG) Ia-afferent fibers to homonymous motoneurons is "topographically weighte...

Evidence for 5-hydroxytryptamine, substance P, and thyrotropin- releasing hormone in neurons innervating the phrenic motor nucleus

Abstract: Retrograde tracing with a fluorescent dye (Fast Blue) combined with immunohistochemistry was used to identify putative neurotransmitter(s) at the phrenic motor nucleus in the cat. Fast Blue was injected bilaterally into the diaphragm of five cats, where each phrenic nerve enters the muscle. Seven days later the animals were perfusion fixed and tissue sections from the fourth, fifth, and sixth cervical spinal cord segments were analyzed using a fluorescence microscope. Retrogradely labeled fluorescent phrenic motor neuron cell bodies appeared in all of the segments but primarily in sections from the fifth segment. The same or adjacent transverse sections were then used for the demonstration of the distribution of the neurotransmitters 5-hydroxytryptamine (5-HT), substance P, and thyrotropin-releasing hormone (TRH) in the area of the phrenic motor nucleus using the indirect immunofluorescence technique. The most conspicuous neurotransmitters found at the phrenic motor nucleus were 5-HT and substance P. We observed dense and diffuse fiber networks throughout the ventral horn which contains the phrenic motor nucleus. These fibers contained varicosities in close proximity to phrenic motor neurons. In addition to 5-HT- and substance P-containing nerve endings, some fibers containing TRH were also found in the area of the phrenic motor nucleus. These results are consistent with earlier physiological data suggesting that 5-HT, substance P, and TRH are important neurotransmitters and/or neuromodulators involved in central control of respiration.

The ultrastructure and synaptic architecture of phrenic motor neurons in the spinal cord of the adult rat.

Abstract: Although light microscopic studies have analysed phrenic motor neurons in several different species, there has never been an ultrastructural investigation of identified phrenic motor neurons. In addition, electrophysiological studies have raised questions relating to the function of phrenic motor neurons which may be answered only by direct electron microscopic investigation. Thus, the present study was carried out to provide a detailed ultrastructural analysis of identified phrenic motor neurons. Phrenic motor neurons in the spinal cord of the rat were labelled by retrogradely transported horseradish peroxidase (HRP) after transecting the phrenic nerve in the neck and applying the enzyme directly to the central stump of the transected nerve. The results showed that the general ultrastructural characteristics of phrenic motor neurons were similar to those previously reported for other spinal motor neurons. However, phrenic primary dendrites appeared to be isolated from all other dendritic profiles in the neuropil. Primary dendrites were not fasciculated. Fasciculation occurred only among the more distal secondary and tertiary phrenic dendritic branches. Direct dendrodendritic or dendrosomatic apposition was rarely seen; gap junctions between directly apposing phrenic neuronal membranes were not observed. The membranes of adjacent phrenic neuronal profiles were most frequently separated by intervening sheaths of astroglial processes. Myelinated phrenic axons and a phrenic axon collateral were identified. The initial portion of the phrenic axon collateral was cone-shaped, lacked myelin, and thus resembled a miniature axon hillock. In one instance, a large accumulation of polyribosomes was observed within the hillock-like structure of a phrenic axon collateral. Eight morphological types of synaptic boutons, M, P, NFs, S, NFf, F, G and C were classified according to criteria used by previous investigators. Most of these endings (M, NFs, NFf, S and F) made synaptic contact with profiles of labelled phrenic somata and dendrites. F, NFf, and S boutons also terminated on phrenic axon hillocks. C and G boutons contacted exclusively phrenic somata and small calibre dendrites, respectively. P boutons established axo-axonic synaptic contacts with the M and NFs bouton. The morphological findings of the present study provide new data that may be related to phrenic synchronized output and presynaptic inhibition of primary afferents terminating on phrenic motor neurons.

The facial nucleus of the rat: representation of facial muscles revealed by retrograde transport of horseradish peroxidase.

Abstract: The representation of facial muscle groups in the facial nucleus of rat was examined by retrograde transport of HRP. Motoneurons supplying muscle groups are arranged in longitudinal columns. Those supplying nasolabial muscles are located in the lateral and ventral intermediate segments, posterior auricular muscles in a medial column, platysma in an intermediate column; the lower lip and ocular muscles are in the ventral and dorsal segments respectively of the intermediate column. The posterior belly of the digastric muscle is supplied by motoneurons extending from the dorsal aspect of the facial nucleus to the caudal pole of the trigeminal motor nucleus.

Neuropeptide proctolin in postural motoneurons of the crayfish.

Abstract: The neuropeptide transmitter candidate proctolin (H-Arg-Tyr-Leu-Pro-Thr-OH) was associated with three of the five excitatory motoneurons innervating the tonic flexor muscles of the crayfish abdomen. Proctolin immunohistochemical staining occurred in cell bodies and axons of these three identified neurons. Stained axon terminals were detected across the entire tonic flexor muscle. Bioassay of extracts of the tonic flexor muscles indicated the presence of 370 fmol of proctolin/muscle or 670 fmol/mg dry weight. Bioactivity was eliminated in muscles in which the tonic flexor motor root was cut 2 months prior to extraction and in muscle extracts pre-incubated with proctolin antiserum. High pressure liquid chromatography purification of tissue extract indicated that all bioactivity in the crude extract was due to authentic proctolin. Our findings suggest that these three cells function as peptidergic motoneurons. A precedent for this is the proctolin-containing postural motoneuron of the cockroach.

The development of the cervical spinal cord of the mouse embryo. II. A Golgi analysis of sensory, commissural, and association cell differentiation

Abstract: The present report describes for the first time the early differentiation of dorsal root ganglion cells and internuncial cells in the spinal cord of mammals using the rapid Golgi technique; embryos of 9-11 days gestation (E9-E11, where E0 = day vaginal plug observed) were used. On E9 dorsal root ganglion cells are relatively undifferentiated and are just beginning to take on a bipolar appearance. A few commissural neurons are differentiating in the alar and basal plates, but their axons have not yet crossed the ventral commissure. Occasionally an ipsilateral funicular or association cell is seen in the alar plate. By E10 bipolar dorsal root ganglion cells send processes both centrally into the spinal cord and peripherally to join the ventral root as the spinal nerve. The relatively few association neurons are intermingled among commissural cells except that they are not found in the region of the ventral horn. The axon of many commissural neurons crosses the ventral commissure. On E11 a thin lateral funiculus is seen as well as the oval bundle of His (presumptive dorsal funiculus). An occasional dorsal root ganglion cell has attained the unipolar stage of differentiation. The commissural and association neurons are still intermingled along the outer regions of the mantle layer, especially in the alar plate region. Both commissural and association cells start to differentiate within the ventricular layer as radially oriented, bipolar neuroblasts. As the cell somas reach the outer region of the ventricular layer the cells become unipolar. The cells reorient more dorsoventrally and a primary dendrite grows first from the pole opposite the axon. This sequence of cell differentiation is in general agreement with what is seen in the differentiation of ventral root motoneurons.

Sensory, interneuronal, and motor interactions within Hermissenda visual pathway

Abstract: The visual pathway of Hermissenda was identified by means of intracellular recordings and iontophoretic injection of the fluorescent dye lucifer yellow. This pathway consisted of five neuron types, namely, type B photoreceptors and the medial type A photoreceptor within each of the two eyes, hair cells in the two statocysts, a group of interneurons in the cerebropleural ganglia, and a putative motor neuron (MN1) in each pedal ganglion. The MN1 cells responded during illumination of the eye with increased impulse and excitatory postsynaptic potential (EPSP) activity. This response was often followed by bursting activity for higher light intensities. The medial type A photoreceptor, which was found to be inhibited by medial and intermediate type B photoreceptors, was demonstrated to excite the MN1 cell indirectly via a group of identified interneurons. Hair cells were also found to excite the MN1 cell indirectly via these interneurons. Among the ipsilateral hair cells, cephalic hair cells were least frequently found to excite the MN1 cell. Among the contralateral hair cells, on the other hand, lateral hair cells were most often found to excite the MN1 cell. Interneurons that were shown to excite the MN1 cell received excitatory input from the medial type A photoreceptor and hair cells. Our observations are consistent with the interpretation that these interactions are mediated by monosynaptic chemical synapses. Electrical stimulation of the MN1 cell with positive-current injection produced turning of the posterior half of the animal's foot to the ipsilateral direction consistent with the animal's turning behavior toward light. The visual pathway identified in this experiment was considered to have some significance in explaining, at least in part, a causal role for changes within type B photoreceptors in producing Hermissenda's modified behavior following associative conditioning.

Central nervous sensitization and dishabituation of reflex action in an insect by the neuromodulator octopamine.

Abstract: Habituation of excitatory synaptic inputs onto identified motor neurons of the locust metathoracic ganglion, driven electrically and by natural stimuli, was examined using intracellular recording. Rapid progressive reduction in amplitude of EPSPs from a variety of inputs onto fast-type motor neurons occurred. The habituated EPSPs were quickly dishabituated by iontophoretic release of octopamine from a microelectrode into the neuropilar region of presumed synaptic action. The zone within which release was effective for a given neuron was narrowly-defined. With larger amounts of octopamine applied at a sensitive site the EPSP became larger than normal, and in many instances action potentials were initiated by the sensitized response. Very small EPSPs onto a motor neuron, which were associated with proprioceptive feedback, and which were originally too small to be detected above the noise, were potentiated to a level of several mV by the iontophoresed octopamine. A DUM neuron (presumed to be octopaminergic) was found, whose direct stimulation was followed by a strong dishabituating and sensitizing action leading to spikes, of inputs to an identified flexor tibiae motor neuron. The action and its time course were closely similar to those evoked by octopamine iontophoresed into the neuropil in the region of synaptic inputs to the motor neuron. It is concluded that DUM (octopaminergic) neurons exert large potentiating actions on central neuronal excitatory synaptic transmission in locusts.

Prevention of natural motoneurone cell death by dibutyryl cyclic GMP.

Abstract: Natural neuronal cell death is a well-described developmental phenomenon common to many nerve centres in a variety of animal species1–8. Neuronal survival has been shown to depend on the presence9–11 and size of the available target tissue12,13 and it has been suggested that neuronal survival is dependent on successful competition for either a limited number of synaptic sites1,14 or a limited amount of trophic factor(s)9,15. In the lateral motor column of the lumbar spinal cord in the chick embryo, the period of axon elongation and innervation of the periphery has been shown to precede that of natural motoneurone cell death16,17. While muscle contractile activity appears to regulate the extent of motoneurone death18, to date the intracellular molecular events that initiate and regulate the developmental process of natural neuronal cell death or, more importantly, neuronal survival are unknown. Our earlier studies suggested that either contact or association between spinal cord processes and muscle cells during neuromuscular junction formation in vivo leads to an increase in cyclic GMP in whole spinal cord19. We now show that treatment of chick embryos with the membrane-permeable cyclic GMP analogue, dibutyryl cyclic GMP during the period of natural motoneurone cell death prevents >58% of natural motoneurone cell death in the lumbar lateral motor column.

Retrograde axoplasmic transport of Adriamycin An experimental form of motor neuron disease

Abstract: Adriamycin (ADM) is a DNA-directed RNA inhibitor. In attempts to produce an experimental form of motor neuron disease, we injected the agent into rat sciatic nerve. Retrograde axoplasmic flow conveyed ADM into soma of the spinal motor neurons, as confirmed by fluorescence microscopy. Motor neuron degeneration, which included nuclear heterochromatinization and diffuse chromatolysis, was observed after 6 to 8 days. After 2 weeks, many neurons that gave rise to sciatic nerve efferents underwent dissolution. Retrograde axoplasmic flow and DNA-injurious substances could affect survival of motor neurons.

Development of the topographical projection of motor neurons to a rat muscle accompanies loss of polyneuronal innervation

Abstract: The rat lateral gastrocnemius muscle receives a topographical projection from lumbar segmental nerves L4 and L5. A study has been made of the development of this projection during the period when polyneuronal innervation is eliminated. The tetanic contraction due to stimulation of each nerve was compared with that due to stimulation of both nerves simultaneously. This percentage of contraction declined from about 90% to 70% for L4 from birth to 2 postnatal weeks; it declined from about 90% to 30% for L5 over the same period. The innervation of about 60% of the cells by both L4 and L5 is therefore eliminated during 2 postnatal weeks. Tetanic and twitch contraction due to stimulation of ventral rootlets of segmental level L4 and L5 was compared to stimulation of the whole muscle directly. There was a general reduction in the size of the motor units from both L4 and L5. A selective reduction in the number of L5 motor units was observed during the first 5 postnatal days. The distribution of L4 and L5 terminals on the dorsal surface of the muscle was determined by intracellular impalement of muscle cells and was determined by recording endplate potentials (EPPs) due to stimulation of L4 and L5. Polyneuronal innervation of the lateral gastrocnemius is eliminated by about 2 postnatal weeks. The percentage of muscle cells innervated by L4 or L5 in each of six equal-size muscle sectors was ascertained during this period. This percentage of innervation of muscle cells by L5 declined in all sectors between 3 days and 2 weeks postnatal.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative study of neck muscle motor neurons in a cricket and a locust

Abstract: The gross morphology of the neck muscles of a cricket (Gryllus campestris) and their innervation are described and compared with a locust (Schistocerca gregaria). The motor neurons innervating the neck muscles were stained in crickets and locusts with cobalt chloride introduced via the nerve endings in the muscle. The two species show overall similarities, not only in position of the neck motor neurons in suboesophageal, prothoracic, and mesothoracic ganglia but also in motor neuron morphology. However, muscle 60 in the cricket is innervated by a unique motor neuron with its axon in prothoracic nerve 3, instead of sharing motor neurons in suboesophageal nerve 8 and mesothoracic nerve 1 with muscle 59, as in locust. Muscle 62 has the same attachments and innervation with similar motor neurons in cricket and locust but a different mechanical function in the two species. The findings are discussed with respect to possible segmental homologies and to the origins of the muscles as either dorso-ventral or longitudinal. As several muscles share the same motor neurons, we suggest that neck muscle function be described in terms of "behavioural units of action."

Rate modification in the heartbeat central pattern generator of the medicinal leech

Abstract: 1. Intracellular recordings from neurons in partially dissected leeches and isolated nerve cords were used to study the effects of temperature, sensory stimulation and locomotory activity on the output of the central pattern generator that drives heartbeat. 2. The rate of motor burst production by the heartbeat oscillator changes on gradual heating or cooling with a Q10 averaging around 2.4. Abrupt cooling of isolated nerve cords can induce additional changes in heart rate that are associated with ‘paradoxical’ firing of motor neurons. 3. Heart rate was accelerated when mechanical stimuli were applied to the body wall of partially dissected preparations or when restrained preparations spontaneously initiated movements. For instance, marked acceleration of heart rate occurred when preparations exhibited body movements and motor neuron activity characteristic of swimming in intact animals. Activation of the motor program for swimming in isolated nerve cords led to acceleration of heartbeat oscillator cycling. Accelerations of heart rate associated with swimming are therefore mediated at least partially through central interactions. 4. Individual identified neurons were tested for their influence on the cycle rate of the heartbeat oscillator in isolated nerve cords. Activity of individual mechanosensory neurons was found to accelerate heart rate. Touch mechanosensory neurons were also found to influence the heartbeat oscillator through a rapidly adapting inhibitory pathway. Activity of individual motorneurons generally did not affect heart rate. Some interneurons and neuroeffector cells known to cause motor activation (e.g. swim initiating interneurons, serotonergic Retzius cells) can also produce acceleration of heart rate.

The development of the cervical spinal cord of the mouse embryo. I. A Golgi analysis of ventral root neuron differentiation.

Abstract: No previous Golgi studies have described early neuron differentiation in mammals near the time of neural tube closure. By leaving embryos in utero and varying the impregnation times, the cells of the cervical spinal cord of mice of 8-11 days of gestation (E8-E11, where E0 = day vaginal plug observed) were stained and analyzed in the present study. The early stages of ventral root motoneuron differentiation described earlier (Wentworth and Hinds, '78), using serial section electron microscopy, have been confirmed in this study and include preaxonic neuroblasts (stage 1 - detached from the ventrical but still attached to the basal lamina), bipolar neuroblasts (stage 2), and unipolar neuroblasts (stage 3). Later stages of differentiation described in this study include young neurons with dendrites: secondary bipolar (stage 4) and multipolar neurons (stage 6). At E8 the neural tube is just closing and consists of undifferentiated ventricular cells. On E9 a few ventral root neuroblasts are differentiating in the cervical spinal cord. The majority of these cells are in the bipolar and unipolar neuroblast stages, although the most advanced cells observed had reached the secondary bipolar neuron stage of differentiation. By E10 a large cluster of ventral root motoneurons is differentiating in the ventrolateral cord. Some cells have reached the bipolar stage, a few cells are becoming multipolar, and a subdivision into dorsolateral and ventromedial subgroups is beginning. By E11 the medial and lateral subdivisions are more obvious, and the most advanced cells are well-differentiated multipolar neurons oriented either mediolaterally or dorsoventrally, although cells in the earlier unipolar and secondary bipolar stages are still observed.

The organization of motoneurons in the turtle lumbar spinal cord

Abstract: The distribution of motoneurons in the lumbosacral spinal cord of the turtle Pseudemys scripta elegans was studied by using the technique of retrograde transport of horseradish peroxidase. A total of 19 different hindlimb muscles were injected with varying amounts of horseradish peroxidase. The resulting distribution of labeled motoneurons was studied in both longitudinal and transverse sections of spinal cord. Motoneurons innervating a particular hindlimb muscle are clustered in longitudinally arranged motorpools. Motorpools of different muscles can show considerable overlap in both the rostrocaudal and transverse planes. The distribution of the various motorpools demonstrates a somatotopic organization of motoneurons within the lumbar spinal cord. Motoneurons innervating more distally positioned muscles are generally found in the more caudal segments, while motoneurons supplying proximal muscles are distributed throughout almost the whole lumbosacral intumescence. Motoneurons innervating anterodorsally positioned muscles are found in the ventrolateral part of area IX in the ventral horn, while more dorsomedially positioned motoneurons innervate the posteroventral muscles. These features are consistent with observations in other tetrapods, although the somatotopic representation of motoneurons is more evident in higher vertebrates such as chicken and cat. The observed motorpool distribution is discussed in relation to the presumed ontogeny of the spinal cord and hindlimb muscles and also in relation to the functions of the investigated muscles.