Direct excitatory interactions between spinal motoneurones of the cat

Spatial and Temporal Arrangement of Recurrent Inhibition in the Primate Upper Limb

Renshaw cells mediate recurrent inhibition between motoneurons within the spinal cord. The function of this circuit is not clear; we previously suggested based on computational modeling that it may cancel oscillations in muscle activity around 10 Hz, thereby reducing physiological tremor. Such tremor is especially problematic for dexterous hand movements, yet knowledge of recurrent inhibitory function is sparse for the control of the primate upper limb, where no direct measurements have been made to date. In this study, we made intracellular penetrations into 89 motoneurons in the cervical enlargement of four terminally anesthetized female macaque monkeys, and recorded recurrent IPSPs in response to antidromic stimulation of motor axons. Recurrent inhibition was strongest to motoneurons innervating shoulder muscles and elbow extensors, weak to wrist and digit extensors, and almost absent to the intrinsic muscles of the hand. Recurrent inhibitory connections often spanned joints, for example from motoneurons innervating wrist and digit muscles to those controlling the shoulder and elbow. Wrist and digit flexor motoneurons sometimes inhibited the corresponding extensors, and vice versa. This complex connectivity presumably reflects the flexible usage of the primate upper limb. Using trains of stimuli to motor nerves timed as a Poisson process and coherence analysis, we also examined the temporal properties of recurrent inhibition. The recurrent feedback loop effectively carried frequencies up to 100 Hz, with a coherence peak around 20 Hz. The coherence phase validated predictions from our previous computational model, supporting the idea that recurrent inhibition may function to reduce tremor.

SIGNIFICANCE STATEMENT We present the first direct measurements of recurrent inhibition in primate upper limb motoneurons, revealing that it is more flexibly organized than previous observations in cat. Recurrent inhibitory connections were relatively common between motoneurons controlling muscles that act at different joints, and between flexors and extensors. As in the cat, connections were minimal for motoneurons innervating the most distal intrinsic hand muscles. Empirical data are consistent with previous modeling: temporal properties of the recurrent inhibitory feedback loop are compatible with a role in reducing physiological tremor by suppressing oscillations around 10 Hz.

Fictive Scratching Patterns in Brain Cortex-Ablated, Midcollicular Decerebrate, and Spinal Cats

Background: The spinal cord’s central pattern generators (CPGs) have been explained by the symmetrical half-center hypothesis, the bursts generator, computational models, and more recently by connectome circuits. Asymmetrical models, at odds with the half-center paradigm, are composed of extensor and flexor CPG modules. Other models include not only flexor and extensor motoneurons but also motoneuron pools controlling biarticular muscles. It is unknown whether a preferred model can explain some particularities that fictive scratching (FS) in the cat presents. The first aim of this study was to investigate FS patterns considering the aiming and the rhythmic periods, and second, to examine the effects of serotonin (5HT) on and segmental inputs to FS.

Methods: The experiments were carried out first in brain cortex-ablated cats (BCAC), then spinalized (SC), and for the midcollicular (MCC) preparation. Subjects were immobilized and the peripheral nerves were used to elicit the Monosynaptic reflex (MR), to modify the scratching patterns and for electroneurogram recordings.

Results: In BCAC, FS was produced by pinna stimulation and, in some cases, by serotonin. The scratching aiming phase (AP) initiates with the activation of either flexor or extensor motoneurons. Serotonin application during the AP produced simultaneous extensor and flexor bursts. Furthermore, WAY 100635 (5HT1A antagonist) produced a brief burst in the tibialis anterior (TA) nerve, followed by a reduction in its electroneurogram (ENG), while the soleus ENG remained silent. In SC, rhythmic phase (RP) activity was recorded in the soleus motoneurons. Serotonin or WAY produced FS bouts. The electrical stimulation of Ia afferent fibers produced heteronymous MRes waxing and waning during the scratch cycle. In MCC, FS began with flexor activity. Electrical stimulation of either deep peroneus (DP) or superficial peroneus (SP) nerves increased the duration of the TA electroneurogram. Medial gastrocnemius (MG) stretching or MG nerve electrical stimulation produced a reduction in the TA electroneurogram and an initial MG extensor burst. MRes waxed and waned during the scratch cycle.

Conclusion: Descending pathways and segmental afferent fibers, as well as 5-HT and WAY, can change the FS pattern. To our understanding, the half-center hypothesis is the most suitable for explaining the AP in MCC.

On the occurrence and enigmatic functions of mixed (chemical plus electrical) synapses in the mammalian CNS

Electrical synapses with diverse configurations and functions occur at a variety of interneuronal appositions, thereby significantly expanding the physiological complexity of neuronal circuitry over that provided solely by chemical synapses. Gap junctions between apposed dendritic and somatic plasma membranes form "purely electrical" synapses that allow for electrical communication between coupled neurons. In addition, gap junctions at axon terminals synapsing on dendrites and somata allow for "mixed" (dual chemical+electrical) synaptic transmission. "Dual transmission" was first documented in the autonomic nervous system of birds, followed by its detection in the central nervous systems of fish, amphibia, and reptiles. Subsequently, mixed synapses have been detected in several locations in the mammalian CNS, where their properties and functional roles remain undetermined. Here, we review available evidence for the presence, complex structural composition, and emerging functional properties of mixed synapses in the mammalian CNS.

Recurrent excitation between motoneurones propagates across segments and is purely glutamatergic

Spinal motoneurones (Mns) constitute the final output for the execution of motor tasks. In addition to innervating muscles, Mns project excitatory collateral connections to Renshaw cells (RCs) and other Mns, but the latter have received little attention. We show that Mns receive strong synaptic input from other Mns throughout development and into maturity, with fast-type Mns systematically receiving greater recurrent excitation than slow-type Mns. Optical recordings show that activation of Mns in one spinal segment can propagate to adjacent segments even in the presence of intact recurrent inhibition. While it is known that transmission at the neuromuscular junction is purely cholinergic and RCs are excited through both acetylcholine and glutamate receptors, here we show that neurotransmission between Mns is purely glutamatergic, indicating that synaptic transmission systems are differentiated at different postsynaptic targets of Mns.

The Preparation of Oblique Spinal Cord Slices for Ventral Root Stimulation

Electrophysiological recordings from spinal cord slices have proven to be a valuable technique to investigate a wide range of questions, from cellular to network properties. We show how to prepare viable oblique slices of the spinal cord of young mice (P2 - P11). In this preparation, the motoneurons retain their axons coming out from the ventral roots of the spinal cord. Stimulation of these axons elicits back-propagating action potentials invading the motoneuron somas and exciting the motoneuron collaterals within the spinal cord. Recording of antidromic action potentials is an immediate, definitive and elegant way to characterize motoneuron identity, which surpasses other identification methods. Furthermore, stimulating the motoneuron collaterals is a simple and reliable way to excite the collateral targets of the motoneurons within the spinal cord, such as other motoneurons or Renshaw cells. In this protocol, we present antidromic recordings from the motoneuron somas as well as Renshaw cell excitation, resulting from ventral root stimulation.

Synaptic Connectivity between Renshaw Cells and Motoneurons in the Recurrent Inhibitory Circuit of the Spinal Cord

Renshaw cells represent a fundamental component of one of the first discovered neuronal circuits, but their function in motor control has not been established. They are the only central neurons that receive collateral projections from motor outputs, yet the efficacy of the excitatory synapses from single and converging motoneurons remains unknown. Here we present the results of dual whole-cell recordings from identified, synaptically connected Renshaw cell-motoneuron pairs in the mouse lumbar spinal cord. The responses from single Renshaw cells demonstrate that motoneuron synapses elicit large excitatory conductances with few or no failures. We show that the strong excitatory input from motoneurons results from a high probability of neurotransmitter release onto multiple postsynaptic contacts. Dual current-clamp recordings confirm that single motoneuron inputs were sufficient to depolarize the Renshaw cell beyond threshold for firing. Reciprocal connectivity was observed in approximately one-third of the paired recordings tested. Ventral root stimulation was used to evoke currents from Renshaw cells or motoneurons to characterize responses of single neurons to the activation of their corresponding presynaptic cell populations. Excitatory or inhibitory synaptic inputs in the recurrent inhibitory loop induced substantial effects on the excitability of respective postsynaptic cells. Quantal analysis estimates showed a large number of converging inputs from presynaptic motoneuron and Renshaw cell populations. The combination of considerable synaptic efficacy and extensive connectivity within the recurrent circuitry indicates a role of Renshaw cells in modulating motor outputs that may be considerably more important than has been previously supposed.

SIGNIFICANCE STATEMENT

We have recently shown that Renshaw cells mediate powerful shunt inhibition on motoneuron excitability. Here we complete a quantitative description of the recurrent circuit using recordings of excitatory synapses between identified motoneuron and Renshaw cell pairs. We show that the excitation is highly effective as a result of a high probability of neurotransmitter release onto multiple release sites and that efficient neurotransmission is maintained at physiologically relevant firing rates in motoneurons. Our results also show that both excitatory and inhibitory connections exhibit considerable convergence of inputs. Because evaluation of the determinants of synaptic strength and the extent of connectivity constitute fundamental parameters affecting the operation of the recurrent circuit, our findings are critical for informing any future models of motor control.

The recurrent case for the Renshaw cell

Although Renshaw cells (RCs) were discovered over half a century ago, their precise role in recurrent inhibition and ability to modulate motoneuron excitability have yet to be established. Indirect measurements of recurrent inhibition have suggested only a weak modulatory effect but are limited by the lack of observed motoneuron responses to inputs from single RCs. Here we present dual recordings between connected RC-motoneuron pairs, performed on mouse spinal cord. Motoneuron responses demonstrated that Renshaw synapses elicit large inhibitory conductances and show short-term potentiation. Anatomical reconstruction, combined with a novel method of quantal analysis, showed that the strong inhibitory input from RCs results from the large number of synaptic contacts that they make onto individual motoneurons. We used the NEURON simulation environment to construct realistic electrotonic models, which showed that inhibitory conductances from Renshaw inputs exert considerable shunting effects in motoneurons and reduce the frequency of spikes generated by excitatory inputs. This was confirmed experimentally by showing that excitation of a single RC or selective activation of the recurrent inhibitory pathway to generate equivalent inhibitory conductances both suppress motoneuron firing. We conclude that recurrent inhibition is remarkably effective, in that a single action potential from one RC is sufficient to silence a motoneuron. Although our results may differ from previous indirect observations, they underline a need for a reevaluation of the role that RCs perform in one of the first neuronal circuits to be discovered.

Abundance of gap junctions at glutamatergic mixed synapses in adult Mosquitofish spinal cord neurons

"Dye-coupling", whole-mount immunohistochemistry for gap junction channel protein connexin 35 (Cx35), and freeze-fracture replica immunogold labeling (FRIL) reveal an abundance of electrical synapses/gap junctions at glutamatergic mixed synapses in the 14th spinal segment that innervates the adult male gonopodium of Western Mosquitofish, Gambusia affinis (Mosquitofish). To study gap junctions' role in fast motor behavior, we used a minimally-invasive neural-tract-tracing technique to introduce gap junction-permeant or -impermeant dyes into deep muscles controlling the gonopodium of the adult male Mosquitofish, a teleost fish that rapidly transfers (complete in <20 mS) spermatozeugmata into the female reproductive tract. Dye-coupling in the 14th spinal segment controlling the gonopodium reveals coupling between motor neurons and a commissural primary ascending interneuron (CoPA IN) and shows that the 14th segment has an extensive and elaborate dendritic arbor and more gap junctions than do other segments. Whole-mount immunohistochemistry for Cx35 results confirm dye-coupling and show it occurs via gap junctions. Finally, FRIL shows that gap junctions are at mixed synapses and reveals that >50 of the 62 gap junctions at mixed synapses are in the 14th spinal segment. Our results support and extend studies showing gap junctions at mixed synapses in spinal cord segments involved in control of genital reflexes in rodents, and they suggest a link between mixed synapses and fast motor behavior. The findings provide a basis for studies of specific roles of spinal neurons in the generation/regulation of sex-specific behavior and for studies of gap junctions' role in regulating fast motor behavior. Finally, the CoPA IN provides a novel candidate neuron for future studies of gap junctions and neural control of fast motor behaviors.

Connexin36 identified at morphologically mixed chemical/electrical synapses on trigeminal motoneurons and at primary afferent terminals on spinal cord neurons in adult mouse and rat

Morphologically mixed chemical/electrical synapses at axon terminals, with the electrical component formed by gap junctions, is common in the CNS of lower vertebrates. In mammalian CNS, evidence for morphologically mixed synapses has been obtained in only a few locations. Here, we used immunofluorescence approaches to examine the localization of the neuronally expressed gap junction forming protein connexin36 (Cx36) in relation to the axon terminal marker vesicular glutamate transporter-1 (vglut1) in the spinal cord and the trigeminal motor nucleus (Mo5) of rat and mouse. In adult rodents, immunolabeling for Cx36 appeared exclusively as Cx36-puncta, and was widely distributed at all rostro-caudal levels in most spinal cord laminae and in the Mo5. A high proportion of Cx36-puncta was co-localized with vglut1, forming morphologically mixed synapses on motoneurons, in intermediate spinal cord lamina, and in regions of medial lamina VII, where vglut1-containing terminals associated with Cx36 converged on neurons adjacent to the central canal. Unilateral transection of lumbar dorsal roots reduced immunolabeling of both vglut1 and Cx36 in intermediate laminae and lamina IX. Further, vglut1-terminals displaying Cx36-puncta were contacted by terminals labeled for glutamic acid decarboxylase65, which is known to be contained in presynaptic terminals on large-diameter primary afferents. Developmentally, mixed synapses begin to emerge in the spinal cord only after the second to third postnatal week and thereafter increase to adult levels. Our findings demonstrate that axon terminals of primary afferent origin form morphologically mixed synapses containing Cx36 in broadly distributed areas of adult rodent spinal cord and Mo5.

Connexin36 in gap junctions forming electrical synapses between motoneurons in sexually dimorphic motor nuclei in spinal cord of rat and mouse

Pools of motoneurons in the lumbar spinal cord innervate the sexually dimorphic perineal musculature, and are themselves sexually dimorphic, showing differences in number and size between male and female rodents. In two of these pools, the dorsomedial nucleus (DMN) and the dorsolateral nucleus (DLN), dimorphic motoneurons are intermixed with non-dimorphic neurons innervating anal and external urethral sphincter muscles. As motoneurons in these nuclei are reportedly linked by gap junctions, we examined immunofluorescence labeling for the gap junction-forming protein connexin36 (Cx36) in male and female mice and rats. Fluorescent Cx36-labeled puncta occurred in distinctly greater amounts in the DMN and DLN of male rodents than in other spinal cord regions. These puncta were localized to motoneuron somata, proximal dendrites, and neuronal appositions, and were distributed either as isolated or large patches of puncta. In both rats and mice, Cx36-labeled puncta were associated with nearly all (> 94%) DMN and DLN motoneurons. The density of Cx36-labeled puncta increased dramatically from postnatal days 9 to 15, unlike the developmental decreases in these puncta observed in other central nervous system regions. In females, Cx36 labeling of puncta in the DLN was similar to that in males, but was sparse in the DMN. In enhanced green fluorescent protein (EGFP)-Cx36 transgenic mice, motoneurons in the DMN and DLN were intensely labeled for the EGFP reporter in males, but less so in females. The results indicate the presence of Cx36-containing gap junctions in the sexually dimorphic DMN and DLN of both male and female rodents, suggesting coupling of not only sexually dimorphic but also non-dimorphic motoneurons in these nuclei.

Requirement of neuronal connexin36 in pathways mediating presynaptic inhibition of primary afferents in functionally mature mouse spinal cord

Electrical synapses formed by gap junctions containing connexin36 (Cx36) promote synchronous activity of interneurones in many regions of mammalian brain; however, there is limited information on the role of electrical synapses in spinal neuronal networks. Here we show that Cx36 is widely distributed in the spinal cord and is involved in mechanisms that govern presynaptic inhibition of primary afferent terminals. Electrophysiological recordings were made in spinal cord preparations from 8- to 11-day-old wild-type and Cx36 knockout mice. Several features associated with presynaptic inhibition evoked by conditioning stimulation of low threshold hindlimb afferents were substantially compromised in Cx36 knockout mice. Dorsal root potentials (DRPs) evoked by low intensity stimulation of sensory afferents were reduced in amplitude by 79% and in duration by 67% in Cx36 knockouts. DRPs were similarly affected in wild-types by bath application of gap junction blockers. Consistent with presynaptic inhibition of group Ia muscle spindle afferent terminals on motoneurones described in adult cats, conditioning stimulation of an adjacent dorsal root evoked a long duration inhibition of monosynaptic reflexes recorded from the ventral root in wild-type mice, and this inhibition was antagonized by bicuculline. The same conditioning stimulation failed to inhibit monosynaptic reflexes in Cx36 knockout mice. Immunofluorescence labelling for Cx36 was found throughout the dorsal and ventral horns of the spinal cord of juvenile mice and persisted in mature animals. In deep dorsal horn laminae, where interneurones involved in presynaptic inhibition of large diameter muscle afferents are located, cells were extensively dye-coupled following intracellular neurobiotin injection. Coupled cells displayed Cx36-positive puncta along their processes. Our results indicate that gap junctions formed by Cx36 in spinal cord are required for maintenance of presynaptic inhibition, including the regulation of transmission from Ia muscle spindle afferents. In addition to a role in presynaptic inhibition in juvenile animals, the persistence of Cx36 expression among spinal neuronal populations in the adult mouse suggests that the contribution of electrical synapses to integrative processes in fully mature spinal cord may be as diverse as that found in other areas of the CNS.

Crossing dendrites of the hypoglossal motoneurons: possible morphological substrate of coordinated and synchronized tongue movements of the frog, Rana esculenta

Application of different fluorescent tracers to the right and left hypoglossal nerve of the frog revealed the extent of dendrites crossing the midline into the territory of contralateral hypoglossal motoneurons. By using confocal microscopy, a large number of close appositions were detected between hypoglossal motoneurons bilaterally, which formed dendrodendritic and dendrosomatic contacts. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. Application of neurobiotin to one hypoglossal nerve resulted in labeling of perikarya exclusively on the ipsilateral side of tracer application, suggesting the absence of dye-coupled connections with contralateral hypoglossal motoneurons. At the ultrastructural level, the dendrodendritic and dendrosomatic contacts did not show any morphological specialization; the long membrane appositions may provide electrotonic interactions between the neighboring profiles. We propose that dendrites of hypoglossal motoneurons that cross the midline subserve one of the morphological substrates of co-activation, synchronization and timing of bilateral activity of tongue muscles during prey-catching behavior of the frog.

Immunohistochemical detection of connexin36 in sympathetic preganglionic and somatic motoneurons in the adult rat

Gap junctional communication in the adult CNS plays an important role in the synchronization of neuronal activities. In vitro studies have shown evidence of electrotonic coupling through gap junctions between sympathetic preganglionic motoneurons and between somatic motoneurons in the neonatal and adult rat spinal cord. Electrotonic transmission of membrane oscillations might be an important mechanism for recruitment of neurons and result in the generation of rhythmic sympathetic and somato-motor activity at the population level. Gap junctions in the adult spinal cord are constituted principally by connexin36 (Cx36). However, the distribution of Cx36 in specific neuronal populations of the spinal cord is unknown. Here, we identify Cx36-like immunoreactivity in sympathetic preganglionic and somatic motoneurons in thoracic spinal cord segments of the adult rat. For this purpose, double immunostaining against Cx36 and choline acetyltransferase (ChAT) was performed on transverse sections (20 μm) taken from spinal segments T6–T8. Cx36 punctate immunostaining was detected in the majority of ChAT-immunoreactive (-ir) neurons from lamina VII [intermediolateral cell column (IML) and intercalated cell group (IC)], lamina X [central autonomic nucleus (CA)] and in ventral horn neurons from laminae VIII and IX. Cx36 puncta were distributed in the neuronal somata and along dendritic processes. The presence of Cx36 in ChAT-ir neurons is consistent with electrical coupling between sympathetic preganglionic motoneurons and between somatic motoneurons through gap junctions in the adult spinal cord.

Dendrodendritic and dendrosomatic contacts between oculomotor and trochlear motoneurons of the frog, Rana esculenta

Gaze fixation requires very fast movements of the eye during body displacement. The morphological and physiological background of the very fine and continuous tuning of gaze fixation is not yet fully understood. In a previous study we have shown that the dendrites of oculomotor neurons form bundles which invade the trochlear nucleus, and vice versa, trochlear dendritic bundles invade the oculomotor nucleus. Earlier physiological observations demonstrating electrotonic coupling between dendrites of spinal motoneurons in the frog suggest a similar mechanism between the oculomotor and trochlear motoneurons. We studied a possible morphological basis of gaze fixation. The experiments were carried out on common water frogs, Rana esculenta. The trochlear and oculomotor nerves were cut, and their proximal stumps were labeled simultaneously with different retrograde fluorescent tracers. Using confocal laser scanning microscope we detected a large number of close contacts in both nuclei, the majority of them were dendrodendritic apposition. The distance between the adjacent profiles suggested close membrane appositions without intercalating glial or neuronal elements. At the ultrastructural level, the dendrodendritic and dendrosomatic contacts did not show any morphological specialization; the long membrane appositions may provide ephaptic interactions between the neighboring profiles. This electrotonic coupling between the oculomotor and trochlear nerve motoneurons may promote the co-activation of the muscles responsible for vertical eye movements.

Stochastic resonance in the motor system: effects of noise on the monosynaptic reflex pathway of the cat spinal cord

In sensory systems, the presence of a particular nonzero level of noise may significantly enhance the ability of an individual to detect weak sensory stimuli through a phenomenon known as stochastic resonance (SR). The aim of this study was to demonstrate if such phenomenon is also exhibited by the motor system; in particular, in the Ia-motoneuron synapse of the cat spinal cord. Monosynaptic reflexes elicited by periodic electrical stimulation to the medial gastrocnemius nerve were recorded in the L(7) ventral root (or in single motoneurons) of decerebrated cats. Random stretches (mechanical noise) were applied to the lateral gastrocnemius plus soleus muscle by means of a closed-loop mechanical stimulator-transducer. In all cats, we observed the SR phenomenon. The amplitude of the monosynaptic reflexes (or number of action potentials recorded in the motoneurons) evoked by the weak electrical stimuli applied to the medial gastrocnemius nerve were an inverted U-like function of the mechanical noise applied to the lateral gastrocnemius plus soleus muscle. A significant maximum value in the amplitude of the monosynaptic responses was reached with a particular noise amplitude value. Numerical simulations on a model of the monosynaptic reflex pathway qualitatively reproduce this stochastic resonance behavior. We conclude that the monosynaptic reflex response elicited by Ia afferents is optimized by the noisy stretching of a synergistic muscle. Our study provides the first direct demonstration that the motor system, and not only the sensory systems, exhibits the SR phenomenon.

Recurrent inhibition of wrist extensor motoneurones: a single unit study on a deafferented patient

In order to document the effects of recurrent inhibition on the firing times of human alpha-motoneurones during natural motor behaviour, a case study was performed on a deafferented patient. The fact that this subject had completely lost the large-diameter sensory afferents provided us with a unique opportunity of selectively stimulating the motor axons in the nerves. The tonic activity of single motor units (n = 21) was recorded in the extensor carpi radialis muscles while applying randomly timed antidromic electrical stimuli to the radial nerve. The peristimulus time histogram analysis showed the presence of biphasic inhibitory effects, including an early, short-lasting component followed by a longer-lasting component occurring 20-40 ms later. The interspike interval (ISI) during which the stimulation occurred was generally lengthened as compared to the previous ISIs. The stimulation was most effective when delivered early (20-30 ms) after a spike. It was also effective, although less so, when delivered at the end of the ISI (70-100 ms after a spike). The lengthening effect sometimes extended over one or two of the subsequent ISIs. The lengthening effect of the motor axon stimulation was followed by an excitatory-like effect, which took the form of a shortening that affected up to five ISIs after the stimulation. The biphasic inhibitory effects and the subsequent facilitatory effects are discussed in terms of the dual nature of the synaptic processes involved in the recurrent inhibitory network, the postactivation facilitation/depression processes and the mutual inhibition occurring between Renshaw cells.

Differential expression of the muscarinic m2 acetylcholine receptor by small and large motoneurons of the rat spinal cord

The purpose of this study was to determine if motoneurons in the spinal ventral horn express the muscarinic m2 acetylcholine receptor. Motoneurons were retrogradely labelled in adult rats by intramuscular injection of Fluorogold and examined with confocal microscopy for evidence of immunoreactivity for the receptor. The cells were also double-labelled for choline acetyltransferase to determine if they were contacted by cholinergic axons. Almost all large motoneurons (diameters greater than 35 microm) displayed intense immunoreactivity for the receptor which was evenly distributed along the plasma membrane. Small cells were immunonegative for the receptor or weakly labelled. As large cells probably correspond to alpha-motoneurons and receive many cholinergic contacts, it is concluded that acetylcholine can influence this type of motoneuron by acting through the m2 receptor.

Recurrent excitation of motoneurons in the isolated spinal cord of newborn rats detected by whole-cell recording

Motoneurons in the isolated spinal cord of newborn rats (1- to 6-day-old) were visualized using infrared videomicroscopy. Whole-cell recordings were performed from the neurons under observation. Stimulation of a sciatic nerve which adjoined ventral roots elicited postsynaptic currents (PSCs) in 28 out of 88 motoneurons. When membrane potentials were changed, some PSCs reversed at around -70 mV, which was compatible with the chloride equilibrium potential calculated. Thus, they were considered to be recurrent inhibitory, namely Renshaw inhibition. On the other hand, we detected PSCs which reversed at +3.3 mV on average. They were interpreted as excitatory based on the level of the reversal potential which was similar to that of orthodromic excitatory PSCs. To determine the origin of the antidromic excitatory inputs, the effect of d-tubocurarine (10 microM) on the PSCs was examined. In three out of five motoneurons, the excitatory currents were eliminated. Therefore, it was concluded that the excitatory inputs, if not all, are mediated via the axon collaterals. Furthermore, it was found that the locations of motoneurons receiving the recurrent inhibitory and the excitatory PSCs were different in the spinal cord.

Dendro-dendritic connections between motoneurons in the rat spinal cord: an electron microscopic investigation

The ultrastructural characteristics of identified dendrite bundles in the rat spinal cord were analyzed following retrograde tracing from the soleus muscles. Dendrite bundles are arranged in networks that are heavily interconnected by means of gap junctions. The bundles are formed by at least 10 crossing dendrites travelling in different focal planes. In between dendrites, elongated gap junctional complexes are frequently found. Dendrite lamellar bodies, recently described to occur in relation with gap junctions in the central nervous system were not observed in the present study.

Lack of biocytin transfer at gap junctions in the chicken vestibular nuclei

In vivo experiments were designed to test for functional gap junctions at 'mixed' synapses that were morphologically characterized between the large-diameter, primary vestibular fibers and second-order vestibular neurons in the chicken, Gallus gallus. In previous intracellular recordings and dye injections into these neurons from brain slice preparations of chick embryos (E15/16) and also newborn hatchlings (HI-2), no evidence was obtained for functional gap junctions. Therefore, biocytin, a low molecular weight tracer that permeates gap junction channels, was extracellularly applied to either the ampullary nerves or to the vestibular ganglion of 3-6 day old hatchlings and adult chickens (9 months). This procedure resulted in the uptake of the dye and heavy staining of both the thick and thin fibers composing the vestibular nerve and in loading of vestibular efferent neurons. However, no dye transfer was observed between the large-diameter, primary vestibular fibers and second-order vestibular neurons. This observation, which was performed using a relatively non-invasive approach on intact animals, suggests that the gap junctions at these mixed synapses are probably not functional under the conditions of these experiments.

H reflex amplitude. Effect of leg muscle activity

The H reflex is a monosynaptic reflex whose latency can be used to evaluate pathology of the S1 nerve root and axon. Little has been written about the use of the H reflex amplitude in evaluating S1 nerve root and axon. Little has been written about the use of the H reflex amplitude in evaluating S1 nerve physiology. Although amplitude measurement of evoked potentials is useful in the evaluation of other sensory and compound muscle action potentials, this single case study shows it to be an unreliable parameter with which to evaluate the integrity of S1 nerve fibers, because muscle activity in antagonist and agonist affects the amplitude and in some instances abolishes it.

Mixed synapses discovered and mapped throughout mammalian spinal cord

Previously, synaptic activity in the spinal cord of adult mammals was attributed exclusively to chemical neurotransmission. In this study, evidence was obtained for the existence, relative abundance, and widespread distribution of "mixed" (chemical and electrical) synapses on neurons throughout the spinal cords of adult mammals. Using combined confocal microscopy and "grid-mapped freeze fracture," 36 mixed synapses containing 88 "micro" gap junctions (median = 45 connexons) were found and mapped to 33 interneurons and motor neurons in Rexed laminae III-IX in cervical, thoracic, and lumbosacral spinal cords of adult male and female rats. Gap junctions were adjacent to presumptive active zones, where even small gap junctions would be expected to increase synaptic efficacy. Two morphological types of mixed synapse were discerned. One type contained distinctive active zones consisting of "nested" concentric toroidal deformations of pre- and postsynaptic membranes, which, because of their unusual topology, were designated as "synaptic sombreros." A second type had gap junctions adjacent to active zones consisting of broad, flat, shallow indentations of the plasma membrane. Morphometric analysis indicates that mixed synapses correspond to 3-5% of all synapses on the somata and proximal dendrites, but, because of their subcellular location and morphology, they could represent 30-100% of excitatory synapses. The relative abundance of mixed synapses on several classes of neurons in spinal cords of adult rats suggests that mixed synapses provide important but previously unrecognized pathways for bidirectional communication between neurons in the mammalian central nervous system.

Cholinergic and electrical synapses between synergistic spinal motoneurones in the Xenopus laevis embryo

1. To investigate central motoneurone synapses within the spinal cord of a simple vertebrate, the Xenopus embryo, simultaneous intracellular recordings were made from fifty-five pairs of spinal motoneurones. 2. Chemical synapses were found between seventeen out of thirty-five pairs on the same side of the spinal cord. Current-evoked spikes in the presynaptic neurone led to fast depolarizing postsynaptic potentials (PSPs) in the postsynaptic neurone at latencies of 0.5-1.5 ms. The PSPs had an average amplitude of 7 mV, a rise time of 8 ms and a half-fall time of 18 ms. 3. The presynaptic motoneurone was always the more rostral of the pair. No excitatory connections were found which crossed the cord. The fast PSPs were blocked by 10 microM mecamylamine but not by 1 mM kynurenic acid, so were mediated by nicotinic acetylcholine receptors (nAChRs). These are the first unitary excitatory postsynaptic potentials (EPSPs) mediated by nAChRs recorded intracellularly within the vertebrate central nervous system. 4. Bidirectional electrical synapses were found between five pairs of motoneurones. All these pairs were on the same side of the spinal cord and less than 70 microns apart. Each neurone responded in a graded manner to either hyperpolarizing or depolarizing current injected into the other. 5. Since motoneurones are rhythmically active during swimming, both chemical and electrical synapses will add to the fast on-cycle excitation underlying spiking activity in other motoneurones. This may increase the reliability and local synchrony of synergistic motoneurone firing during locomotion.

Changes in sciatic nerve and spinal cord function induced by a CNS viral infection

Electrophysiologic response characteristics of mouse sciatic nerve and spinal cord were investigated following CNS infection with the temperature-sensitive (ts) vesicular stomatitis virus (VSV) mutant G31 KS5. Measurements were obtained before clinical symptoms of the virus-induced CNS disease appeared. Sciatic nerve peak conduction velocities were not different between virus and control inoculated groups. For all control groups, sciatic nerve response (SNR) recovery, characterized by the amplitude ratio of double pulse-evoked responses, followed a facilitation-depression time course. By 4 days after VSV inoculation, the time course of SNR recovery changed with the SNR amplitude ratios significantly depressed compared to control. Crossed spinal responses (CSRs) were measured from one sciatic nerve in response to stimulation of the contralateral sciatic nerve. For all control groups, CSR recovery, as characterized by area ratios calculated from single and double pulse evoked responses, followed a facilitation-depression time course. By 5 days after VSV inoculation, the time course of CSR recovery changed with the CSR area ratios significantly depressed compared to control. The results show that simple electrophysiologic techniques can be used to detect virus-induced changes in sciatic nerve and spinal cord previously undetectable by clinical measures.

GABA-like immunoreactive innervation and dendro-dendritic contacts in the ventrolateral dendritic bundle in the cat S1 spinal cord segment: an electron microscopic study

The motoneurons (MNs) in the ventrolateral nucleus (VLN) of the upper sacral spinal cord segments in the cat supply the external sphincters and the ischiocavernosii muscles. The dendrites of the MNs in the VLN are arranged into rostro-caudally oriented bundles (ventrolateral dendritic bundle, VLB). In this study we describe the distribution and synaptic arrangement of gamma-aminobutyric acid-immunoreactive (GABA-IR) axonal bouton profiles innervating the VLB. This was accomplished using the peroxidase-antiperoxidase technique and a polyclonal antibody raised against glutaraldehyde-conjugated GABA. The VLN receives an extensive innervation of GABA-IR axonal bouton profiles that surround both cell bodies and dendrites. Twenty-five per cent of the total number of vesicle-containing axonal profiles in the VLN neuropil were estimated to be GABA-IR. On cell bodies in the alpha-motoneuron size-range, the membrane covering of GABA-IR bouton profiles was about 18% and they constituted about 29% of the total membrane covering of axonal bouton profiles. Quantitative analysis of GABA-IR bouton profiles on dendrites revealed membrane covering figures rather similar to those on the cell bodies. They were not randomly distributed within the dendritic arborisations. Instead, they were very infrequent (2.5% of the covering) on small calibre dendrites (< 1 microns) as compared to larger dendrites (> 1 microns, 14-18.5% of the covering), although the total membrane covering of axonal bouton profiles was rather similar for all dendrites (42-52%). The data on membrane covering by GABA-IR boutons presented here may be low estimates due to technical limitations, indicating that the GABAergic input to this region might be even more extensive. A frequent finding was that one and the same GABA-IR bouton made synaptic contact with two to three adjacent dendrites. This type of synaptic arrangement among the VLN MNs indicates a divergence of the GABAergic input at the terminal level. In addition, the postsynaptic dendrites involved in such arrangements often disclosed dendro-dendritic contacts. In total, 44% of the bundled dendrites in the VLN disclosed direct dendro-dendritic contact regions. These contacts were most often of the puncta adherentia type, while desmosome-type contacts were less frequent. None of the dendro-dendritic contacts studies had the characteristics of a gap junction. Taken together, the present results indicate that GABA may be a transmitter substance in a large fraction of the synaptic input to the VLN MNs.(ABSTRACT TRUNCATED AT 400 WORDS)

AAEM Minimonograph #13: H reflexes and F waves: physiology and clinical indications

Motoneurons can be activated both reflexly and antidromically following electrical stimulation of peripheral nerves. These H reflexes and F waves are clinically useful responses which interface at the level of the peripheral nerves and the spinal cord. Because these responses are commonly employed in the electrodiagnostic evaluation of patients, an understanding of their physiology and clinical applications is important. These are reviewed. Reasoning from the physiology, both the value and limitations of H-reflex and F-wave studies are considered for disorders of peripheral nerves, roots, and the central nervous system. Theoretical concepts about the physiology and pathophysiology of the nervous system based on H-reflex and F-wave data are also discussed.

Firing rate interactions among human orbicularis oris motor units

Groups of human motor units from the same muscle exhibit joint fluctuations in firing rate during voluntary muscle contraction. In an effort to determine whether similar behavior would be observed in a muscle lacking muscle spindles, motor unit firing behavior was examined in the human orbicularis oris inferior (OOI) during mild voluntary effort. Motor unit activity was recorded with a quadrifilar needle inserted in the OOI. Firing occurrences were identified using a motor unit decomposition procedure. Cross-correlation of motor unit firing rates revealed a tendency for motor unit firing rates to covary, although the effect was somewhat more variable than that observed previously in other skeletal muscles. There was also a statistically significant tendency for pairs of motor units to fire at simultaneous or near-simultaneous (+/- 5 ms) intervals (synchronization). Firing rate variability in OOI motor units was not significantly different (p > .05) from that observed in the FDI. Thus, the present results suggest that the common drive of human motor unit activity may not depend on the presence of muscle spindles.

Postnatal changes in motoneurone electrotonic coupling studied in the in vitro rat lumbar spinal cord

1. Electrotonic coupling between motoneurones innervating ankle flexor and extensor muscles, as well as between unidentified lumbar motoneurones, was studied using intracellular recordings in an in vitro spinal cord-hindlimb preparation isolated from rats between birth (P0) and 13 days (P13). 2. Graded ventral root stimulation could elicit graded, short latency depolarizations (SLD) which preceded, coincided with, or followed the antidromic action potential. These SLDs were identified as electrotonic junctional potentials by their latency, relative insensitivity to changes in membrane potential and their resistance to one or more of the following: (1) high-frequency stimulation, (2) collision with a somatofugal action potential, (3) removal of Ca2+ from the bathing solution. 3. SLDs were studied in 162 neurones and were identified in 77.2% of the cells in preparations from P0 to P3 rats (n = 57), but only in 30.8% at P8 to P13 (n = 39). 4. SLDs were largest in the youngest animals (P0 to P3), decreasing from a mean of 1.31 mV (+/- 0.17, n = 34) to 0.56 mV (+/- 0.10, n = 7) at P8 to P13. The SLDs comprised two to eight (4.3 +/- 0.36) all-or-none components as determined from twenty collision experiments. 5. Electrotonic coupling between motoneurones was specific. SLDs could be elicited in given motoneurones by stimulation of their homonymous but never of their antagonistic muscle nerves. 6. These results indicate that electrotonic coupling between lumbar motoneurones in neonatal animals exhibits a high degree of specificity and that its significance, as judged by the amplitude and frequency of occurrence of SLDs, decreases postnatally at a rate that can be correlated with the functional maturity of the motoneurones and the muscular system.

Plasticity of recurrent inhibitory reflexes in cat spinal motoneurons following peripheral nerve injury

Chronic axotomy of a peripheral motor nerve in cat causes a gradual reduction in the number of intramedullary axon collaterals originating from the axotomized motoneurons (Havton and Kellerth 1984, 1989). This axon collateral elimination would be expected to reduce the amount of recurrent inhibitory reflex actions mediated by these cells. The aim of the present study was to investigate the recurrent inhibition originating from axotomized motoneurons and, also, to see whether the elimination of axon collaterals from the axotomized neurons might induce secondary compensatory changes in the recurrent inhibitory pathways originating from synergistic non-lesioned motoneurons. The results, which were obtained by means of intracellular recordings and monosynaptic reflex testing, indicate that postoperative enhancement of reflex actions may take place in the recurrent inhibitory pathways persisting in the axotomized motoneurons as well as in those originating from synergistic nonlesioned motoneurons. It is suggested that the site of compensatory enhancement is at the synaptic reflex contacts between the motoraxon collaterals and the inhibitory Renshaw interneurons.

Physiological and Ultrastructural Characterization of a Central Synaptic Connection between Identified Motor Neurons in the Locust

An excitatory connection between an extensor and several flexor tibiae motor neurons that innervate antagonistic muscles in the hind leg of a locust has been characterized using physiological and ultrastructural methods. Simultaneous intracellular recordings from the single fast extensor (FETi) motor neuron and up to three flexor motor neurons show that a spike in FETi is followed by a short latency depolarizing synaptic potential in the flexors that is powerful enough to evoke a burst of flexor spikes. The chemically mediated excitatory postsynaptic potential (EPSP) is caused centrally as it persists when sensory feedback from the leg is removed, and has a latency of 1.6-2.0 ms depending upon the position of the recording electrodes in the somata or neuropilar segments of the pre- and postsynaptic neurons. The amplitude of the EPSP declines gradually in a saline containing no calcium but high magnesium, indicating that no spiking interneuron is interposed in the pathway. With repetitive stimulation, the EPSP decrements markedly so that at intervals of 50 ms the second EPSP of a pair is reduced by 90%. The amplitude of the EPSP is also dependent on the amplitude of the presynaptic spike. The physiological evidence suggesting a monosynaptic connection is directly confirmed by electron microscopy of ganglia in which FETi and a flexor were both labelled with horseradish peroxidase. Direct chemical synapses between the two identified neurons, in which FETi is the presynaptic element, occur in three regions of the neuropil examined. At a synapse, the flexor motor neuron may be the only postsynaptic neuron or it may be one element in a dyad. The synaptic arrangements between the two neurons are complex with serial synapses through unlabelled processes linking FETi to flexor motor neurons and with frequent reciprocal synaptic connections between FETi and unlabelled processes. Unidentified processes also make input synapses on both neurons close to the synapses from FETi. The behavioural significance of the connection lies in the mechanical requirements for kicking and jumping. To prepare for these powerful movements the extensor and flexor tibiae muscles must co-contract. The connection from FETi enhances the depolarization and frequency of spikes in the flexors during the co-contraction.

Excitatory interactions between phrenic motoneurons: intracellular study in the cat

1. Intracellular recordings were made from 220 Phrenic Motoneurons (PM) in anaesthetized, spontaneously breathing cats, deafferented from C3 to C7, in order to look for somatic events related to the Recurrent Responses (RR) evoked in PM axons by repetitive stimulation of the phrenic nerve. RR appear sporadically at a constant latency, originate from a spinal nicotinic mechanism and can be evoked in a PM without the presence of an antidromic volley in its axon (Khatib et al. 1986). 2. Using stimuli effective for eliciting RR in axons, we failed to observe intracellularly somatic events corresponding to RR after the occurrence of an antidromic action potential. RR were observed extracellularly in two cases, but in both cases the recording originated from axons. 3. We attempted to elicit somatic RR without a preceding antidromic action potential, using either parathreshold stimulation of the impaled PM, or suprathreshold stimulation of a phrenic strand which excluded the axon of the impaled PM. In both cases, RR-like events, with very stable latencies, appeared sporadically in 4/142 and 2/15 PMs respectively. 4. Parathreshold stimuli or stimulation of a strand were coupled with averaging of the synaptic noise in order to look for small events temporally related to the stimuli. Short latency small depolarizations, looking-like recurrent EPSPs, were revealed in 22/142 and 5/15 PMs respectively. 5. These results confirm the existence of interrelations between PMs, providing for re-excitation and coupling within the phrenic pool, in addition to centrally imposed synchronization.

F response abnormality in Parkinson's disease

The F response was recorded from the 1st dorsal interosseous muscle (1st DI) and two antagonistic muscles, the flexor digitorum brevis (FDB) and the extensor digitorum brevis (EDB), in 16 patients with Parkinson's disease and 22 age-matched normal controls. In Parkinson's disease, the F responses from the 1st DI and the FDB occurred more frequently with a longer duration and a greater number of phases in comparison with the normal controls. In addition, the coefficient of variation of onset latency was significantly smaller in Parkinson's disease. On the other hand, the F response recorded from the EDB showed the same tendency as that from the FDB, but not to a significant extent. These findings suggest that the excitability of the spinal motor neurons is enhanced in rigid parkinsonian patients. It thus seems reasonable to assume that the main underlying cause of rigidity is an excessive supraspinal drive to the spinal motor neurons, including an increase in motoneuron excitability.

Electrical activity of rat ocular motoneurons recorded in vitro

The electrophysiological properties of rat oculomotor neurons were studied in an in vitro slice preparation. Motoneurons were identified by their antidromic response to third nerve rootlet stimulation, and by their orthodromic responses to medial longitudinal fasciculus and reticular stimulations. Passive membrane properties showed the existence of an inward rectification mechanism in all the recorded motoneurons. The action potential is comprised of several distinct components. The fast initial spike, composed of an initial segment spike and a somatodendritic spike, is followed by a delayed depolarization, an afterhyperpolarization and a late afterdepolarization. The afterhyperpolarization has a maximum duration of 55 ms. The late afterdepolarization is a voltage-dependent mechanism that produces an oscillatory behavior in depolarized cells. Two types of motoneurons were distinguished on the basis of their response to long-lasting depolarizing current pulses. The intensity-frequency curves show the existence of a primary and secondary range of discharge and the study of the interspike intervals points to specific properties of the conductance underlying the afterhyperpolarization. It is concluded that large, stellate motoneurons of the brainstem maintained in vitro retain specific electrophysiological properties, comparable to those described in vivo and which differentiate the ocular motoneurons from spinal motoneurons.

The dendrites of single brain-stem motoneurons intracellularly labelled with horseradish peroxidase in the cat. An ultrastructural analysis of the synaptic covering and the microenvironment

Two laryngeal motoneurons intracellularly stained with horseradish peroxidase were studied ultrastructurally. The precise position of the ultrastructural observations made along the dendrites was obtained from the computer-reconstruction of the motoneurons in three dimensions. The shape and the size of the synaptic boutons, the percentage of membrane covered by bouton appositions and active zones, the number of boutons per 100 microns2 (packing density) were analysed on the soma and on the labelled dendrites at different distances from the soma up to 1000 microns. The results revealed no important regional differences in the mean length of synaptic apposition. The packing density was in the range of 9.3-14.9 boutons per 100 microns2 and was not correlated with the distance from the soma. The percentage apposition covering was higher on the soma and the proximal part of the dendrites than on the remaining part of the dendritic arborization. Close appositions between labelled dendrite and unlabelled somata and/or dendrites together with dendro-dendritic synapses suggested the possibility that the dendrites may be involved in local cell-to-cell communication. Microdendrites emerging from the soma or the proximal dendrites were contacted by synaptic boutons which may be more efficient as revealed by computation.

The dendrites of single brain-stem motoneurons intracellularly labelled with horseradish peroxidase in the cat. Morphological and electrical differences

The geometrical differences between individual dendrites of a given motoneuron were investigated in the cat. We chose two brain-stem motoneurons involved in different motor activities. One abducens and one laryngeal motoneuron were selected from two series of experiments which had combined intracellular recording and horseradish peroxidase staining. Three-dimensional reconstructions were made using a computer-aided microscope to obtain high-resolution measurements from serial histological sections. Each dendrite was characterized by computer dissection. Comparisons between dendrites were made on the basis of the following parameters: spatial projections, length, diameters, tapering, branching pattern, daughter--branch ratio and branching power. The present findings show that each dendrite projects to specific terminal fields for both motoneurons and are different in the complexity of their geometry and branching structure. The consequences of this complexity for the cable properties of the motoneurons were analysed. The dendrites of the two motoneurons were partitioned into a series of contiguous regions deemed short enough to be considered an isopotential cylinder and the steady-state properties were calculated for each segment. The properties of each segment were then combined for each dendrite for the following parameters: electronic distance, somatopetal and somatofugal voltage attenuation, input resistance and charge transfer effectiveness ratio. The present results show significant differences in the electrical behaviour of individual dendrites. Branch-to-branch computation reveals low attenuation pathways between branches suggesting the possibility of local influences within the distal branches of the dendritic arborization. It is proposed that the individual dendrites of the motoneuron function as distinct channels and/or integrators for afferent inputs.

Electrophysiological observations on the human pudendo-anal reflex

A reproducible electrophysiological technique is described to determine the latency of reflex contraction of the external anal sphincter in response to stimulation of the dorsal genital nerve: the pudendo-anal reflex. This was studied in 38 asymptomatic control subjects and 20 women with neurogenic faecal incontinence, supplemented by determination of the mean motor unit potential duration (MUPD) of the external anal sphincter and anorectal manometry. The reflex latency in the control group was 38.5 +/- 5.8 (SD) ms and appeared to be independent of age or sex. Three patients with faecal incontinence had absent reflexes; the remainder showed significant prolongation of latency (56 +/- 12.2 SD ms) and diminution of amplitude. MUPD was prolonged in incontinence and showed significant correlation with the corresponding reflex latency determination (tau = 0.56, p less than 0.001). The latency of this polysynaptic spinal reflex hence provides a reliable index of neuropathy of the external anal sphincter.

Motoneurone activity in an isolated spinal cord preparation from the adult mouse

This paper describes an isolated, hemisected preparation of adult mouse spinal cord, in which motoneurones remain viable. At 18-22 degrees C both orthodromic synaptic activation and antidromic invasion of populations of motoneurones could be demonstrated by extracellular recording of ventral root reflexes and ventral horn field potentials. Motoneurones had resting potentials of -55 to -65 mV and input resistances of 5-30 M omega, and, following ventral or dorsal root stimulation or during outward current injection, they generated action potentials which resembled those recorded from adult motoneurones in vivo. Recurrent inhibitory synaptic potentials followed antidromic spikes, demonstrating viability of the Renshaw cell pathway.

Excitatory interactions between phrenic motoneurons in the cat

Interactions between phrenic motoneurons have been analysed in anaesthetized, paralyzed cats after C3 to C7 deafferentation. Effects of electrical stimulation of the C5 phrenic axons have been studied on thin filaments dissected from the stimulated nerve. Repetitive stimulation could elicit, after the primary direct response of the stimulated axons, a secondary response named Recurrent Response, RR. RRs have been obtained in 117/186 phrenic axons. They appear sporadically (mean occurrence: 3.75 RRs elicited by 100 shocks of stimulation) at a constant latency. They originate from a spinal mechanism since they persist after C2 transection and disappear after section of the ventral roots. The mechanism responsible for RR shows spatial and temporal facilitation. The RR probability increases with the number of antidromically invaded motoneurons as revealed by changes either of stimulation intensity or of central respiratory drive. However, RR could be evoked in a motoneuron without an antidromic volley in its axon. Systemic injections of nicotinic blocking drugs such as dihydro-beta-erythroidine or mecamylamine decrease or suppress the occurrence of RR; therefore, cholinergic synapses are involved in the RR generating process. RR are assumed to be due to direct excitatory interactions between homonymous motoneurons. Recurrent axon collaterals impinging directly on neighbouring motoneurons would link together the different motoneurons of the phrenic pool. The functional significance of this phenomenon is discussed.

The morphology and distribution of neurons containing choline acetyltransferase in the adult rat spinal cord: an immunocytochemical study

A monoclonal antibody to choline acetyltransferase (ChAT), the acetylcholine (ACh)-synthesizing enzyme, has been used to localize ChAT within neurons in immunocytochemical preparations of adult rat spinal cord. Morphological details of known cholinergic spinal neurons are presented in this study, and previously unidentified ChAT-containing neurons are also described. Immunoreaction product was present within cell bodies, dendrites, axons, and axon terminals, thereby allowing comprehensive descriptions of the distribution of ChAT-positive neurons and the interrelationships of their processes. In the ventral horn, ChAT-positive motoneurons were located in the medial, central, and lateral motor columns, and their dendrites formed elaborate longitudinal and transverse ChAT-positive bundles. These bundles were present throughout the rostrocaudal extent of the spinal cord. In the central gray matter, small ChAT-positive cell bodies were clustered around the central canal. Small longitudinal fascicles of immunoreactive processes were also observed in this region adjacent to the ependymal layer. The intermediate gray matter of virtually the entire spinal cord was spanned by medium to large ChAT-positive multipolar cells termed partition neurons. At autonomic spinal levels, partition neurons were intermingled with other immunoreactive cells that were identified as preganglionic sympathetic or parasympathetic neurons because of their locations and morphological characteristics. In the sympathetic system, four groups of ChAT-positive neurons were observed; the principal intermediolateral nucleus (ILp) in the lateral horn, the central autonomic cell column (CA) dorsal to the central canal, the intercalated nucleus (IC) located between ILp and CA, and the funicular intermediolateral neurons (ILf) in the white matter lateral to the ILp. The dendrites of ILp and CA neurons formed substantial longitudinal bundles within each group, as well as transverse bundles between the groups that resembled the rungs of a ladder. ChAT-positive cell bodies were also present in the dorsal horn, and those located in laminae III-V extended dendrites dorsally into a longitudinal plexus within lamina III.

Is there electrical coupling between phrenic motoneurons in cats?

The possibility of electrical coupling between phrenic motoneurons was studied in anesthetized cats. In animals with C4-C7 dorsal roots cut (spinal cord intact), no sign of short-latency increase in the firing probability was observed in one phrenic rootlet following stimulation of the other phrenic rootlet. Intracellular recording from 21 phrenic motoneurons, in cats spinalized at the C1 segment, revealed no graded, short-latency antidromic depolarizations, even when the collision technique was used. Conditioning depolarizations of the impaled motoneurons never resulted in an increase of cell firing following test antidromic activation of adjacent motoneurons. It is concluded that the short-term discharge synchronization, observed within the phrenic nucleus by other authors, must be due to the action of chemical synapses.

On re-excitation of feline motoneurones: its mechanism and consequences

Conditions required for re-excitation of lumbosacral motoneurones, i.e. for double impulses in the motor axons associated with a single soma-dendritic action potential, were examined in cats anaesthetized with pentobarbitone and paralysed with gallamine triethiodide. Simultaneous recording from a motoneurone (intracellular, and in some experiments also extracellular), and from its axon in a ventral root, was used to assess the relations between the soma and the double axonal action potentials. Action potentials (greater than 70 mV) evoked by brief depolarizing current pulses applied intracellularly were never observed to cause re-excitation. Re-excitation could, however, be regularly induced by procedures which increased the delay between the initial segment and soma-dendritic components of these potentials. Re-excitation could be evoked (i) when brief hyperpolarizing pulses were applied before the onset of the soma-dendritic spikes, (ii) when the depolarizing pulses were applied on a background of long hyperpolarizing pulses or (iii) when two action potentials were evoked in a quick succession (by two brief depolarizing pulses). No relationship was found between the presence of re-excitation of motor axons and the presence of the delayed depolarization which follows the soma-dendritic spikes. Neither re-excitation nor delayed depolarization were found to be dependent upon re-excitation of the initial segment. These observations are thus at variance with previous suggestions that the initial segment spikes induce the re-excitation of motor axons and that the initial segment spikes cause the delayed depolarization following soma-dendritic spikes. Since re-excitation of a motor axon occurred without any signs of a second initial segment spike, it is concluded that it is initiated at the level of the axon, most likely at the first node of Ranvier. Re-excitation of motor axons was also observed during repetitive firing induced by intracellular current injection. However, it occurred then only occasionally, and only under strong depolarizing drive. It is thus not expected to be a common phenomenon under natural conditions of repetitive firing.

Firing of spinal motoneurones due to electrical interaction in the rat: an in vitro study

The excitatory interaction between spinal motoneurones was investigated by means of electromyogram (EMG) recordings from hindlimb muscles as well as intracellular ones from their innervating motoneurones in the isolated preparation of immature rats. Stimulation of the muscle nerve to biceps femoris or medial gastrocnemius or of the L5 ventral root evoked early and late EMG responses in the muscle of the preparations with the dorsal roots cut. The early response was produced directly by volleys in the motor nerve. The late response was of spinal origin, since it disappeared after the severance of the ventral root. The thresholds and the conduction velocities of nerve fibres, which conducted the centripetal impulse causing the late response, were compatible with those of motor nerve fibres. The amplitude of the late response was 5-10% of that of the maximum early EMG response. Intracellular recordings from spinal motoneurones revealed that stimulation of the ventral root elicited the double discharge composed of antidromic and delayed spike potentials. The delayed spike was never evoked after the spike potential elicited directly by a short depolarizing pulse. The double discharge was observed in about 6% of the motoneurones examined. The threshold of the stimulus intensity evoking the double discharge was in the range of those of motor nerve fibres. The latencies of the delayed excitation were 7.0-9.0 ms, comparable to the intraspinal delays of the late EMG response. Stimulation of the ventral root at intensities subthreshold for antidromic activation was found to produce a small depolarizing potential in about 60% of the motoneurones examined. The amplitudes were 0.5-5.0 mV, and the onset and the peak latencies 2.0-7.0 ms and 5.0-8.0 ms, respectively. The potential was unaffected by the deficiency of calcium ions in the perfusing medium and persisted after the degeneration of the afferent fibres in the ventral root.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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.

Anal sphincter responses after perianal electrical stimulation

By perianal electrical stimulation and EMG recording from the external anal sphincter three responses were found with latencies of 2-8, 13-18 and 30-60 ms, respectively. The two first responses were recorded in most cases. They were characterised by constant latency and uniform pattern, were not fatigued by repeated stimulation, were most dependent on placement of stimulating and recording electrodes, and always had a higher threshold than the third response. The third response was constantly present in normal subjects. It had the longest EMG response and the latency decreased with increasing stimulation to a minimum of 30-60 ms. This response represented the clinical observable spinal reflex, "the classical anal reflex". The latencies of the two first responses were so short that they probably do not represent spinal reflexes. This was further supported by the effect of epidural anaesthesia which left the first responses unaffected but abolished the classical anal reflex. The origin of the two first responses is discussed and models involving antidromal impulse propagation in the efferent fibre as the afferent limbs of the responses are proposed.

Electron microscopic observations of axon collateral synaptic endings of cat oculomotor motoneurons stained by intracellular injection of horseradish peroxidase

Motoneurons in the cat oculomotor nucleus have been identified electrophysiologically and stained by intracellular injection of horseradish peroxidase. Axon collateral arborizations with preterminal and terminal boutons identified by light microscopy corresponded to synaptic endings observed by electron microscopy. Despite variations in size and shape, synaptic endings showed similar ultra-structural features and established asymmetrical predominantly axodendritic synaptic contacts usually characterized by the presence of subjunctional dense bodies underlying the postsynaptic membrane densification.