Electrical interaction between antidromically stimulated frog motoneurones and dorsal root afferents: enhancement by gallamine and TEA

1. Electrical interactions have been studied in the isolated frog spinal cord preparation. It is found that gallamine and tetraethylammonium chloride (TEA) markedly enhance all non-cholinergic synaptic interactions, including the electrical interaction between motoneurones (VR-VRP). In addition, in the presence of either of these drugs, a short-latency interaction is seen to exist between antidromically stimulated motoneurones and dorsal root afferents (early VR-DRP). The early VR-DRP is rarely seen in the absence of gallamine or TEA.2. The early VR-DRP is of the same short latency as the VR-VRP and fulfils the same criteria for electrical interaction: it increases in amplitude with cooling from 17-10 degrees C, it is not blocked by a wide variety of pharmacological blocking agents, and it is suppressed by both Mg(2+) and Ca(2+), with no antagonism of action between the two.3. The early VR-DRP appears as a cluster of unitary events: all-or-none spikes conducted out the dorsal root fibres. No initial graded slow potentials are seen. Often there are two peaks in the response.4. The early VR-DRP is facilitated by a dorsal root volley, with a time course normally intermediate between that of the orthodromic ventral root potential (DR-VRP) and the dorsal root potential (DR-DRP). This orthodromic facilitation apparently is achieved by increasing invasion of motoneurone dendritic trees and depolarization of dorsal root afferents toward threshold.5. If the same ventral root is stimulated twice, or adjacent roots stimulated at different intervals, the second early VR-DRP, like the VR-VRP, is seen to be occluded for 10-20 msec, then facilitated to supranormal amplitudes. It is concluded that motoneurone dendrites are presynaptic to both interactions.6. Evidence is presented that gallamine and TEA act by increasing the duration of activity both in axon terminals and in antidromically invaded motoneurones. Often second or multiple spikes result. The increased duration of depolarization can increase transmitter release at terminals and increase coupling at electrical junctions.7. Possible morphological correlates for the two electrical interactions are discussed. It is speculated that the motoneurone interaction arises at numerous areas of close apposition between dendrites in dendritic ;thickets', and that the early VR-DRP is mediated by fewer, lower-resistance, electrical junctions between dendrites and afferent nerve terminals.     

The effects of gallamine on field and dorsal root potentials produced by antidromic stimulation of motor fibres in the frog spinal cord

Summary The effects of gallamine on the intraspinal field potentials and the dorsal root potentials produced by antidromic stimulation of motor fibres were studied in the isolated frog spinal cord preparation. After gallamine (10^-3 M), the duration of the negative field potential produced by antidromic activation of motoneurons (N₁ response) was increased often without changing its amplitude. This resulted in an increased passive spread of the antidromic action potential towards the dorsal dendritic regions, where afferent fibres terminate.In the untreated spinal cord, stimulation of motor axons produced a late negative dorsal root potential (VR-DRP) which was depressed after gallamine administration. Abolition of the VR-DRP was frequently associated with the appearance of a short latency, conducted response, in the dorsal roots (EVR-DRP). The earliest component of the EVR-DRP had a latency ranging between 0.5 and 2.5 ms measured after the peak of the N₁ response recorded at the motor nucleus. Such a brief latency of the EVR-DRP suggests that this response results from electrical interaction between motoneurons and afferent fibres. After gallamine, the primary afferent depolarization produced by orthodromic stimulation of sensory nerves facilitates the EVR-DRP without necessarily increasing the amplitude or duration of the N₁ response. Also, gallamine appears to increase directly the excitability of the afferent fibre terminal arborizations.The nature of the electrical interaction between motoneuron dendrites and afferent fibre terminal arborizations is discussed in terms of two hypotheses: interaction by current flows and by electrical coupling.     

Extracellular potassium accumulation and transmission in frog spinal cord

In the isolated frog spinal cord, intracellular recordings from motoneurons and neuroglia and extracellular recordings from dorsal and ventral roots were used to compare electrophysiological changes produced by tetanic stimulation of dorsal roots with those resulting from increasing extracellular potassium concentration, [K⁺]₀. Many of the after-effects of tetanic dorsal root stimulation could be mimicked by increasing [K⁺]₀. These include the following: depolarization of motoneurons and neuroglia, prolongation and depression of excitatory postsynaptic potentials, depression of dorsal root potentials, facilitation and depression of ventral root potentials, as well as increases in spontaneous synaptic activity and depression of antidromic spikes recorded from motoneurons. The levels of [K⁺]₀ necessary to produce effects comparable to those following maximal dorsal root stimulation are about twice those estimated from measurements with K-selective microelectrodes or glial depolarization, suggesting that both these methods underestimate the amount of potassium which accumulates in the narrow intercellular spaces between neurons and glia in the intermediate region of the spinal cord.It is concluded that, at least within the isolated frog spinal cord, K⁺ accumulation is a significant factor modifying transmission following dorsal root tetanic stimulation and that its distribution is inhomogeneous.     

Dendritic responses of frog motoneurons produced by antidromic activation

Responses to ventral root stimulation were studied in spinal cords in situ in unanesthetized frogs. Extracellular as well as intracellular recordings from motoneurons indicated that considerable depolarization of the dendrites occurred in the response to ventral root volley. Active components of this dendritic depolarization could also be observed. Extracellularly, negative field potentials were recorded both in the vicinity of motoneuron cell bodies and in areas occupied mostly by motoneuron dendrites. The refractory period of the negativity recorded at the vicinity of dendrites was longer than in the vicinity of somata. Changes in antidromic excitability were studied by the double volley technique. Augmentation of the field potential to a test volley was observed during the period of dendritic depolarization, followed by a longer-lasting period of depression of the test response. It is concluded that an action potential in frog motoneurons induces depolarization of the dendrites. The depolarized dendrites can generate local action potentials and can produce negative field potentials remotely from the somatic pool. The response of dendrites to stimulation of the ventral root has particular importance in the recurrent facilitation of the frog motoneurons.     

Studies on convulsants in the isolated frog spinal cord. II. Effects on root potentials.

1. In the isolated frog spinal cord picrotoxin, bicuculline, and strychnine were evaluated for their effects on synaptically induced root potentials recorded by the sucrose gap technique. 2. Picrotoxin (greater than 10- minus 4 M) completely blocked the dorsal root potential (DRP) elicited by stimulating the ventral root of the same segment (VR-DRP). Although picrotoxin antagonized the DRP elicited by stimulation of either an adjacent dorsal root (DR-DRP) or the lateral column (LC-DRP), a slower component to these potentials appeared and increased in size as the concentration of picrotoxin was increased. Thus picrotoxin brings out a later, picrotoxin resistant component to the DR-DRP and LC-DRP. 3. Strychnine (10- minus 8-10- minus 5 M) reduced and abolished the VR-DRP without prolongation and progressively increased and prolonged the DR-DRP (and LC-DRP) and the DR-VRP. Strychnine in higher concentrations (greater than 10- minus 4 M) also reduced the amplitude and prolonged the duration of the compound action potential of afferent fibres. 4. These results combined with those presented in the preceding paper (Barker, Nicoll & Padjen, 1975) suggest that (1) a GABA-like transmitter mediates the final step in the DR-DRP and LC-DRP pathways and that (2) either taurine or beta-alanine may mediate the last step in the VR-DRP pathway.     

Asphyxial potentials of spinal grey matter, and of ventral and dorsal roots

1. Asphyxial potentials of short latency (from a few to about 10 sec) were recorded with mono- and bipolar electrodes from the cat's spinal cord. Monopolarly, a zone of maximum negativity was found somewhat dorsal of the central canal in the dorsal horn. With bipolar leads potentials of opposite polarity were observed in the dorsal and ventral horns. In the dorsal horn the more ventral electrode tip became negative with respect to the more dorsal one, in the ventral horn the more ventral tip became more positive. In the centre of the cord where the monopolar potential showed a maximum the bipolar potentials were small in either direction, or reversed during asphyxiation.2. These observations can be explained by the development of two independent dipoles of opposite polarity, located in the dorsal and ventral horn respectively, oriented with their negative poles towards the centre of the cord.3. In the ventral as well as in the dorsal root a negativity of a proximal electrode with respect to a more distal one developed during asphyxiation after the same latency as the asphyxial cord potentials. The asphyxial root potentials continued to grow during periods of asphyxiation as long as 30 min, and recovered promptly upon re-oxygenation.4. Ventral and dorsal root potentials were abolished by asphyxiation of the cord for a period of 60 min, 2 weeks previously. This procedure destroys practically all the neurones, but not the dorsal root fibres. The dorsal root potential, but not the ventral one, was abolished by extradural sectioning of the roots, 2 weeks previously.5. The asphyxial ventral and dorsal root potentials were interpreted as the result of depolarization of the intraspinal part of the motoneurones and primary afferent endings respectively, conducted electrotonically along the roots. The short latency of these potentials suggests that an early depolarization of motoneurone, and of the primary afferent end knobs occurs. The latter, which may have some relation to presynaptic inhibition, explains the early failure of synaptic conduction during acute asphyxiation.     

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.     

The organization of primary afferent depolarization in the isolated spinal cord of the frog

1. The organization of primary afferent depolarization (PAD) produced by excitation of peripheral sensory and motor nerves was studied in the frog cord isolated with hind limb nerves.2. Dorsal root potentials from sensory fibres (DR-DRPs) were evoked on stimulation of most sensory nerves, but were largest from cutaneous, joint and flexor muscle afferents. With single shock stimulation the largest cutaneous and joint afferent fibres gave DR-DRPs, but potentials from muscle nerves resulted from activation of sensory fibres with thresholds to electrical stimulation higher than 1.2-1.5 times the threshold of the most excitable fibres in the nerve. This suggests that PAD from muscle afferents is probably due to excitation of extrafusal receptors.3. Dorsal root potentials produced by antidromic activation of motor fibres (VR-DRPs) were larger from extensor muscles and smaller or absent from flexor muscles. The VR-DRPs were produced by activation of the lowest threshold motor fibres.4. Three types of interactions were found between test and conditioning DRPs from the same or different nerves. With maximal responses occlusion was usually pronounced. At submaximal levels linear summation occurred. Near threshold the conditioning stimulus frequently resulted in a large facilitation of the test DRP. All three types of interactions were found with two DR-DRPs, two VR-DRPs or one DR-DRP and one VR-DRP.5. The excitability of sensory nerve terminals from most peripheral nerves was increased during the DR-DRP. The magnitude of the excitability increase varied roughly with the magnitude of the DR-DRP evoked by the conditioning stimulus.6. There was a marked excitability increase of cutaneous and extensor muscle afferent terminals during the VR-DRP. Flexor muscle afferent terminals often showed no excitability changes to ventral root stimulation. In those experiments where afferent terminals from flexor muscles did show an excitability increase, the effects were smaller than those of cutaneous and extensor terminals.7. The VR-DRPs appear to reflect activity of a negative feed-back loop from extensor motoneurones on to sensory fibres from cutaneous and extensor muscles. This system may have a role in modulating the ballistic movement of the frog. DR-DRPs, on the contrary, are widespread in origin and distribution. PAD from sensory fibres may function to sharpen contrast between incoming afferent information.     

Centrifugal dorsal root discharges induced by motoneurone activation

1. It has been confirmed that antidromic stimulation of motoneurones in the cat lumbar cord can induce, when properly conditioned, a centrifugal discharge in dorsal root afferent fibres.2. The effective conditioning can be (a) an orthodromic volley to the same or an adjacent dorsal root, (b) a volley to the dorsal column one or two segments above the tested level, or (c) a natural stimulus applied to the ipsi- or contralateral hind limb.3. The conditioning stimulus acts by increasing presynaptic excitability; the peak of its effect (maximum presynaptic depolarization) occurs 7-10 msec after the arrival of the conditioning volley to the cord and then quickly decays.4. A large antidromic field potential in the ventral horn is not necessary for the production of a centrifugal dorsal root discharge. Activation of a ventral root filament of approximately 100 mu in diameter can still induce such a discharge in a single dorsal root fibre. Furthermore, antidromic stimulation of the remaining fibres of the same ventral root cannot affect the terminals activated by the thin ventral root filament.5. The phenomenon of motoneurone-presynaptic interaction was obtained in different types of experimental preparations: acute and chronic spinal, anaemic and midcollicular decerebrate, animals with intact supraspinal centres, and one animal without acute laminectomy.     

Effects from the sensorimotor cortex on the spinal cord in cats with transected pyramids

Summary Effects of stimulation of the sensorimotor cortex on activity of the lumbosacral cord were studied in pyramidotomized cats. The following actions initiated by corticofugal volleys were found: 1. postsynaptic effects on motoneurones, mainly excitatory in flexor motoneurones and inhibitory or excitatory in extensor motoneurones, 2. facilitation of spinal reflexes to motoneurones at an interneuronal level, 3. depolarization of presynaptic terminals of group Ib and cutaneous fibres. The latencies of the earliest cortical effects on motoneurones as indicated by modification of monosynaptic reflexes or PSPs were 9–12 msec. Experiments with lesions of different spinal tracts suggest that the effects on motoneurones are mediated mainly by pathways in the ventral part of the lateral funiculus (probably reticulospinal), the facilitation of reflex transmission by pathways in the dorsal part of the lateral funiculus (probably rubrospinal) and primary afferent depolarization by both the former and the latter pathways. The strongest cortical effects were evoked by stimulation of an area around the postcruciate dimple.JBRO-Fellow     

Intracellular autogenetic and synergistic effects of muscular contraction on flexor motoneurones

1. Intracellular records have been taken from cat motoneurones innervating flexor muscles of the hind limb. Contractions of the ankle flexors tibialis anterior and extensor digitorum longus were elicited by stimulation of the peripheral end of the cut L 7 ventral root and the reflex effects of these contractions were recorded in silent and repetitively firing motoneurones.

2. Contraction usually produces a hyperpolarizing response inside flexor motoneurones. This hyperpolarization is tension-sensitive in the sense that when, at constant muscle extension, the strength of the contraction is increased, the magnitude of the inhibitory response is augmented.

3. Increasing the resting length of the muscles, while using a stimulus of constant strength to the ventral root, causes this inhibitory response to increase in some cells. More often, however, the hyperpolarization caused by contraction is gradually reduced in duration and/or amplitude as the muscles are extended.

4. Even with the muscles slackened, so that they develop no tension at their ends, contraction usually produces prominent hyperpolarization of the motoneurones.

5. By passing polarizing currents or injecting chloride ions through the intracellular micro-electrode, the hyperpolarizing potentials produced by contraction of the slack and extended muscles are shown to be, at least in part, genuinely post-synaptic inhibitory events.

6. When the neurone is fired repetitively by injected current, the `silent period' in contraction corresponds to the hyperpolarization of the post-synaptic membrane.

7. Monosynaptic testing of the flexor motoneurone pool has been used to confirm the essential features of the intracellularly recorded activity.

8. Acutely spinalizing the animal increases the magnitude of the inhibitory responses caused by contraction.

9. Recordings from dorsal root fibres show that Golgi tendon organs of the ankle flexors are very sensitive to contraction and are indeed often activated by the internal forces developed in a contracting slack muscle.

10. A number of muscle spindles of the ankle flexors are activated by stimulation of the ventral root at a strength submaximal or just maximal for the α-motor fibres, despite the simultaneous unloading effect of the contracting extrafusal fibres. This spindle activation, which occurs mainly during the phase of tension development in contraction, is favoured by an increased extension of the muscle. Attempts were made to establish whether it is due to α-motor innervation of the receptors or to some mechanical interaction between the intra- and extrafusal muscle fibres.

11. The possible central and peripheral causes of the changes in motoneurone excitability produced by flexor muscle contraction are discussed. It is suggested that tendon organs of flexor muscles strongly inhibit flexor motoneurones and that α-motor innervation of muscle spindles is likely to play a more prominent role in flexors than in extensor muscles.

     

Medullary control of the pontine swallowing neurones in sheep

Summary The origin of the inputs from the medullary swallowing centre (dorsal region including the nucleus of the solitary tract, or ventral region corresponding to the reticular formation surrounding the nucleus ambigous) to the pontine swallowing neurones (PSNs) was studied in sheep anaesthetized with halothane.Out of 101 PSNs located in the posterior part of the trigeminal (Vth) motor nucleus, 46 were activated by stimulating either the dorsal (21 neurones) or the ventral (25 neurones) region of the ipsilateral medullary swallowing centre, 3–4 mm rostral from the obex. Thirty-one neurones out of the 46 were identified as a motoneurones supplying swallowing muscles (mylohyoïd, anterior body of digastric and medial pterygoïd). Their average activation latency through stimulation of the dorsal medullary region was about 1 ms longer than through stimulation of the ventral region (3.63 ms±0.81 versus 2.72 ms±0.32).To determine the origin of the medullary input to the PSNs, we tried to activate the medullary swallowing neurones (MSNs) antidromically through stimulating the posterior part of the Vth motor nucleus, which contains the swallowing motoneurones. Seventy-three MSNs were tested (25 located in the dorsal and 48 in the ventral region). None of the dorsal neurones tested could be antidromically activated by pontine stimulation: 15 ventral neurones showed a clear antidromic response (collision test) with an average latency of 2.5 ms±0.73. These neurones, which send their axons into the pons, were all located in the reticular formation, above the nucleus ambiguus, 3–4 mm rostral from the obex.These results suggest that MSNs in the ventral reticular formation connect the medullary swallowing centre to the Vth motor nucleus. They also suggest that during swallowing, inputs originating from the dorsal region of the medullary centre (interneurones programming the motor sequence) are relayed in the ventral region (reticular formation adjacent to the nucleus ambiguus) before reaching the PSNs.This work was supported, in part, by grants from CNRS (LA 205), INRA and M.R.I. (82 E 0685)     

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

Summary 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. It was thus concluded that the depolarizing potential was generated by electrical synapses between motoneurones.In a few motoneurones the electrical synaptic potential was found to elicit spike potentials. Latencies of these spikes were similar to those of the delayed excitation in motoneurones with the double discharge. The time course of changes in the excitability in these motoneurones showed that the delayed excitation, hence the late EMG response, was also caused by the electrical synaptic potential.     

The pharmacology and ionic dependency of amino acid responses in the frog spinal cord

1. The isolated frog spinal cord was used to study the action of amino acids and their antagonists on primary afferent terminals and motoneurones. The direct effects of these substances were observed by bathing the cord in 20 mM magnesium sulphate (thus blocking synaptic transmission) and recording the polarization level of the dorsal and ventral roots.2. gamma-Aminobutyric acid (GABA) and glutamic acid depolarized the dorsal root and reduced dorsal-root potentials, while glycine produced only weak and variable effects. Glutamic acid also depolarized the ventral root; GABA usually produced either a hyperpolarization or had little effect, while glycine caused variable effects.3. Bicuculline and picrotoxin antagonized all the synaptic potentials recorded on the dorsal root, as well as the GABA responses on both dorsal and ventral roots.4. All the synaptic potentials examined remained and were markedly prolonged in the absence of external chloride except the ventral root-dorsal root potential. Replacement of the physiologic complement of chloride during chloride-free perfusion restored the potentials to their original time courses.5. Depolarizing amino acid responses remained in the absence of external chloride, while hyperpolarizing responses were reversed into depolarizations. Return to normal Ringer solution re-established the hyperpolarizations.6. Removal of external sodium reversibly abolished the amino acid depolarizations but had little effect on the depolarizations in response to applications of high external potassium concentrations.7. The results support the hypotheses (a) that GABA mediates presynaptic inhibition by depolarizing primary afferent terminals and (b) that the GABA-mediated depolarization is sodium dependent.8. The results also indicate that GABA utilizes different ionic mechanisms to mediate presynaptic inhibition (sodium) and post-synaptic inhibition (chloride) in the amphibian (and presumably in the mammal).     

Monosynaptic excitatory connexions of reticulospinal neurones in the nucleus reticularis pontis caudalis with dorsal neck motoneurones in the cat

Summary 1. Projections of reticulospinal neurones (RSNs) in the nucleus reticularis pontis caudalis (N.r.p.c.) to dorsal neck motoneurones supplying splenius (SPL, lateral head flexor) and biventer cervicis and complexus (BCC, head elevator) muscles were studied in the cat anaesthetized with pentobarbiturate or -chloralose. 2. Threshold mapping for evoking antidromic spikes revealed that most of RSNs tested projecting down to brachial segments but not to lumbar segments (C-RSNs) gave off collaterals to the gray matter of the upper spinal cord in C2–C3 segments. 3. Spike triggered averaging showed that negative field potentials were evoked after firing of a single C-RSN (single fibre focal synaptic potentials, FSPs) in the region of C2–C3 where large antidromic field potentials from nerves supplying SPL or BCC muscles were evoked. The single fibre FSPs ranged between 1 and 10 V in amplitude and had latencies between 0.7 and 1.2 ms from the onset of the triggering spike. In most cases, a presynaptic spike preceded the negative potential by 0.3 ms. These results indicated that C-RSNs project to the SPL or BCC motor nucleus. 4. Spike triggered averaging of postsynaptic potentials revealed EPSPs (single fibre EPSPs) in 36 dorsal neck motoneurones, predominantly in SPL (25) and less in BCC (11) motoneurones, evoked from 15 C-RSNs. The amplitude of the single fibre EPSPs ranged from 5 to 310 V, and had latencies of 0.8–2.0 ms from the onset of the triggering spikes of C-RSNs, or 0.3–0.5 ms from the presynaptic spike when recorded. The results indicated monosynaptic excitatory connexions of C-RSNs to dorsal neck motoneurones. 5. Single fibre EPSPs from a C-RSN were usually recorded from either BCC or SPL motoneurones but not from both types of motoneurones, when tested in many motoneurones. This showed that connexions of C-RSNs with dorsal neck motoneurones were muscle specific. 6. RSNs projecting down to the lumbar segment (L-RSN) also showed branching in C2–C3 segments. Excitatory monosynaptic connexion of L-RSNs with neck motoneurones were demonstrated by recording single fibre postsynaptic population potentials (p.s.p.p.s.) from the C2 ventral root perfused with sucrose. The probability of evoking monosynaptic single fibre p.s.p.p.s. was less (19%) than for C-RSNs (59%).     

The effect of temperature on electrical interactions between antidromically stimulated frog motoneurons and dorsal root afferent axons

The effect of temperature on electrical interactions between antidromically stimulated motoneurons and dorsal root afferents was studied in the isolated and hemisected spinal cord of the frog, superfused with Ringer in which Ca2+ was equimolarly replaced by Co2+ or Mn2+ to suppress chemical synaptic transmission. Suction electrodes were used for stimulating and/or recording from dorsal and ventral roots from segments IX or X. Intrafibre recordings from sensory fibres were made at their point of entry into the spinal cord. Supramaximal ventral root stimuli elicited two distinct responses in the segmental dorsal root. First a brief short-latency depolarizing potential. Second, at temperatures below 11 degrees C, a second depolarizing root potential appeared following the short-latency depolarizing potential-I. Amplitude and duration of short-latency depolarizing potential-II reversibly increased as the bath temperature was decreased, reaching a maximum at 3 degrees C. Between 8 and 3 degrees C, short-latency depolarizing potential-II increased in amplitude by 20%/degrees C. In contrast short-latency depolarizing potential-I did not show substantial changes with temperature. The short-latency depolarizing potential-II, unlike short-latency depolarizing potential-I showed stepped fluctuations in amplitude, and appeared to be composed of unitary events. Intrafibre records revealed that the unitary events corresponded to action potentials on individual dorsal root fibres. Double shocks applied to the ventral root, at constant bath temperatures (below 11 degrees C), revealed facilitation of the short-latency depolarizing potential-II, which was maximal between 50 and 80 ms and lasted about 200 ms. Neither the antidromic motoneurone field potential nor the short-latency depolarizing potential-I were facilitated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhancement of synaptic transmission by dendritic potentials in chromatolysed motoneurones of the cat

1. Monosynaptic transmission in cat lumbosacral motoneurones undergoing chromatolysis was studied by intracellular recording from 7 to 20 days after section of the appropriate ventral roots.2. The average input resistance measured by passing polarizing currents across the cell membrane showed no significant difference between normal and chromatolysed motoneurones. Average rheobasic current for chromatolysed motoneurones was significantly lower (by about 30%) than that for normal motoneurones.3. Spike-like partial responses were commonly superimposed on monosynaptic EPSPs in chromatolysed motoneurones. These responses could be eliminated by stimulation of the bulbar inhibitory reticular formation, but could not be blocked by hyperpolarization applied to the motoneurone soma.4. The spike-like partial response in chromatolysed motoneurones showed a refractory period following (i) the antidromic invasion of the neurone generated by ventral root stimulation, and (ii) in response to two successive afferent stimuli. The refractory period ranged from 5 to 13 msec.5. Initiation of the partial response had no direct relation with the amplitude of the underlying EPSP. The partial response could be evoked by small EPSPs of about 0.5 mV.6. The action potential of a chromatolysed motoneurone arose from the partial response at different levels of depolarization, showing multiple trigger zones for spike initiation. Occasionally, chromatolysed motoneurones discharged in response to stimulation of a single afferent fibre.7. In neurones where more than one spike-like response was obtained, interaction between dendritic responses showed no refractoriness.8. It is concluded that the partial response is an all-or-none event originating at some discrete site on dendrites, and that its presence increases the efficacy of synaptic excitation in chromatolysed motoneurones.     

Synaptic actions produced by individual ventrolateral tract fibres in frog lumbar motoneurones

Excitatory post-synaptic potentials (EPSPs) were evoked in lumbar motoneurones of the isolated frog spinal cord by impulses in single ventrolateral tract fibres. In a few cases after recording an EPSP the fibre and the motoneurone involved were both filled with horseradish peroxidase (HRP) and the synaptic connexion between them was studied histologically. Monosynaptic EPSPs produced by direct stimulation of supraspinal (mainly reticulospinal) or unidentified (presumably propriospinal) fibres are mediated via chemical and, less frequently, dual-action synapses. The shape indices of chemical single-fibre EPSPs varied considerably in different connexions being, as a whole, similar to those of chemical components of EPSPs at synapses between primary afferents and motoneurones. Quantal analysis of the single-fibre EPSPs yielded quantal unit amplitude 18-113 microV and mean quantum content ranging from 1.14 to 16.4, the applicability of both Poisson and binomial models to transmitter release was revealed. Descending fibres electrically coupled with lumbar motoneurones were found to generate a depolarizing response to dorsal root stimulation. They were also characterized by a larger depolarization to superfused glutamate. The presence of electrical junctions between descending axons and spinal motoneurones suggests that the depolarization seen in these axons in response to synaptic excitation and glutamate could be the result of passive flow of depolarizing current from motoneurones electrically coupled to them. gamma-aminobutyric acid (GABA) did not produce conspicuous actions in axons forming both chemical and dual-action synapses. Axons injected with HRP have been followed to their site of termination in the lateral motor column. Synaptic boutons and varicosities were found to form contacts predominantly with dendrites of target motoneurones.     

On the control of myotomal motoneurones during “fictive swimming” in the lamprey spinal cord in vitro

Intracellular recordings have been made from myotomal motoneurones during “fictive swimming” in the in vitro preparation of the lamprey spinal cord, while monitoring the efferent burst activity in the ventral roots. The pattern of rhythmic activity in the motoneurones is described, as well as how synaptic inputs from the premotoneuronal level exert their control of motoneurone activity. (1) All motoneurones investigated displayed rhythmic, symmetric oscillations of their membrane potential during “fictive swimming”. The period of depolarization occurred in phase with the burst discharge in the ventral root containing the motoneurone axon. (2) About one-third of the cells fired bursts of action potentials during the depolarized phase, while the remaining motoneurones exhibited subthreshold oscillations. (3) Intracellular injection of chloride ions reversed the sign of the hyperpolarized phase, demonstrating phasic active inhibition of the motoneurones during rhythmicity. (4) The depolarized phase was unaffected after chloride injection, showing that the motoneurones also received phasic active excitation. (5) “Pre-triggered” averaging of the motoneurone recording (using the ventral root spikes from other motoneurones for triggering), revealed that some degree of synchronous excitation of several motoneurones occurred, suggesting common excitation from the same premotor-interneurones. It is concluded that the rhythmic oscillations of membrane potential in lamprey myotomal motoneurones during “fictive locomotion” depend on phasic excitation alternating with phasic active inhibition. The premotoneuronal mechanism responsible for this control may consist of reciprocally organized groups of excitatory and inhibitory interneurones.     

Effects of NH4+ on reflexes in cat spinal cord

1. In deeply barbiturate-anesthetized animals. NH4+ decreases spinal excitatory synaptic transmission by neuronal depolarization and subsequent block of conduction of action potentials into presynaptic terminals of low-threshold (presumably Ia-) afferents. Because barbiturates by themselves depress excitatory synaptic transmission and may have modified the effects of NH4+, this study examines the effect of NH4+ on excitatory synaptic transmission in the unanesthetized animal. 2. The effects of NH4+ on monosynaptic and polysynaptic excitatory reflexes as well as di- and polysynaptic inhibition were investigated in the spinal cord of the decerebrate and unanesthetized cat in vivo. 3. The monosynaptic excitatory reflex (MSR) elicited by muscle nerve stimulation and polysynaptic excitatory reflexes elicited by muscle (MSR-PSR) or cutaneous nerve stimulation (Cut-PSR) were recorded from the ventral roots L7 or S1. The P-wave was recorded from the cord dorsum. Di- and polysynaptic inhibition was elicited by muscle nerve stimulation and measured as decrease of the MSR. 4. Intravenous infusion of ammonium acetate (AA) decreased MSR and the monosynaptic motoneuron pool excitatory postsynaptic potential (EPSP) recorded from the ventral root (VR-EPSP). Decrease of MSR and VR-EPSP was accompanied by an increase of the intraspinal conduction time in presynaptic terminals. The maximal decrease of the MSR was preceded by a period of transient increase of the MSR and reflex discharges from previously subthreshold VR-EPSPs. 5. The effects of NH4+ on MSR and VR-EPSP are consistent with those in barbiturate-anesthetized animals and suggest that NH4+ also decreases monosynaptic excitation in unanesthetized animals by depolarization and subsequent conduction block for action potentials in presynaptic terminals. 6. Decrease of the MSR was accompanied by a decrease of the P-wave, indicating that NH4+ simultaneously decreases mono- and oligosynaptic excitatory synaptic transmission as well as presynaptic inhibition. 7. Decrease of the MSR was accompanied by increases of MSR-PSR and Cut-PSR and decreases of di- and polysynaptic postsynaptic inhibition. 8. The neuronal circuits underlying MSR-PSR and Cut-PSR include presynaptic inhibition of group I and II afferents as well as postsynaptic inhibition of motoneurons. It is suggested that increases of MSR-PSR and Cut-PSR are contributed to by decreases of pre- and postsynaptic inhibition and neuronal depolarization by NH4+. These effects increase afferent input to motoneurons, permit uncontrolled discharge of motoneurons, and initiate reflex discharges by previously subthreshold excitatory postsynaptic potentials.