Article Figures & Data
Figures
- Fig. 1.
A, Recordings from the femorotibialis muscle nerve (Fem), the LS7 ventral root (LS7), and the contralateral VLF during an evoked episode of rhythmic activity are shown. B, A cobalt gel was introduced around the caudal cord (LS3–LS7) to block synaptic transmission. Inset, The recording arrangement is shown. Approximately 30 min after the gel was applied, the activity of motoneurons under the gel (LS7) was depressed, indicating the efficacy of the blockade. The amplitude of the VLF potentials was slightly increased in the presence of the gel (possibly because of an increase in the space constant of axons within the gel). The decrease of Fem discharge after the gel was applied was probably caused by leakage of Co2+ from the rostral edge of the gel that abuts the femorotibialis pool of motoneurons (black bar on cord in inset).cVLF, Contralateral VLF.
- Fig. 2.
Examples of the activity patterns of ventrally located interneurons during an episode of rhythmic activity. The resting membrane potential is indicated at the beginning of the intracellular record. A, Irregularly discharging cell type. B, Interneuron with a firing behavior like flexor, sartorius motoneurons. Arrows indicate the pause in each cycle of discharge. The asterisk indicates the postepisode hyperpolarization seen in some cells. C, Interneuron with a firing behavior like extensor, femorotibialis motoneurons. D, The relationship between resting membrane potential and the amplitude of the peak synaptic drive for a population of 97 ventrally located interneurons. Theinset indicates symbols used for the different types of interneuron. VR, Ventral root; fem, femorotibialis muscle nerve.
- Fig. 3.
A, The effect of DC current injection on the synaptic drive of a ventrally located interneuron that received rhythmic synaptic potentials but did not fire at the resting membrane potential (−50 mV). The rhythmic drive potentials reversed close to −35 mV. B, Plot showing the relationship between the peak amplitude of the rhythmic synaptic drive and membrane potential for eight interneurons.
- Fig. 4.
A, Distribution of interneurons recovered after filling with neurobiotin. The lateral motor column (LMC) is indicated in A–C.B, Morphology of a ventrally located lumbosacral interneuron with an extensive axonal arborization within the lateral motor column. C, Morphology and location of a ventrally located interneuron identified by antidromic activation from the VLF. The dotted line demarcates the gray–white matter boundary.
- Fig. 5.
A, Comparison of the timing of discharge in muscle nerves [sartorius (sart) and femorotibialis (fem)] and the rostral VLF (rVLF) during a spontaneous episode of activity. The discharge was filtered at 100 Hz to 3 kHz, rectified, and integrated (τ = 20 msec). The dotted lines are aligned to the onset of discharge in the femorotibialis muscle nerve.B, Timing of slow-potential activity recorded from muscle nerves and the VLF at the onset of a spontaneous episode.Inset, The recording arrangement and the approximate location of the sartorius (open vertical bar) and femorotibialis (black vertical bar) motoneuron pools. The VLF slow potentials [rVLF (DC)] were recorded with a bandwidth of DC to 3 kHz, and therVLF discharge [rVLF(AC)] was recorded with a bandwidth of 100 to 3 kHz and further amplified. The muscle nerve recordings were obtained atDC to 3 kHz. The rapid rise of the VLF potential at the onset of the episode and the slow ramp in the sartorius record are indicated by arrows.
- Fig. 6.
A, B, Comparison of the timing of the slow potentials recorded in the VLF and the ventral root with the intracellularly recorded membrane potential of two ventrally located interneurons (A, B) at the onset of a spontaneous episode of rhythmic activity. Thearrowheads identify the time of the first spike in the intracellular recording. C, An averaged set of traces (5 interneurons) synchronized to the peak ventral root activity. Thethick line of each color is the mean, and the thin lines are the SEM.
- Fig. 7.
Cross-correlograms of the femorotibialis muscle nerve with the intracellularly recorded interneuronal rhythmic drive potential and population potentials recorded from the VLF. Thered dashed lines indicate 3 SEM. A,C, D, Correlograms are illustrated for three different neurons. B, The tracesshow the last three cycles of the potentials recorded during an episode of activity that were used to generate the cross-correlogram inA.
- Fig. 8.
Comparison of spontaneously occurring episodes recorded from LS3 ventral root (LS3 VR) and the VLF (VLF T7) before and after bath application of drugs [mecamylamine (50 μm); atropine (2 μm); and the 8–37 fragment of human α-CGRP (1 μm)] to block the synaptic action of recurrent motoneuron collaterals. A, Comparison of spontaneously occurring episodes before and after application of the drugs (control,black line; drugs, red line).B, Recording of the synaptic potentials in the LS3 ventral root after stimulation of the LS2 ventral root before (black line) and after (red line) application of the drugs. C, D, The effects of the drugs on the timing of VLF and ventral root activity at the start of spontaneously occurring episodes. The recordings inC were made before the drugs were applied and show three successive episodes (black, red, andblue lines) superimposed. The recordings inD were made after application of the blockers and show three successive episodes superimposed.
- Fig. 9.
A, The effects of graded VLF stimulation on evoked ventral root potentials are shown. The current intensity was increased from 16 to 31 μA in 1 μA increments. Thearrows indicate when amplitude measurements were made on the short- and long-latency components and after an episode had been triggered (episode). The blue asterisk indicates the appearance of the long-latency component in the blue traces. The red traces indicate activation of an episode. B, Plot of the normalized amplitude of the three components (short, long, and episode) as a function of the stimulus current is shown. C, Potentials are recorded from the sartorius muscle nerve in response to graded stimulation of the VLF. The numbers on the individualtraces indicate the stimulus current (in microamperes), and S indicates the stimulus artifact. In this example, the VLF was stimulated suprathreshold for triggering of an episode (20 μA; episode threshold, 7 μA, indicated by an arrow) and evoked a brief, high-intensity discharge in the muscle nerve (referred to as the synchronous spike; marked by anasterisk). D, The synchronous spike evoked in motoneurons was resistant to high-frequency (30 Hz) stimulation. The horizontal arrow identifies the first synchronous spike. The first stimulus artifact is marked byS and an arrowhead. Subsequent artifacts are indicated by arrowheads.
- Fig. 10.
Synaptic potentials evoked in individual interneurons in response to VLF stimulation. A, Synaptic potentials recorded in a ventrally located interneuron in response to VLF (upper trace) and ventral root (lower trace) stimulation. Notice that the interneuron is depolarized by the ventral root stimulus presumably by synaptic release of acetylcholine from recurrent motoneuron collaterals. Consistent with this interpretation, these synaptic potentials are blocked by mecamylamine (P. Wenner and M. J. O’Donovan, unpublished observations). B, VLF-evoked intracellular potential recorded in another ventrally located interneuron located in LS2.C, VLF-evoked synaptic potential recorded in a sartorius motoneuron. stim., Stimulation.
- Fig. 11.
Physiological evidence that the short-latency synchronous spike evoked in motoneurons by VLF stimulation is mediated by short (<5 segments) propriospinal rather than descending, ascending, or long-range propriospinal axons. A, The spinal cord was stimulated at several different levels (the rostral cut end at T1; the VLF at T1, T4, and T7; see D). The response in motoneurons was monitored from the femorotibialis muscle nerve (Fem) and the afferent volley (C,D, VLF volley) recorded by a glass electrode inserted into the VLF near T7 or LS1 (D).B, Records were obtained during a train at 20 Hz applied to the VLF. Notice the rapid growth in the amplitude of the synchronous response when the electrode is moved from T4 to T7. S, Stimulus artifact.
- Fig. 12.
A, Diagrams of the hypothesized synaptic organization and recruitment of VLF interneurons during graded stimulation of the VLF. B, Electrical responses recorded from the ventral roots during graded stimulation of the VLF (data from Fig. 9A). Three conditions are illustrated.Left, Low-threshold stimulation evoking only short-latency responses in motoneurons. Middle, An intermediate intensity of stimulation evoking long-latency responses.Right, Suprathreshold stimulation that triggers regenerative recruitment of the whole network and causes an episode to occur. The color code in A illustrates the neurons we hypothesize are recruited under the three stimulus conditions. The VLF is indicated by the parallel linessurrounded by the ellipses. The red lines(and cell bodies) indicate which axons have been activated directly by the stimulus. Notice that the number increases from left to right as the stimulus intensity is increased. Gray lines andcells are not activated directly by the stimulus. At the lowest stimulus intensity, it is assumed that the synaptic responses recorded in the ventral roots (shown in B) are mediated primarily by direct, monosynaptic contacts between the VLF axons and motoneurons. Notice in A, left, that the cell body of one neuron is antidromically activated but that this does not spread to recruit any of the other VLF neurons to which it is connected. At an intermediate intensity of stimulation (middle), several axons are directly activated (red), and these stimulated neurons synaptically activate other neurons (pink). These recurrently connected, synaptically activated neurons underlie the long-latency responses (asterisk). At the highest intensity of stimulation shown (right), three neurons are stimulated directly (red), and this results in regenerative recruitment of the whole network, and an episode occurs. InB, right, two stimulus intensities are illustrated. The lowest intensity (black trace) is just suprathreshold for an episode and evokes a long-latency response (atsmall arrow) that develops into an episode (atlong arrow). When the stimulus intensity is well above threshold (blue trace), an episode is triggered without the intervening recruitment of the long-latency response (see text for details). S, Stimulus artifact.
