Spinal neurons bursting in phase with fictive scratching are not related to spontaneous cord dorsum potentials.

Spontaneous cord dorsum potentials (spontaneous CDPs) are produced by the activation of dorsal horn neurons distributed along the L4 to S1 spinal cord segments, in Rexed's laminae III-VI, in the same region in which there are interneurons rhythmically bursting during fictive scratching in cats. An interesting observation is that spontaneous CDPs are not rhythmically superimposed on the sinusoidal CDPs generated during fictive scratching episodes, thus suggesting that the interneurons producing both types of CDPs belong to different spinal circuits. In order to provide experimental data to support this hypothesis, we recorded unitary activity of neurons in the L6 spinal cord segment. We found that the neurons firing rhythmically during the sinusoidal CDPs associated with the extensor, flexor or intermediate phases of scratching were not synchronized with the spontaneous CDPs. Moreover, we found that the neurons firing during the spontaneous CDPs were not synchronized with the sinusoidal CDPs. These results suggest that the neurons involved in the occurrence of spontaneous CDPs are not part of the spinal cord central pattern generators (CPGs). This study will be relevant for understanding the relationships between the spinal cord neuronal populations firing spontaneously and the CPGs, in the intact and injured spinal cord.

GABAA receptors mediate motoneuron tonic inhibition in the turtle spinal cord.

GABA(A) receptors mediating tonic inhibitory currents are present in neurons from hippocampus, cerebellum, sensory cortex and thalamus. These receptors located at peri- and extra-synaptic sites are constituted mainly by α(4/6) and α(5) subunits which confer them high affinity for GABA and low desensitization. Immunohistochemical and in vitro hybridization studies have shown the expression of these subunits, while functional studies have reported the presence of GABAergic tonic currents in spinal dorsal horn neurons. However, the presence of this inhibitory current has not been documented in motoneurons. In addition, we previously reported that the monosynaptic reflex is facilitated by furosemide, an antagonist of the α(4/6) GABA(A) receptors, without affecting the dorsal root potential, which suggests the presence of a GABAergic tonic inhibitory current in motoneurons. The aim of this work was to investigate the presence of high affinity GABA(A) receptors in motoneurons. By intracellular recordings made with sharp electrodes and the whole-cell patch clamp recording technique we show here that the membrane input resistance and the monosynaptic excitatory post-synaptic potential (EPSPs) are significantly increased by bicuculline. Likewise, the depression of the EPSPs and the input membrane resistance normally induced by muscimol was partially reverted by 20 µM bicuculline and abolished when the concentration of the antagonist was raised to 100 µM. Last, bicuculline at low concentration did not affect the holding current as occur with the high concentration that block the tonic inhibitory GABAergic current. Together these results suggest that the excitability in motoneurons may be tonically inhibited by high affinity GABA(A) receptors.

Cholinergic facilitation of erection and ejaculation in spinal cord-transected rats.

Penile reflexes (PRs) were monitored in chronic spinal cord-transected rats by identifying them visually, and at the same time they were recorded as the electromyographic activity of bulbospongiosus muscles. Intraperitoneal injection of the agonist muscarine (10 microg) produced a facilitation of PRs. A decrease in the latency, an increase in the number of clusters and often an increase in the duration of cups were found after muscarine. In addition, 66% (six out of nine) of the animals ejaculated after muscarine. These results suggest that cholinergic receptor stimulation may be involved in erectile and ejaculatory mechanisms mediated by the spinal cord.

Dorsal root potential produced by a TTX-insensitive micro-circuitry in the turtle spinal cord.

1, The mechanisms underlying the dorsal root potential (DRP) were studied in transverse slices of turtle spinal cord. DRPs were evoked by stimulating one filament in a dorsal root and were recorded from another such filament. 2. The DRP evoked at supramaximal stimulus intensity was reduced but not eliminated after blockade of GABAA receptors. The remaining component was eliminated by blocking NMDA and AMPA receptors. 3. The DRP was reduced but not eliminated after blockade of AMPA receptors. The early component of the remaining DRP was dependent on GABAA receptors and the residual component on NMDA receptors. 4. The DRP was reduced but not eliminated by TTX. GABAA, NMDA and AMPA receptors contributed to the generation of the TTX-insensitive DRP. The early component of the DRP in the presence of TTX depended on GABAA receptor activation, and the late component mainly on the activation of NMDA receptors. 5. Our results show that part of the DRP is generated by a TTX-resistant, probably non-spiking micro-circuit with separate components mediated by GABA and glutamate.

Oscillatory interaction between dorsal root excitability and dorsal root potentials in the spinal cord of the turtle.

The response to dorsal root stimulation, at one to two times threshold, was investigated in the isolated cervical enlargement of the turtle spinal cord. At frequencies near 10 Hz the synaptic response in motoneurons and the cord dorsum potential, after an initial lag time, oscillated in amplitude with a period of more than 1 s. The mono- and polysynaptyic postsynaptic response in motoneurons, the pre- and postsynaptic component of the cord dorsum potential and the dorsal root potential oscillated in synchrony. These oscillations were only observed with stimulus frequencies in the range 9-11 Hz. The oscillating response could only be evoked from stimulus sites to which dorsal root potentials were conducted from the spinal cord (2-3 mm). At more distant stimulus sites cyclic variations in amplitude of the cord dorsum potential and the synaptic response in motoneurons were not observed. During an oscillating spinal response to a stimulus train in one dorsal root filament, the response evoked by a stimulus in another short filament (2-3 mm) from the same root varied in amplitude with the induced oscillation. The spinal response to a stimulus in a longer filament (i.e. more than 3 mm) did not oscillate. It is argued that the oscillating responses described rely on interactions between distributed elements rather than on unit oscillators. We also show that primary afferent transmission is unaffected by the substantial variations in dorsal root potentials during oscillations.

Local facilitation of plateau potentials in dendrites of turtle motoneurones by synaptic activation of metabotropic receptors.

1. The spatial distribution of synaptic facilitation of plateau potentials in dendrites of motoneurones was investigated in transverse sections of the spinal cord of the turtle using differential polarization by applied electric fields. 2. The excitability of motoneurones in response to depolarizing current pulses was increased following brief activation of either the dorsolateral funiculus (DLF) or the medial funiculus (MF) even when synaptic potentials were eliminated by antagonists of ionotropic receptors. 3. The medial and lateral compartments of motoneurones were differentially polarized by the electric field generated by passing current between two electrodes on either side of the preparation. In one direction of the field lateral dendrites were depolarized while the cell body and medial dendrites were hyperpolarized (S- configuration). With current in the opposite direction the cell body and medial dendrites were depolarized while lateral dendrites were hyperpolarized (S + configuration). 4. Following brief activation of the DLF the excitability and the generation of plateau potentials were facilitated during differential depolarization of the lateral dendrites but not during differential depolarization of the cell body and medial dendrites. Following brief activation of the MF the excitability and generation of plateau potentials were facilitated during differential depolarization of the cell body and medial dendrites but not during differential depolarization of the lateral dendrites. 5. It is concluded that the synaptic facilitation of the dihydropyridine-sensitive response to depolarization is compartmentalized in turtle motoneurones.

Metabotropic synaptic regulation of intrinsic response properties of turtle spinal motoneurones.

1. The effect of a brief train of electric stimuli in the dorsolateral funiculus on the intrinsic response properties of turtle motoneurones was investigated in transverse sections of the spinal cord in vitro. 2. Even when glutamatergic, GABAergic and glycinergic ionotropic synaptic transmission was blocked by antagonists of AMPA, NMDA, glycine and GABA receptors, dorsolateral funiculus (DLF) stimulation induced a facilitation of plateau potentials during current clamp and the underlying inward current in voltage clamp. This facilitation lasted more than 10 s. 3. The plateau potential and the facilitation by DLF stimulation was absent in the presence of 10 microM nifedipine. The DLF-induced facilitation was reduced by antagonists of 5-HT1A, group 1 metabotropic glutamate receptors and muscarine receptors. 4. These findings suggest that the intrinsic properties of spinal motoneurones are dynamically regulated by afferent synaptic activity. These afferents can be of spinal and extraspinal origin. Continuous regulation of intrinsic response properties could be a mechanism for motor flexibility.

Methods to find aponeurosis and tendon stiffness and the onset of muscle contraction.

A method to measure small movements of living tissues either large or small is presented. The method is based on the detection of changes in reflected infrared light. An optocoupler (coupled photodiode and photodetector) and a small (< 1 cm2) mirror were used. The optocoupler (OC) has a low cost and it can be calibrated easily. It can be also used as the transducer of a strain-gage. Three different uses are shown: (a) as a strain-gage transducer; (b) detection of tendon and aponeurosis movements in large muscles (cat soleus); (c) detection of the onset of muscle contraction. Movements of less than 1 microm can be detected with the aid of automatic averaging of the signals. Concerning the second use (b), it permits the estimation of tendon stretch. Concerning the third use, the onset of muscle movement precedes by at least 2 ms that of the force recorded at the tendon.

The cat pudendal nerve: afferent fibers responding to mechanical stimulation of the perineal skin, the vagina or the uterine cervix.

Some afferent fibers from the pudendal nerve of the female cat were stimulated by pressing on the perineal skin, the vagina or the uterine cervix. Three different types of skin mechanoreceptors were found: (1) with low threshold (< 20 mg) and slow-adapting discharges; (2) with high threshold (0.1-0.5 g) and slow-adapting discharges; and (3) with low threshold and fast-adapting discharges. Most of these receptors increased their firing frequency as the velocity of skin indentation was increased (velocity detectors). The average conduction velocity of the skin afferents was 29 +/- 9 m/s. The receptors located at the vagina showed a fast-adapting response to probing and were sensitive to the velocity of the probe movement. Most of these receptors, however, showed a slow adaptation when the vaginal wall was distended with a balloon. The conduction velocity in vaginal afferents was 37 +/- 16 m/s. Those receptors responding to pressure on the uterine cervix adapted slowly to constant pressure but were sensitive to the velocity of the pressure pulses. The conduction velocity in the afferents from the uterine cervix was 31 +/- 9 m/s.

Prolonged inhibition of the flexor reflex by probing the cervix uteri in the cat.

In decerebrate or spinal cats, sustained mechanical stimulation of the cervix uteri inhibited the flexor reflex elicited by electrical stimulation of the foot pad during the probing period (160 s). After probing, 3-15 min were required for reflex recovery. No additional inhibition was produced if probing was repeated before recovery, but instead the reflex was facilitated. When probing was applied 5-10 min after reflex recovery the reflex was again abolished. The recovery, however, occurred earlier and was followed by facilitation. Probing the cervix with single mechanical pulses inhibited transiently (140-200 ms) the short latency reflex components, but the components with longer latencies are unaffected or facilitated. Distension of the vaginal wall with a balloon also inhibited the flexor reflex, but a transient, mild facilitation appeared several seconds after the distension. In general, whenever the inhibition decreases, the facilitation predominates. Our findings suggest that cervical probing or vaginal distension triggers both a long-lasting inhibition and a concomitant facilitation in different intraspinal flexor reflex pathways.

Sustained activation of the triceps surae muscles produced by mechanical stimulation of the genital tract of the female cat.

In decerebrate cats, controlled mechanical stimulation of the perivulvar skin, the vaginal wall or the cervix uteri induced visible hind limb extension. Pressing on the cervix uteri produced the greater response. To quantify these responses, the EMG activity and the tension developed by the normally inserted triceps surae muscles were recorded. The activity induced in these muscles by stimulation of the genital canal outlasted the stimulus by many seconds or a few minutes. These effects disappeared after spinalization at the T12 level. We propose that stimulation of the vaginal canal in the female cat may induce bistability of triceps surae motoneurones.

The influence of peripheral connections on wallerian degeneration.

Following electrophysiological techniques we investigated whether wallerian degeneration of the cat sural nerve may be influenced by (1) its peripheral connection and (2) the distance from the cell body. Distal stumps connected to their innervation sites peripherally showed less degeneration than proximal ones when isolated between two sections. However, when two isolated adjacent segments were produced by making 3 sections, the resulting degeneration was more pronounced in the distal segment. Thus, both the factors mentioned above appear to influence wallerian degeneration.