Renshaw cells and recurrent inhibition: comparison of responses to cyclic inputs

Effects of evoked pain on the electromyogram and compound muscle action potential of the brachial biceps muscle

Muscle pain is often accompanied by a feeling of muscle fatigue and weakness. We examined the effect of experimental muscle pain on the electromyogram (EMG) during maximal voluntary contraction (MVC), and on the compound muscle action potential (CMAP) of the brachial biceps muscle. Twenty-one healthy subjects were injected intramuscularly with 0.1 ml/30 microg of the vanilloid receptor agonist (capsaicin) or 0.2 ml of 5% hypertonic saline. A Teflon-coated cannulated EMG needle was used to record the EMG interference pattern (IP) at MVC. The CMAP of the brachial biceps muscle was obtained by stimulation of the musculocutaneous nerve at the axilla using surface electrodes. Amplitude, mean frequency of the power spectrum, and turns/s of the interference pattern were reduced after pain induced by capsaicin or hypertonic saline. Latency, amplitude, and area-under-curve of the CMAP did not change after injection of either substance. Acute stimulation of muscle nociceptors thus produced a fatiguelike change in the interference pattern during MVC, possibly due to a decrease in motoneuron firing rate and increased muscle fatigability.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Effects of neurochemically excited group III-IV muscle afferents on motoneuron afterhyperpolarization

When humans voluntarily and maximally contract a muscle under isometric conditions, the average firing rate of motor units decreases from an initially high value over several tens of seconds. The mechanisms underlying the rate reduction are probably manifold. One mechanism could involve changes in the motoneuron afterhyperpolarization, another reflex effects of group III-IV muscle afferents that are excited during developing muscle fatigue. It appears possible that changes in motoneuron afterhyperpolarization are mediated by these afferent inputs. We therefore studied effects on motoneuron afterhyperpolarization of small-diameter muscle afferents excited by intra-arterially injected metabolites such as bradykinin and serotonin. In decerebrate and mostly spinalized cats, lumbosacral alpha-motoneurons were recorded intracellularly. Current pulses were injected to test for input resistance and elicit action potentials and afterhyperpolarizations. Afterhyperpolarizations were averaged from c. 10 successive stimulus repetitions. Measurements were taken of afterhyperpolarization amplitude, half-width and area; and exponential functions were fitted to the afterhyperpolarization decay phase to determine afterhyperpolarization decay time-constants. In selected cases, the entire afterhyperpolarization trajectory was fitted with a sum of two exponentials to assess more precisely changes in afterhyperpolarization trajectory. Small catheters were inserted into side-branches of the sural artery and the accompanying vein to apply substances like bradykinin, serotonin and KCl to the calf muscles. Concentrations were in the range of those used by other workers. Intra-arterial injection of bradykinin and serotonin usually decreased blood pressure, which may at times have affected mean motoneuron membrane potentials. Afterhyperpolarization amplitude usually changed with membrane potential in a way expected from ensuing changes in driving potential. Whenever excitation of group III-IV muscle afferents caused moderate to strong increases in motoneuron synaptic noise, afterhyperpolarization amplitudes were reduced, usually in parallel to decreases in input resistance. Afterhyperpolarization half-widths were mostly unaffected, but occasionally decreased. There was a significant trend for afterhyperpolarization decay time-constants to increase during increased synaptic noise, this increase being inversely correlated with the reduction in afterhyperpolarization amplitude. The reduction in input resistance was associated with a decrease in the membrane time-constant, which could therefore not account for the prolongation of the afterhyperpolarization decay time-constant. The afterhyperpolarization area decreased, indicating that the reduction of afterhyperpolarization amplitude outweighed the prolongation of afterhyperpolarization decay time-constant. During a prolonged fatiguing muscle contraction group III-IV afferents become increasingly excited, produce augmenting synaptic inputs in motoneurons, and will change afterhyperpolarization properties. On average, these changes per se tend to diminish the effect of afterhyperpolarization on motoneuron discharge.

On the role of recurrent inhibitory feedback in motor control

This article reviews presumed roles of recurrent inhibition in motor control, that have been proposed over the past five decades. The discussion is structured in an order of increasing complexity. It starts out with the simplest and earliest circuit, that is recurrent self-inhibition of skeleto-motoneurons, and related functions. It soon becomes clear that in order to understand recurrent inhibition, we must look beyond the simple self-inhibitory CNS circuit. First, recurrent inhibition must be seen in the context of other neural circuits. Second, some quantitative features appear to be correlated with features of the neuromusculo-skeletal periphery. Third, the aspect of lateral inhibition between different members of a motoneuron pool as well as between different motoneuron pools points to the essential multiple input-multiple output structure of recurrent inhibition that again can be understood only by correlating it with features of the neuromusculo-skeletal periphery. Another extension results from the discovery that recurrent inhibition affects not only skeleto-motoneurons, but also gamma-motoneurons, Ia inhibitory interneurons mediating reciprocal inhibition between antagonist motoneurons, other Renshaw cells and cells of origin of the ventral spinocerebellar tract (VSCT). Then the view broadens again, investigating the potential role that recurrent inhibition plays in two far-ranging theories of motor control, the inverse-dynamics approach and the equilibrium-point hypothesis. Finally, the present author tries to formulate, in broad strokes, a personal functional interpretation of recurrent inhibition. All the functional considerations, right or wrong, should yield ideas for new experiments, and this then is the last objective of this review.