During the course of work on the effects of polarizing currents on the electrical activity of the rat cerebral cortex, we found that prolonged changes in the level of cortical activity could be produced by relatively brief periods of polarizing. This paper describes the after-action of transcortical polarizing current upon the activity evoked by stimulating the forepaw and upon spontaneous firing in cortical neurones. METHODS Experimental AnimalsAbout 200 albino rats of 200-250 g body weight were used. Each rat was anaesthetized with intraperitoneal urethane (360O solution, 0 5 c.c./100 g) and fixed in a rigid head-holder. The cortical surface was exposed by making a trephine hole 4 mm dia. and attaching a polyethylene cup to the skull as described by Bindman, Lippold & Redfearn (1962a) or by drilling 500 , dia. holes with an electric drill. Body temperature was maintained within 0.20 C of a given temperature, usually 350 C, with a 12 V heater driven by an OC 35 transistor, controlled by a thermistor in the rectum.Stimulation and polarization For evoked potential studies, stimuli were delivered at 2 sec intervals to the skin of the forepaw by means of two fine stainless steel needles inserted beneath the skin. The pulse generator gave 200 ,Lsec square waves which were isolated from earth and fed to the preparation by a 1:1 transformer.,The polarizing circuit consisted of a battery across which was connected a potentiometer. A series swamping resistance of 10 MQ was used in order to minimize any effects due to variation in resistance of the electrodes or the preparation. A galvanometer having a deflexion of 4 cm/,uA enabled the current flowing in the circuit to be measured. ElectrodesGlass micro-pipettes, usuallyfilled with 10%/' NaCl solution (and some filled with 1.8 % NaCl and some with 11 O0 KCI), were connected to the pre-amplifier by Ag-AgCl wire. Tip diameters were from 0 5 to 20 , according to requirements. In some experiments non-polarizable wick electrodes or agar gel ones were used for polarizing. * M. R. C. Scholar. 24Physiol. 172

SUMMARY1. During fatigue from a sustained maximal voluntary contraction (m.v.c.) the mean motoneurone discharge rates decline. In the present experiments we found no recovery of firing rates after 3 min of rest if the fatigued muscle was kept ischaemic, but near full recovery 3 min after the blood supply was restored.2. Since 3 min is thus sufficient time for recovery of any central changes in excitability, the results support the hypothesis that, during fatigue, motoneurone firing rates may be regulated by a peripheral reflex originating in response to fatigue-induced changes within the muscle.

SUMMARY1. Tungsten micro-electrodes have been used to record the electrical activity of single motor units in the human adductor pollicis during maximal voluntary contractions. The potentials were characteristic of those from single muscle fibres.2. In brief maximal contractions, the firing rates of over 200 motor units were obtained from five normal subjects. Four subjects had a similar range (mean 26-4 + 6-5 Hz) while the fifth was slightly higher (35 + 7 4 Hz).3. When maximal voluntary force was sustained for 40-120 s, there was a progressive decline in the range and mean rate of motor-unit discharge. In the first 60 s, mean rates fell from about 27 Hz to 15 Hz. There was some evidence to suggest that those units with the highest initial frequencies changed rate most rapidly.4. It is suggested that this decline in motor unit discharge rates is not responsible for force loss, but that it may enable effective modulation of voluntary strength by rate coding to continue during fatigue.

Measurements were made from the human adductor pollicis muscle of force, contractile speed, and electromyographic activity (EMG) before, during, and after maximal isometric voluntary contractions sustained for 60 s. The use of brief test periods of maximal nerve stimulation with single shocks or trains of shocks enabled various muscle mechanical properties to be studied throughout each contraction. Electrical activity was measured after rectification and smoothing of the surface potentials and also by counting the total number of potentials per unit time from a population of motor units using fine wire intramuscular electrodes. During a 60-s maximal voluntary contraction, the force fell by 30-50%. Throughout the experiment the voluntary force matched that produced by supramaximal tetanic nerve stimulation. This indicated that, with sufficient practice, full muscle activation could be maintained by voluntary effort. However, the amplitude of the smoothed, rectifed EMG and the rate of spike counts declined. Since no evidence for neuromuscular block was found, the decline in EMG and spike counts was attributed to a progressive reduction of the neural drive from the central nervous system, despite maintained maximum effort. After the prolonged voluntary contractions twitch duration was prolonged, mainly as a result of slowing in relaxation rate. Twitch summation in unfused tetani increased. Both the maximum rate of relaxation and the time course of force decay declined by 50-70%. Similar changes were seen in both voluntary contractions and in test periods of stimulation. The percentage change in muscle contractile speed measured by these parameters approximately equaled the percentage change in the surface EMG measured simultaneously. It is concluded that 1) during a 60-s sustained maximal voluntary contraction there is a progressive slowing of contraction speed such that the excitation rate required to give maximal force generation is reduced, 2) the simultaneous decline in EMG may be due to a continuous reduction in motoneuron discharge rate, and 3) the EMG decline may not necessarily contribute to force loss.

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