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During fatiguing submaximal contractions a constant force production can be obtained at the cost of an increasing central command intensity. Little is known about the interaction between the underlying central mechanisms driving motor behaviour and cognitive functions. To address this issue, subjects performed four tasks: an auditory choice reaction task (CRT), a CRT simultaneously with a fatiguing or a non‐fatiguing submaximal muscle contraction task, and a fatiguing submaximal contraction task alone. Results showed that performance in the single‐CRT condition was relatively stable. However, in the fatiguing dual‐task condition, performance levels in the cognitive CRT deteriorated drastically with time‐on‐task. Moreover, in the fatiguing dual‐task condition the rise in force variability was significantly larger than during the fatiguing submaximal contraction alone. Thus, our results indicate a mutual interaction between cognitive functions and the central mechanisms driving motor behaviour during fatigue. The precise nature of this interference, and at what level this interaction takes place is still unknown.

Kernell, D. The limits of firing frequency in cat lumbosacral motoneurones possessing different time course of after hyperpolarization. Acta physiol. scand. 1965. 65, 87–100. — Long‐lasting repetitive discharges were set up in cat lumbosacral motoneurones by steady currents injected through the tip of an intracellular micro‐electrode. The upper and lower limits of discharge frequency of the various motoneurones were significantly correlated with the time course of their afterhyperpolarization the latter being measured from single spike discharges (generally antidromic). From previous work it is known that the relation between discharge frequency and current strength (the “f‐I relation”) is linear over a certain range from the minimal firing rate and upwards (Granit, Kernell and Shortess 1963, Kernell 1965 b). In the present paper it was shown that the maximal discharge frequency within this linear part of the f‐I relation was also significantly correlated with the time course of afterhyperpolarization. The findings are discussed with regard to problems of regulation of muscle tension by repetitive firing on the part of the motoneurones.

Kernell, D. High‐frequency repetitive firing of cat lumbosacral motoneurones stimulated by long‐lasting injected currents. Acta physiol. scand. 1965. 65. 74–86. Repetitive discharges were initiated in motoneurones by steady currents injected through an intracellular micro‐electrode. At weaker currents, producing firing rates up to on the average 51 (30–84) imp/sec, the steady discharge frequency was approximately linearly related to current strength (cf. Granit, Kernell and Shortess 1963 a). Several cells were capable of setting up steady discharges only within this so‐called “primary” range. In many other cells, steady repetitive firing was obtained even with stronger currents. There were in the latter two ranges of steady firing, the primary one referring to weak and a “secondary” referring to stronger stimulating currents, each approximately fitted by a separate straight line in the graphs relating impulse frequency to current strength. The straight line wich referred to steady firing within the secondary range had a 2–6 times steeper slope than the one characterizing the primary range. Motoneurones capable of maintained firing within the secondary range could reach steady firing rates of the order of 125 (88–195) imp/sec. The maximal impulse frequency within the primary range was about the same initially in the discharge as later on. The findings are discussed also with regard to the repetitive firing of reflexly activated motoneurones.

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SUMMARY1. The arm area of the baboon's precentral motor cortex was stimulated by brief surface-anodal pulses, and the discharge of the corticospinal tract (the 'pyramidal tract waves') was recorded by an electrode resting on the dorsolateral surface of the cervical spinal cord.2. Some properties of the pyramidal tract waves were described, and they were also studied in relation to the firing of single cortico spinal fibres.3. The results led to the conclusion that the later pyramidal tract waves (the 'I waves') were almost exclusively due to a semi-synchronous repetitive discharge of the same fast cortico spinal fibres as those responsible for the initial wave (the 'D wave').4. Some problems concerning the origin and significance of the I waves were discussed.

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