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CLASSICAL PERSPECTIVES |
1 Department of Anaesthesia, The Copenhagen Muscle Research Centre, Rigshospitalet, University of Copenhagen, Denmark
Email: nhsecher{at}rh.hosp.dk
Even one small step for man represents a giant challenge for integration of not only motoneurons directly involved in the movement, but also for those involved in regulating ventilation and circulation to enhance oxygen delivery to the muscles. Ventilation and circulation are kept at a low level at rest to allow graded increases to occur during exercise as controlled by feedback from the muscles (‘the muscle pressor reflex’) as well as by a central nervous influence, termed ‘central command’. Such feed-forward control of oxygen transport is important because anaerobic capacity is small as illustrated by reptiles who, after moving very fast from one place to the other, need to sit still in order to eliminate the oxygen deficit because their lungs represent little more than balloons encompassing only few alveoli.
For humans it takes a few minutes for steady-state oxygen uptake to be established during exercise illustrating that either feed-forward control of oxygen delivery is a weak influence or that there are considerable energy stores in skeletal muscle. Yet, feed-forward control was addressed by J. E. Johansson (1862–1938) in Stockholm as early as 1893 and from careful experiments in the rabbit, he found convincing evidence for feed-forward control of both heart rate and ventilation (Johannsson, 1983). It had to wait 20 years before the subject was addressed in humans and then through an international collaboration. Krogh & Lindhard (1913) could not monitor heart rate in Copenhagen. In Oxford Miss Buchanan, however, was in possession of an ECG apparatus. In a separate effort both the initial responses of heart rate and ventilation could then be reported and it became clear that the responses were very fast, and variables might even increase in anticipation of exercise. Interestingly, Miss Buchanan was not a co-author on the resulting paper and one can only speculate whether gender bias was of significance in that regard.
In numerous follow-up papers, it has been shown that central command is operative at the onset of exercise (Secher, 1999). Partial as well as full neuromuscular blockade, regional anaesthesia and both passive and imagined exercise have been applied as experimental models to separate the central and reflex influences on the investigated variables and in many of these investigations J. H. Mitchell in Dallas has played an important role. It is now established that the first increase in heart rate stems from the central nervous system but that the reflex influence makes its contribution quickly, within the last one third of the first heart beat (Williamson et al. 1995). The responses at the onset of exercise are, however, more complicated. The increase in heart rate, even within the first beat, is proportional to the force developed and a decrease in heart rate is noted at the onset of very weak contraction as exemplified during shooting (Fig. 1). There may also be a transient decrease in blood pressure similar to that seen when humans stand up, and most people have experienced blurred vision in that situation.
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Obviously, it was not possible to place the electrodes at will, and the authors had to use the placement that was chosen on medical grounds. Thereby there is no claim that the site of central command is identified, but that areas of the brain that are or are not important relay stations are defined. Refined and often non-invasive techniques allow for human studies on integrative aspects of physiology that previously could be thought of only in an animal preparation. As illustrated in the papers by Green et al. non- or minimally invasive observations can be supplemented by unique observations in patients as pioneered for the study of the central nervous system when W. Penfield (1891–1976) mapped the somatosensory cortex by electrical stimulation during neurosurgical procedures carried out with a local anaesthetic. Penfield is quoted to state that he had ‘an experimental preparation’ that could talk back to him!
References
Green AL, Wang S, Owen SLF, Xie K, Liu X, Paterson DJ, Stein JF, Bain PG & Aziz TZ (2005). Deep brain stimulation can regulate arterial blood pressure in awake humans. Neuroreport 16, 1741–1745.[CrossRef][Medline]
Green AL, Wang S, Purvis S, Owen SLF, Brian PG, Stein JF, Guz A, Aziz TZ & Paterson DJ (2007). Identifying cardiorespiratory neurocircuitry involved in central command during exercise in humans. J Physiol 578, 605–612.
Johansson JE (1983). Ueber die Einwirkung der Muskelth
tigkeit auf die Athmung und die Hrzthtigkeit. Skand Arch Physiol 5, 20–66.
Krogh A & Lindhard J (1913). The regulation of respiration and circulation during the initial stages of muscular work. J Physiol 47, 112–136.
Nowak M, Holm S, Biering-Sørensen F, Secher NH & Friberg L (2005). "Central command" and insular activation during attempted foot lifting in paraplegic humans. Hum Brain Mapp 25, 259–265.[CrossRef][Medline]
Secher NH (1999). Cardiovascular function and oxygen delivery during exercise. In Physiological Determinants of Exercise Tolerance in Humans, ed. Whipp BJ & Sargent AJ, pp. 93–113. Portland Press, Colchester.
Williamson JW, Nobrega AC, Winchester PK, Zim S & Mitchell JH (1995). Instantaneous heart rate increase with dynamic exercise: central command and muscle-heart reflex contributions. J Appl Physiol 78, 1273–1279.
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