Circulatory and respiratory changes in response to muscular exercise in man

Defining the neurocircuitry of exercise hyperpnoea

One hundred years ago in this journal, Krogh and Lindhard published a seminal paper highlighting the importance of the brain in the control of breathing during exercise. This symposium report reviews the historical developments that have taken place since 1913, and attempts to place the detailed neurocircuitry thought to underpin exercise hyperpnoea into context by focusing on key structures that might form the command network. With the advent of enhanced neuroimaging and functional neurosurgical techniques, a unique window of opportunity has recently arisen to target potential circuits in humans. Animal studies have identified a priori sites of interest in mid-brain structures, in particular the subthalamic locomotor region (subthalamic nucleus, STN) and the periaqueductal grey (PAG), which have now been recorded from in humans during exercise. When all data are viewed in an integrative manner, the PAG, in particular the lateral PAG, and aspects of the dorsal lateral PAG, appear to be key communicating circuitry for 'central command'. Moreover, the PAG also fulfils many requirements of a command centre. It has functional connectivity to higher centres (dorsal lateral prefrontal cortex) and the basal ganglia (in particular, the STN), and receives a sensory input from contracting muscle, but, importantly, it sends efferent information to brainstem nuclei involved in cardiorespiratory control.

Claude Gordon Douglas, 1882-1963

Claude Gordon Douglas was born in Leicester on 26 February 1882, the son of Claude Douglas, F.R.C.S., Honorary Surgeon to the Leicester Royal Infirmary, and Louisa Bolitho Peregrine of London. His elder brother, J. S. G. Douglas, D.M., was Professor of Pathology at the University of Sheffield. Douglas was unmarried and for the last part of his life he lived with his younger sister, Miss Margaret Douglas, who died in 1960. Besides his father, both grandfathers and one great-grandfather were in medical practice and another great-grandfather was a keen student of natural history. From Arlington House, his preparatory school in Brighton, he won open scholarships in classics to Radley College, which he did not accept, and to Wellington College, which he accepted. He left Wellington College at the age of sixteen in order to study science at Wyggeston Grammar School, Leicester. In 1900 he went up to New College, Oxford, with an open exhibition in Natural Science (Biology) but in his first term won an open demyship in the same subject at Magdalen College, whither he migrated for the remainder of his time as an undergraduate. As an undergraduate he owed much to the personal friendship and encouragement of Professor Francis Gotch, F.R.S., and from an early stage he was especially influenced by the teaching of Dr J. S. Haldane, F.R.S. His undergraduate friendships with G. R. Girdlestone, later the eminent orthopaedic surgeon, and with A. S. (later Sir Arthur) MacNalty, were maintained throughout his life. MacNalty recounts how he provided notes on the professor’s five o’clock lectures for Douglas and Girdlestone when they were delayed on the golf course.

Muscle fibre types, ubiquinone content and exercise capacity in hypertension and effort angina

The composition of skeletal muscle fibre expressed as a percentage of slow twitch (ST), type I or "red" and fast twitch (FT), type II or "white" were determined in patients with hypertension (HT) or with severe ischaemic heart disease (IHD) and compared to age matched controls. Similarly, exercise capacity expressed as the cycle intensity eliciting a blood lactate concentration corresponding to 2.0 mmol x 1-1 were compared with healthy controls. Both patient groups had a higher percentage of FT fibres with relatively lower exercise capacities than their controls. The exercise capacities were reduced even when the relationship of decreased capacity with the percentage of increased FT was considered. There was an increase IHD but not in HT in patients with fibre subgroup FTc, which most probably reflected fibre trauma. Both patient groups were low in the skeletal muscle mitochondrial electron carrier and unspecific antioxidant ubiquinone, coenzyme Q10 or CoQ10. Patients with IHD but not HT showed, however, a faster fall in the ratio CoQ10 over ST% the higher the percentage value of ST. The ratio reflects the antioxidant activity related to CoQ10 in the fibre hosting most of the oxidative metabolism. A low ratio indicates a risk of metabolic lesion and cell trauma. This could explain fibre plasticity and offer an alternative cause to heredity in elucidating in deviating muscle fibre composition in patients with HT and IHD.

The effect of treadmill speed on ventilation at the start of exercise in man

1. The change in ventilation at the start of exercise was determined during both hyperoxic rebreathing and air breathing in four volunteers. 2. In order to differentiate between the effects of limb-movement frequency and exercise load in terms of oxygen uptake, three treadmill exercises were tested: E1, at an oxygen uptake of 1 l/min on a level treadmill; E2, at 2 l/min on an inclined treadmill at the same speed as E1; E3, at 2 l/min on a level treadmill at a higher speed. All of the exercises were performed at a walking pace. 3. Prior to rebreathing, hyperventilation for 5 min to 20 mmHg was used to reduce carbon dioxide to below the central chemoreceptor threshold. From eleven to fourteen rebreathing experiments were done on each volunteer for each of the three exercises, with the treadmill started at carbon dioxide levels which ranged from 36 (below threshold) to 58 mmHg (above threshold). 4. Ten experiments were performed on each volunteer for each of the three exercises during air breathing, with the treadmill started after 5 min of rest. 5. In both the rebreathing experiments and the air breathing experiments it was found that the change in ventilation at the start of exercise was the same for exercises E1 and E2, and significantly greater for exercise E3. 6. It was concluded that the frequency of limb movement, rather than exercise load (oxygen consumption) is a determinant of the change in ventilation at the start of exercise.

Can lactate-evoked cardiovascular responses be used to identify muscle ergoreceptors?

The increase in blood pressure and heart rate which accompanies muscular exercise is in part a reflex mediated by afferent nerve fibres in the group III and IV (small myelinated and unmyelinated) range. It has been reported that perfusion of lactate ions into hind limb muscles is an effective stimulus for these reflex responses. To investigate this hypothesis further, and to test adequacy of the controls used, a solution containing 15 mM lactic acid was perfused through a hind limb of urethane-anaesthetised rats, the leg's circulation being isolated from the rest of the body. During lactate perfusion, increases were seen in arterial blood pressure and heart rate. Denervation of the entire leg abolished the responses. To locate the receptors involved in the reflex, selective denervations of skin or muscle were performed. Clear responses were never seen when the leg was skinned or denervated by section of cutaneous nerves. On the other hand, responses to lactate perfusion were still seen following section of all nerves supplying the muscles of the leg, leaving the cutaneous innervation largely intact. It is concluded that perfusion of a hindlimb with lactate solutions is not an adequate technique to identify ergoreceptors in muscle.

Immediate ventilatory response to sudden changes in venous return in humans

We changed venous return transiently by postural manoeuvres, and by lower body positive pressure, to see what happened simultaneously to ventilation. Cardiac output was measured by a Doppler technique. In seven subjects, after inflation of a pressure suit to 80 and 40 mmHg at 30 deg head-up tilt, both cardiac output and ventilation increased. Ventilation increased rapidly to a peak in the first 5 s, cardiac output more slowly to a steady state in about 20 s, at 80 mmHg inflation. After inflation to 80 mmHg in six subjects at 12.5 deg head-up and 30 deg head-down tilt, cardiac output did not change in the first, and fell in the second case. There were no significant changes in ventilation. On release of pressure there were transient increases in both cardiac output and ventilation, with ventilation lagging behind cardiac output, in contrast to (2) above. In five subjects, elevation of the legs at 30 deg head-up tilt caused a rise in both cardiac output and ventilation, but in two subjects neither occurred. In all seven subjects there was a transient increase in cardiac output and ventilation when the legs were lowered. Ventilation and cardiac output changes were approximately in phase. We were therefore unable to dissociate entirely increasing cardiac output from increasing ventilation. The relation between them was certainly not a simple proportional one.

Cardiovascular control during exercise: central and reflex neural mechanisms

Both reflex neural and central command mechanisms have been postulated to explain the cardiovascular responses that occur during exercise. The 2 mechanisms appear to affect the same neural circuits and to be capable of working either in conjunction with one another or independently.

Responses in muscle afferent fibres of slow conduction velocity to contractions and ischaemia in the cat

The aim of the study was to find out to what extent muscle receptors with slowly conducting afferent fibres (group III and IV) are activated by muscular contractions of moderate force, and what kind of muscle afferents could mediate the pain of ischaemic exercise. In chloralose-anaesthetized cats, the impulse activity of single afferent units from the triceps surae muscle was recorded from dorsal root filaments during muscular contractions with intact blood supply and after occlusion of the muscle artery. Two types of responses were observed to contractions without muscular ischaemia. One was characterized by sudden onset and a graded response amplitude to contractions of increasing force. In most cases stretching the muscle was also an effective stimulus. Units showing this response behaviour were labelled c.s.m (contraction-sensitive with mechanical mechanism of activation). The other response type had a more delayed onset and often outlasted the exercise period; because of the unknown mechanism of activation, units of this kind were labelled c.s.x. The proportion of c.s.m receptors was significantly higher amongst group III than amongst group IV units. During ischaemic contractions of comparable force the c.s.m and c.s.x receptors exhibited an unchanged or a decreased response amplitude. Under these conditions another receptor type (N, for nociceptive) was activated which did not respond to contractions with intact blood supply. Vigorous activations during ischaemic work were only observed in group IV receptors. The majority of the 131 group III and IV units tested did not respond to contractions at all. These contraction-insensitive (c.i.) endings probably comprised different receptor populations (nociceptors, thermoreceptors, low-threshold mechanoreceptors). It is concluded that the various central nervous effects of muscular exercise without ischaemia which are known to be due to raised activity in thin muscle afferents (e.g. cardiopulmonary adjustments, spinal locomotor reflexes) are probably produced by the c.s.m and c.s.x types. The pain of ischaemic contractions is most likely mediated by the N receptors most of which possess non-myelinated afferent fibres.

Somato-autonomic reflexes in anesthetized and unanesthetized dogs

Blood pressure and heart rate alterations were induced in anesthetized and unanesthetized mongrel dogs. Pressor responses were brought about in the anesthetized group by bilateral, high intensity stimulation of the sciatic nerves. In these animals, bilateral section of the dorsolateral sulcus area (DLS) in the lumbar spinal cord completely eliminated both blood pressure and heart rate responses to this stimulation. Baroreceptor-mediated bradycardia induced by pressor doses of phenylephrine was attenuated by sciatic nerve stimulation. Obliteration of this baroreceptor-somatic afferent interaction required ventrolateral extension of the bilateral spinal lesions to include both the DLS and the dorsolateral funiculus (DLF). Blood pressure and heart rate increments in unanesthetized dogs were induced by treadmill running. The pressor response to exercise was markedly increased by transient hind limb arterial occlusion during the course of the run. Surgical interruption of the ascending limb of the somato-autonomic reflex in the spinal cord of these animals (L1-L2 combined DLS and DLF lesion) significantly reduced the blood pressure response to simultaneous exercise-occlusion. These spinal lesions also reduced the heart rate response to treadmill running without occlusion. The descending pathways involved in autonomic reflexes appeared intact as the spinal lesions did not alter the blood pressure or heart rate response to bilateral carotid artery occlusion.

Gas exchange during bicycle exercises preceded or not by loadless pedalling in female and male subjects

The effects on ventilation and oxygen uptake of pedalling with no load on the ergometer before exercise have been investigated in 10 subjects (6 males and 4 females). It has been found that: (1) the O2-deficit calculated at the beginning of exercise is less variable and generally reduced when exercise has been preceded by a loadless pedalling period; (2) for a given steady-state VO2, the O2-deficit is larger in female than in male subjects; (3) the respiratory equivalents for oxygen and for carbon dioxide are not affected by loadless pedalling; they are generally larger in female than in male subjects; (4) the increase in ventilation at the beginning of exercise is smaller after loadless pedalling but this seems to be apparent only in male subjects; (5) the pattern of breathing and heart rate are not affected by loadless pedalling. It is concluded that pedalling with no load on the ergometer before the exercise prepares the subject to deliver oxygen more rapidly to the muscles. It then reduces the variability in gas exchanges and the O2-deficit. Several factors which may account for this better efficiency of oxygen delivery are discussed.

Responses of group IV afferent units from skeletal muscle to stretch, contraction and chemical stimulation

In an attempt to differentiate between nociceptive group IV muscle receptors and "ergoceptive" ones, the discharges of single group IV fibres from skeletal muscle in response to local pressure, sustained stretch, repetitive contraction and intra-arterial injections of bradykinin, 5-hydroxytryptamine (5-HT), potasssium, phosphate, and lactate were studied in anesthetized cats.

Role of muscular contraction in the reflex vascular responses to stimulation of muscle afferents in the dog

The effect of stimulation of somatic afferents from muscle was investigated in 24 anesthetized dogs. The carotid sinuses were isolated and kept at constant pressure, the vagi were cut, and the left iliac artery was isolated and perfused at constant flow. Muscle afferents were stimulated by either inserting electrodes into the muscles of the right thigh or stimulating the central end of the right femoral nerve. Muscle stimulation at 5 Hz decreased perfusion pressure in the iliac artery 76 ± 6 mm Hg (mean ± SE ); the decrease was not prevented by paralysis of the stimulated muscles, administration of atropine or antihistamines, or -receptor blockade. Muscle stimulation at 40 Hz increased perfusion pressure in the iliac artery 28 ± 3 mm Hg. This increase was prevented or changed to a depressor response by muscle paralysis. Similar effects were obtained with femoral nerve stimulation. Both depressor and pressor responses persisted after the leg had been skinned and the muscles had been separated from their tendons, but they were abolished by left sympathectomy. Thus, skeletal muscle contraction is necessary for activation of the muscle afferents which cause reflex vasoconstriction but not for activation of those which cause reflex vasodilatation.

The reflex nature of the pressor response to muscular exercise

1. In anaesthetized cats tetanic contraction of the hind-limb muscles, elicited by stimulating the ventral roots L6-S1, caused a rise of arterial blood pressure, usually accompanied by small increases in heart rate and pulmonary ventilation: in decerebrate cats, all components of the response were much increased.2. With tetani of different strengths, obtained by stimulating with different intensities at the same frequency, the pressor response increased with increasing tension.3. When muscle contraction had been abolished by gallamine, or when dorsal roots L6-S1 had been sectioned, ventral root stimulation no longer caused a pressor response. The response is therefore a reflex, initiated in the exercising limb.4. The pressor response was not affected by section of all articular nerves to knee and ankle joints, or by section of the vagi. The stimulus therefore originates in the contracting muscles alone.5. The pressor response is potentiated by occluding the circulation through the working muscles. Reasons are discussed for concluding that the stimulus is chemical rather than mechanical, and that the ;metabolic receptors' for this exercise reflex are the free endings of group III and IV sensory nerve fibres located around the blood vessels.