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In 1970 Paintal described the J reflex, the inhibition of somatic muscle by stimulation of type J receptors in the alveolar walls of cats. Twenty years earlier he had begun studying these receptors, and named them 'juxta-pulmonary capillary receptors', or J receptors. They are now usually called pulmonary C fibre receptors, since they are one group in a population of airway receptors with non-myelinated afferent fibres, a population that includes bronchial, tracheal and laryngeal groups, probably together with nasal groups, and possibly pharyngeal and oral receptors.
The evidence for the J reflex in cats is compelling. Deshpande & Devanandan (1970) showed that stimulation of C fibre receptors in cats depressed the somatic muscle stretch reflex. Kalia (1973) and Ginzel et al. (1971) showed that stimulation of C fibre receptors, whether the cats were anaesthetized, conscious or mesencephalic, led to a profound decrease in skeletal muscle stretch reflex (Fig. 1) and, if the cats were walking, to its attenuation or abolition. Paintal (1970) suggested that the reflex was present in man, and speculated that it would be a major factor in limiting severe exercise. The evidence was indirect: in cats the reflex is known to inhibit muscle tone and the pulmonary receptors are stimulated by an increase in pulmonary capillary pressure and flow. These pulmonary vascular changes also occur in human exercise and, therefore, it is argued, skeletal muscle force would be inhibited in exercising humans by the J reflex, and therefore limit the exercise. This important mechanism has never been established or refuted, until the paperby Gandevia et al. (1998) in this issue of The Journal of Physiology. They show that activation of C fibre receptors in humans by injections of lobeline, claimed to be a selective stimulant, increased the size of the H reflex in soleus and the size of EMG responses evoked in arm muscles by transcranial magnetic stimulation. The injections neither decreased the force of sustained contractions of the elbow flexors at submaximal or maximal levels, nor disrupted walking. The injections were known to be effective because they caused severe respiratory discomfort and cardiorespiratory responses including cough. In other words the skeletal muscle responses were precisely the opposite of what had been shown in the cat and what had been anticipated for human exercise.
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A, the normal knee jerk elicited every 2 s by applying a pulse pressure to the patella tendon. Saline (1·5 ml) injected at the arrow into the right atrium caused no change in knee jerk amplitude. B, phenyl diguanide (Pdg, 150 µg) injected at the arrow caused a marked depression of the knee jerk due C fibre receptors stimulation. Upper traces, aortic blood pressure. From Kalia (1973). | ||
Our understanding of the C fibre reflex in exercising humans is made more complicated by the extensive studies of Coleridge & Coleridge (1984). Working mainly with anaesthetized dogs and cats, they and others showed that activation of the receptors causes powerful reflex broncho- and laryngoconstriction, airway mucus secretion and mucosal vasodilatation, as well as confirming the hypopnoea, hypotension and bradycardia reported by Paintal (1970). And yet very few of these changes are seen in human exercise, even when extreme. In fact ventilation, blood pressure and heart rate all increase, laryngoconstriction seems unlikely, and there is no evidence for mucus secretion or mucosal vasodilatation. Another paradox relates to cough. This has never been shown with C fibre stimulation in experimental animals, anaesthetized or not; in fact in cats the cough reflex is inhibited (Tatar et al. 1988). Yet cough is a characteristic response to C fibre stimulation in relaxed humans, following some seconds after what is often intense laryngeal discomfort. This pattern suggests it may be a behavioural response to discomfort rather than a direct reflex. In exercise the cough may be suppressed or even absent. The tightness, even pain, felt in the chest during severe exercise, at least in unfit physiologists, may indicate that C fibre receptors are being stimulated.
A further problem lies in the fact that, both in experimental animals and humans, the stimuli to C fibre receptors have mostly been intravascular injections of drugs, such as phenyl diguanide, capsaicin and lobeline. These drugs are not specific and probably not very selective. There are many examples of their different actions on lung receptors and reflexes in cats, rabbits and dogs. It is therefore quite unjustifiable to assume any similarity between their actions in humans and in cats.
A clue to the different responses in cats and humans may lie in a neglected observation by Kalia (1970). She showed that in cats the J reflex was abolished by bilateral lesions involving the caudate nuclei, but if both cingulate gyri were destroyed the reflex inhibition was consistently reversed to an augmentation. Thus the cat brain has reflex pathways, activated by C fibre receptors, which can either increase or decrease the strength of skeletal muscle contractions. The balance between these two pathways may determine whether stretch reflexes are enhanced or inhibited; perhaps more appropriate to an experimental approach, different physiological conditions, e.g. other afferent nervous activities, could adjust the balance. The search for the J reflex in man should move towards situations where these tuning processes can be tested, as Gandevia et al. (1998) suggest.
In 1868, Breuer & Hering described the inflation and deflation reflexes that bear their names; they also thought they had identified a third reflex from the lungs which was not volume dependent but which increased the frequency of breathing. They stated their intention to study this reflex further, but if they did they never published their results. (Breuer took up psychoanalysis and Hering was too busy as a Head of Department.) Fifty-five years ago Paintal studied this reflex and stimulated a vast flow of research that established its complexity and importance. It is good that there are still problems to be solved.
| Breuer, J. & Hering, E. (1868). Sitzungsberichte der Akademie der Wissenschaften in Wien 58, 909-937. | |
| Coleridge, J. C. C. & Coleridge, H. M. (1984). Reviews of Physiology, Biochemistry and Pharmacology 99, 1-110. | [Medline] |
| Deshpande, S. S. & Devanandan, M. S. (1970). The Journal of Physiology 206, 345-357 | [Medline] |
| Gandevia, S. C., Butler, J. E., Taylor, J. L. & Crawford, M. R. (1998). The Journal of Physiology 511, 289-300. | [Abstract/Full Text] |
| Ginzel, K. H., Eldred, Watenabe, S. & Grover, F. (1971). Neuropharmacology 10, 77-91 | [Medline] |
| Kalia, M. (1973). Pflügers Archiv 343, 297-308 | [Medline] |
| Paintal, A. S. (1970). In Breathing: Hering- Breuer Centenary Symposium, ed. Porter, R., pp. 59-71, Churchill, London. | |
| Tatar, M., Webber, S. E. & Widdicombe, J. G. (1988). The Journal of Physiology 402, 411-420 | [Abstract] |
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