Análisis del segundo objetivo

The behaviour of respiratory neurones during the pulmonary chemoreflex in rabbits IS

in general accord with that previously described in rats and cats. This suggests that species differences are not as great as previously imagined. The great similarity between Head’s paradoxical reflex and the pulmonary chemoreflex is explained by the strong post- inspiratory increase that these reflexes manifest. It is suggested that there is neither a peripheral nor a central difference between the reflexes in rabbits and cats. Indeed the only paradoxical feature of Head’s paradoxical reflex is that there is no paradox at all (this statement of course, constitutes a failure in the logic of language). It is of particular interest that amongst the various theories of respiratory rhythmogenesis only the three phase theory (Richter et al. 1987) can account for the apparent peculiarity of the rabbits pulmonary chemoreflex. This theory satisfactorily explains the cardioinhibitory changes. The common practice of referring to the inspiratory apneusis of the rabbit’s pulmonary chemoreflex is now shown to be incorrect. What appears as apneusis is in reality a fast inspiratory/post-inspiratory cycling of the respiratory rhythm generator with "fusion" of the phrenic impulses and tetany of the diaphragm. The upper limit on the speed of the respiratory rhythm generator or clock is unknown, however the rabbits diaphragm can respond to rapid signalling. Head (1889) noted that if a cut distal end of a phrenic nerve be placed on the beating heart the diaphragmatic slip contracts with the electrical activity of each heartbeat. The question arises: why does the rabbit choose to hold its chest at elevated functional residual capacity (FRC), when other species have expiratory apnoeas? The rise in venous return through what is essentially a Mueller’s manoeuvre must be counter productive during this defensive response. Coleridge and Coleridge (1984) have speculated that the tonic diaphragmatic response constitutes a respiratory adaptation. Their argument runs as follows:the chest wall of a rabbit is frail, and there is small FRC at the position of rest; prolonged apnoea in this position might lead to collapse of alveoli, requiring very large inspiratory efforts to reinflate the lungs. This explanation is probably incorrect for two reasons: first, Crosfil^and Widdicombe (1961) have compared various respiratory mechanical parameters across species (rat, cat and rabbit), although the rabbit has very compliant lungs there is little difference between the chest wall compliance of

a rat and a rabbit when expressed with respect to body weight. Second, rabbits do exhibit prolonged expiratory apnoea during upper airway stimulation with smoke ( Dr. F. Kratschmer 1870). Kratschmer referred to this as an ’’expiratory tetanus”. This yields quite a different pattern in the phrenic nerve of the rabbit to that of lower airway C-fibre stimulation (personal observation). Perhaps it is the classic mistake of a respiratory physiologist to assume respiratory modifications always serve a respiratory role. The tonic drive to diaphragm and chest wall may constitute an important element of the highly developed freezing response of rabbits. It is quite striking to observe how the rib cage and abdomen appear motionless during the pulmonary chemoreflex. This playing dead response may be confusing to predators who finally alight on their prey after a prolonged chase. The J-receptors of Paintal should certainly be stimulated during the pulmonary blood flow increases observed during vigorous exercise (Anand & Paintal 1980).

What is the central mechanism that permits this unique respiratory control in rabbits? The fact that there is a considerable post-inspiratory input to the rabbits diaphragm at rest and during the pulmonary chemoreflex makes the lagomorph invaluable in studies of respiratory rhythmogenesis and control. However the numerous studies in the past that have attempted to quantify respiratory cycle length (inspiratory time, expiratory time) with respect to the rabbits diaphragm or phrenic nerve must have yielded highly misleading data. It is of interest that while the dorsal respiratory group of the cat and dog contains predominantly inspiratory neurones and only 4-6% expiratory neurones (Cohen & Feldman 1984;Berger 1977) the dorsal respiratory group of the rabbit is reported to contain 44% expiratory neurones (Jiang et al. 1986;Jiang & Shen 1991). This is also the case when vagal afferents are eliminated (Wei et al. 1984). In addition the dorsal medulla of the rabbit contains post-inspiratory cardiac vagal preganglionic neurones which have B-fibre axons and are indistinguishable from their counterparts around the nucleus ambiguus (Jordan et al. 1982). It has been assumed that these represent neurones that failed to migrate ventrolaterally during embryogenesis. Is the dorsal medulla of the rabbit the source of the powerful post-inspiratory drive to the diaphragm? Have the neighbouring cardiac vagal preganglionic neurones assumed the firing characteristics of more ventrally placed neurones, not because they failed to migrate, but that their dendritic fields have become influenced by the powerful enveloping post-inspiratory

activity? In this regard the rabbits diaphragm more closely resembles a branchiomeric structure than the diaphragm of a cat. The degree of preceding speculation simply reflects an overwhelming ignorance concerning the medulla of a fascinating creature.

5. Role of the hvpothalaniic defence area during the pulmonary chemoreflex in the