On the transmission of the stimulating effects of carbon dioxide to the muscles of respiration.

Abstract

1. Electromyography was used to measure the response of the diaphragm and intercostal muscles to CO2 in artificially ventilated decerebrate cats. 2. Hypocapnia produced tonic activity in either inspiratory or expiratory muscles or both, according to the preparation. 3. A graded effect of CO2 on both rhythmic and tonic activity was observed and for the latter this could be seen at as low as 10 torr PA,CO2. 4. In one human subject tonic firing of expiratory motoneurones was also induced by hypocapnia and this activity showed a graded increase with increasing (CO2. 5. A saggital incision of the medulla aimed at interrupting inspiratory bulbospinal axons abolished activity in inspiratory muscles and at eupnoeic levels of CO2 converted the activity of expiratory muscles from a periodic to a topic firing pattern. 6. Following such lesions the threshold for rhythmic excitation of expiratory muscles was elevated and this revealed that the graded effect of CO2 on tonic expiratory activity extends to as high as 60 torr. 7. The tonic activation of respiratory muscles in response to CO2 ceased after cervical cord transection or when the saggital incision in the medulla was extended caudally to the first cervical segment. 8. It is concluded that the CO2 dependent activation of spinal respiratory motoneurones is conveyed by bulbospinal axons which decussate in the vicinity of the obex and that this activation can be rhythmic or tonic. 9. It is suggested that the rhythmic excitation of expiratory muscles derives from a periodic inhibition of expiratory bulbospinal neurones which are subjected to a tonic CO2 dependent excitation which is continuously variable over the physiological range.

Respiratory mechanical advantage of the canine external and internal intercostal muscles

“…The muscles also appeared to display greater inspiratory activity in the dorsal than in the ventral portion of the rib cage and greater inspiratory activity in the rostral than in the caudal segments. In contrast, the internal intercostals were active during expiration and displayed greater activity in the caudal than the rostral segments (Bainton et al 1978;De Troyer & Ninane, 1986;Greer & Martin, 1990). In view of the topographic distributions of mechanical advantage among the muscles, such distributions of activity suggest Figure 5.…”

“…The main implication of the present studies, therefore, is that the actions of the external and internal intercostals on the lung during breathing are largely determined by the topographic distribution of neural drive among the muscles. A number of electrical recordings from intercostal muscles and nerves in anaesthetized cats (Sears, 1964;Bainton et al 1978;Kirkwood et al 1982Kirkwood et al , 1984Greer & Martin, 1990) and dogs (De Troyer & Ninane, 1986) have shown that the external intercostals are active during inspiration. The muscles also appeared to display greater inspiratory activity in the dorsal than in the ventral portion of the rib cage and greater inspiratory activity in the rostral than in the caudal segments.…”

Variations in the time course of the synchronization of intercostal motoneurones in the cat

“…For hypocapnic apnoea in general, especially in light anaesthesia, expiratory bulbospinal neurones still provide some excitation of motoneurones (Bainton et al 1978), but at 2-4 % end-tidal CO2 the bulbospinal neurones are either silent or almost so (Bainton & Kirkwood, 1979), and the discharge of the motoneurones must be maintained almost entirely by other inputs (cf. Sears, 1964 b).…”

“…D, a third preparation (decerebrated under halothane anaesthesia), rhythmic discharges, end-tidal CO2 4 0 %, spontaneous respiration, efferents recorded as e.m.g. signals (a few motor units in each recording channel) from T6 (reference spikes) and T8 (data from the recordings of Bainton et al 1978). A, B scaled in proportion to the base line counts.…”

The effects of opiates on the respiratory activity of thoracic motoneurones in the anaesthetized and decerebrate rabbit.

“…Studies of the transition from the state of apnoea to rhythm generation are therefore of relevance. In the decerebrate cat, Bainton, Kirkwood & Sears (1978) showed that during hypocapnic apnoea respiratory motoneurones are not quiescent; instead, expiratory a and fusimotor neurones show a tonic discharge (cf. Sears, 1963) which is graded in intensity with increasing fractional end-tidal CO2 (FET CO) until rhythm generation ensues.…”