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Inspiration Expiration

FIGURE 3 Ventilatory volume changes and the pattern of motor nerve activity to an expiratory muscle (internal intercostal, recorded from 10th thoracic spinal nerve), and two inspiratory muscles (external intercostal, recorded from 5th thoracic spinal nerve; and diaphragm, recorded from phrenic or 5th cervical spinal nerve). (Nerve activity after Hlastala and Berger, Physiology of respiration, Oxford, UK: Oxford University Press, 1996.)

Inspiration Expiration

FIGURE 3 Ventilatory volume changes and the pattern of motor nerve activity to an expiratory muscle (internal intercostal, recorded from 10th thoracic spinal nerve), and two inspiratory muscles (external intercostal, recorded from 5th thoracic spinal nerve; and diaphragm, recorded from phrenic or 5th cervical spinal nerve). (Nerve activity after Hlastala and Berger, Physiology of respiration, Oxford, UK: Oxford University Press, 1996.)

expiration by passive elastic recoil of the lung and chest wall (see Chapter 19). Figure 3 shows this pattern of activity for the phrenic nerve (lower trace), which is the motor nerve innervating the diaphragm. The rhythm from the central pattern generator is synaptically transmitted to phrenic motor neurons at the third to fifth cervical levels of the spinal cord (C3-C5). Injuries that disrupt this normal flow of information between the medulla and C3-C5 spinal cord result in respiratory paralysis, so high quadriplegics require artificial ventilation. Phrenic nerve activity reflects basic features of the central pattern generator, including (1) a sudden onset of inspiratory activity, (2) a ramp-like increase in activity during inspiration, and (3) a relatively sudden termination of activity at the onset of expiration. Low levels of phrenic activity at the onset of expiration (Fig. 3) allow the diaphragm to smooth the transition to passive expiration.

Inspiratory and expiratory intercostal muscles are innervated by spinal nerves from all levels of the thoracic spinal cord (T1-T11). The pattern of electrical activity in the external intercostal nerves is similar to that in the phrenic nerve, whereas the internal inter-costals are activated during expiration (see Fig. 3). Remember that expiration is passive at rest, so ventilation is elevated in Fig. 3 to illustrate intercostal activity. Electrical activity in lower thoracic and upper lumbar spinal nerves, which innervate the abdominal expiratory

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