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Chapter 11. Central and Peripheral Neural Controls of Respiration

A semi-comatose 80-year-old male is brought to the ER on a respirator. Tests show he has developed bilateral hemorrhagic infarcts of his lateral medulla. He has no noticeable respiratory efforts. This effect is best explained by necrosis of which of the following structures?

a. Neurons generating the "wakefulness stimulus"

b. Lower motor neurons in nucleus ambiguus innervating the diaphragm

c. Central chemoreceptors in the ventral medulla

d. Lower motor neurons of the major respiratory pump muscles

e. The central locus for controlling the rhythm of respiration

The most correct answer is e.

The central locus for controlling the rhythm of respiration; bilateral lesions of the ventrolateral medulla induce respiratory arrest, most probably by disrupting the central rhythm generator. The "wakefulness stimulus" dominates respiratory behavior when awake, but respiration is maintained without it during sleep and coma (answer a). Motor neurons for the diaphragm originate in the ventral horn of cervical levels 3-5 (answer b). Stimulation of central chemoreceptors induces hyperpnea; their distribution is widespread and total ablation would be very rare (answer c). Motor neurons for the diaphragm, intercostal and abdominal muscles, and accessory muscles of respiration are in the spinal cord, while those innervating the pharyngeal and laryngeal muscles are in the nucleus ambiguus (answer d).

A 57-year-old woman enters the emergency room displaying 'bluish' lips and complaining of difficulty in catching her breath walking up stairs. The attending physician admits the woman for observation. Several hours later the resident places the patient on FIO2 = 1.00 at 4.0 L/min via a nasal cannula hoping to rectify her cyanotic lips, but within two minutes the patient stops breathing. What is the most likely explanation for this patient's apnea?

a. The patient crossed her hyperoxia apnea threshold.

b. The nasal cannula has accidentally obstructed the patient's pharynx.

c. An infarct in the lateral medulla has destroyed her pre-Bötzinger complex.

d. The patient has attempted suicide by holding her breath until she dies.

e. Central chemoreceptors have been stimulated by her high PaCO2.

The most correct answer is a.

The patient was cyanotic with an unknown %SaO2; placing her on FIO2 = 1.00 exceeded her hyperoxic threshold and induced an apnea. Even if the nasal cannula ...

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