Control of Ventilation

Control of Ventilation

Early Experiments and Medullary Control

  • In the 1940s, Lundstedt conducted an experiment involving a pontomedullary transection (separation of the pons and medulla oblongata).
  • Lundstedt observed that breathing continued, but with an abnormal gasping pattern.
  • Later observations indicated that relatively normal breathing could occur after transection at this level.
  • The gasping observed by Lundstedt was likely due to surgical damage to the respiratory center in the medulla.
  • Most neurophysiologists believe the primary respiratory center is in the medulla.
  • Other sites, including the pons, primarily shape the respiratory output to produce a normal breathing pattern.

Fine-Tuning of Respiratory Rhythm

  • The medulla can generate a basic respiratory rhythm, but higher CNS centers and sensory inputs fine-tune it.
  • The pons contains neurons that influence respiratory output.
  • A mid-pons transection has a modest effect, leading to:
    • Increase in tidal volume.
    • Slight decrease in respiratory rate.
  • Bilateral vagotomy (interrupting the vagus nerves):
    • The vagus nerves carry sensory information from pulmonary stretch receptors.
    • Bilateral vagotomy has a similar, but smaller, effect compared to a mid-pons transection.
  • Combining mid-pons transection with bilateral vagotomy causes:
    • Prolonged inspiratory effort, termed inspiratory apneosis.
    • This is terminated by a brief expiration.

Brainstem Transections and Pontine Centers

  • Brainstem transection above the pons does not alter basic respiratory patterns.
  • These observations led to the hypothesis that:
    • The caudal pons contains an apneutic center, which can cause apneosis.
    • The rostral pons contains a pneumotaxic center, which prevents apneosis.
  • It was believed that the medulla, apneutic center, and pneumotaxic center are all required for normal breathing.
  • This viewpoint, while common in literature, is not universally accepted among respiratory physiologists.

Respiratory Centers in the Brainstem

  • Dorsal Respiratory Group (DRG):
    • Primarily contains inspiratory neurons.
    • Extends about one-third the length of the medulla.
    • Located bilaterally (on both sides) around the nucleus tractus solitarius.
    • The nucleus tractus solitarius receives sensory input from all viscera of the thorax and abdomen.
    • Plays a role in the control of the autonomic nervous system.
    • The nucleus tractus solitarius is viscerotopically organized.
    • The respiratory portion is ventrolateral to the tractus solitarius, beneath the floor of the fourth ventricle.
    • Neurons in the nucleus tractus solitarius and adjacent neurons in the dorsal medulla make up the DRG.
  • Ventral Respiratory Group (VRG):
    • Contains both inspiratory and expiratory neurons.
    • Ventral to the DRG, about midway between the dorsal and ventral surfaces of the medulla.
    • Lies within and around a series of nuclei that form a column of neurons.
    • Extends from the pons nearly to the spinal cord, making it considerably longer than the DRG.