Neural Control of Respiration
Neural Control of Respiration
Overview
Medulla
- The medulla is the primary brainstem mediator of respiration.
- Via the dorsal respiratory group (DRG), the dorsal (posterior) medulla controls sensory integration.
- For its location, think: solitary tract nucleus.
- Via the ventral respiratory group (VRG), the ventral (anterior) medulla controls motor output.
- For its location, think: nucleus ambiguus.
Phrenic nerve
- C3, C4, C5 supply the phrenic nerve, which innervate the diaphragm: C3, C4, C5 "keep the diaphragm alive".
Brainstem circuitry
Ventral respiratory group (VRG)
- Within the medulla, anteriorly, lies the ventral respiratory group (VRG), which lies within the ventrolateral medulla.
- It provides innervation for motor output.
- It is involved in the activation of both inspiration and expiration.
Dorsal respiratory group (DRG)
- Within the dorsal medulla (in the solitary tract nucleus), lies the dorsal respiratory group (DRG).
- It provides sensory integration.
- It receives sensory input related to the inspiration phase of respiration.
Peripheral chemoreceptors
Peripheral chemoreceptors act on the dorsal respiratory group
- Peripheral innervation involves the aortic bodies (shown here in the arch of the aorta) and the carotid bodies in the carotid bifurcation.
- The carotid and aortic bodies are chemoreceptors.
- They respond to levels of arterial oxygen and carbon dioxide levels and blood acidity.
Innervation to the brainstem respiratory center
- Both cranial nerves 9 and 10 (we treat them jointly for simplicity) pass through the jugular foramen within the skull base across from the brainstem to innervate the dorsal respiratory group.
secondary inspiratory muscles
Key structures
- Anterior face, tongue, pharynx, and larynx.
Innervation
- Cranial nerves 9, 10, and 12 innervate the secondary inspiratory muscles (again, we treat them jointly for simplicity).
VRG innervation of CNs 9 and 10
- The ventral respiratory group acts upon these cranial nerves.
primary inspiratory muscles
Key structures
- Thoracic cage, diaphragm, and intercostal muscles.
Innervation
- The ventral respiratory group innervates C3, C4, C5 motor neurons in the anterior horn of the spinal cord gray matter, which supply the phrenic nerve, which innervates the diaphragm (again: C3, C4, C5 "keep the diaphragm alive").
Intercostal nerves
- Intercostal nerves innervate the intercostal muscles.
Detailed anatomy of the DRG & VRG
Dorsal respiratory group (DRG)
- The dorsal respiratory group lies within the solitary tract nucleus of cranial nerves 9 and 10.
Ventral respiratory group (VRG)
Simplification of VRG microanatomy
- First, add nucleus ambiguus of CNs 9 and 10.
- This will help us continue to recall the important of CNs 9 and 10 and thus the medulla itself in respiratory control.
- There are many subnuclei that constitute the ventral respiratory group; we'll only address the Bötzinger nuclei, here.
- The Bötzinger complex lies within the superior aspect of the ventral respiratory group (some authors distinguish it from the ventral respiratory group, entirely).
- The pre-Bötzinger complex is considered the "respiratory pacemaker."
- Notably, it contains mu receptors, which makes it sensitive to opioids.
- Thus, we can see one of the ways in which opioids (such as morphine) can depress our drive to breathe.
Clinical correlation: Ondine's curse
- Ondine's curse is the clinical eponym for the failure of automatic breathing during sleep.
- It typically occurs from lower medullary or high cervical spinal cord lesions.
- These patients are dependent on a ventilator when they sleep to survive.
pontine respiratory control centers
Apneustic center
- The apneustic center is a nonspecific region in the posterior lower pons.
- It promotes apneusis: a prolonged inspiratory pause.
- It comprises diffuse lower pontine nuclei.
Pneumotaxic center
- The pneumotaxic center (aka the pontine respiratory group).
- It prevents apneusis: it promotes regular breathing.
- The pneumotaxic center comprises the medial parabrachial nucleus and the Kölliker-fuse nucleus.
Functions
Whereas these centers where formerly thought to be well-defined and to perform unique functions, now they are understood to be diffuse and their functions are no longer thought to be unique (the pneumotaxic center is not the only site to prevent apneusis, for instance*).
Breathing Patterns
We can use breathing patterns in comatose patients to localize the level of the CNS lesion, as follows:
Patterns
- Cerebral hemispheric lesions cause Cheyne-Stokes respirations.
- Midbrain lesions cause hyperventilation.
- Illustrate hyperventilation as rapid, deep breathing.
- High pontine lesions cause apneustic breathing.
- Illustrate apneustic breathing as periods of long inspiratory pauses before release of air.
- Low pontine lesions cause cluster breathing.
- Illustrate cluster breathing as irregular clusters of breaths.
- Medullary lesions cause ataxic breathing.
- Illustrate ataxic breathing as a completely irregular breathing pattern.
Limitations
- Although these localizations are notoriously unreliable, they still give us a simple heuristic to follow when we examine comatose patients, which is essential.