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Respiratory Center

centrum respiratorium

For medical students2 min readUpdated 2026-10-10

The respiratory center is a complex hierarchical system of central nervous system structures spanning from the cerebral cortex to the spinal cord. This system generates the basic respiratory rhythm, coordinates the phase transition between inspiration and expiration, and finely adapts breathing to the metabolic, emotional, and behavioral demands of the body.

Rhythm GeneratorLocated in the medulla oblongata (medullary center)

Key Study MethodBrainstem transection experiments (Lumsden, 1924)

Diaphragm InnervationSupplied by cervical motor segments ($C_3-C_5$)

ApneaComplete cessation of breathing following spinal cord transection at the C1 level

Hierarchical Organization of the Respiratory Center

The respiratory center follows a strict hierarchical principle: higher levels exert more complex modulation over external respiration, allowing for dynamic adaptability.

Cerebral Cortex (especially prefrontal areas): Exerts voluntary control, adjusting breathing to behavioral tasks and social or emotional stimuli.
Cerebellum: Synchronizes respiratory movements with somatic motor activity.
Hypothalamus and Reticular Formation: Integrates respiration into autonomic and emotional responses.
Pons: Contains two critical centers. The upper portion houses the pneumotaxic center, which controls the respiratory rate and regulates the transition from inspiration to expiration. The lower portion contains the apneustic center, which promotes inspiration.
Medulla Oblongata (Medullary Center): Serves as the primary generator of the respiratory rhythm. It contains clusters of inspiratory and expiratory neurons, notably within the nucleus tractus solitarius and nucleus retroambiguus.
Spinal Cord (Motor Centers): The final common pathway. Cervical segments ($C_3-C_5$) innervate the diaphragm via the phrenic nerve. Thoracic segments ($Th_1$ and below) control the intercostal muscles.

Effects of Brainstem Transections (Lumsden Experiments)

In 1924, physiologist Thomas Lumsden demonstrated the functional distribution of the respiratory network by performing serial transections of the brainstem at various levels:

Upper border of the midbrain: Breathing remains regular and smooth, proving that the cortex and diencephalon merely modulate the process and are non-essential for basic rhythmogenesis.
Upper border of the pons: The pneumotaxic center remains intact, and breathing is preserved. However, if the transection isolates the apneustic center from superior pontine inhibition, breathing becomes characterized by prolonged inspiratory gasps (apneusis).
Upper border of the medulla oblongata: Breathing persists (as the medullary center is autonomous), but the pattern changes dramatically to gasping—an irregular, convulsive breathing pattern driven solely by the isolated medulla.
Upper border of the spinal cord (C1): Results in complete respiratory arrest (apnea), as the brainstem respiratory center is disconnected from all spinal motoneurons.
Between cervical and thoracic spinal segments: Intercostal muscle function is lost, but diaphragmatic breathing is preserved because the connection between the brainstem and the phrenic motor nucleus ($C_3-C_5$) remains intact.

Automacity and Classification of Respiratory Neurons

Medullary neurons fire rhythmically, strictly phase-locked to either inspiration or expiration with characteristic bursts of action potentials. These neurons form an interconnected network via excitatory, inhibitory, and reciprocal feedback loops.

Neurons are classified based on their timing within the respiratory cycle:

Early neurons: Fire at the very beginning of the inspiratory or expiratory phase.
Late neurons: Fire toward the end of the phase (e.g., peak firing of late inspiratory neurons occurs just before expiration begins).
Full-phase neurons: Active throughout the entire duration of inspiration or expiration.
Transition neurons (inspiratory-expiratory and expiratory-inspiratory): Fire at phase junctions to ensure smooth cyclical transitions.
Tonic (background) neurons: Display continuous baseline activity regardless of the respiratory phase.

Mnemonic

Remember the brainstem hierarchy from top to bottom: Pneumotaxic (pons) → Apneustic (lower pons) → Medullary center (medulla oblongata, the core baseline generator).

Frequently asked questions

What are the primary functional and anatomical groups of neurons in the medullary respiratory center?

Medullary respiratory neurons are organized into two main anatomical groups and further classified by their phase activity:

Dorsal Respiratory Group (DRG): Composed predominantly (~90%) of inspiratory neurons, located within the nucleus tractus solitarius.
Ventral Respiratory Group (VRG): Contains a mix of inspiratory and expiratory neurons located near the nucleus ambiguus and retroambiguus.

Classified by timing:

Early and Late inspiratory and expiratory neurons.
Full-phase inspiratory and expiratory neurons.
Phase-transition neurons (active at junctions).
Tonic (continuous) neurons — maintain constant baseline activity.

Which receptors provide afferent input to the respiratory center?

Afferent input is delivered primarily by peripheral mechanoreceptors of the airways/lungs and peripheral chemoreceptors:

Pulmonary stretch receptors: Located in the smooth muscle of the trachea and bronchi, they respond to lung inflation and send inhibitory signals via afferent fibers of the vagus nerve (n. vagus) to limit inspiration.
Peripheral chemoreceptors: Located in the carotid bodies (at the carotid bifurcation) and aortic bodies, they monitor blood gas changes ($PO_2$, $PCO_2$, and $pH$).
Central chemoreceptors: Located on the ventrolateral surface of the medulla oblongata, they respond directly to changes in CSF pH and $PCO_2$ (mediated locally by $CO_2$ diffusion across the blood-brain barrier), representing a direct humoral rather than peripheral reflex input.

What is the Hering-Breuer reflex and how does it affect the respiratory center?

The Hering-Breuer inflation reflex prevents over-inflation of the lungs. Excessive lung distension stimulates pulmonary stretch receptors, sending afferent signals via the vagus nerve (n. vagus) to the medullary respiratory center to prematurely terminate inspiration and trigger expiration. A deflation reflex promotes inspiration when the lungs deflate rapidly. Together, these reflexes regulate respiratory depth and frequency via negative feedback.

Which humoral factors exert the greatest influence on the respiratory center?

Carbon dioxide, hydrogen ions, and oxygen are the primary humoral regulators:

Carbon dioxide ($CO_2$): The primary physiological stimulant of ventilation, which crosses the blood-brain barrier with high efficiency.
Hydrogen ions ($H^+$): Formed locally within the brainstem extracellular fluid from hydrated $CO_2$, acting directly on central chemoreceptors.
Oxygen ($O_2$): Hypoxia ($PO_2$ drop) is detected exclusively by peripheral chemoreceptors to drive reflex hyperventilation when arterial $O_2$ tension falls significantly.

Where is the primary respiratory rhythm generator located?

The primary pacemaker is localized in the medulla oblongata (medullary center). Key structures include the nucleus tractus solitarius and the nucleus retroambiguus.

What happens if the spinal cord is transected at the upper cervical level (C1)?

Complete respiratory arrest (apnea) occurs because the brainstem respiratory center is completely disconnected from the motor neurons supplying the diaphragm and intercostal muscles.

What is the function of the pneumotaxic center?

Located in the pons, the pneumotaxic center regulates the respiratory rate and controls the timely switching mechanism from inspiration to expiration.

Go deeper

Mechanisms of reciprocal inhibition between inspiratory and expiratory neurons
The role of the nucleus tractus solitarius (NTS) in respiratory control
Hypothalamic modulation of breathing during emotional and behavioral states
Pathophysiology of gasping and abnormal breathing patterns associated with isolated medullary centers