Hierarchical organization of the respiratory center
The respiratory center is built on a strict principle: the higher the structure is located, the more complex its connection with external respiration. Higher-level structures influence lower-level ones, ensuring respiratory adaptability.
- Cerebral cortex (especially prefrontal areas): provides higher control. It regulates breathing in accordance with behavioral activity and socially significant reactions.
- Cerebellum: "coordinates" (synchronizes) respiratory movements with motor activity.
- Hypothalamus and reticular formation: integrate respiration into emotional and autonomic responses.
- Pons: contains two important centers. The upper part houses the pneumotaxic center, which guides respiration and regulates the switch from inspiration to expiration. The lower part contains the apneustic center, which stimulates inspiration.
- Medulla oblongata (bulbar center): serves as the primary generator of the respiratory rhythm. Inspiratory and expiratory neurons are localized here (key clusters include nucleus tractus solitarius and nucleus retroambiguus).
- Spinal cord (motor centers): the effector link. Cervical segments ($C_3-C_5$) innervate the diaphragm via the n. phrenicus. Thoracic segments ($Th_1$ and below) control the intercostal muscles.
Effect of transections on respiration (Lumsden experiments)
In 1924, the physiologist Lumsden proved the distribution of roles within the respiratory center by performing brain transections at different levels:
- Upper border of the midbrain: respiration remains regular and smooth. This proves that the cortex and diencephalon merely modulate the process and are not critical for the basic rhythm.
- Upper border of the pons: the pneumotaxic center remains intact, and respiration is preserved. If the transection affects the pons in such a way that the apneustic center dominates, breathing becomes rhythmic but with long pauses and an expiratory delay.
- Upper border of the medulla oblongata: respiration is preserved (the center is autonomous), but the pattern changes dramatically. Gasping occurs—convulsive breathing of the isolated bulbar center.
- Upper border of the spinal cord (C1): complete cessation of breathing (apnea) occurs. The brainstem respiratory center is cut off from spinal motoneurons, and impulses do not reach the muscles.
- Between the cervical and thoracic segments: the intercostal muscles are shut down, but diaphragmatic breathing is preserved because the connection between the brainstem and the phrenic nerve nuclei ($C_3-C_5$) is not disrupted.
Automatism and classification of respiratory neurons
Neurons of the bulbar center function rhythmically. Their activity is strictly tied to the phases of the respiratory cycle (inspiration or expiration) and is manifested by an increase in discharge frequency. Respiratory neurons are integrated into a network via feedforward and feedback connections, excitatory interactions, and reciprocal inhibition.
Classification of neurons by timing of activity:
- "Early": activated at the very beginning of the inspiratory or expiratory phase.
- "Late": switch on at the end of the phase (for example, the peak activity of late inspiratory neurons occurs just before expiration).
- "Full": active throughout the entire act of inspiration or expiration.
- Transitional (inspiratory-expiratory and expiratory-inspiratory): activated at the junction of phases, ensuring a smooth transition between cycles.
- Background ("continuous"): possess tonic activity and function without pauses regardless of the current respiratory phase.
Briefly: answers to topical questions
What are the main functional groups (complexes) into which the neurons of the bulbar respiratory center are divided?
Neurons of the bulbar respiratory center are divided into two main anatomical groups and are also classified according to the phases of the respiratory cycle.
- Dorsal group — consists of 90% inspiratory neurons.
- Ventral group — contains the majority of expiratory neurons located near the nucleus ambiguus.
According to the time of activation, the following are distinguished:
- Early and late inspiratory and expiratory neurons.
- Full inspiratory and expiratory neurons.
- Inspiratory-expiratory and expiratory-inspiratory (transitional at phase junctions).
- Background (continuous) neurons — possess constant tonic activity.
Which receptors provide afferent input to the respiratory center?
Afferent input to the respiratory center is provided by peripheral receptors: mechanoreceptors of the airways/lungs and peripheral chemoreceptors.
- Mechanoreceptors of the lungs and airways respond to stretching of lung tissue; slowly adapting stretch receptors are located in the smooth muscles of the trachea and bronchi and are activated during inspiration. Impulses from stretch receptors travel along afferent fibers of the vagus nerve to respiratory neurons.
- Peripheral chemoreceptors are located in the carotid bodies in the area of the carotid artery bifurcation and in the aortic arch/aortic bodies. They respond to changes in the gas composition of the blood: $O_2$, $CO_2$, $pH$; sources also specify $H^+$, $pCO_2$, and a decrease in $pO_2$.
- Central chemoreceptors of the medulla oblongata relate to direct humoral influences on the respiratory center rather than to afferent input from peripheral receptors.
What is the Hering-Breuer reflex and how does it affect the respiratory center?
The Hering-Breuer reflex consists of the reflex inhibition of inspiration upon lung inflation.
Artificial lung inflation excites stretch receptors, from which afferent impulses travel along the fibers of the vagus nerve (n. vagus) to the respiratory neurons. As a result, inspiration is inhibited and transitions into expiration. Conversely, a sharp decrease in lung volume inhibits expiration and activates inspiration. This reflex controls the depth and rate of breathing via a feedback mechanism and protects the lungs from overexpansion during forced breathing.
Which humoral factors have the greatest effect on the activity of the respiratory center?
The greatest humoral influence on the activity of the respiratory center is exerted by carbon dioxide, hydrogen ions, and oxygen.
- Carbon dioxide is the main stimulant of respiratory center activity, easily crossing the blood-brain barrier.
- Hydrogen ions are formed locally in the extracellular fluid of the brainstem from carbon dioxide and serve as the true active factor exciting central chemoreceptors.
- Oxygen — its deficiency (hypoxia) in the blood is detected by peripheral chemoreceptors and stimulates the respiratory center, increasing pulmonary ventilation.
The hierarchy of brainstem centers is easy to remember from top to bottom: Pneumotaxic center (pons) → Apneustic center (lower pons) → Bulbar center (medulla oblongata, base).
Class and exam questions
Where is the main respiratory rhythm generator located?
The primary rhythm generator is localized in the medulla oblongata (bulbar center). Key structures here are the nucleus tractus solitarius and the nucleus retroambiguus.
What happens if the spinal cord is injured at the upper cervical segments (C1)?
Complete cessation of breathing (apnea) will occur. The brainstem respiratory center will lose connection with all motor neuron effectors of the diaphragm and intercostal muscles.
What is the function of the pneumotaxic center?
The pneumotaxic center is located in the pons and is responsible for the timely switching of respiratory phases — transitioning from inspiration to expiration.
What else to study on this topic
- Mechanisms of reciprocal inhibition between inspiratory and expiratory neurons
- The role of the nucleus tractus solitarius in the regulation of respiration
- The influence of the hypothalamus on respiration during various emotional reactions
- The pattern of gasping and convulsive breathing with an isolated bulbar center
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