Involuntary control of respiration

From Academic Kids

The body's involuntary control of respiration is mediated by the brain's respiratory center located in the brainstem, particularly in the medulla oblongata and pons. The respiratory center regulates the rhythmic, alternating cycles of inspiration and expiration.

From within the medulla, graded action potentials are discharged in a cyclic pattern, and act to excite respiratory muscles. Bilaterally paired aggregations of neurons called the dorsal respiratory group and the ventral respiratory group, act in inspiration and expiration (respectively), are mutually inhibitory and act with symmetry (facilitated by cross communication). The dorsal respiratory group is active during inspiration although they are not the source of rhythmic respiration. The ventral respiratory group is involved in inspiration and expiration; these neurons play an important role in respiratory muscle contraction. The Pre-Bo Complex is responsible for the generation of respiratory rhythm, via pacemaker I neurons, located within the medulla.

Receptors play important roles in the regulation of respiration; central and peripheral chemoreceptors, and mechanoreceptors.

Central chemoreceptors of the central nervous system, located on the ventrolateral medullary surface, are sensitive to the pH of their environment. These act to detect a change in pH of the cerebral spinal fluid. An increase in carbon dioxide tension of the arteries will indirectly cause the blood to become more acidic; the cerebral spinal fluid pH is closely comparable to the plasma pH, as carbon dioxide easily diffuses across the blood/brain barrier. The detection of variation in the arterial carbon dioxide tension acts as a quick-response-system, useful in short term regulation. This system utilizes a negative feedback system, therefore if the pH of the cerebral spinal fluid does not compare to an ideal “set” level, then the receptor will send an error signal to the effectors and appropriate action may be executed.

Peripheral chemoreceptors act most importantly to detect variation of the oxygen in the arterial blood, in addition to detecting arterial carbon dioxide and pH. These nodes, called aortic and carotid bodies, are located on the arch of the aorta and on the common carotid artery, respectively. A continual signal is sent, via cranial nerves IX and X, from the peripheral chemoreceptors. With a decrease in arterial oxygen tension, the signal intensifies, calling for an increase in respiration.

Mechanoreceptors are located in the airways and parenchyma, and are responsible for a variety of reflex responses. These include the Hering-Breuer reflex that terminates inspiration to prevent over inflation of the lungs, and the reflex responses of coughing, airway constriction, and hyperventilation. The upper airway receptors are responsible for reflex responses such as, sneezing, bradycardia, coughing, closure of glottis, and hiccups. The spinal cord reflex responses include the activation of additional respiratory muscles as compensation, gasping response, hypoventilation, and an increase in breathing frequency and volume.

In addition to involuntary control of respiration by the respiratory center, respiration can be affected by conditions such as emotional state, via input from the limbic system, or temperature, via the hypothalamus. Voluntary control of respiration is provided via the cerebral cortex, although chemoreceptor reflex is capable of overriding conscious control.

References

  • Paul, Anthony D., et al., Neuronal Connections of a Ventral Brainstem Respiratory Chemosensitive Area: “Ventral Brainstem Mechanisms and Control of Respiration and Blood Pressure” [1] (http://www.oakwood.edu/ocgoldmine/adoc/faculty/apaul/)
     
  • Rabbany, Sina Y., “Breathing Coordination”, Hofstra University [2] (http://people.hofstra.edu/faculty/sina_y_rabbany/engg81/breathingcoordination.html)
     
  • Webber, Charles L., Jr., Ph.D, Pulmonary Curriculum Function:“Neural Control of Breathing”, Stritch School of Medicine, Loyola University-Chicago [3] (http://www.meddean.luc.edu/lumen/MedEd/medicine/pulmonar/physio/pf11.htm)
     
  • Rhoades, Rodney, Ph.D., Pflanzer, Richard, Ph.D, (2003) Human Physiology, 4th ed., Thompson Learning, Inc: Brooks/Cole, pp. 672-676
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