6.3 Additional neuronal control
The lungs also receive innervation from the autonomic nervous system (Figure 17). The sympathetic innervation originates from the thoracic portion of the spinal cord and synapses onto the bronchiolar smooth muscle. Stimulation of these nerves causes bronchodilation.
Question 13 Bronchodilation
What happens to airway resistance during bronchodilation?
Answer
It decreases. Dilation will increase the diameter of the bronchioles, so the resistance to airflow will decrease.
In parallel, the vagus nerves (or cranial nerve X) synapse onto the bronchi and pulmonary blood vessels as part of the parasympathetic innervation. Activity of these neurons counterbalances the sympathetic response by stimulating constriction of the bronchi. Activation of these pathways is involved in the ‘fight or flight’ response.
Question 14 Hyperventilation
Sarah has a panic disorder and frequently experiences panic attacks that cause her to hyperventilate (i.e. breathe more rapidly than normal) and feel dizzy. What branch of the autonomic nervous system is activated during the panic attack?
Answer
Activation of the sympathetic nervous system will cause the bronchioles to dilate to meet the demands of increased inspiration and expiration.
What will happen to the PCO2 levels in the alveoli during hyperventilation?
Answer
They will drop. The rapid breathing causes more CO2 to be expired, so the PCO2 in the alveoli will be lower than normal.
Sarah finds that if she breathes into a paper bag during hyperventilation, her breathing returns to normal more quickly than when she just waits for the attack to pass. Why do you think this is?
Answer
During hyperventilation, PCO2 in the alveoli will be lower than normal. Decreased CO2, in combination with the decreased acidity of the blood, will shift the oxygen–haemoglobin dissociation curve to the left, increasing the affinity of Hb for O2 and making it harder for O2 to diffuse into the tissue (which partly explains why she feels dizzy).
Breathing into a bag concentrates the gases that are breathed out, including CO2. Re-breathing the expired, concentrated CO2 will lower the pH, reduce the activity of the respiratory neurons and restore the homeostatic breathing rate.
Finally, if you play a musical wind instrument, you know that some aspects of breathing can be controlled voluntarily. This ‘override’ of the autonomic breathing system involves the motor cortex, thalamus and cerebellum, which are also involved in breath control during speech and behavioural tasks that modify breathing by learning and experience.
OpenLearn - Blood and the respiratory system
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