Download Where does breathing start?

WHERE DOES BREATHING START?
The impulse to breathe, the inhale, is a reflex action that is generated in the lower brainstem. So
one could say that the prime mover for breathing is the brain. But where does the stimulus for the
reflex come from?
It comes from the respiratory centres called medulla oblongata and the pons which are located in
the lower brainstem. The medulla oblongata contains the Ventral Respiratory Group (VRG) and the
Dorsal Respiratory Group and the pons contains the Pneumotaxic (PNG) and the Apneustic
centres (APN). The four respiratory centres function through the interaction of neurons. Neurons
have an action potential that makes them send electrochemical signals via the brain stem down the
spinal cord into spinal nerves. In breathing they innervate the main breathing muscles: the
diaphragm and the intercostals.
The diaphragm and the intercostals are skeletal muscle and are innervated by the somatic nervous
system which controls motor and sensory nerves. The diaphragm is innervated by the phrenic
spinal nerve (C3 - C5) and the intercostals by the thoracic spinal nerve (T1 - T11). We can move
skeletal muscle voluntarily, we have, in part, conscious control over it.
The signals that neurons send to the diaphragm and the intercostals function as if they were
autonomic, even though the respiratory centres are not part of the autonomic nervous system. This
is due to the interplay of the neurons of the individual centres. The neural impulses of each centre
have their own intrinsic firing pattern. The firing patterns of each centre play off each other in a
reverberating circuit, which makes them autonomous. We certainly can‘t control them.
The muscles of the airways, of the trachea and the bronchi, are smooth muscle. Smooth muscle
(as well as cardiac muscle) is innervated by the autonomic nervous system. We cannot will smooth
muscle to do anything. Our gut is made of smooth muscle, for example.
The autonomic nervous system, which is also located in the medulla oblongata, sends its signals
via the cranial and spinal nerves (C3,7,9,10).
The respiratory centres in the brain adjust what signals they send to the breathing muscles. But the
signals they send depend on what signals they are getting. The brain is susceptible to input from
all other areas and systems of the body, for example, reflexes from mechanoreceptors (stretch),
chemoreceptors (gases) and baroreceptors (blood pressure).
Mechanoreceptors in the muscle tissue of the lungs and the airways, for example, detect
contraction and relaxation and send signals to the brain via motor (efferent) and sensory (afferent)
nerves in the vagus nerve (C10).
Chemoreceptors in the carotid bodies in the division of the carotid arteries detect changes in
arterial blood oxygen and send signals to the brain via the glossopharyngial cranial nerve (C9).
Baroreceptors in the aortic bodies in the aortic arch detect changes in oxygen and carbon dioxide
concentration and send signals to the brain via the vagus nerve (C10).
Emotions and thoughts are translated in the limbic system which is connected to the respiratory
centres.
And these are just the most relevant examples of inter-signalling for breathing. So where does
breathing start? It‘s a chicken or egg question. There are many theories about where the stimulus
comes from, but no one can say for sure.
© Nicola Caroli 2015