Neural control of breathing

The muscles that make us breathe need to work or ‘contract’ both when we are awake and asleep to bring oxygen into the lungs (and remove carbon dioxide) to keep us alive. This contraction of our breathing or ‘respiratory’ muscles occurs because signals from the brain travel to the ‘motoneurones’ which are specialized cells in the spinal cord that cause muscles to contract. This ‘neural drive’ can come from different places in the brain, but the main area that controls breathing during rest (i.e. not exercising) and sleep is the brain stem, which is at the base of the brain. At the top of the brain is the somatosensory cortex, a region involved in making voluntary movements. The somatosensory cortex is the part of the brain that sends neural drive to the muscles in your shoulder, arm and hand when you reach for that last piece of chocolate cake! But there are also pathways from this region to the respiratory muscles. Those pathways are used when you need to modify your breathing, for example to sniff or take a larger breath in before you speak1, 2.

There is more neural drive to the respiratory muscles in older people

If we measure the electrical signals in muscles, using a method called electromyography (EMG), we can work out how much neural drive is going to those muscles. For example, the amount of EMG in the main breathing muscle, called the ‘diaphragm’, is usually higher in people aged over 50 years of age3. We think this is due to changes in the respiratory system and loss of muscle strength as people get older. However, because the EMG measurement is taken at the muscle, it cannot tell us which part of the brain is controlling the muscle. This is important if we want to know which part of the brain is controlling the respiratory muscles. In our recent paper, published in the Journal of Physiology4, we used a specialised method that allowed us to work out where in the brain the neural drive to the respiratory muscles was coming from. Therefore, we should be able to work out where in the brain the increased neural drive to the diaphragm3 comes from.

Does neural drive come only from the brain stem that controls automatic breathing, or is it also from the somatosensory cortex that controls voluntary breathing (e.g. sniffing and speaking)?

Using electroencephalography (EEG) to measure if the somatosensory cortex controls breathing

The somatosensory region at the top of the brain that controls voluntary movements is close to the skull, and therefore we can measure its activity with electroencephalography (EEG) electrodes on the scalp. Although the electrical signals are very small, if we take measurements during lots of breaths (e.g. 50-100 breaths) and measure what happens to the combined or averaged EEG signal ~1-2 seconds before all these breaths, we can measure what is happening in the somatosensory cortex. What we are looking for is a slow negative change in the electrical activity of the brain called a Bereitschafts, or readiness potential. This readiness potential tells us if the somatosensory cortex was involved in a breath (or any movement for that matter).

In our paper4, we measured a group of older people with lung disease (chronic obstructive pulmonary disease; COPD), a group of older people without lung disease and a group of young people without lung disease. In our experiment, we used EEG to look for readiness potentials when people were sniffing. We use a sniff to confirm our measurement is working because it is a voluntary movement and usually generates a readiness potential. Then, we look to see if there is a readiness potential in resting breathing. In resting breathing in young, healthy participants, there are usually no readiness potentials. This is because the neural drive to the respiratory muscles for resting breathing control comes from the brain stem, not the somatosensory cortex.

Lung disease did not affect neural control!

If there were more readiness potentials during resting breathing in the older people with and without lung disease, compared to the young healthy people, this would mean that somatosensory cortex is giving more neural drive to the respiratory muscles than normal. We expected that the somatosensory cortex would control breathing in more people with COPD because the disease damages the respiratory system and so more neural drive is needed to make sure the lungs are bringing in enough oxygen and removing carbon dioxide5.

To our surprise, the number of readiness potentials that we measured was much higher in both the older group with COPD and the group of healthy older people without lung disease. We also found that there was no relationship between the amount of damage to the lungs or how strong the respiratory muscles were, with the number of readiness potentials. This means that ageing was the main reason that the somatosensory cortex was helping to drive the breathing muscles. Lung disease seemed to have no effect!

So by measuring the activity of the brain using EEG, we were able to work out that the increased neural drive to the breathing muscles in older people3 is probably coming from both the brain stem and the somatosensory cortex, compared to only the brain stem in young people. We think that this occurs because the brain stem, which controls automatic breathing, is affected by ageing4.

Glossary

Contract – the shortening of a muscle to make a movement.

Respiratory muscles – the muscles in our chest, abdomen and neck that contract to make us breathe.

Neural drive – electrical signals sent to muscles for movement control.

Motoneurones – cells in the spinal cord that control muscles.

Electromyography (EMG) – is the measurement of the electrical activity of a muscle.

Diaphragm – the main respiratory muscle.

Electroencephalography (EEG) – is the measurement of the electrical activity of brain cells.

Bereitschafts (readiness) potential – movement-related electrical activity in the brain that is recorded using EEG.

 

References
  1. Hudson AL, Navarro-Sune X, Martinerie J, et al. (2016). Electroencephalographic detection of respiratory-related cortical activity in humans: from event-related approaches to continuous connectivity evaluation. J Neurophysiol 115, 2214-23.
  2. Tremoureux L, Raux M, Ranohavimparany A, et al. (2014). Electroencephalographic evidence for a respiratory-related cortical activity specific of the preparation of prephonatory breaths. Respir Physiol Neurobiol 204, 64-70.
  3. Jolley CJ, Luo YM, Steier J, et al. (2009). Neural respiratory drive in healthy subjects and in COPD. Eur Respir J 33, 289-97.
  4. Nguyen DAT, Boswell-Ruys CL, McBain RA, et al. (2018). Inspiratory pre-motor potentials during quiet breathing in ageing and chronic obstructive pulmonary disease. J Physiol, DOI:10.1113/JP275764.
  5. De Troyer A, Leeper JB, McKenzie DK, Gandevia SC (1997) Neural drive to the diaphragm in patients with severe COPD. Am J Respir Crit Care Med 155, 1335-40.
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