[ C H R O N I C L E ]
Relaxation activities, such as yoga and meditation, are increasingly popular. What do they have in common? Controlling breathing to calm oneself and reduce anxiety and stress. As well as its vital function, breathing seems to have an effect on our behaviour and our emotions. How can such a simple and natural act as breathing affect our psychological state to such an extent? In fact, what may seem natural in our daily experience is not necessarily so from a neurobiological viewpoint. Breathing is not just a sequence of inspirations and exhalations, but combines with other motor behaviours such as speaking, laughing, crying or eating without becoming breathless. This activity requires fine control from our brains which use numerous neural circuits to achieve it.
In mammals, respiratory rhythm and motor control are governed by the medulla oblongata and the brain-stem bridge, located at the base of the skull. In particular, a region in the medulla oblongata called the pre-Bötzinger Complex, discovered twenty-seven years ago, contains neurons responsible for the respiratory reflex, i.e. the ability to breathe without thinking. These neurons, called pacemakers, activate in rhythm with the cycles of inspiration. Any injury to this neural complex results in deregulation of the respiratory rhythm!
In mice, neuroscientists at the American Stanford University recently identified a subgroup of neurons in this complex, whose function is to send respiratory information to other areas of the brain. They are particularly connected to the locus coeruleus, a structure in the brainstem involved in waking, vigilance and the emotions. Better still; they activate certain neurons in the locus coeruleus. The faster the rate of respiration, the more these neurons activate, and conversely. There is therefore a neurobiological link between respiration and regulation of the state of waking, vigilance and the emotions.
The scientists wanted to understand the role on behaviour of this neural subgroup in the pre-Bötzinger Complex. They used genetic techniques to remove it. In mice deprived of this neural subgroup, breathing was normal. But they were calmer, moved less and spent much more time preening, a behaviour associated with a reassuring environment. This state continued even when they were placed in a new environment, usually a source of stress for rodents. Without the activity in this group of neurons in the pre-Bötzinger Complex, some neurons in the locus coeruleus no longer activated, reducing the mice’s state of wakefulness and vigilance; they became calmer. This is exactly what would have happened had they slowed their own breathing - they would reduce activity in this neural group and calm down. However, this is not absolute serenity. The mice retained the ability to react to a more stressful event, such as exposure to very strong light, an aversive stimulant for these nocturnal animals. The locus coeruleus therefore remains sensitive to the environment, probably by means of other cerebral pathways.
The existence of a similar circuit in humans would explain why we can calm ourselves down by controlling the rhythm of our breathing. Panic attacks, characterised by over-alertness, can be treated with breathing exercises. This discovery comes in addition to another circuit responsible for sighing in mice. There are probably others. What additional behaviours could they regulate? Are there other types of cell able to regulate different functions? Could respiratory regulation have a direct effect on our immune system, childbirth or even the aging process? The answer to these questions could take our breath away!
Mariana Alonso is a neuroscientist at the Laboratory of Perception and Memory at Institut Pasteur. Her work looks into neurogenesis and brain plasticity in adults, the neuronal bases of smell linked to behaviour and the relationships between the microbiota, immunity and the brain.