The Control of Breathing Mechanisms in Humans

If the spinal cord is cut in the lower cervical region, the intercoastal muscles are paralysed but breathing continues as normal due to the diaphragm. If the spinal cord is cut above the phrenic nerve, all respiratory muscles will be paralysed. This shows that there must be control from the brain. When a cut is made through the brain stem so that the medulla oblongata is connected to the spinal cord but separated from the rest of the brain, breathing will continue regularly, so the cells controlling the breathing are in the medulla and are called the respiratory centre. As well as the medulla, the pons and midbrain have also been associated with the control of breathing mechanism as housing the pneumotaxic and apneustic centres. To get an idea of where these parts of the brain are located, they are indicated on the diagram below:

The medulla oblongata looks like a swollen tip to the spinal cord. Nerve impulses arising here:

> rhythmically stimulate the intercostal muscles and diaphragm

> regulate heartbeat

> regulate the diameter of arterioles thus adjusting blood flow.

Therefore destruction of the medulla causes instant death.

The pons, as well as serving as a relay station carrying signals, also participates in the reflexes that regulate breathing.

It is thought that there are at least three kinds of cell in the respiratory centre: some are active throughout inspiration, some start to produce action potentials late in inspiration and some are active in expiration. Breathing occurs automatically through involuntary reflex action controlled by motorneurones in the spinal cord which in turn are controlled by the control centre which continually adjusts the rate of breathing to meet the body's immediate needs. For example, when at rest the breathing rate may be at normal. Then when performing activities the breathing rate increases to provide enough extra oxygen required for respiration to carry out the activity. In cases of extreme activity such as physical exercise, the lungs ventilate even more deeply and rapidly. Faster and deeper breathing results in and increased supply of oxygen and eliminates more of the carbon dioxide produced by the body. Adjustments to the intrinsic rhythm are made as part of a homeostatic mechanism by feedback control.

When ventilation increases, the concentration of CO2 falls but when the co2 concentration rises, the ventilation again increases and so the cycle goes on. Therefore, if a graph of ventilation against CO2 concentration is drawn, the lines will have opposite slopes depending on whether the ventilation is altered and this changes the CO2 conc. or whether the CO2 conc. changes and this alters the ventilation. This is a general property of control systems working by negative feedback. The point where the two lines cross will be the values that the variables have normally.

We know that when more physical activity takes place, so respiration increases and also CO2 concentration as a waste product. It has been found that by applying solutions containing a raised partial pressure of carbon dioxide to localised areas of the medulla oblongata, the breathing can be increased, so this shows that CO2 chemoreceptors are found there. However, the way in which these receptors work is more complex than as meets the eye. To understand how they work, we need to know that the cerebral capillaries are very impermeable. There is a blood-brain barrier through which even small ions like hydrogen or hydrogen carbonate cannot pass. However, carbon dioxide, which is lipid soluble, can easily cross the barrier. The ventricles of the brain are filled cerebrospinal fluid and molecules can pass easily between the cerebrospinal fluid and the extracellular fluid in the brain. If either of these fluids has a high concentration of CO2 then ventilation will increase. So the central chemoreceptors are influenced by the composition of these two fluids.

When CO2 dissolves in these fluids the following reaction takes place:

CO2 + H2O ? H+ + HCO3-

As you can see, the protons produced when CO2 dissolves in water decrease the pH of the solution as it becomes more acidic. So in fact, it is the pH which is sensed by the receptors and not directly the CO2 concentration.

Basically, falls in pH increase the breathing and rises in pH decrease it; when the pH falls, the increase in ventilation reduces the CO2.

Carbon dioxide is therefore not directly detected and so there are some consequences to this system. As carbon dioxide can diffuse freely across the blood-brain barrier, which hydrogen ions cannot pass, the CO2 in the blood plasma and the cerebrospinal fluid will be the same, but the pH's of the two may differ. Therefore the blood-brain barrier has made a hydrogen ion receptor specific to CO2.