The centre which regulates the respiratory movements is situated in the floor of the fourth ventricle, at the upper and back part of the medulla. Flourens long since showed that injury of this spot - the vital point - was followed by almost instant cessation of respiration.
This is a good example of a so-called automatic centre; that is to say, the blood flowing through the medulla and nourishing the cells suffices to supply them with the energy necessary for their activity. Even slight variations in the quality or temperature of the blood reaching this part modifies the activity of the cells." The less oxygen and waste products contained in the blood, the more powerfully does it act as a stimulant on the centre.
Although we take the respiratory centre as an example of an automatic centre, its working is arranged by afferent impulses, so that the normal rhythm of breathing is regulated by reflex action. The mechanical state of the lungs - whether distended as in inspiration or contracted as in expiration - seems to excite the terminals of certain fibres of the vagus, which carry impulses to the centre, and thus excite or restrain movements.
This automatic centre can also be influenced by the higher centres of the brain, for by our will we can regulate our breathing movements or stop breathing altogether for a time. Independent of volition the higher centres control the respiratory rhythm, as seen in sleep, when their action is partially in abeyance while the vagi are active, and respiration becomes periodic, or when the brain functions are impaired and respiration becomes intermittent (Cheyne-Stokes respiration). Further, the action of the respiratory centre can be altered by impulses arriving from the surface, as may be seen by the gasping inspirations which involuntarily follow the sudden application of cold.
Again, the activity of the centre may be altered by stimulations of certain parts of the air passages; so much so, that convulsive actions of the respiratory muscles are brought about, which induced some to speak of a sneezing centre and a coughing centre in the medulla. But sneezing and coughing may be equally well explained as a peculiar form of activity of the respiratory centre, or a reflex alteration in the respiratory rhythm, caused by irritation of the nasal or laryngeal mucous membranes.
Though the action of the respiratory centre can be modified by (1) the will and (2) various peripheral stimulations, and is habitually regulated from the periphery through (3) the vagi by the state of the lungs, the condition of the blood supplied to the centre may be such that these remoter influences are quite powerless. This uncontrollable condition of the centre is established when the blood flowing through it is abnormally venous and the cells become over-stimulated. We know how short a time we can hold our breath by voluntary checking of the centre, and most people have had occasion to observe the inordinate and painful efforts of a person whose respiration is interfered with by disease. When the dyspnoea becomes intense, nearly all the muscles in the body are called into action. Thus, in quiet breathing comparatively few nerve cells in the medulla carry on the work of respiration, but under certain emergencies they can call to their aid the entire motor areas of the gray substance of the spinal cord, and thus give rise to a general effort. Hence, we often hear of a convulsive centre in the medulla being placed in close relation to the respiratory centre. In some cases, irrita- tion of the air passages or imperfect oxidation of the blood, the convulsive centre comes under the command of the cells of the respiratory centre, which can then excite coughing, sneezing or convulsive inspiratory effort.
As already mentioned, the convulsions of asphyxia may be explained by the impure blood acting as a stimulus on the cells of the cord itself.