It has been found that stimulation of the cervical portion of the spinal cord causes quickening of the heart beat. This occurs even after the possibility of increase of blood pressure has been removed by section of the splanchnic nerves, and the tonic inhibition of the vagi has been cut off by their section. In the cervical portion of the spinal cord nerve channels must exist which are capable of stimulating the muscle fibres of the heart, so as to cause it to beat more quickly. These accelerator fibres pass from the cord through the communicating branches to the last cervical or first dorsal sympathetic ganglion, and thence to the heart. Stimulation of the ganglia, or of the branches passing thence to the heart, quickens its beat. The effect of stimulus applied to these nerves does not begin to show itself until a comparatively long time after it has been applied, and the acceleratory effort continues for a considerable time after the stimulus is removed. Stimulation of the accelerator fibres has less effect than the inhibition of the vagus, which follows stimulation whether the accelerators are stimulated or not, while 24 the action of the accelerators is suspended so long as the vagus is being stimulated.
An analogy exists between the nervous mechanism of the heart and that of the blood vessels (to be described in a future chapter) which may help in their better comprehension. Both the heart and vascular muscles can work automatically; though no ganglionic cells can be found in the latter. Both are regulated by central influences. The heart receives constant inhibitory dilator impulses by the vagus, and occasional motor (accelerator) impulses by the sympathetic. The vessels receive constant motor (constrictor) impulses by the sympathetic and occasional inhibitory (dilator) impulses from other nerves.
The motor influences are supposed to act by increasing the chemical activity of the tissue (anabolic action), while the inhibitory impulses lessens the tissue change (katabolic action).