Vasomotor Tone

The muscles of the blood vessels need to be given a constant nervous stimulation to maintain a resting level of contraction. This phenomenon is known as vasomotor tone. It maintains the diameter of the blood vessels and therefore is important in maintaining blood pressure. Any changes in it tends to have an effect on the cardiac output and is also a complication in the various heart disorders and diseases.

Control of the Vasomotor Tone

Narrowing down of arteries is a cause for concern, as this leads to a reduction in blood supply and consequently a reduction in oxygen supply to the various organs. Arteries contain muscles that contract and relax in order to maintain the blood pressure. This contraction and relaxation is also controlled by certain factors, such as different chemicals released by the endothelium of the blood vessels. The endothelium has shown to control the tone to a great extent. The endothelial layer releases nitric oxide that diffuses into the vessel and plays a major role in its relaxation.

Another mode of control is the change in the electrical potential across the cells of the vessel walls. Studies have shown that change in electrical potential is passed through cells via gap junctions in order to control the contraction at a resting level. Some studies also state that in order to maintain the tone the nitric oxide and the change in electrical potential work in a synergistic manner. Injury to the endothelium affects the vasomotor control and can have grave effects in maintenance of blood pressure.

Inadequate Vasomotor Tone

This condition arises when there is an injury to the endothelial layer. As the endothelial layer can no more release vasoactive substances such as nitric oxide, the tone becomes inadequate and maintenance of the appropriate diameter of blood vessels becomes a problem. This has an adverse effects on organs to which the affected blood vessels supply blood. Many a time, as an adverse effect, a cardiac problem arises. A loss of tone is commonly seen in patients of atherosclerosis. In atherosclerosis the endothelium is damaged due to which release of vasoactive substances is difficult, leading to a progressive decline in the vasomotor functioning. This reaches a stage where there is a complete loss of vasodilation due to atherosclerotic arteries.

Certain studies have also shown that in the absence of these vasoactive substances, the gap junctions compensate the lack of these. This conclusion was derived when a group of scientists showed that in the absence of vasoactive substances certain amount of tone is maintained. This was attributed to the gap junctions. However, once these gap junctions were disrupted, there was a complete loss of the tone.

Factors Affecting It

Apart from the gap junctions and nitric oxide various other factors may also affect vasomotor tone. These factors can be hormones or other chemical transmitters that lead to a change in the basal or resting levels of contraction. Acetylcholine for one is known to cause relaxation of the vessels. Phenylephrine has shown to have positive effects on the blood vessels when tone is lost.

Thus vasomotor tone is of critical importance in the maintenance of the smooth muscles function of the blood vessels. Any alteration in the tone has an adverse effect on the blood supply to various organs. Modulation of this is possible by application of different stimuli. This tone along with various other factors effectively play an important role in the regulation of cardiac output.