Answer
The term "tone" in " sympathetic tone" or "parasympathetic tone" refers to the normal level of activity of the relevant branch of the autonomic nervous system and its maintenance of the normal state or level of activity of an organ, structure or organ system
Parasympathetic tone:
Parasympathetic system activity determines the normal levels of activity of the GI and urinary tract. This system also slows the heart rate if tachycardia tends to develop because of over-excitement or over-activity. However, in stressful situations, the control of the parasympathetic system can be over-ridden by the sympathetic system.
Sympathetic tone is important for maintaining the normal vasomotor tone of the circulatory system which maintains normal blood pressure. Normally, sympathetic stimulation keeps the muscles of the blood vessels in a state of partial contraction. When blood pressure falls too low, sympathetic neurons fire more rapidly ; this increases constriction of blood vessels and raises blood pressure. If blood pressure rises too high, sympathetic fibers decrease their rates of firing ; as a result blood vessels dilate and blood pressure falls back to within the normal range.
Work Step by Step
The term tone in this question means normal level of activity or tension of an organ or organ system which is maintained by the normal level of activity of the parasympathetic or sympathetic branch of the autonomic nervous system
Each branch of the ANS maintains the normal level of activity of several system, and can keep their activities within normal ranges by increasing or decreasing the rate of stimulation of the relevant organ or structure.
Examples:
1 . The parasympathetic system maintains or sustains normal GI and urinary tract motility. This
branch of the ANS also monitors and controls the heart rate, If the heart begins to
beat too rapidly(tachycardia), the rate is lowered by increased parasympathetic stimulation.
2. The sympathetic system maintains normal vasomotor tone of blood vessels. Normally, sympathetic
neurons in blood vessel muscles fire at a rate that keeps blood vessels partially constricted. When
blood pressure falls too low to maintain normal blood flow, the sympathetic neurons in the blood
vessel walls fire more frequently; this increases the constriction of the blood vessels. The result is
elevation of the blood pressure into the normal range. On the other hand, if the blood pressure
rises out of normal range (temporary hypertension(, sympathetic neurons (fibers) respond by firing
more slowly. The blood vessels respond by dilating, and the result is that the blood pressure falls
back into normal range. Thus the sympathetic system maintains the normal vasomotor tone.
