Control Without Dual Innervation
Dual innervation is not always necessary for the ANS to produce opposite effects on an organ. The adrenal medulla, piloerector muscles, sweat glands, and many blood vessels receive only sympathetic fibers. The most significant example of control without dual innervation is regulation of blood pressure and routes of blood flow. The sympathetic fibers to a blood vessel have a baseline sympathetic tone which keeps the vessels in a state of partial constriction called vasomotor tone (fig. 15.10). An increase in firing rate causes vasoconstriction by increasing smooth muscle contraction. A drop in firing frequency causes vasodilation by allowing the smooth muscle to relax. The blood pressure in the vessel, pushing outward on its wall, then dilates the vessel. Thus, the sympathetic division alone exerts opposite effects on the vessels.
Sympathetic control of vasomotor tone can shift blood flow from one organ to another according to the
Figure 15.9 Dual Innervation of the Iris. Shows antagonistic effects of the sympathetic and parasympathetic divisions.
Figure 15.9 Dual Innervation of the Iris. Shows antagonistic effects of the sympathetic and parasympathetic divisions.
Saladin: Anatomy & I 15. The Autonomic Nervous I Text I © The McGraw-Hill
Physiology: The Unity of and Visceral Reflexes Companies, 2003 Form and Function, Third Edition
- 578 Part Three Integration and Control
Figure 15.10 Sympathetic Tone and Vasomotor Tone.
(a) Vasoconstriction in response to a high rate of sympathetic nerve firing. (b) Vasodilation in response to a low rate of sympathetic nerve firing.
10. What are the two ways in which the sympathetic and parasympathetic systems can affect each other when they both innervate the same target organ? Give examples.
11. How can the sympathetic nervous system have contrasting effects in a target organ without dual innervation?
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