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

Iris Parasympathetic Innervation
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

578 Part Three Integration and Control

Sympathic Vasoconstriction

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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  • Feaven
    How does vasomotor tone controls vasoconstriction and vasodilation of blood vessels?
    8 years ago
  • Stefanie
    How can sympathetic nervous system have contrasting effects in a target organ without dual?
    8 years ago
    What happens without dual innervation?
    5 years ago
  • Quintina
    Why do some organ not receive dual innervation?
    4 years ago
  • Jose
    How is blood pressure regulated with the absense of dual innervation?
    4 years ago
  • gundobad
    How do organs without dual innervation work?
    3 years ago
  • Marko
    How does sympathetic division have contrasting effects on an organ without dual innervation?
    2 years ago
  • Kerstin
    Which organ is not dual innervated?
    6 months ago
  • fnan
    How is the iris dual innervation?
    4 months ago
  • Eden
    How do organs that do not have dual innervations maintain autonomic tone?
    4 months ago

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