Autonomic Nervous System (ANS) Innervation

Autonomic Nervous System (ANS) Innervation

• Most organs in the body receive dual innervation from both divisions of the ANS.
• Some organs, however, are supplied by only one division of the ANS:
  • Iris sphincter muscle (circular) is supplied by parasympathetic fibers (M3).
  • Iris dilator muscle (radial) is supplied by sympathetic fibers (a1).
  • Arrector pili muscle (pilomotor muscle) is supplied by sympathetic fibers (a1), causing involuntary hair erection in response to stress, cold, or fear.
  • Thermoregulatory (eccrine) sweat glands are supplied only by sympathetic fibers, but through cholinergic (parasympathetic) receptors (M3).
  • Blood vessels: vascular smooth muscle is primarily innervated by the sympathetic, with indirect non-innervation by the parasympathetic.

Central Nervous System (CNS) in Autonomic Function

• The ANS is controlled by streams of afferent impulses originating in the viscera and other autonomically innervated structures.
• These impulses travel to integrating centers in the CNS, such as the hypothalamus, medulla oblongata, and spinal cord.
• The CNS responds to stimuli by sending out efferent reflex impulses via the ANS.

Reflex Arc

• A reflex arc is a neural pathway controlling an action reflex.
• Most afferent impulses are involuntarily translated into reflex responses.
• Example: a fall in blood pressure triggers a reflex response, increasing sympathetic output to the heart and vasculature, and decreasing parasympathetic output to the heart, resulting in a compensatory rise in blood pressure and tachycardia.