The precapillary sphincter is innervated by the sympathetic portion of the Autonomic Nervous System (ANS).

• A. True
• B. False
• C. Not specified
• D. Both sympathetic and parasympathetic

Answer: (A)

Precapillary sphincters are smooth muscle rings at the entrance to capillary beds that regulate blood flow into the capillaries. They are controlled mainly by sympathetic vasomotor nerves, which constrict these sphincters via norepinephrine acting on vascular smooth muscle receptors. This sympathetic control reduces capillary perfusion when needed. Parasympathetic input to most vascular beds is minimal, so these sphincters are not typically innervated by parasympathetic fibers. Therefore, the statement is true. Local metabolic factors can further modulate flow, but the predominant extrinsic innervation of these sphincters is sympathetic.