Final answer:
Sympathetic impulses inhibit digestion by causing the constriction of sphincter muscles in the GI tract, reducing motility and secretion as part of the fight-or-flight response.
Step-by-step explanation:
The sympathetic impulses in the autonomic nervous system are known to inhibit digestion and this includes contractions of the sphincter muscles in the gastrointestinal (GI) tract. When the sympathetic system is activated, it creates a fight-or-flight response, leading to the constriction of the sphincters. This action decreases GI motility and secretion, effectively slowing down the digestive process. On the other hand, the parasympathetic system, often referred to as the rest-and-digest system, works to increase GI secretion and motility, thus promoting digestion. However, when substances or situations result in equal activation of both sympathetic and parasympathetic systems, the opposing effects may cancel each other out, causing no significant change in the digestive system's function.