Internal vs external anal sphincter anatomy lesson! The internal anal sphincter (IAS) is an involuntary muscle while the external anal sphincter (EAS) is voluntary. The anorectal inhibitory reflex is where the IAS relaxes in response to rectal stretch via fecal matter, and then EAS pressure automatically increases. The brain can override this impulse by contracting the EAS to hold and store, which in turn facilitates the IAS to contract again. If the EAS is weak or has poor endurance/tone, this