Simultaneously, the internal urethral sphincter opens. This is called the micturition reflex.

同时，括约肌内壁开始打开。这叫做排尿反射。

These parasympathetic neurons make the rectum contract and the internal anal sphincter relax.

这些副交感神经元可以使直肠收缩，并使肛门内括约肌松弛。

Typically, that internal sphincter muscle opens up when the bladder is about half full.

一般当膀胱充盈量达到一半时，内括约肌会开放。

In men and women, around the internal urethral orifice, the detrusor muscle thickens to form the internal sphincter.

无论男性女性，尿道内口的逼尿肌增厚，形成尿道内括约肌。

Simultaneously, the internal urethral sphincter opens.

同时，尿道内括约肌打开。

And it causes an increase in parasympathetic stimulation and decrease in sympathetic stimulation which makes the detrusor muscle contract and the internal sphincter relax.

脊髓反射导致副交感神经兴奋，而交感神经兴奋受到抑制，造成逼尿肌收缩而内扩约肌松弛。

This sets off a reflex arc called the micturition reflex which causes contraction of the bladder and relaxation of the internal sphincter and external sphincter.

这就触发了一个反射弧，叫做排尿反射，导致膀胱收缩、尿道内外括约肌扩张。

The first door is the internal sphincter muscle, which is made of smooth muscle and is under involuntary control, meaning that it opens and closes automatically.

一个阀门是内括约肌，由平滑肌组成，不受意识支配，意味着它可以自行开合。