Final answer:
The last step to occur before urine is released is the relaxation of the external urethral sphincter, which is inhibited by the somatic nervous system. This relaxation allows urine to be expelled from the body after the activation of the micturition reflex and the contraction of the detrusor muscle.
Step-by-step explanation:
The last step to occur before urine is released from the body is the relaxation of the external urethral sphincter.
This sphincter is controlled by the somatic nervous system and it must be inhibited to relax, allowing urine to pass through.
During micturition, or urination, the detrusor muscle of the bladder contracts and the internal urethral sphincter opens as part of an involuntary process.
However, urine does not leave the body until we voluntarily relax the external urethral sphincter.
The process of urination begins with the sensation to void when the bladder reaches a certain volume.
As the bladder continues to fill, the urges become stronger. Eventually, if voluntary control (preventing relaxation of the external urethral sphincter) fails due to high bladder volume or decision to void, urination occurs. The conscious decision to urinate initiates the activation of the micturition reflex, which involves several steps, including contraction of the detrusor muscle and opening of the internal urethral sphincter.
Inhibition of the somatic motor neurons follows, resulting in the relaxation of the external urethral sphincter, and finally, urine is expelled.