The bladder serves to store urine until an appropriate time for urination. The wall of the bladder is made of the detrusor muscle, which is smooth muscle, and an overlying mucosal layer which is lined by uroepithelium. The detrusor muscle is essentially what most people refer to as “the bladder.” The detrusor muscle relaxes to allow urine to fill the bladder and stretches to hold more urine. When it is time to urinate, the detrusor muscle contracts to push urine out of the bladder.
The detrusor muscle impacts urination. It can malfunction in two main ways. It can be overactive (contract too much, when not needed) or underactive (doesn’t contract when it should). If the detrusor muscle is overactive, it contracts to squeeze the urine out even when the person is not ready to urinate. This can cause a sense of urgency or severe desire to urinate. When this urge occurs but the patient doesn’t loose urine, we call it overactive bladder / overactive detrusor muscle. If the squeeze of the detrusor muscle is strong enough, it can push urine out even if the patient is ready to urinate. When you lose urine because of an overactive detrusor muscle, it is called urge incontinence.
If the detrusor muscle does not contract (squeeze) or is overstretched, it can be difficult to get urine out even when you are trying to urinate. Because of that, you will retain urine, called urinary retention.
The bladder is connected to the outside by the urethra. In females, the urethra is fairly short, while in males, the urethra is longer and divided into 3 segments (the prostatic urethra, the membranous urethra, and the penile urethra). Two sphincters control the exit of the bladder into the urethra. The internal urethral sphincter is a thickening of the smooth muscle at the bladder neck. The external urethral sphincter is skeletal muscle located in the pelvic floor.
Innervation of the bladder.
The bladder is controlled by the central nervous system. The smooth muscles (i.e. the detrusor muscle and the internal urethral sphincter) are controlled by the two different divisions of the autonomic nervous system. The parasympathetic nervous system stimulates the detrusor muscle to contract, causing the person to urinate. The sympathetic nervous system stimulates contraction of the internal urethral sphincter, causing the person to hold urine. Somatic efferent neurons stimulate contraction of the external urethral sphincter, which is skeletal muscle.
“The walls of the bladder stretch as the bladder expands during bladder filling. This is sensed by bladder afferent neurons whose sensory dendrites are located in the bladder wall. Bladder afferents project to the spinal cord and to various regions in the brain that are responsible for coordinating the efferent output to the bladder and urethra.” Washington.edu
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Note: Afferent and efferent with regards to nerves take the CNS as their reference point.
Reference
https://courses.washington.edu/conj/bess/urination/urination.html