How your bladder works
Your bladder is controlled by a complex system of nerves from your autonomic nervous system (parasympathetic and sympathetic), as well as by your spinal nerves. Some of these allow you conscious control, but others cause unconscious actions (things you cannot control).
Which nerves control what?
This is important for people living with spinal injury, as nerve damage can result in many changes to bladder function and control.
The nerves that control your bladder can be described as follows:
- Parasympathetic nerves from the S2, S3 and S4 levels of your spinal cord cause the upper part of your bladder to contract and your bladder neck to relax, assisting in the process of micturition (urination).
- Sympathetic nerves from the T11-L2 levels of your spinal cord do the opposite, causing the upper section of the bladder to relax and the bladder neck to contract, ensuring you can store urine.
Internal urethral sphincter nerves
The nerves that control your urethral sphincter can be described as follows:
- Parasympathetic nerves from the S2, S3 and S4 levels of the spinal cord control the internal sphincter, causing it to relax to allow urine to pass out of the bladder.
- Sympathetic nerves from the T11-L2 levels of the spinal cord cause the internal urethral sphincter to tighten, helping to hold stored urine in the bladder.
- Both of these functions are involuntary. This means that they operate in an automatic or reflex way, beyond your control.
External urethral sphincter nerves
Nerves from the S2-S4 levels of your spinal cord control your external urethral sphincter. This sphincter is able to be voluntarily or consciously controlled.
Filling and emptying your bladder
When the amount of urine in your bladder reaches around 250ml, sensors in your bladder muscle are stimulated. Your bladder signals your brain, and you will feel a slight urge to pass urine. Once you have around 400-500 ml in your bladder, this urge grows in intensity and you need to empty your bladder.
When full, the stretch receptors in your bladder stimulate nerves to initiate the subconscious reflex called the micturition reflex. The final stage of urination remains in your conscious control, until you can access an appropriate place and relax the external sphincter.