Normal bladder function
If the bladder is empty, the bladder muscle (detrusor) relaxes and the sphincter contracts – the bladder is shut.
The nerves connected to the bladder wall register the bladder stretching, and as it fills, they carry a message to the brain. During urination, the sphincter muscle relaxes deliberately to allow the opening of the urethra.
What is the difference with a spastic bladder?
Any information concerning the filling of the bladder is registered by the stretch receptors at the bladder wall. These receptors send information through the bladder nerves (sacral nerves S2 – S4) to the spinal cord. These nerves enter the spinal cord below the twelfth thoracic vertebra – this area is called the micturition centre.
The information sent by the micturition centre does not reach the brain and thus the brain cannot send back signals to the micturition centre. In this case, the information “full bladder” is sent back immediately to the bladder muscle as a command and thus triggers urination (so-called reflex arc). Muscle activity follows promptly in response to a stimulus. This is comparable to putting your hand on a hot stove and pulling it away as a reflex, even before you have registered that the stove is hot.
The bladder is emptied whenever the stretch receptors are stimulated enough to send the mes-sage. However, the point of stimulation varies: sometimes the reflex arc is triggered at a level of 200 ml, other times at only 80 ml. Therefore the maximum filling volume of the reflex bladder varies each time.
Furthermore, the bladder muscle is stimulated to contract and thus start urination, but at the same time, the sphincter muscles will not relax. Result: bladder muscle and sphincter muscles no longer work with each other, but against each other. This can lead to a high pressure in the bladder, which could cause incontinence and over time a reflux1 can lead to kidney damage.
What is the difference with a flaccid bladder?
A spinal cord injury below the twelfth thoracic vertebra usually causes a flaccid bladder.
The bladder nerves (sacral nerves S2 – S4) transmit information from the spinal cord to the bladder and vice versa. If the spinal cord injury affects the nerves of the bladder, little or no information can be sent to the brain. No information exchange can take place between the bladder and the brain – as a result, urination can no longer be controlled. Although the bladder itself is intact, it can no longer react – it is flaccid.
The technique you now use to empty your bladder when you have a spinal cord injury depends on the nature of the cystoparalysis.
1 Reflux = Urine reflux into the kidneys