Innervation of the bladder
The bladder, sphincters and pelvic floor are all under nervous control.
In this section we are going to look at the nerves involved in bladder control and maintenance of continence, the micturition control centres within the brain and spinal cord and the response of the neurotransmitters (the molecules that carry the signals in the nervous system).
Bladder innervation is a complex process and involves both parts of the nervous system:
- Central nervous system
- Peripheral nervous system
Central nervous system
The central nervous system consists of the brain, brain stem, spinal cord and sacral reflexes. It has sensory nerves to collect information and motor nerves to send messages to the muscles, like the detrusor muscle of the bladder.
Peripheral nervous system
The peripheral nervous system is the nervous system outside the brain, its main function is to connect the central nervous system to the limbs and organs, relaying messages between the brain and spinal cord and the rest of the body.
The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.
The somatic nervous system’s main function is voluntary control to the pelvic floor and external sphincter. This enables us to do pelvic floor exercises.
The autonomic nervous system regulates the function of internal organs, like the bladder. And, as its name suggests, it acts ‘automatically’ and subconsciously.
The autonomic nervous system is made up of the sympathetic nervous system (activated by flight or fight) and parasympathetic nervous system (activated during rest).
These two systems have antagonistic/opposing effects on the bladder.
Figure 2.9: How the bladder and sphincter are innervated.
The pelvic nerves, which originate at the S2-S4 level sacral level of the spinal cord, are the main parasympathetic nerves and they ‘make you pee’, they cause contraction of the detrusor muscle and relaxation of the internal sphincter.
The hypogastric nerves, which originate at the T10-12 level of the spinal cord, are the main sympathetic nerves and they ‘stop you peeing’, they promote detrusor relaxation and internal sphincter contraction.
The pudendal nerves, which originate at the S2-S4 sacral level of the spinal cord, and are the main somatic nerves, innervate the striated muscle of the pelvic floor and the external sphincter.
These nerves are under voluntary control and responsible for the contraction of the external sphincter during the filling phase and the relaxation of the external sphincter during voiding. These are the muscles we use when doing pelvic floor exercises.
The pelvic, hypogastric and pudendal nerves all also have sensory cells that transmit information, such as the feeling of bladder fullness and pain, from the lower urinary tract up the spine to the higher centres in the brain.
Neurotransmitters are chemical messengers that enable signals to pass from one nerve cell to another.
Hypogastric nerves neurotransmitter
As we already know, the main function of the hypogastric nerves is to inhibit detrusor activity (remember sympathetic - ‘stops you peeing’) and in men it stimulates the smooth muscle of the internal sphincter to prevent retrograde ejaculation.
The neurotransmitter for the hypogastric nerves is noradrenaline (also know as epinephrine).
Pelvic nerves neurotransmitter
As we also know, the pelvic nerves cause contraction of the detrusor muscle and inhibit the urethral smooth muscle promoting relaxation of the internal sphincter and allowing urine to be eliminated.
The neurotransmitter for the pelvic nerves is acetylcholine.
Did you know? This neurotransmitter is what many ‘anticholinergic’ drugs are designed to inhibit, because they work by reducing the number and intensity of detrusor contractions and thus reduce the urge to void.
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