Diagram of Micturition Reflex

The micturition reflex is a reflex pathway by which urination or micturition occurs. This reflex is induced by the stimulation of stretch receptors present on the wall of the bladder and urethra. The intravesical pressure increases when the bladder collects about 300 to 400 mL of urine. This stretches the wall of the urinary bladder and thus results in stimulation of stretch receptors and the creation of sensory impulses leading to urination. In healthy adults, this process of micturition is under voluntary control.

Let’s look at the detailed pathway of the micturition reflex with a well-labelled diagram.

Well-labelled Diagram of Micturition Reflex

Micturition Reflex Process

• When the bladder is full, it sends the signal to the brain for the process of emptying. The bladder emptying phase is called micturition, and it involves the coordinated reflexes of the outer and inner urethral sphincter under somatic and sympathetic regulation, respectively.
• At first, the afferent impulses or the sensory impulses from the receptors reach the spinal cord through the sensory fibres of the pelvic nerve (parasympathetic nerve).
• The motor impulses created in the spinal cord run through the motor fibres of the pelvic nerve towards the bladder and the internal sphincter.
• These motor impulses (efferent impulses) create contraction of the detrusor muscle and also the relaxation of the internal sphincter. Thus, the urine enters the urethra from the urinary bladder.
• Once urine reaches the urethra, the stretch receptors present in the urethra are stimulated, and they send afferent impulses to the spinal cord through pelvic nerve fibres.
• Now the impulses created from the spinal centres obstruct the pudendal nerve. This leads to the relaxation of the external sphincter, and micturition occurs.
• Facilitatory centres for micturition are present in the pons, and some are even in the cerebral cortex. It facilitates micturition through spinal centres.
• Inhibitory centres for micturition are present in the cerebral cortex and midbrain. It inhibits the micturition by repressing spinal micturition centres.

The micturition reflex process is self-regenerative. This is because the initial contraction of the urinary bladder activates the receptors to create an increase in sensory impulses from the urethra as well as the bladder. These impulses, in turn, create a further increase in reflex contraction of the urinary bladder. This cycle of events repeats until the force of contraction of the urinary bladder reaches the maximum, leading to the voiding of urine or the micturition process. During micturition, the urine flow is facilitated by the increase in abdominal pressure. This is due to the voluntary contraction of the muscles in the abdomen.

The micturition reflex process can be summarised as follows –

1. Filling of the urinary bladder
2. Stimulation of stretch receptors
3. Afferent impulses pass through the pelvic nerve and reach the spinal cord
4. Efferent impulses through the pelvic nerve
5. Contraction of the detrusor muscle and relaxation of the internal sphincter
6. The flow of urine into the urethra and stimulation of stretch receptors
7. Afferent impulses through the pelvic nerve
8. Inhibition of pudendal nerve
9. Relaxation of the external sphincter
10. Voiding of the urine or micturition

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