Micturition- Definition, Components & Process

Micturition: Urine is the excretory waste fluid that needs to be thrown out of the body. But how? If not thrown out, it may accumulate in the body which may cause different disorders. You must have made the mattresses wet when you were a child! But now as an adult, you don’t. Do you know why? Is it a voluntary process or involuntary? Are Micturition and urination the same? Get all your answers in this article below.

Introduction to Micturition

Kidney has two parts- the outer cortex and inner medulla. Malphigian capsule, Proximal convoluted tubule and distal convoluted tubule lie in the cortex while loop of Henle remains in the medullary region. Distal convoluted tubule opens in collecting duct. Urine from many collecting ducts enters a common tube called Duct of Bellini which is opening into the narrow part of the pyramid or medullary pyramid. From the medullary pyramid, urine is collected by Minor Calyx and then Major Calyx. Then it enters the renal pelvis and then to ureter. From the ureter, it finally enters the Urinary bladder. It is stored here in the urinary bladder till a voluntary signal is given by the CNS.

What is Called Micturition?

Micturition is the emptying of the bladder once it gets filled or the act of voiding urine according to the will of the person. Bladder fills until pressure crosses the threshold triggering the micturition reflex which empties it.

Urinary Bladder and Urethra

1. Urinary bladder and urethra are the main organs that are involved in micturition. Urinary bladder has a body where urine is filled and a neck that continues with the urethra.
2. Neck region consists of a smooth triangular region called trigone which consists of openings of the ureters.
3. The wall of the bladder has a smooth muscle called the detrusor muscle.
4. When detrusor muscle contracts, the bladder pressure increases.
5. Internal urethral sphincter is present in the neck of the bladder which is involuntary in function.
6. External urethral sphincter is voluntary in action.
7. Urethra is longer in male \(\left( {{\rm{20 \;cm}}} \right)\) than females \(\left( {{\rm{4 \;cm}}} \right){\rm{.}}\) That is why females are more prone to urinary tract infection (UTI).
8. When the bladder is half or slightly more than half full, stimulus is generated and sent to muscles of the urinary bladder. Then detrusor muscles contract and sphincters relax and urine is released. This micturition occurs involuntarily in children upto age of \(3-5\) years after which it occurs voluntarily.

Nerve Supply to Urinary Bladder and Sphincters

Receptors present on bladder
(I) \({\rm{M3}}\) receptor
(II) \({\rm{\beta 3}}\) receptor
Receptor on internal sphincter
(I) \({\rm{\alpha 1}}\) receptor
Receptor on external sphincter
(I) Nicotinic receptor

Nervous system controls these receptors.
1. Pelvic nerves– It has sensory and parasympathetic nerves fibres. It comes from the sacral region of the spinal cord. It is not under our control. It releases acetylcholine neurotransmitter which binds to the \({\rm{M3}}\) receptor. It will cause contraction of detrusor muscle resulting in emptying the bladder.
2. Pudendal nerve– It is a somatic nerve and is in our control. It also releases acetylcholine which binds to nicotinic receptors found on external sphincters. This leads to contraction in external sphincters resulting in holding the urine. This is the second nerve arising from the sacral region of the spinal cord.
3. Hypogastric nerve– It arises from the thoracic lumbar area. It is a sympathetic nerve.
It releases nor-adrenaline. Hypogastric nerve targets two things:-
a. When noradrenaline binds to the \({\rm{\beta 3}}\) receptor of detrusor muscle, it causes its relaxation.
b. When it binds to the \({\rm{\alpha 1}}\) receptor on the internal sphincter, it will cause its contraction.
Due to which the Hypogastric nerve is responsible for holding urine.

Fig:  Neural control of Micturition

What are the Steps of Micturition?

Neural control of Micturition can be better understood by these two cases of the bladder: –
When the bladder is empty
In his case, there is little urine in the bladder and so there is little or no stretching of the bladder. Due to which the sensory pelvic nerve will send slow impulses towards the spinal cord.
1. This neuron will stimulate the sympathetic hypogastric nerve from the thoracic and lumbar area which will stimulate the \({\rm{\alpha 1}}\) receptor in the internal sphincter causing it to contract.
2. Also, \({\rm{\beta 3}}\) receptor gets activated by this hypogastric nerve which causes relaxation of the detrusor muscle.
3. Pelvic afferent nerve will give a negative effect to the bladder so that there is no contraction of detrusor muscle, which ultimately does not allow the emptying of the bladder.
4. Pudendal nerve stimulates the nicotinic receptor of the external sphincter which causes its contraction causing holding of urine.
5. So, as a whole it can be said that the sympathetic nerve allows it to hold the urine when the bladder is empty or has little urine in it.

When bladder is full
1. When the bladder is full, its muscles stretch. Sensory pelvic nerves of the detrusor muscle send fast signals to the sacral region of the spinal cord.
2. This signal also reaches the pontine micturition centre of Pons.
3. Nerves arising from the pontine centre will inhibit the hypogastric nerve. Due to which there will be no relaxation of detrusor muscle and internal sphincter.
4. Nerves from the pontine centre will also stimulate pelvic afferent nerves which cause contraction of detrusor muscle by acting on the M3 receptor.
5. It also inhibits the pudendal nerves due to which no contraction but relaxation of exterior sphincter occurs resulting in MICTURITION.

What are the Phases of Normal Micturition?

Micturition can also be divided into these two distinct phases:

VOIDING PHASE
1. When the bladder is full, then muscles of the bladder get stretched which is sensed by the nerves and the brain.
2. At that time the brain signals the detrusor muscles to contract and the external sphincter of the urethra to relax so that urine can exit out of the body through the urethra, called micturition.

Fig: Cycle of bladder filling and emptying

Micturition Reflex

When we start peeing, we do not want to stop till the bladder empties. This is due to micturition reflex. The pelvic nerve fibres coming out of the detrusor muscle will keep sending impulses to the sacral region of the spinal cord. Interneuron also works which will connect with the parasympathetic afferent nerve fibre and when parasympathetic afferent fibres get stimulated, it will cause contraction of the detrusor muscle. This makes us pee continuously till external sphincters of the urethra contract again.

Micturition- Flow Chart

Fig: Flow chart of micturition

Micturition- Associated Disorders

1. Urinary incontinence- In this there is loss of control in urination.
2. Detrusor instability- This is a condition where the detrusor muscle contracts without any apparent reason and this involuntary contraction of detrusor muscles leads to incontinence.
3. Polyuria- In this disorder, there occurs more urination than normal.
4. Bladder infections- It refers to infection in the bladder due to bacteria.
5. Urinary Retention- It is inability to completely or partially empty the bladder.  It can be caused by a variety of conditions, such as Benign prostatic hyperplasia (BPH),  Nerve dysfunction, Infection – e.g. UTI, Constipation, Drugs.

Summary

It is the act of voiding urine according to the will of the person. Bladder fills until a pressure crosses the threshold triggering the micturition reflex which empties it. Urinary bladder and urethra are the main organs which are involved in micturition. The wall of the bladder has smooth muscle called detrusor muscle. When the bladder is half or slightly more than half full, stimulus is generated and sent to muscles of the urinary bladder.

Then detrusor muscles contract and sphincters relax and urine is released. This micturition occurs involuntarily in children upto age of \(3-5\) years after which it occurs voluntarily. Somatic nerves, parasympathetic nerves and sympathetic nerves are involved in controlling micturition. When we start peeing we do not want to stop till the bladder empties. This is due to micturition reflex.

Frequently Asked Questions (FAQs) on Micturition

The answers to the most commonly asked questions about Micturition are provided here:

We hope this detailed article on Micturition helps you in your preparation. If you get stuck do let us know in the comments section below and we will get back to you at the earliest.