Physiology of Micturition

Summary

This video explains the physiology of micturition, detailing the organs and structures involved in urine production and release. It highlights key components such as the kidneys, bladder, urethra, and muscles like the detrusor and sphincters. The video explores the role of different nerve systems—parasympathetic, sympathetic, and somatic—in controlling the bladder and sphincters, both involuntarily and voluntarily. It distinguishes between male and female urinary tracts, focusing on receptors, nerve signals, and the nervous system's involvement in holding and releasing urine. Lastly, it outlines the reflexes triggered during the voiding process.

Takeaways

• 🔬 The micturition process involves several key organs: kidneys, ureters, bladder, and urethra.
• 🧠 The central nervous system, particularly the pons, controls the micturition process.
• 🟡 The bladder stores urine and is composed of the detrusor muscle, which can contract and relax.
• 🚻 Males have an internal sphincter, prostate gland, and a longer urethra, while females lack an internal sphincter and have a shorter urethra.
• 🔗 The pelvic, pudendal, and hypogastric nerves play crucial roles in controlling the contraction and relaxation of bladder muscles and sphincters.
• 🧪 The receptors involved in micturition include muscarinic M3, beta-adrenergic B3, alpha-1, and nicotinic receptors, each affecting different parts of the urinary tract.
• 💡 The sympathetic nervous system facilitates urine retention by relaxing the detrusor muscle and contracting the internal sphincter.
• 🔄 The parasympathetic nervous system is responsible for voiding by contracting the detrusor muscle.
• 📶 Sensory (afferent) pelvic nerves respond to bladder stretching and help signal the need to urinate.
• 🔄 When the bladder is full, the pontine micturition center inhibits sympathetic activity and stimulates parasympathetic activity, allowing for urination.

Q & A

What is the primary function of the bladder in the process of micturition?

The primary function of the bladder in micturition is to store urine until it is ready to be expelled from the body. When the bladder contracts, it releases urine, which then travels through the urethra and exits the body.

What role does the detrusor muscle play in micturition?

The detrusor muscle is a muscle in the bladder that contracts to expel urine during micturition. It relaxes to allow the bladder to fill with urine and contracts to facilitate urination.

What are the key differences between the male and female urinary tracts?

The key differences are that males have a longer urethra, an internal sphincter, and a prostate gland, while females have a shorter urethra and no internal sphincter or prostate. This shorter urethra in females increases the likelihood of urinary tract infections (UTIs).

How does the parasympathetic nervous system influence micturition?

The parasympathetic nervous system controls voiding (urination). Specifically, the pelvic nerve, which releases acetylcholine (ACh), stimulates the M3 receptors on the detrusor muscle, causing the bladder to contract and expel urine.

What is the role of the hypogastric nerve in micturition?

The hypogastric nerve, part of the sympathetic nervous system, is responsible for urinary retention. It releases noradrenaline (norepinephrine) that binds to beta-3 receptors on the detrusor muscle to relax it and to alpha-1 receptors on the internal sphincter to contract it, preventing urination.

How does the pudendal nerve function in the control of urination?

The pudendal nerve is a somatic nerve that is under voluntary control. It releases acetylcholine (ACh) to stimulate nicotinic receptors on the external sphincter, causing the sphincter to contract, which helps in holding urine and preventing urination when necessary.

What triggers the voiding reflex during urination?

The voiding reflex is triggered when the bladder becomes full and stretches, activating the pelvic afferent (sensory) nerves. These nerves send signals to the sacral region of the spinal cord, which leads to the stimulation of parasympathetic efferent nerves, causing the detrusor muscle to contract and the sphincters to relax, allowing urination.

What role does the pons play in the micturition process?

The pons, specifically the pontine micturition center, is crucial in controlling the micturition process. It coordinates signals from the central nervous system, including the inhibition of the sympathetic nervous system (to allow urination) and the stimulation of the parasympathetic system (to trigger detrusor muscle contraction and sphincter relaxation).

How do sensory pelvic nerves contribute to the process of micturition?

Sensory pelvic nerves detect the stretching of the bladder as it fills with urine. When the bladder is full, these nerves send fast signals to the sacral region of the spinal cord, which triggers the micturition reflex, resulting in the bladder contracting and the sphincters relaxing.

What happens during micturition when the bladder is empty?

When the bladder is empty, the sensory pelvic nerves send slow impulses to the sacral region of the spinal cord, which stimulates the hypogastric nerve to contract the internal sphincter and relax the detrusor muscle. This process, along with the voluntary contraction of the external sphincter via the pudendal nerve, helps retain urine in the bladder.

Outlines

00:00

🩺 Physiology of Micturition: Organs and Structures

The video introduces the physiology of micturition, starting with the organs involved, such as the kidneys, ureters, bladder, and urethra. Urine is produced in the kidneys and travels through the ureters to the bladder, which stores it until released through the urethra. The male urinary system includes the prostate gland and internal and external sphincters, whereas females lack an internal sphincter and have a shorter urethra, making them more prone to urinary tract infections (UTIs). The detrusor muscle in the bladder controls contraction and relaxation. The paragraph sets the stage for a detailed exploration of the male urinary system and its functioning.

05:01

💡 Nerves and Receptors in Micturition

This section explains the role of various nerves in controlling the urinary system, highlighting three key efferent nerve fibers: the pelvic nerve (parasympathetic), pudendal nerve (somatic), and hypogastric nerve (sympathetic). Each nerve has a different function: the pelvic nerve contracts the detrusor muscle, the pudendal nerve controls the external sphincter voluntarily, and the hypogastric nerve is responsible for bladder relaxation and urinary retention. Additionally, there is an afferent pelvic nerve that senses bladder stretching, sending signals to trigger the micturition process. The discussion focuses on how the nervous system regulates urine storage and release.

10:02

🔄 Empty Bladder: Mechanisms of Urine Retention

This paragraph explores the mechanics of urinary retention when the bladder is empty. The sensory pelvic nerve sends slow impulses to the sacral region of the spinal cord, stimulating the hypogastric nerve to contract the internal sphincter and relax the detrusor muscle, allowing urine retention. Additionally, the pudendal nerve contracts the external sphincter voluntarily, helping hold urine in. The parasympathetic efferent nerve is inhibited, preventing bladder contraction. This section emphasizes the role of the sympathetic nervous system in urine retention and the mechanisms that prevent involuntary urination when the bladder is not full.

15:03

🌊 Full Bladder: Initiation of Micturition

The focus shifts to what happens when the bladder is full. The pelvic afferent nerve detects bladder stretching and sends fast signals to the sacral spinal cord, bypassing the thoracic-lumbar region to stimulate the pontine micturition center. This inhibits the hypogastric nerve, relaxing the internal sphincter, while stimulating the pelvic efferent nerve to contract the detrusor muscle. The pudendal nerve is inhibited, allowing the external sphincter to relax. The result is the release of urine through the urethra as the coordinated muscle movements trigger micturition.

🔁 The Voiding Reflex: Continuation of Urine Release

The final paragraph describes the voiding reflex, which keeps the urine flow continuous once urination starts. Sensory impulses from the pelvic afferent nerve are sent to the sacral region, where they activate an interneuron connected to parasympathetic efferent fibers. This loop ensures the continued contraction of the detrusor muscle, allowing the bladder to empty fully. The video concludes by summarizing how this reflex sustains urination and thanking the audience for watching.

Keywords

💡Micturition

Micturition refers to the process of urination or voiding. In the video, it is the central theme and involves the coordination of various muscles, sphincters, and nerves to release urine from the bladder. The video explains how the nervous system controls micturition by regulating bladder contraction and sphincter relaxation.

💡Detrusor muscle

The detrusor muscle is the smooth muscle that forms the wall of the bladder. It contracts to expel urine and relaxes to allow the bladder to fill. The video discusses how acetylcholine, through parasympathetic nerves, causes contraction of the detrusor muscle during micturition.

💡Internal and External Sphincters

These are two muscles that control the release of urine from the bladder. The internal sphincter is under involuntary control, while the external sphincter is controlled voluntarily. The video highlights how these sphincters work together to retain or release urine, with the internal sphincter playing a significant role in males, while the external sphincter functions similarly in both sexes.

💡Pelvic nerve

The pelvic nerve is a crucial nerve involved in micturition. It has both sensory (afferent) and motor (efferent) fibers. In the video, the pelvic nerve's sensory fibers detect bladder stretch, while its motor fibers trigger bladder contraction through acetylcholine release.

💡Pudendal nerve

The pudendal nerve controls the external sphincter and is under voluntary control. The video explains how this nerve allows us to consciously hold in urine by stimulating the nicotinic receptors on the external sphincter, causing it to contract.

💡Hypogastric nerve

The hypogastric nerve is part of the sympathetic nervous system and is involved in urinary retention. In the video, this nerve is shown to cause relaxation of the detrusor muscle and contraction of the internal sphincter, helping the body hold urine in the bladder.

💡Muscarinic receptor (M3)

M3 receptors are found on the detrusor muscle and are activated by acetylcholine from parasympathetic nerves. The video explains that when these receptors are activated, they cause the detrusor muscle to contract, initiating the process of urination.

💡Alpha-adrenergic receptor (alpha-1)

Alpha-1 receptors are located on the internal sphincter and are activated by the sympathetic nervous system. The video explains how activation of these receptors by the hypogastric nerve causes contraction of the internal sphincter, preventing urine from leaking.

💡Pontine micturition center

This center, located in the pons of the brainstem, plays a critical role in controlling micturition. The video highlights how signals from the pontine micturition center regulate bladder contraction and sphincter relaxation during the process of urination.

💡Voiding reflex

The voiding reflex is the body's feedback loop that ensures urination continues once it starts. The video explains that once the bladder begins to empty, sensory signals trigger parasympathetic activity, ensuring continuous contraction of the detrusor muscle and complete bladder emptying.

Highlights

The kidneys produce urine, which travels through the ureters to the bladder for storage.

The bladder contracts to release urine through the urethra, which is longer in males and surrounded by spongy tissue.

The detrusor muscle of the bladder can contract and relax, playing a crucial role in the micturition process.

Males have both internal and external sphincters, while females have only an external sphincter.

The prostate gland is located between the internal and external sphincters in males, contributing to sperm activation.

Prostate hyperplasia can obstruct the urethra, causing difficulty in urination.

The shorter urethra in females makes them more prone to urinary tract infections (UTIs).

The central nervous system, particularly the pons, contains the micturition center that controls urination.

The parasympathetic pelvic nerve stimulates the detrusor muscle for bladder contraction during urination.

The somatic pudendal nerve controls the external sphincter, allowing voluntary control over urination.

The sympathetic hypogastric nerve is responsible for urinary retention by relaxing the detrusor muscle and contracting the internal sphincter.

Afferent sensory nerves in the detrusor muscle send signals to the spinal cord when the bladder is stretched.

When the bladder is full, the pontine micturition center inhibits the sympathetic system, allowing bladder contraction and sphincter relaxation.

The voiding reflex ensures continuous urination once initiated, through a loop between sensory and parasympathetic nerves.

Micturition involves complex coordination between parasympathetic, sympathetic, and somatic nerves to control bladder function.

Transcripts

In this video we're going to talk about

the physiology of micturition

In order to understand the physiology of micturition,

we have to know about the organs

and the structures

involved in the micturition process.

The kidneys are the site where urine is produced.

The urine will travel down

these muscular tubes called

ureters

and into the bladder.

The bladder stores the urine

When needed, the bladder will contract and release the urine.

The urine will travel down through the urethra, and then outside

This particular urethra I am drawing is a male urethra

and so part of the urethra is surrounded by the penis, spongy tissue.

Going back to the bladder,

the bladder is made up of a muscle called the detrusor muscle

and because it is a muscle, it can contract -and- it can relax

There are also 2 important sphincters, which play an important role in holding or retaining urine in the bladder

Now these are the internal sphincters and the external sphincters

Between these 2 sets of sphincters is the prostate gland

The prostate is only found in males and has an important role in activating sperm

When there is a prostate hyperplasia, this can cause obstruction in the urethra and thus problems with urinating

The external sphincter is within an area known as the urogenital diaphragm

This urogenital tract I am drawing was the male's tract, which is very different to the female's

It is important to look at the female one as well

So, females also have the kidneys, which connect to the bladder via the ureters. The bladder is composed of the detrusor muscle

Now, the female doesn't actually have an **internal** sphincter BUT females do have an external sphincter

Also the urethra of the female is a lot shorter than the males'

So again, the major difference is that the females' urinary tract are shorter- a shorter urethra

and they have NO internal sphincter or prostate

And because the females' urethras are shorter, this would mean that there is a more likely chance that they will get

a urinary tract infection (UTI).

Now in this video we will mainly focus on the male urinary tract

because it is better to understand the micturition process this way, not being biased

Let us review the receptors found within the urinary tract

that play a role in micturition

and holding in urine

receptors found on the bladder include the muscarinic receptor, M3

and the beta-adrenergic receptor, or B-3

we can find the alpha-adrenergic receptor on the internal sphincter muscle

and on the external sphincter muscle, we can find the nicotinic receptor

Ok, so now we know the structures and the receptors involved in micturition

But what *controls* these things?

Well the nervous system controls these things

So here I am drawing the central nervous system (CNS), which includes the brain, the brainstem

Which is made up of the midbrain, the pons, and the medulla

The most important structure is the pons because the pons is where the micturition center is located

This is where the micturition center is located

and then we have the spinal cord-- the cervical region, thoracic, lumbar, and sacral

Signals from the central nervous system (CNS) will be sent to the urinary tract via the nerves

We will look at 3 important *efferent* nerve fibers

The first nerve, comes from the sacral region of the spinal cord

This nerve is the PELVIC nerve, which is a parasympathetic nerve

It is NOT under our control

The pelvic nerve releases acetylcholine (ACh)

which binds onto the M3 receptor

When ACh binds to the M3 receptor, on the detrusor muscle, it will cause contraction of the detrusor muscle

So that is why I am drawing a positive sign here, for contraction

The second nerve, coming out of the sacral region is the PUDENDAL nerve, which is a somatic nerve

and it is under our (voluntary) control

The pudendal nerve also releases ACh, and acts on the nicotinic receptor found on the external sphincter

When ACh from the pudendal nerve binds on the nicotinic receptor, it will cause the external sphincter to contract

We are firing our pudendal nerve when we are trying to hold in our urine

The third nerve, is part of the sympathetic nervous system

The pre-sympathetic nerve fibers that target the urinary tract comes from actually, the thoracic-lumbar area

and synapses at a (pelvic) ganglion here

secretes neurotransmitters and will pass on the information to a post-sympathetic fiber

known as the hypogastric nerve

Now, the hypogastric nerve targets 2 things:

and because it is a post-sympathetic nerve, it releases the neurotransmitter noradrenaline (norepinephrine)

When noradrenaline binds to the beta-3 (B3) receptor on the detrusor muscle, it causes relaxation of the detrusor muscle

So it is a negative sign here for relaxation

When noradrenaline binds to the alpha-1 receptor, on the internal sphincter, it will cause contraction of the internal sphincter

So positive effect

So we can safely say that the hypogastric nerve or the sympathetic nervous system altogether is responsible for urinary retention

holding in urine

Whereas the parasympathetic nervous system is responsible for voiding or peeing

Also, there's actually a 4th neuron that I have not drawn, which is an afferent nerve

It is a sensory nerve fiber

It comes actually from the detrusor muscle

Its called also the pelvic nerve, but it is an afferent nerve

And it responds or is stimulated when the bladder is stretched

Ok now that we know the nerves, the receptors, and the structures involved in micturition, let us see how it all works

By putting it altogether

Let us see what happens when we have an empty bladder, and what happens when we have a full bladder

So going over the structures again, we have the detrusor muscle, the internal sphincter, the external sphincter, the prostate, the urethra...

...then we have the receptors on the detrusor muscles, the M3 and B3

the alpha-1 receptor on the internal sphincter, and the nicotinic receptor on the external sphincter

and here we have the CNS that controls the peeing

So the higher brain center, the pons, which is important because it contains the micturition center

then we have the thoracic-lumbar level of the spinal cord, and then we have the sacral level of the spinal cord

Ok, so now we have a sort of empty bladder

so we don't have much urine in the bladder here

Now this means

that we don't have that much stretching of the bladder

when we don't have much stretching of the bladder, the sensory pelvic nerve will only send slow impulses toward the spinal cord

sacral region

the slow impulses from the sensory pelvic nerve will send signals to a neuron at the sacral level

and this neuron will actually

stimulate

the hypogastric nerve from the thoracic and lumbar area

so it will stimulate the sympathetic, hypogastric nerve

when the hypogastric nerve is stimulated, it will stimulate the alpha-1 receptor in the internal sphincter causing the internal sphincter to contract

also, the stimulated hypogastric nerve will act on the beta-3 receptor

so positive effect

and when the beta-3 receptor is activated

this will cause relaxation of the detrusor muscle

so negative effect here

in the sacral level of the spinal cord, there is also some inhibitory activity

here which I will talk about later

Anyway, the brain and the pons know

when we have an empty bladder

and they will actually then send a signal

in the thoracic level

to stimulate the hypogastric nerve

so that we can retain urine

also there are some signals being sent to the sacral level

so there is a signal being sent to the pelvic *efferent* nerve

the signal being sent to the pelvic efferent nerve

is actually being inhibited

at this level

however, there are signals that will stimulate the pudendal nerve, which is under our voluntary control

when the pudendal nerve is stimulated, this will stimulate the nicotinic receptors on the external sphincter

causing contraction

and so we are able to basically hold our urine

in volantarily

so the take-home message from this diagram is

that the sympathetic nerve is stimulated allowing us

to hold our urine

in the bladder

the parasympathetic nerve motor (efferent) fiber is inhibited, so we don't contract the detrusor muscle

and the pudendal nerve is stimulated so we can hold our urine in voluntarily

So what happens when we have a full bladder?

Well, let's see

I'm going to draw the exact same diagram as I did

So here we have the bladder, the urethra, the external sphincter, and the internal sphincter

and here we have the receptors on the detrusor muscle

the M3, the beta-3

and on the internal sphincter, the alpha-1

and then we have a nicotinic receptor on the external sphincter

and here I'm drawing the structures that control all that

which are the nervous system, we have the sacral region of the spinal cord

the thoracic-lumbar region, and we have

the pons, which contains the micturition center

so when we have a full bladder

we have a lot of urine in the bladder

and this will cause a lot of stretching in the bladder

this will stretch our detrusor muscle

we have sensory nerves in our detrusor muscle

called the pelvic (afferent/ sensory) nerve, remember?

and when there's a lot of stretching in this pelvic nerve, the pelvic nerve increases firing

and it will send out a lot of fast signals, not slow signals

it will send fast signals to the sacral region of the spinal cord

and when the pelvic nerve sends fast signals, a lot of impulses to the sacral region of the spinal cord, this will cause a new neuron to basically bypass the thoracic-lumbar area

and go straight to the pontine micturition center

and this will stimulate actually, the nerve, some neurons there

when the neurons in the pontine micturition center are stimulated,

they will cause a few things

one of which is inhibiting the hypogastric sympathetic nerve

Now, when the hypogastric sympathetic nerve is inhibited, we have no relaxation of the detrusor muscle

and we have relaxation, or no function of the alpha-1 receptor

and so we have relaxation of the internal sphincter

also, the neurons coming from the pontine micturition center

will stimulate the pelvic *efferent* nerves

when the pelvic efferent nerves are stimulated,

it will cause contraction of the detrusor muscle

by working on the M3 (muscarinic) receptor

also, the neurons coming from the pontine micturition center will inhibit the pudendal nerve

and so this is under our (voluntary) control

and when the pudendal nerve is inhibited, there is no contraction of the external sphincter

and thus it is relaxed

and so altogether what we have is

we have no relaxation of the detrusor muscle

we have contraction of the detrusor muscle

and we have relaxation of the internal and external sphincters

and therefore urine is able to go out through the urethra

and basically we pee out

voiding

the process is called voiding

and when we begin to void, the voiding reflex begins

so when we start peeing, we sort of continue to pee

right?

so this is our voiding reflex

and let us just see how this works

well in the voiding reflex,

here I am drawing the exact same structure

the pelvic *afferent* nerve fibers are sensory fibers coming out of the detrusor muscle

will keep sending impulses to the sacral region of the spinal cord

it then works on an interneuron on this level

as well

which will connect with the parasympathetic *efferent* nerve fibers

and when the parasympathetic nerve fibers are stimulated,

it will just cause contraction of the detrusor muscle

so it's like a loop

sensory sends signals to the pelvic efferent nerve, which will cause contraction of the detrusor muscle, and this is why we continue to pee

I hope that all made sense

and I hope you enjoyed this video on micturition

Thank you for watching!