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!