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!
