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Poliomyelitis pathophysiology

Video transcript

so how does the polio virus cause polio myelitis so how does it infect a person and end up causing the symptoms that we see which is muscle damage muscle paralysis that sort of thing well first of all the polio virus is what's considered an enterovirus in taro by risk and an enterovirus is a sort of virus that likes to replicate in the intestines so this little virus will end up getting into the mouth of the patient will be swallowed and get into the intestines and once it's in the intestines it'll replicate it will duplicate itself until there's more and more virus in the body now this entero virus polio polio virus can actually replicate in the throat as well so you see these two sites of replication and once it has replicated to a certain number so there's a large amount of the virus in the body the virus is somehow able to spread to the central nervous system so it actually gets to the spinal cord right here's the spinal cord right here and attacks the cells of the spinal cord actually specific cells of the spinal cord which I'll get to in a moment but how does it actually get to the spinal cord well that's actually not entirely known there's - there's two theories of how it can get there one is very Mia which is the virus gets into the blood amia refers to anything associated with being in the blood so the virus gets into the blood and then from the blood it somehow finds its way to its specific target in the spinal cord these neurons in the spinal cord so that's one theory the other theory is that there is something called retrograde axonal transport retrograde means it goes backwards and it's transported backwards and axonal refers to the axon of an nerve so generally what a nerve looks like is a cell body right this is where the nucleus the cytoplasm and the other organelles or aspects of the cell that allow it to grow and survive and all of that jazz you know every cell has a cell body but what's special about neurons is they have this long tail or this little long pathway that we call an axon so this axon is how nerves which are collections of these neurons transport information they send electrical impulses down the axon and at the terminal of the neuron there's a some sort of chemical reaction that occurs to pass this information on to you know perhaps muscles of the body or potentially even to other nerves so the idea here is the poliovirus is somehow able to get into this axon and somehow travels up to the cell body where it does its replication and damage of the neuron so it's possible ated that neurons serve as a highway from the site of replication to the spinal cord where they do most of their damage just to recap polio myelitis is a motor neuron disease so motor neurons are the target of the polio virus now really quick people who are infected don't necessarily show symptoms in fact 90 to 95 percent of people are asymptomatic even after being infected and they usually recover however even patients who are asymptomatic who are infected by polio can still transmit this virus and shed it out of their intestines through their feces in fact people can be asymptomatic not have any symptoms but shed this disease for up to four to six weeks so the virus spreads and continues through the population so you can see that even though many people recover and don't experience symptoms of polio at some point it can get to somebody who will experience the symptoms and it's really kids about less than six months old that are most at risk and the problem is these kids you know they wear diapers and in areas of improper sanitation this virus can just get spread all over throughout the community now taking a step back to the pathophysiology I have this cross section of the spinal cord drawn out because I want to emphasize something the motor neurons are the cells that are targeted by polio and they are in a part of the spinal cord right this is the entire spinal cord known as the anterior horn anterior means in the front and horn is just kind of the way it looks so these are the cells in the front of the spinal cord now just a quick recap of the spinal cord this that I have outlined here is the grey matter grey matter is where the cell bodies are this white surrounding area known as the white matter is composed of axons so we've got the white matter here so it's these cell bodies of the anterior horn which so happened to be the motor neurons of the body that are affected by the polio virus so you've got all these little neurons and they connect to different muscles of the body so if I show it on our little guy here there's neurons that are all throughout this spinal cord neuron cell bodies and they connect to different muscles and here I'll actually let's draw it in in this purple color so you can see a little bit better so these neurons right here are connecting to the biceps now when polio kills some of these neurons might kill some of them but not all of them you have decreased output to the muscle so let's say these neurons are killed off you no longer have innervation or support provided to the muscles they're no longer getting this electrical output from that neuron it's been killed off so patients will experience weakness and not be able to use their muscles effectively so the symptoms you're seeing are from damage to these motor neurons and sometimes they're actually referred to as the lower motor neurons remember they're connect between neurons so there are neurons in the brain that connect down into the spinal cord and then they connect with this other cell body of a neuron that goes and provides innervation to the muscle so what I was talking about over here saying there's a connection to another cell body of a neuron okay so there's this lower motor neuron disorder and damage thankfully there's a way that the body can compensate even though this neuron right here might be damaged right it's no longer able to provide any input to the muscle other neurons which have axons that extend to that muscle - may sprout additional axons to provide innervation to the area where the old neuron used to provide innervation so this single neuron here is now controlling more of the muscle movement so some patients get compensation and recovery from polio through this reinnervation of collateral sprouting as it's called however as a patient gets older they can experience something called post-polio syndrome and this occurs years and years later after the initial acute infection with polio post-polio syndrome is defined as on set of functional deterioration so the muscles are getting worse after a long period of stability so even after this recovery years later the patient may continue to experience weakening of the muscles now why does that happen is the infection staying in there and continuing to damage other neurons no no what's actually happening is through the aging process these neurons just start to die this is the normal aging process and you lose function of neurons now normal people have a lot of different neurons serving their muscles so they don't experience this weakness as severely but since you have now maybe only a few neurons that are trying to provide a lot of support to a single muscle or muscle group when these cells die just through the normal process of aging a patient will experience continued and worsening weakness of the muscles that were originally affected so neurons all throughout the body are dying but it's these neurons that are having this additional extra effort to support the muscles that when they go the patient really starts to notice that weakness so again post-polio syndrome can occur years after the original infection of polio