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- [Voiceover] So how does the polio virus cause poliomyelitis? 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 anterovirus, anterovirus. And an anterovirus is a virus that likes to replicate in the intestines. This little virus will end up getting into the mouth of a patient; will be swallowed and get into the intestines. Once it's in the intestines, it'll replicate. It'll duplicate itself until there's more and more virus in the body. Now, this anterovirus, polio, polio virus can actually replicate in the throat as well. So you see these two sites of replication. 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. It actually gets to the spinal cord. 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 two theories of how it can get there. One is viremia, which is the virus gets into the blood. Emia 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 the nerve. Generally, what a nerve looks like is a cell body. This is where the nucleus, the cytoplasm, or the other organelles, or aspects of the cell that allow it to grow and survive and all of that jazz. 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. 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 some sort of chemical reaction that occurs to pass this information on to, perhaps, muscles of the body, or potentially even to other nerves. The idea here is the polio virus 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. It's postulated 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, poliomyelitis is a motor neuron disease. 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% 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. The virus spreads and continues through the population. 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. It's really kids about less than six months old that are most at risk. The problem is these kids 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 this spinal cord drawn out because I wanna emphasize something. The motor neurons are the cells that are targeted by polio. They are in a part of the spinal cord; 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. These are the cells in the front of the spinal cord. Now to just recap of the spinal cord, this, that I have outlined right here is the gray matter. Gray 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. It's these cell bodies of the anterior horn which so happen to be the motor neurons of the body that are affected by the polio virus. You've got all these little neurons and they connect to different muscles of the body. If I show it on our little guy here, there's neurons that are all throughout the spinal cord. Neuron cell bodies and they connect to different muscles. Here, actually let's draw it in this purple color, so you can see it a little bit better. 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 muscles. Let's say these neurons are killed off. You know longer have innervasion or support provided to the muscles. They're no longer getting this electrical output from that neuron. It's been killed off. Patients will experience weakness and not be able to use their muscles effectively. The symptoms you're seeing are from damage to these motor neurons. Sometimes they're actually referred to as the lower motor neurons. Remember there're connections 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, it's no longer able to provide any input to the muscle, other neurons, which have axons that extend to that muscle too may sprout additional axons to provide innervation to the area where the old neuron used to provide innervation. This single neuron here is now controlling more of the muscle movement. 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. This occurs years and years later after the initial actute infection with polio. Post-Polio Syndrome is defined as an onset of functional deterioration, so the muscles are getting worse after a long period of stability. Even after this recover, 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 cell die just through the normal process of aging, a patient will experience continued and worsening weakness of the muscles that were originally affected. 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. Again, Post-Polio Syndrome can occur years after the original infection of polio.