hi everyone dr mike here in this video i want to talk to you about motor neurons reflexes and damage to motor neurons so the first thing we need to talk about is how does as an example our leg contract we want the muscles of our leg to contract so we can tell that leg to move well it all begins up in the brain specifically begins in the cortex so they add a few millimeters of the brain in an area called the motor cortex now we have a map of our body on our motor cortex so there's a part of our brain map to the leg so if we want that leg to move it's going to begin here and then what it does is through a neuron it sends a signal down through the brain into the brain stem at the medulla the lowest part of the brain's then it crosses to the other side continues to move down down down down down and at the level in which it wants to exit here is where it synapses with another neuron it synapses with a second neuron and this second neuron will leave the spinal cord at that level and go to the muscle and tell that muscle to contract now you can see it's a two neuron system one neuron from the brain to the level in which it wants to exit and the second neuron at the level that exit go into that muscle to tell it to contract this neuron the first neuron is called the upper motor neuron and this second neuron is called the lower motor neuron really important now let's just focus on this lower motor neuron for a sec it's coming straight out of the spinal cord and go into that muscle to tell it to contract now interestingly what we have is in our muscles we have stretch receptors in our tendons we have stretch receptors and if you were to stretch the muscle too much what does it want to do it doesn't want to over stretch if it over stretches it's going to snap so it wants to reflexively contract so if the muscle stretches too much the lower motor neuron says i don't like this let's contract and all of this is happening at the level of the spinal cord and therefore it's called a reflex it's happening reflexively so there's going to be receptors here that's going to send a signal back into the spinal cord and go back to that lower motor neuron and say contract now an example of this is the patellar tendon reflex little hammer hits just below the patella the kneecap it stretches the tendon which stretches the quadricep this receptor goes oh i'm stretching and it sends the signal into the spinal cord to the lower motor neuron it says oh i don't want to stretch too much and sends a signal out and the muscle contracts perfect that's a reflex alright this also highlights to us that the lower motor neuron likes to contract that's its job it wants to contract wants to contract this is important because the upper motor neuron while it plays an important role in initiating the contraction for a particular area it also plays an important inhibition role it actually tells this lower motor neuron to stop all right now what we can talk about is if there's damage to the upper motor neuron or damage to the lower motor neuron what is the effect going to be all right let's think about it let's just say the spinal cord is damaged here now you can see that's the upper motor neuron is affected the upper motor neuron so let's write this upper motor neuron what do you expect to see if the upper motor neuron is damaged what's going to happen here at the leg you can see the lower motor neuron is intact and what that means is the lower motor neuron can continue to innervate or speak to the muscles of the leg and neurons release certain what we call trophic or growth factors that allow for that muscle to maintain its mass and because it's intact the mass loss that this muscle gets is minimal compared to if it was an upper motor neuron if it was a lower motor neuron injury so what we find is that the muscle mass decreases okay it decreases but compared to a lower mode neuron injury you'll see in a sec it's minimal what about the power well there is still power that's present right because this neuron is intact it can still tell the muscle to contract but the power is obviously reduced so mass is reduced power is reduced what happens if i were to test that reflex okay stretches stimulates and goes through and it continues to do this contract contract contract because the upper motor neuron is not intact it can't stop it so what you get is hyper reflexia hyper means over the top reflexia so your reflexes are over the top but in addition to that because there's no inhibition this muscle is just going to remain contracted this is called hypertonia hypertonia sometimes known as spasticity so if it's an upper motor neuron issue and the lower motor neurons intact hyperreflexia and spasticity is going to be common but what if it's a lower motor neuron issue so let's just say this is intact but the lower motor neuron is damaged so let's just say the damage is here what's going to be the issue all right let's have a look let's write it up here lower motor neuron well firstly because the lower motor neuron's damaged there's no signal going to the muscle so the muscle doesn't get any growth signals so the mass loss is significant compared to the upper motor neuron so you have a lot more loss in mass of that muscle you have a lot more loss in power of that muscle what about the reflex stretch it goes back to the spinal cord nothing's coming out so there's no reflexes or hyporeflexia and what about that muscle is that muscle contracting is it doing the spasticity hypertonic contraction no it's actually flaccid so it's hypotonic also known as flaccid and that's a lower mode neural now let's think about what happens in real life with actual spinal cord injuries well let's think about it right let's draw this up again everything's intact now let's just say we've got the arm here okay so we're going to have a upper motor neuron that's coming down for the arm as well right so let's just say it's speaking here it's coming down coming down goes crosses over goes down it synapses here with its lower neuron neuron for the arm and tells the arm to move all right and then this one for the leg continues down let's just say there's a spinal cord injury at the neck the cervical region here what can you see has happened you can see that the lower motor neuron for the arm is affected but the upper motor neuron for the leg is affected so what that basically means is for a cervical injury what happens to the arm it's a lower motor urine issue so loss in mass loss in power loss in reflexes and flacidity of the arm but what about the leg it's an upper motor neuron issue so the loss in mass is less pronounced the loss in power less pronounced it's hyper reflexive and you can have the spasticity or hypertonia and so this is a nice run-through of upper and lower motor neurons and reflexes and also some lesions that can occur