0:00 In this video we will talk about motor disorders and what I mean by disorders 0:12 is disorders affecting 0:14 somehow your skeletal muscles, your voluntary muscles, will be comparing upper 0:20 motor neuron 0:21 lesions to lower motor neuron lesions and what each of these mean. 0:28 In order to understand upper and lower motor neuron lesions we must recap the 0:32 motor pathways 0:33 looking at the pyramidal system which is the system of voluntary movement. 0:39 Normal motor function depends on the transmission of signals from the brain to 0:43 brain stem or 0:43 spinal cord by the upper motor neurons and then from there to skeletal muscles 0:48 by lower 0:49 motor neurons. 0:51 Here is a cross section of the primary motor cortex of the brain. 0:57 The outer gray matter of the cortex have areas where neurons arise from which 1:01 are associated 1:02 to a particular area in the body so the feet, hands and the face. 1:06 The white matter consists mainly of neurons axons which travel through the 1:11 brain. 1:12 Motor neurons will pass through the internal capsule situated near the thalamus 1:16 and basal 1:16 ganglia. 1:18 Here is the brain stem which is made up of three parts, the midbrain. 1:22 One area here is the red nucleus and cruceribri and the pons not drawn as well 1:29 as the medulla 1:30 oblongata here. 1:33 Here is a cross section of the spinal cord where the communication between 1:37 upper and 1:38 lower motor neurons occur. 1:40 We will learn about this soon. 1:42 The spinal cord have tracts which are where neurons travel through. 1:47 Motor tracts in the pyramidal system are the lateral corticospinal tract and 1:51 the anterior 1:52 corticospinal tracts. 1:54 The target organ of the motor neurons are the skeletal muscle cells. 2:00 Let's create a scenario where we want to move our right thumb. 2:05 The upper motor neurons arise from the left primary motor cortex, pass through 2:10 the internal 2:11 capsule, through the brain stem passing the cruceribri, passing through the p 2:17 ons and then 2:18 passing through the medulla, the area called the pyramids, hence it's called 2:22 the pyramidal 2:24 system. 2:26 The vast majority of upper motor neurons that project from the motor cortex to 2:30 the spinal 2:31 cord are contained in the lateral corticospinal tracts. 2:36 The neurons that will travel through the lateral corticospinal tracts will 2:41 cross over at the 2:42 medulla of the brain stem. 2:44 Once the upper motor neurons reaches the correct spinal level, it will sign up 2:50 with the lower 2:50 motor neurons at the anterior horn of the spinal cord. 2:55 The lower motor neurons will then target the skeletal muscles responsible for 3:00 movement 3:01 of that body part involved. 3:04 So in this scenario, it is the muscles involved in the right thumb movement. 3:09 Majority of upper motor neurons are contained, as mentioned, in the lateral 3:13 corticospinal 3:13 tract. 3:14 However, there are those that project to the proximal muscles, and these have 3:20 axons that 3:20 do not cross over at the medulla, but actually travel in the anterior corticosp 3:26 inal tract 3:27 in the spinal cord. 3:29 Those of the anterior corticospinal tract will cross over when they reach the 3:34 level of the 3:34 spinal cord, where they will form synapses with the lower motor neurons. 3:40 Hope that all made sense. 3:41 Now it's important to realize two things here. 3:44 The first is the general rule, in which motor neurons activated on one side of 3:50 the brain 3:51 will control movement of the opposite side of the body. 3:56 One thing to remember is that the spinal motor tracts we discussed are under 4:01 voluntary control. 4:03 There is another motor tract, which is also under voluntary control, called the 4:09 rubrospinal 4:09 tract. 4:10 Now, the rubrospinal tract originates in the red nucleus of the midbrain. 4:16 It decosates and then descends in the lateral aspect of the spinal cord, where 4:21 it will sign 4:22 apps with a lower motor neuron; the rubrospinal tract targets lower motor 4:27 neurons, which facilitates 4:29 in muscle flexion and inhibits neurons in muscle extension. 4:35 Now in summary, the upper motor neuron transmits information from the brain to 4:40 the spinal 4:41 cord or the brainstem. 4:42 Lower motor neurons transmit information from the brainstem or spinal cord to 4:47 the skeletal 4:48 muscles. 4:50 All findings on examination can indicate whether a motor disorder is due to an 4:55 upper or a lower 4:57 motor neuron lesion. 4:59 However, before going into the clinical examination, let's look at some causes 5:04 of upper and lower 5:05 motor neuron lesions, beginning with the upper motor neurons. 5:09 Remember, the upper motor neuron transmits information from the brain to the 5:14 spinal cord 5:15 or the brainstem. 5:18 For really any injury to the brain, such as a stroke, infection, or tumus, can 5:23 cause 5:23 upper motor neuron lesions. 5:26 Similarly, any injury to the brainstem or spinal cord, specifically the white 5:32 matter 5:33 of the spinal cord on the outside here, where the upper motor neurons are 5:37 traveling through 5:38 can cause upper motor neuron lesions. 5:44 Lower motor neurons are the neurons which transmit signals from the spinal cord 5:49 or brainstem 5:50 to the skeletal muscle. 5:52 Lower motor neuron lesions are caused by any damage along this tract. 5:56 So for example, injury to the axons leaving the spinal cord can cause lower 6:01 motor neuron 6:02 lesions. 6:04 Injury to the spinal cord itself as well, particularly to the ventral gray 6:09 matter of the spinal cord, 6:10 the anterior one on the spinal cord, because this is where the lower motor 6:15 neuron starts. 6:17 It's quite amazing because through doing a proper neurological examination, you 6:21 can sort 6:22 of figure out whether a motor disorder is due to an upper or lower motor neuron 6:28 lesion. 6:29 Let's look at some of these signs on examination, beginning with upper motor 6:33 neuron lesion signs. 6:34 Again, the signs elicited on examination, of course, depends on what area of 6:38 the motor 6:39 pathway is affected, which side of the brain, the specific spinal cord level 6:44 affected, but 6:45 we won't go into that detail. 6:47 Just remember these overall signs. 6:50 With upper motor neuron lesions, you would have minimal muscle atrophy. 6:54 You would have weakness as the other neurons from other areas can sort of help 6:58 with muscle 6:59 movement still. 7:03 When you test the deep tendon reflexes such as the patellar reflex here, you 7:06 would be hyperreflexic 7:09 The reason is a bit complicated, but essentially with a deep tendon reflex 7:14 where the spinal 7:15 level is unaffected, the reflex is amplified because there are no upper motor 7:21 neurons which 7:21 are regulating the reflex at that level. 7:25 However, in upper motor neuron lesions, there is diminished or absent 7:30 superficial reflex. 7:32 The Babinski sign is positive, where if you scrape the sole of the foot, what 7:36 will happen 7:37 is dorsiflexion and fanning of the toes. 7:43 With lower motor neurons, there is muscle atrophy because the actual neurons 7:48 which supply the 7:49 muscles are not getting there. 7:51 Flacid paralysis is present. 7:54 Normal or no planter response because no signal are arriving at the muscles. 8:00 Similarly, there is absent tendon reflexes. 8:07 You can also see fasciculations of the affected muscle. 8:11 This is where single muscle fibers are still stimulated. 8:14 The reason for this is because there is still some transmission, some signals 8:18 from lower 8:19 motor neurons which are not fully injured. 8:23 So that was the overview for upper and lower motor neuron lesions. 8:27 Motor disorders or deficits can also arise from other problems. 8:30 For example, disorders or problems of the skeletal muscle specifically. 8:36 An example of this includes necrotizing myopathies or electrolyte changes such 8:41 as hyperchlemia 8:42 or hypochlemia. 8:45 Motor disorders can also arise from neuromuscular junction disorders. 8:50 The neuromuscular junction is where the lower motor neurons will release 8:53 chemicals which 8:54 will stimulate the muscle fibers. 8:57 All of neuromuscular junction disorders include myasthenia gravis, botulism and 9:03 amino glycoside 9:04 toxicity. 9:06 If you want to learn more about the pyramidal system or other neurology topics, 9:10 check out 9:11 the playlist or visit armandoh.org.
