Descending Pathways: Corticospinal Tracts

00:00 Welcome to this presentation on the spinal cord. During this presentation, you’ll learn more about the ascending and descending tracts that are associated with the spinal cord. First, we’re going to take a look at descending pathways. When you look at the descending pathways, you are looking at corticospinal tracts.

00:27 This is a nice cross section of the spinal cord and tracts are highlighted here in various colors. One of the corticospinal tracts is the lateral corticospinal tract that you see in through here. This is a fairly large descending pathway. Descending pathways are coming from, in this case, the cortex of the cerebrum down to the spinal cord. The prefix will always tell you the starting point of the pathway. The suffix will tell you the destination of the pathway. Cortico, starting point is the cortex; spinal, destination is the spinal cord.

01:11 There’s also an anterior corticospinal tract. That is shown here in a much more anterior location.

01:20 It is not nearly as large as the lateral corticospinal tract. The purpose of this slide is to help you understand the two neurons that make up this particular pathway and what happens as they leave their starting point and reach their destination. First is that you have an upper motor neuron shown right in through here.

01:52 This resides in the primary motor cortex and specifically, this is in the precentral gyrus. This upper motor neuron will send its axon downwards in a descending manner. It will pass through the internal capsule.

02:12 Then, it’s going to pass through the cerebral peduncle in the midbrain. Now, it has reached the medullary pyramids.

02:25 Some of the descending fibers will remain ipsilateral and some of them are going to decussate.

02:33 90% percent of the axons that we see here, 90% percent will decussate within the medullary pyramids and go over to the opposite or contralateral side. We started on the left side. Now, 90% of those axons are decussating over into the right medullary pyramid. Then they’ll continue their journey down in the lateral corticospinal tract. As you may remember from the previous slide, the lateral corticospinal tract is much larger than the anterior corticospinal tract. The lateral corticospinal tract is going to supply distal musculature.

03:22 The 10% of the axons that do remain ipsilateral are going to form the anterior corticospinal tract.

03:33 Then they will decussate at the level of the spinal cord. Then within the spinal cord, lower motor neurons will be activated. That’s the second neuron in this pathway. It will send axons out to the skeletal musculature.

03:55 Again, the lower motor neurons of the spinal cord that are in the lateral corticospinal tract do the distal musculature. Then the lower motor neurons that are in the anterior corticospinal tract are going to do more proximal musculature. That would be the axial musculature of the body.