 Let's talk about the cortical spinal tract, truly important and critical for voluntary willed motor control, whether it's over your cranial nerves, such as my vocal cords as I speak, my tongue, my face, so that I can put out the words in the right intonation as well as the right volume, as well as all of my muscles controlled by my spinal cord. Imagine a neuron sitting here, it's axon. This is all the same axon, traveling down through the internal capsule, down deep here you can see how white it is. This is the midbrain level. You can see the substantia nigra on either side and it enters into the cerebral peduncle, which we already were introduced to when we were looking at the sections of the midbrain. This is midbrain because you see the aqueduct, you see the tectum or the caliculi, and these are the cerebral peduncles. Here we have our cortical spinal tract in the middle part of these cerebral peduncles. No synapse, the same axon is going to travel. It comes down through the pons, you can see these fibers kind of broken up here, right here if we're on the left side, and now they're going to coalesce and form the ventral pyramid in the medulla. If I hear they're starting to form the pyramid, if I turn it over, I can see right here, here's one pyramid and here's the other. Perhaps you can see it better here, it's a little darker, here's one pyramid, sort of pyramidal shape, right, and here's the other pyramid. This is still the same axon. So now if we look at the ventral surface with the pons here, here are those two pyramids emerging from the pons, and right about here there's a little bit of a kink in this median fissure, and there's some fibers that cross here. This is incredibly important. All the fibers that were on the left side in this pyramid, almost all of them are going to cross to the other side. For all intents and purposes, as a clinician, this is where the left brain fibers cross to the right side, and left brain from here on affects right body, right musculature. So your left brain controls your little pinky on the right side of your body. These axons then travel down to the appropriate level of whatever muscle they are innervating. Let me show you a cross section. Here is a cross section at the cervical level. It's easier to see because there's more white and gray matter at this level. The white is really the gray matter, remember, and this big ventral or anterior horn is where the lower motor neurons are that are going to go to the muscles. So where is that cortical spinal tract after it crosses? It's here. It's out here laterally in the spinal cord. On the right side now, it's going to end on neurons in the anterior horn. They're going to send their axons out as ventral roots. Now we have two clinical terms we use. We call the neurons that start in the cortex and run down through the internal capsule, through the midbrain, through the pons, through the medulla, crossing and controlling and traveling in the lateral part of the spinal cord as the lateral cortical spinal tract. These are all called we call upper motor neurons. Why upper? Because they were the first ones. They started in the cortex and then went all the way down into the spinal cord. Neuron number one, upper motor neurons. The ones that live in the cord, in the ventral cord, in the anterior horn cells, the number two guys are called the lower motor neurons. Let's summarize this important motor pathway. The cortical spinal tract, sometimes called the pyramidal tract, goes not only to the spinal cord but to the brainstem motor nuclei associated with certain cranial nerves. The upper motor neuron originating in the cortex and crossing to the opposite side of the spinal cord is the neuron which originates in the cerebral cortex. The lower motor neuron or the anterior horn cell or ventral horn cell in the spinal cord or in the brainstem is the second neuron and it goes out to the striated muscle. Diseases of this pathway are divided into upper motor neuron diseases and lower motor neuron diseases. Each has its own characteristics. Upper motor neuron diseases are characterized by spasticity or a change in muscle tone, increased deep tendon reflexes or hyperreflexia and loss of voluntary control. Lower motor neuron diseases are characterized by a reflexia, a loss of reflexes, a flaccidity and severe atrophy of the muscle due to the loss of trophic factors coming from the lower motor neurons.