So here we have encircled there the descending systems.
So far what we've looked at is the lower motor neurons and the local circuit neurons which we discussed how they mediate actual muscular movement. The higher brain centers that control movement send signals via the upper motor neurons to these lower motor neurons/local circuit neurons.
Descending tracts : Pyramidal = corticospinal ( control body muscles; anterior for proximal and lateral for distal muscles, and corticobulbar (control muscles of the face)
Extra-pyramidal = vestibulospinal (balance) and reticulospinal (consciousness and posture)
RAW VIDEO TRANSCRIPT: i didnt manage to get around to making notes (lots of typos as word made this)
The relationship between the upper motor neuron with the lower motor neuron is not one where the upper motor neuron necessarily micromanages the function of the lower motor neuron. Because, as we said, the lower motor neuron is the final path to movement. So the relationship between the upper motor neuron and the lower motor neuron is more concerned with communication of the intention of movement as well as plans of that movement. Thus, upper motor neurons communicate intention of movement- they initiate movement, they are involved in planning of movement and they pass that information down to lower motor neurons and local circuit neurons.
Upper motor neurons involve both the motor cortex and the cerebrum - which is more concerned with the skilled movements and movements of the distal parts of the limbs. But upper motor neurons or descending systems are also found in brain stem centers -and these are more concerned with the control of balance and posture as well as stereotyped movements or rhythmic movement.
We shouldn't forget that the upper motor neurons in the descending systems mainly innovate the local circuit neuron. That's a very important point. They mainly innovate the local circuit neurons, which are the interneurons that we find in the spinal cord - rather than going directly onto the lower motor neurons. But we do have just a little bit of direct innovation of the lower motor neurons by the upper motor neuron.
Another thing that we already know that is worth revisiting at this point is the somatotopic organization of the lower motor neuron and the ventral horn of the spinal cord. And this is something that we looked at in our last lecture when we were looking at lower motor neurons. We've already said that the medial ventral horn contains lower motor neurons that govern posture and balance - and these medial motor neurons receive descending input from pathways that originate mainly in the brainstem. Which pass through the anterior medial white matter of the spinal cord and then terminate bilaterally (by bilaterally I mean they innovate both sides of the spinal cord).