MODULE 5
THE CORTICOSPINAL TRACT and the CORTICOBULBAR PATHWAY
- - While we will take up many pathways in this module, it is essential keep in mind that the corticospinal tract is by far the most important one. Some would say that this tract IS the motor system of the brain, and from a clinical point of view that's not far from the truth. Figure 5-2 gives a broad outline of the course this descending tract takes, passing from the cortex through the brainstem to reach the spinal cord. The neurons giving rise to this pathway all have their cell bodies in lamina 5 of the cerebral cortex. The axons of these neurons descend through the internal capsule, cerebral peduncle, basilar pons and pyramid to reach the caudal medulla. At this point the majority of the axons (about 85%) cross the midline in the pyramidal decussation to enter the lateral corticospinal tract and continue descending into the spinal cord About 40% of the neurons contributing to the tract have their cell bodies in the parietal lobe. These include cells in the postcentral gyrus (areas 3, 1, and 2) and the superior parietal lobule (areas 5 and 7). About 30% of the axons in the tract have their cell bodies in the precentral gyrus (area 4), an area also known as the primary motor cortex. The remaining 30% of the axons have their cell bodies in area 6, lying just anterior to area 4 in regions called the premotor and supplementary motor cortices.
- - Since we tend to regard the corticospinal tract as a motor pathway, the origin of the majority of the descending axons in the frontal lobe is to be expected, but the fact that a significant numbe originate in the sensory cortex of the parietal lobe seems puzzling. When these axons are traced into the spinal cord, the majority terminate in the dorsal horn. Thus, their role appears to be to modulate incoming sensory information - not to bring about movement.
- - It should be noted that since this pathway is defined as containing all the axons that pass through the pyramids of the medilla, it is also commonly known as the PYRAMIDAL TRACT. It is convenient to think of the motor system as con sisting of an upper motor neuron, in the motor cortex, sending an axon caudally to terminate on a lower motor neuron in the spinal cord which, in turn, sends an axon out in a peripheral nerve to innervate muscle fibers. A few of the upper motor neurons in area 4 are quite large - as much as 100um in diameter. They are called Betz cells and at one time they were thought to be a major contributor of axons to the corticospinal tract. Meticulous cell and axon counts of the 1940’s howerver revealed that no more that1-2% of the axons in the tract are derived from Betz cells. The diameter of axons in the tract to think of fact that early anatomists used the term "bulb" to describe the brainstem. It is logical to consider these pathways together. In diagrams and slide descriptions we will often refer to the corticobulbar fibers as the "head" component of the pathway.
- - Strictly speaking, the corticospinal tract (all those descending fibers that pass through the pyramids of the medulla) originates in a broad area lying in front of and behind the central sulcus. We show this in Figure 5-8 and Blumenfeld's Figure 6.9A gives a similar view. That part of the tract which starts out in the parietal lobe (blue) appears to be concerned with modulating sensory input rather than initiating movement and will not be considered in detail in this module. The precentral component of the tract - the part responsible for motor function - is organized in a somatotopic manner at most levels and the next slide Figure 5-9 shows the color scheme that will be used in subsequent views. The next views shows the course of the tract as it passes through the corona radiata Figure 3-7 and the internal capsule Figure 3-6 to reach the cerebral peduncle Figure 5-10. The last view of this series, Figure 5-2, summarizes the regions through which the pathway passes. Blumenfeld's Figure 6.9B and Figure 6.10 also show the somatotopic organization of the pathway.
- - We're going to spend the rest of our time tracing these pathways on three different sets of slides. They are:
----- Horizontal sections through the whole brain. These are useful in tracing descending (and ascending) pathways as the traverse the interval between the surface of the hemisphere and the brainstem. They are particularly valuable in looking at the regional anatomy of the internal capsule. You have labeled copies of them in your "Atlas of Whole Brain Sections", so refer to it if you want help in identifying structures.
----- Frontal, or coronal, sections through the whole brain. These are useful in relating the brainstem set of sections to the hemisphere as a whole. In part that's because they were cut in about the same plane as the brainstem slides. For example, if you look at slide #30 in the brainstem series, you get a good view of the thalamus; if you then look the equivalent section in the "Atlas of Whole Brain Sections", you will see the relationship of the thalamus to surrounding structures such as the ventricles, the basal ganglia and the surface of the hemisphere. Labeled copies can be found in the same "Atlas of Whole Brain Sections".
----- "Cross sections" through the brainstem. Labeled copies of these are found in the "Atlas of Brainstem Sections". As you know (take another look at Figure 2-18), some of the sections in this series are true cross sections, and others are cut in an oblique plane.
- - Let's start to trace the corticospinal tract, and related corticobulbar pathway, by looking at three horizontal sections. Call them up by using Figure 5-11. In real life, you will be looking at CT and MRI images rather than histological sections, so as you look at the sections below, compare them with views in Blumenfeld's Chapter 4.
- - Next, look at some frontal or coronal sections through the whole brain. To see them, use Figure 5-12.
- - Finally, let's trace the pathway through the brainstem, by looking at several slides of the Cornell series. On these views, both the corticospinal tract and the corticobulbar pathway are labeled. Call them up using Figure 5-13.
- - The corticobulbar pathway breaks away from the corticospinal one at the midbrain level and moves dorsally to descend through the tegmentum of the pons and medulla, reaching as far caudally as the cervical cord. We will discuss the corticobulbar system in greater detail when we take up the cranial nerves.
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