BASIC MOTOR PATHWAY
A. An overview:
The motor pathways are pathways which originate in
the brain or brainstem and descend down the spinal cord to control
neurons. These large neurons in the ventral
horns of the spinal cord send their axons out via the spinal roots
and directly control the muscles. The motor pathways can control
posture, reflexes, and muscle tone, as well as the conscious voluntary
movements that we think of when we hear "motor system".
The most famous pathway is the so called "pyramidal system",
which begins with the large pyramidal neurons of the motor cortex,
travels through the pyramids of the brainstem, (somewhere in here
there is a coincidence), and finally ends on or near the a-motor
neurons. This system is extremely important clinically, as strokes
often affect the motor system. Therefore it is crucial to understand
the anatomy of the motor pathway.
B. The anatomy:
The primary motor pathway is also called the corticospinal
pathway. As all such pathways are named from beginning to end,
this pathway starts in cortex and ends in the spine. Specifically,
it starts in the precentral gyrus, the fold of cortex just
anterior to the central sulcus.
The precentral gyrus has many names: primary motor
cortex, Brodmann's area 4, M1, etc. It provides the bulk of the
corticospinal tract, but other cortical areas contribute as well.
One such area is area 3a, part of primary somatosensory
cortex, which is hidden down inside the central sulcus.
If we take a section through the central sulcus,
we can see subtle differences between the pre- and post-central
gyri. All areas of cerebral cortex have six varying layers of
cells, from the most superficial and cell-free layer I to the
deep layer VI. Each layer has a slightly different cellular makeup,
and the thicknesses of the layers vary with cortical area. The
postcentral gyrus, also known as primary somatosensory cortex,
has a distinct layer IV - across cortical areas, layer IV generally
receives sensory information. In the precentral gyrus, the motor
cortex doesn't receive much in the way of sensory input, so layer
IV is indistinct. However, layer V, which is generally responsible
for sending information down to the brainstem and beyond, is very
prominent in motor cortex.
The corticospinal tract originates as the axons of
pyramidal neurons in layer V of (mainly) primary motor cortex.
In general, the farther the axon has to go, the larger the neuron.
In the precentral gyrus you can see some especially large neurons,
visible even at low magnification. These neurons are called Betz
cells, and were once thought to be the sole source of the
corticospinal tract. We now know that they are only a subset
of the pyramidal neurons which make up the tract. Pyramidal neurons,
incidentally, occur in all types of cortex - it is just a morphological
name for large neurons that are triangular in shape, pointing
a long apical dendrite toward the surface of the brain.
Once the axons leave the pyramidal cells, they enter the white matter just below layer VI. Every gyrus in the brain has this core of white matter, which contains all of the axons entering or exiting the gyrus. As you get deeper into the brain, all of these slips of white matter coalesce to form one large body of axons, the corona radiata, "radiating crown". As you get still deeper into the hemispheres, the corona radiata dives into the deep nuclei of the brain, the caudate and putamen, splitting them in two. At this point, all of these axons are called the internal capsule.
The internal capsule is a major two-way highway,
and very vulnerable to strokes. Sensory information travels up
it on the way from the thalamus to the cortex, and motor information
travels through on the way down to the spine. As you saw in the
horizontal sections at the beginning of the course, the internal
capsule has an anterior and posterior limb. The motor and somatosensory
information travels through the posterior limb.
If you were to follow horizontal sections down through
the brain, at around the level where the midbrain begins,
you would see the internal capsule coalesce into a tight bundle
to exit the cerebral hemispheres. At this point the axons are
called the cerebral peduncles, or the "stalks"
of the cerebrum. The peduncles make up the floor of the midbrain,
and contain all of the descending axons going to the brainstem
or spine. The peduncles, unlike the internal capsule, are largely
one-way; most of the axons in them are heading south. The ascending,
sensory axons take other routes to get to the thalamus.
Once midbrain gives way to pons, two things happen to the peduncles. One, many of the axons from cortex were actually headed for the pons (the "corticopontine" fibers, of course), so they get off and synapse. Two, the remaining corticospinal axons get a little fragmented in the pons, so they are no longer visible as a nice tight bundle. They can be seen as several smaller bundles, though.
In the medulla, the fibers come together again as the pyramids. The pyramids were actually named as landmarks on the surface of the brainstem - on a human brainstem you can clearly see them as two ridges running down the ventral midline. The pyramids run the entire length of the medulla, large uninterrupted axon tracts on the ventral surface.
At the very caudal-most end of the medulla, right about at the point where you have to start calling it cervical spinal cord, the fibers in the pyramids cross. The cerebrum controls the opposite side of the body, so it had to happen sometime, didn't it? The crossing event is called the decussation of the pyramids, and you can identify it by the way the midline groove is suddenly way off the midline. In any single section, it looks like only one side is crossing, but this is a sectioning artifact. Large bundles of axons take turns crossing, much like your fingers interlace when you clasp your hands. As you section down through the decussation, the midline fissure will bend first to one side, then the other.
As each individual fiber crosses (remember, these are still the same axons we started with, from the pyramidal neurons of area 4!), where does it go? The pyramids do not merely exchange places. Instead each crossing axon takes up residence in the lateral white matter of the spinal cord. By the time the decussation is completed, the corticospinal fibers reside in this new location, now called the lateral corticospinal tract.
From this position they dive into the grey matter
of the spinal cord at their target levels. Those fibers controlling
the arms, for example, get off in the cervical levels of the cord.
Once in the ventral horn they synapse either on interneurons
(most common) or directly on the a-motor
neurons. They preferentially innervate the limbs and distal muscles.
C. A lesson in nomenclature:
As you may have learned in anatomy, if there is a greater horn of Ubu, there is probably also a lesser horn of Ubu. (Fictional, don't look it up.) Likewise, if there is a lateral corticospinal tract, there must also be a medial one. Indeed there is, only it is called the anterior corticospinal tract. Take comfort in the fact that it does actually lie medially. These fibers were part of the original corticospinal tract, and made up 15-20% of the pyramids, but at the decussation, they did not cross. As a result, in the spinal cord they are still sitting right where the pyramids were - ventrally, on either side of the midline.
These fibers also dive into the ventral horns at
appropriate levels, and they tend to innervate the muscles of
the trunk. Fine finger movements and tap dancing are the exclusive
domain of the lateral corticospinal tract, but push-ups and hula-hooping
rely more on the anterior corticospinal tracts.
D. In summary:
Here is a diagram summarizing the two main pathways
of the voluntary motor system:
E. Clinical notes:
In motor cortex, the body is mapped out across the extent of the gyrus. Control of the feet lies near the midline at the top of the gyrus, whereas the lateral side of the gyrus controls the hands and face. Because the body parts are spread so widely in cortex, cortical strokes are rarely big enough to affect an entire half of the body. Instead, the hands or face, which take up the most cortical area, are preferentially affected. Strokes in the internal capsule are a different story. Fibers are bundled together so closely that a small stroke can in fact paralyze an entire side of the body - this is called hemiplegia (half-paralysis) or hemiparesis (half-weakness). If a stroke occurs in the brainstem, affecting the pyramids, the hemiparesis will often be accompanied by other symptoms or "neighborhood signs"; the brainstem is a very crowded place.
Although the cortex is primarily concerned with the
contralateral (opposite) side of the body, some muscle groups
get innervation from both hemispheres. This bilateral innervation
protects the muscle groups in case of stroke. Most of the muscles
in the trunk are bilaterally innervated, so that even hemiplegic
patients can hold their torsos upright. For some reason, the
forehead also receives bilateral innervation, which can be a key
clue in determining if a patient's facial paralysis is due to
a cerebral stroke or a peripheral nerve injury. In a cerebral stroke patient, only
the lower half of the face (on one side) will be affected. In
a patient with Bell's palsy (a temporary inflammation of the facial
nerve), the entire side of the face will be affected.