Download Seminars of Interest

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This is a great resource for acclimating yourself to anatomy. It’s very straightforward.
Also, make sure to read the book chapters too.
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“So, in summary, here are the level cues so far: wide flat cord, lots of white matter,
ventral horn enlargements = cervical. Round cord, ventral horn enlargements =
lumbar. Small round cord, almost no white matter = sacral. And the remaining level,
thoracic, is the easiest of all. Notice the pointed tips which stick out between the
small dorsal and ventral horns. This extra cell column is called the intermediate horn,
or the intermediolateral cell column. It is the source of all of the sympathetics in the
body, and occurs only in thoracic sections.” –from thalamus.wustl.edu
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This slide is meant to orient you as you begin studying the pathways throughout the
brainstem. We’re looking at sagittal crossections of the brain and/or brainstem.
Basically, imagine that you’re looking through a person’s ear and the side of their
head into their brainstem. We know that these images are oriented with the
posterior/dorsal region to the right because of the cerebellum, which is located at the
nape of the neck. In contast, the ‘bulge’ of the pons is always facing the anterior or
ventral side of the brainstem.
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You are looking at the ventral surface of the brainstem. How do we know that? We
can see the medullary pyramids (which carry corticospinal axons to brainstem), and
we know those travel on the ventral surface of the brainstem. We can also see only
the ‘front’ part of the cerebellum, which is located on the dorsal side of the
brainstem.
For this image, imagine that you are looking at a person face to face. If you could see
through all the skin and tissue, this is the view of the brainstem that you would see.
**This diagram has more detail than you’ll probably need to retain. I’ve highlighted
some of the important structures in green, and also labeled the midbrain, pons, and
medulla. Remember that corticospinal fibers travel through the cerebral peduncle
within the midbrain, they ‘break up’ a little within the pons, and refasciculate in the
medulla to form the pyramids. You should also think back to the dorsal
column/medial lemniscus pathway and understand where those fibers are traveling.
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Figure 17.8
The corticospinal tract. Neurons in the motor cortex give rise to axons that travel
through the internal capsule and coalesce on the ventral surface of the midbrain,
within the cerebral peduncle. These axons continue through the pons and come to lie
on the ventral surface of the medulla, giving rise to the pyramids. Most of these
pyramidal fibers cross in the caudal part of the medulla to form the lateral
corticospinal tract in the spinal cord. Those axons that do not cross (not illustrated)
descend on the same side and form the ventral corticospinal tract (see Figure 17.6).
The axons that terminate in the reticular formation of the pons and medulla comprise
components of the corticobulbar tract.
From: The Primary Motor Cortex: Upper Motor Neurons That Initiate Complex
Voluntary Movements
Copyright © 2001, Sinauer Associates, Inc.
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Primary motor cortex and somatosensory cortex are separated by the central sulcus,
which divides the frontal and parietal lobes. Motor cortex is located anterior to the
central sulcus (in the frontal lobe) while the somatosensory ctx is located in the
parietal ctx
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Figure 17.9
Topographic map of the body musculature in the primary motor cortex. (A) Location
of primary motor cortex in the precentral gyrus. (B) Section along the precentral
gyrus, illustrating the somatotopic organization of the motor cortex. The most medial
parts of the motor cortex are responsible for controlling muscles in the legs; the most
lateral portions are responsible for controlling muscles in the face. (C) Disproportional representation of various portions of the body musculature in the motor
cortex. Representations of parts of the body that exhibit fine motor control
capabilities (such as the hands and face) occupy a greater amount of space than
those that exhibit less precise motor control (such as the trunk).
From: Functional Organization of the Primary Motor Cortex
Copyright © 2001, Sinauer Associates, Inc.
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Start trying to recognize the different sections of brain or spinal cord. You can
recognize the midbrain and pons by their distinctive shapes. Note the ‘crinkly’ look of
the pons. This is formed by a bunch of fibers that cross the midline at the level of the
pons.
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Both of these sections are from the medulla. It’s readily apparent because of the
pyramids on the ventral surface. In the top section you can also identify the medulla
because of the olivary nuclei (the zigzaggy formation near the pyramids)
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This spinal cord section is oriented so that dorsal is up. Note this is opposite of how
we learned about the spinal cord in class. We can quickly know the orientation of the
spinal cord in two ways. The first is to identify the ventral horns with in the gray
matter. They will signify the ventral side of the spinal cord. Additionally, note that the
posterior horns actually trail off through the white matter to the edge of the spinal
cord. This is where the axons from the dorsal root ganglia enter the spinal cord.
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Figure 17.12
Directional tuning of an upper motor neuron in the primary motor cortex. (A) A
monkey is trained to move a joystick in the direction indicated by a light. (B) The
activity of a single neuron was recorded during arm movements in each of eight
different directions (zero indicates the time of movement onset, and each short
vertical line in this raster plot represents an action potential). The activity of the
neuron increased before movements between 90 and 225 degrees (yellow zone), but
decreased in anticipation of movements between 0 and 315 degrees (purple zone).
(C) Plot showing that the neuron's discharge rate was greatest before movements in a
particular direction, which defines the neuron's “preferred direction.” (D) The black
lines indicate the discharge rate of individual upper motor neurons prior to each
direction of movement. By combining the responses of all the neurons, a “population
vector” can be derived that represents the movement direction encoded by the
simultaneous activity of the entire population. (After Georgeopoulos et al., 1986.)
From: Functional Organization of the Primary Motor Cortex
Copyright © 2001, Sinauer Associates, Inc.
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Remember that experiment in class where the pyramid tract was lesioned unilaterally
(on one side, in this case we’ll say the right) in a monkey? The monkey lost fine
control of his left hand. Why the left hand? The lesion occurred above the pyramidal
decussation, where the corticospinal fibers cross, so a lesion on the right pyramid
would affect the left side. A lesion below the site of decussation would affect fine
motor movement on the same side as the lesion.
What can we conclude? We learn that the corticospinal tract must carry information
about fine motor control, but there must be other brain centers that can also mediate
motor output, or the monkey would have been paralyzed on one side. Instead, he
was still able to move his arm as a single unit, he just lost the ability to pick up single
raisins from a cup, for example. In particular, the rubrospinal pathway (right diagram
above) mediates voluntary movements.
Two other pathways, the tectospinal pathway and the vestibulospinal pathway, are in
charge of head and body orientation under special circumstances as outlines above,
in your notes, and in the book.
Figure 17.2
Descending projections from the brainstem to the spinal cord. Pathways that
influence motor neurons in the medial part of the ventral horn originate in the
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reticular formation, vestibular nucleus, and superior colliculus. Those that influence
motor neurons that control the arm muscles originate in the red nucleus and
terminate in more lateral parts of the ventral horn.
From: Descending Control of Spinal Cord Circuitry: General Information
Neuroscience. 2nd edition.
Purves D, Augustine GJ, Fitzpatrick D, et al., editors.
Sunderland (MA): Sinauer Associates; 2001.
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