Neuro: 11:00 – 12:00
Scribe: Strud Tutwiler
Monday, January 8, 2009
Proof Ryan O’Neill
Dr. Baños
The Spinal Cord
Page 1 of 2
*This is a continuation from the previous transcript from 10-11am (the same lecture).
*Correction to previous lecture= In the cervical, C1-C7 nerves go above the vertebrae. C8, thoracic, and the remaining
nerves exit below their corresponding vertebrae. This does not have much impact on clinical symptoms though.
I. Corticospinal Tract [S37, 38]
a. There are four major pathways. The first is the corticospinal pathway.
b. It is in the motor pathways. The first order neuron (an upper motor neuron) starts in the precentral gyrus.
c. It goes downward and passes through the internal capsule, a white matter area with top to bottom pathways
between the thalamus and basal ganglia.
d. When it gets to caudal (lower) medulla, 90% decussates (crosses over) to form the lateral corticospinal tract.
e. 10% remains undecussated and forms the anterior corticospinal tract.
f. In the spinal cord, it synapses on the second order neuron in the ventral gray of the cord (duck feet).
g. The second order neuron innervates the muscle.
II. “HAL” the Motor Homunculus [S39, S40]
a. Homunculi are various representations of the nervous systems and the precentral gyrus (the motor cortex).
b. The cortex is devoted to different aspects of motor functions, and it corresponds to how much nerve power you
need to control that part of body. The nerve power needed forms sort of a person on the brain.
c. Why does he have huge hands? Similar to cervical enlargement. You have many fibers that are used in the
hands, lips, tongue, and face (fine motor coordination).
d. When you look at how these things are mapped on the precentral gyrus (coronal slice), you can see HAL (Head,
Arms, & Legs). His legs are going down the longitudinal fissure.
e. Motor control of the genitals is below the legs; there is not much representation.
f. There is somatotopic organization, but the body parts are not in correct order (i.e. tongue is out of mouth).
III. Corticospinal Tract [S41, S42]
a. Practice drawing these pathways. The neuroaxis is lying on its side. Thalamus (baby blue ovals), midbrain,
pons, medulla, and spinal cord are shown.
b. Shown is the base of the corticospinal tract with cross-sections at each level showing pathways.
c. Lectures on brainstem are next week.
d. It starts in the motor cortex (HAL).
e. First order neuron goes down through the internal capsule (near the thalamus).
f. When it gets to the lower medulla, ~90% decussates and runs the lateral cortical spinal, then only ~10 remains
lateral. The big lateral ones are 90% decussated and anterior cortical spinal are ~10% nondecussated.
g. This is important to know, especially where the pathways are located in the spinal cord.
IV. Upper & Lower Motor Neurons [S43 – S47]
a. Starting in the motor cortex, the upper motor neuron (UMN) goes to ventral grey horn and synapse on the lower
motor neuron (LMN). The LMN goes to the muscle.
b. A sensory feedback neuron (stretch receptors in the muscle) provides feedback to cells in the ventral grey horn.
This becomes part of the basic stretch reflex arc, and it is involved in the maintenance of motor tone.
c. Tone is not only related to muscles. Tone involves the muscle getting a small amount of tonic innervations to
keep it slightly activated so it is ready to react quickly. Stroke patients lack tone (looks like jello).
d. The LMN supplies tonic stimulation while the UMN inhibits or dampens the stimulation.
e. The reflex arc is when there is a sudden stretch of the muscle. The strong signal overrides the UMN. There is
still slight modulation from the UMN though.
f. You can consciously override reflexes. If you think about it, your cortex will provide a much stronger inhibitory
signal to override the reflex.
g. You can have distinct lesions based on where you hit the UMN or LMN.
i. Classic UMN Signs are spastic paresis, hypertonia (much stronger muscle tone), hyperreflexia
(exaggerated reflexes), no muscle atrophy, & a positive Babinski (rubbing bottom of feet - neurological
test).
ii. The lightning bolt on the figure is the lesion hitting the UMN. This eliminates the inhibitory signal from
cortex, allowing the LMN to run uncontrolled. The tone increases steadily, which progresses to
contracture. An example is a stroke victim with contracted arm.
iii. Lower LMN Signs are flaccid paresis (muscle loses all tone), muscle fasciculations (tiny twitches due to
spontaneous firing), hypotonia (no tone signal is sent to muscle), hyporeflexia (diminished or nonexistent
reflexes), muscle atrophy, and negative Babinski.
V. Babinski’s Sign [S48]
a. Take an object and rub it along sole of the foot. The normal response is for toes to curl downward.
b. A positive Babinski means there was an abnormal finding (the toes curled upward).
c. Common misconception: LMN means lower body, and UMN means upper body. The real answer is the there
are LMN and UMN all over the body; ex. Ocular muscles have LMN and UMN.
Neuro: 11:00 – 12:00
Scribe: Strud Tutwiler
Monday, January 8, 2009
Proof Ryan O’Neill
Dr. Baños
The Spinal Cord
Page 2 of 2
VI. Related Terms [S49]
a. Spasticity is increased muscle tone and reflex contraction.
b. Clonus is the rhythmic contractions and relaxations when spastic muscle stretches. Some people in a
wheelchair have to strap their legs down because of clonus.
VII. Basics of Localization [S50]
a. If all the limbs are checked for upper and lower motor signs, you can start to localize a lesion.
b. You can have left vs. right differences. For example: LMN signs in arm and UMN signs in leg, you know the
lesion is high up hitting the LMN of the arms. LMN signs in the leg, there is probably a lesion around the lumbar
enlargement.
VIII. Dorsal Column/Medical Lemniscus [S51 – S53]
a. Discriminative Touch and Conscious Proprioception
b. First order neuron begins in the receptor out in the skin. It enters the spinal cord at the tract of Lissauer.
c. Lower extremities form a fasciculus called the fasciculus gracilis. Upper extremities run in fasciculus cuneatus
or “cone shaped.”
d. Fasciculus gracilis synapses on nucleus gracilis, and fasciculus cuneatus synapses on nucleus cuneatus in the
caudal medulla.
e. A second order neuron originates there, decussates immediately, and runs to the thalamus and the medial
lemniscus.
f. The third order neuron runs from the thalamus to the postcentral gyrus.
g. In the lower extremities, information comes in and forms the fasciculus gracilis.
h. Higher up, the information forms the fasciculus cuneatus (the bigger or wedge shaped one).
i. In the caudal medulla, nuclei correspond to each one. The fiber exits, decussates, hits the thalamus, and finally
reaches the cortex.
j. Know where these fibers/pathways decussate to help localize a lesion.
k. Medial Lemniscus gets its name from being ribbon shaped.
IX. Spinocerebellar Tracts [S54]
a. Unconscious Proprioception
b. There are two spinocerebellar tracts, but one is more important (the posterior tract).
i. Doral (Posterior) Spinocerebellar Tract [S55, S56]
1. The posterior tract is complex and involves Clark’s Column, the row of longitudinal cells in the lumbar
area.
2. Clark’s Column does preliminary processing of information before sending it to the cerebellum.
3. Three scenarios: nerves can enter below, at, or above Clark’s Column (all end in the same place).
a. Nerves below Clark’s Column run with f. gracilis, synapse in Clark’s Column, join dorsal
spinocerebellar tract, and enter the cerebellum.
b. If they enter at Clark’s Column, they synapse in the column, join the dorsal spinocerebellar
tract, and enter the cerebellum.
c. If the nerves enter above Clark’s Column, they run with f. cuneatus, synapse in the lateral
cuneate nucleus (caudal medulla), and project to the ipsilateral cerebellum.
d. The lateral cuneate nucleus is doing the job of Clark’s Column for the nerves entering above.
ii. Ventral (Anterior) Spinocerebellar Tract [S57, S58]
1. Not quite as important. Supplements the Dorsal Spinocerebellar.
2. It brings in information from more diverse array of receptors.
3. Originates from scattered cells in the intermediate gray.
4. Crosses over twice to end up in the ipsilateral cerebellum.
X. Spinothalamic Tract [S59 – S63]
a. Pain and Temperature
b. First order neurons that originate in pain receptors, enter cord at tract of Lissauer, and synapse in substantia
gelatinosa or nucleus proprius.
c. Second order neurons cross at the anterior white commissure, rise 1 or 2 cord levels in the process, and form
the contralateral spinothalamic tract.
d. A third order neuron from the thalamus goes to the cortex. It is technically not part of the spinothalamic tract
(spine to thalamus), but it is in the pain and temperature system that takes it to the cortex.
e. OMIT SLIDE [S61]
f. Figure showing entry to different levels. What is unique about this pathway? This is the only pathway that
decussates nerve by nerve as you go along the spinal cord.
g. Other pathways decussate in either one or two spots.
[Finished 21:40]