NB7-3 - Organization of the Motor System and DLAs

Question 1

Answer 1

E

Question 2

What are the corticospinal and corticobulbar tracts principally involved in?

Answer 2

The corticobulbar tract contains neurons that generally carry voluntary motor information to the cranial nerves

The corticospinal tract contains neurons that generally carry voluntary motor information to the rest of the body

Question 3

Give a general overview of how the motor system carries out a planned action.

Answer 3

1. You choose to perform a motor action with your prefrontal cortex which sends that information to the premotor cortex 2. The premotor cortex has the motor memory (instructions on how to do a learned action). It will relay this information to the primary motor cortex. 3. The primary motor cortex will recruit the motor neurons needed to do the action but first sends that information to the basal ganglia for modulation. 4.** After modulation, the basal ganglia sends the motor information back to the primary motor cortex via the thalamus. **5.** The primary motor cortex now recruits upper motor neurons which synapse onto lower motor neurons in the brainstem or spinal cord to initiate the action. **6.** Sensory information about the initial movement gets sent back to the brainstem and passed on to the cerebellum. **7. If the action is not following the “action plan” then the cerebellum will send new corrective information to the primary motor cortex via the thalamus.

Question 4

Answer 4

D

Question 5

Answer 5

C

Question 6

List the motor pathways and their starting points. Indicate which pathways are the most clinically significant.

Answer 6

Question 7

Describe the location of lower motor neuron cell bodies in relation to their function.

Answer 7

All cell bodies are located within the ventral horn

• Flexor neuron somas are located more dorsally
• Extensor neuron somas are located more ventrally
• Somas for distal muscle neurons are located more laterally
• Somas for proximal muscle neurons are located more medially

Refer to image

Question 8

Describe the correlation that exists between motor tract location and muscle action. What is the major exception to this correlation?

Answer 8

Tracts that carry mostly neurons for flexor muscles are usually located within the lateral columns while tracts that carry mostly neurons for extensor muscles are usually located within the ventral columns.

The major exception to this are the reticulospinal tracts which descend bilaterally within the ventral column and control mostly flexor activity

Question 9

Answer 9

Question 10

Answer 10

B

Question 11

Answer 11

D

Question 12

What is ALS?

Answer 12

Amyotrophic Lateral Sclerosis, aka Lou Gehrig’s disease, is a progressive degenerative disease of the motor system that causes hardening of the lateral columns of the spinal cord (lateral sclerosis) which leads to degeneration of anterior horn cells, the motor components of CNs V, VII, X, and XII, parts of the pyramidal tracts, and parts of the primary motor cortex. This all leads to a very visible atrophy of muscle (amyotrophy).

Question 13

What causes ALS?

Answer 13

Exact causes are not all known but there does appear to be a hereditary form that is autosomal dominant

Question 14

What are the clinical features of ALS and how does it typically progress?

Answer 14

Beginning in the lower limbs and progressing upwards you will typically see atrophy, weakness, fasciculations (micro spasms), spasms, and cramping.

Once the disease progresses into the brainstem you will begin to see decreased couch reflex (aspiration risk), difficulty swallowing and speaking, and weakness/atrophy/fasciculations of the tongue, pharynx, and larynx. Death will usually occur from respiratory insufficiency and aspiration pneumonia.

Emotional and Cognitive deficits may present as well

Question 15

What does a lesion of the anterior spinal artery affect in the CNS?

Answer 15

The anterior spinal artery supplies the anterior 2/3 of the spinal cord. A lesion of this artery will most notably affect the ventral horns, anterior corticospinal tract, the lateral spinothalamic tract (anterolateral system), and the anterior commissures (both grey and white) at the levels of the lesion.

Question 16

What typically causes anterior spinal artery syndrome and what are its clinical features?

Answer 16

Most commonly caused by an infarction, neoplasm, trauma, aortic and/or connective tissue diseases, or a disk herniation. It typically presents with paraparesis (lower limb paralysis), bilateral extensor planter response (Babinski reflex), bilateral loss of pain/temp below the lesion (can’t decussate), touch pathway remains intact, urine retention, and sexual dysfunction.

Question 17

Answer 17

C

Question 18

Answer 18

A

Question 19

List the functional landmarks of the motor system, the muscles used for each function, and the spinal segment corresponding to each function.

Answer 19

Question 20

List the reflex testing landmarks.

Answer 20

Question 21

What is the scale for reflex testing?

Answer 21

Question 22

What type of reflex response often indicates disease?

Answer 22

Unequal reflex responses

Question 23

How is the plantar response evoked? What responses are considered normal and abnormal? What spinal segments does this test?

Answer 23

When the lateral aspect of the sole is stroked from the heel to the ball of the foot with a semi-sharp object (refer to image), a brisk plantarflexion of all toes should be seen in adults or children over 2yo. This assesses the nociceptive fibers and alpha motor neurons of the L5 and S1 spinal segments.

An abnormal response, or Babinski Sign, is when the great toe dorsiflexes while the other toes fan out. This indicates an upper motor neuron lesion

Question 24

What is a myotatic reflex? What is it aka? What are the afferent and efferent neurons in this reflex? Is this reflex mono- or polysynaptic?

Answer 24

The myotatic reflex, or stretch reflex, is a tonic contraction of muscle groups in response to a stretching of that muscle.

Afferent limb - Type Ia fibers from spindle

Efferent limb - Alpha motor neuron

Monosynaptic

Question 25

What is the purpose of the myotatic reflex? How is this reflex modulated?

Answer 25

It helps to maintain muscle length and, therefore, posture. In addition to the spinal reflex pathway of the myotatic response, there is also a long loop pathway that sends stretch information to the supraspinal regions (notably the motor cortex) which will then send efferent fibers that synapse on the alpha motor neurons, modulating their activity.