Week 3: The Corticospinal Tract

Question 1

Supplementary Motor Cortex + Premotor Cortex

Answer 1

i. Functions include: anticipation of movement, prepares primary motor cortex for planned movement, and is where mirror neurons are located
ii. Influences motor behavior by innervating the primary motor cortex (indirect) and sending axons to descending pathways (direct)

Question 2

Primary Motor Cortex

Answer 2

i. Separated from the somatosensory cortex by the central sulcus
ii. Controls individual or sequences of movements that require the activity of multiple muscle groups
iii. Neurons in this region encode the following aspects of movement
Force, direction, extent, speed
iv. Can be distinguished from other motor areas by: Cytoarchitecture and physiology: low intensity of current necessary to elicit movements by electrical stimulation

Question 3

Central Gray Matter in Spinal Cord

Answer 3

The spinal cord consists of a butterfly shaped central gray matter, which contains cell bodies, and a surrounding white matter that consists of the ascending and descending axonal pathways

Question 4

the Ventral Horn

Answer 4

Motor axons that will leave the spinal cord have their cell bodies located on the ventral side, in a region called the ventral horn

Question 5

Ventral Nerve Roots

Answer 5

The motor axons that leave the spinal cord are called ventral nerve roots

Question 6

ratio of gray to white matter

Answer 6

• The ratio of gray matter to white matter in spinal cord depends on which level of the spinal cord you are looking at. This is because each region of the body is innervated by a specific section of the spinal cord, so a section of the spinal cord is representative of the information going into/out of the spinal cord.
• Upper levels of the spinal cord have more white matter because all of the ascending (sensory) fibers have already entered the cord and the descending motor pathways have not left yet

Question 7

The general descending motor pathway from the cerebral cortex to the spinal cord (or brainstem) is:

Answer 7

Upper motor neurons from the cerebral cortex carry motor system outputs to the lower motor neurons in the spinal cord and brainstem. These lower motor neurons then project to muscles in the periphery

Question 8

Lateral Motor Systems vs. Medial Motor Systems

Answer 8

Lateral motor systems – control of movement in the extremities and they descend contralaterally, meaning the axons from a given hemisphere cross over to the opposite side of the spinal cord
Medial motor systems -Important for postural tone, balance and automatic gait related movements. This tract is ipsilateral or bilateral

Question 9

Lateral Corticospinal Tract Step 1

Answer 9

Step 1: Neurons in the primary motor cortex (majority), supplementary motor area and the premotor cortex send their axons (some parietal neurons as well)

Question 10

Lateral Corticospinal Tract Step 2

Answer 10

Step 2: the axons pass through the corona radiata, which is a region within the cortex that contains the ascending and descending axons tracts

Question 11

Lateral Corticospinal Tract Step 3

Answer 11

Step 3: the axons continue to the internal capsule, which is lateral to the thalamus. The somatotopic map for motor control is maintained at this level

Question 12

Lateral Corticospinal Tract Step 4 (midbrain components)

Answer 12

Step 4: the axons continue into the midbrain cerebral peduncles

Question 13

Lateral Corticospinal Tract Step 5 (medulla components)

Answer 13

Step 5: from the midbrain, the axons continue on to the medulla to form the medullary pyramids
About 85% of the axons now cross over to the opposite side of the medulla and into the spinal cord, which is termed the pyramidal decussation (within the medulla/s.c. junction)

Question 14

Lateral Corticospinal Tract Step 6 (region of decussation)

Answer 14

Step 6: the axons in this decussation are located between medulla and the spinal cord, at a region called the cervicomedullary junction. This is significant for because the foramen magnum is present here.

Question 15

Lateral Corticospinal Tract Step 7 (key splits in axons)

Answer 15

Step 7: at the point of decussation, there are several key “splits” within the axons
Axons that crossed over and become contralateral are officially called the corticospinal tract, and these contain commands for limbs
The 15% of axons that stayed ipsilateral are called the medial corticospinal tract and control the trunk muscles

Question 16

Lateral Corticospinal Tract Step 8 (spinal cord)

Answer 16

Step 8: the axons then synapse onto lower motor neurons in the spinal cord (or via an interneuron intermediate synapse). Note: this is the first synapse in the corticospinal pathway

Question 17

Reflex Motor Pathway

Answer 17

Motor reflexes involve activation of one muscle group via the sensory neuron to motor neuron pathway and inhibiting the opposite muscle

1) For example, activation the bicep to contract while relaxing the triceps
2) If both antagonistic pairs of muscles were active at the same time, the limb would be locked into place
3) Inhibition of the opposite muscle occurs with the use of an inhibitory interneuron, which when activated by sensory neuron, inhibits a motor neuron

Question 18

Polio Virus

Answer 18

The polio virus damages lower motor neurons, causing weakness, paralysis, or death

Question 19

Amyotrophic Lateral Sclerosis

Answer 19

Amyotrophic lateral sclerosis (ALS) is caused by death of upper and lower motor neurons

Question 20

Damage to Upper Motor Neurons

Answer 20

1) Signs include muscles weakness and increased muscle tone

2) t is possible with upper motor neuron damage to still have reflexive (involuntary) movement

Question 21

Damage to lower motor neurons

Answer 21

1) Signs include muscle weakness, atrophy, muscle twitches, and decreased muscle tone
2) Damage to lower motor neurons could eliminate reflexive (involuntary) movement