Descending Tracts in the Spinal Cord

Question 1

What is the corticospinal tract the main tract for?

Answer 1

nearly all voluntary muscle activity

Question 2

Where does the corticospinal tract originate?

Answer 2

the precentral gyrus in the frontal lobe (area 4)

Question 3

Where are the cell bodies of the corticospinal tract located?

Answer 3

at the giant pyramidal neurons of Betz (100µm in diameter)

Question 4

Where do axons of the corticospinal tract dessucate?

Answer 4

in the pyramids in the medulla oblongata

Question 5

Where are neurons that supply the leg and foot located?

Answer 5

on the medial aspect of the hemisphere

Question 6

Where are neurons that supply the hand and face located?

Answer 6

on the lateral aspect of the hemisphere

Question 7

Describe the pathway of axons of the corticospinal tract

Answer 7

• pass down through the internal capsule into the cerebral peduncles in the midbrain
• continue down the brainstem to reach the pyramids in the medulla oblongata
• cross to the opposite side and continue as the lateral corticospinal tract in the spinal cord
• at each level of the spinal cord, axons peel off and enter the ventral horn
• terminate by synapsing with lower motor neurons

Question 8

What are the ventral corticospinal tract axons?

Answer 8

axons that do not cross over the medulla and continue down on the same side to enter white columns of the spinal cord before crossing lower down

Question 9

What percentage of all corticospinal fibres do not cross at all?

Answer 9

3%

Question 10

Where do axons that do not cross at all end?

Answer 10

in synapses on the ipsilateral side as the motor cortex

Question 11

What do the lateral and ventral corticospinal tracts consist primarily of?

Answer 11

a single, uninterrupted neuron called upper motor neurons

Question 12

Where do upper motor neurons synapse?

Answer 12

on lower motor neurons

Question 13

How do lower motor neurons send their axons out of the spinal cord?

Answer 13

via the ventral roots into peripheral nerves to supply voluntary muscles

Question 14

What do suppressors of the corticospinal tract do?

Answer 14

prevent the lower motor neurons from over-discharging, e.g. they prevent excessive reflex contraction of a muscle during the stretch reflex

Question 15

What happens if the suppressors of the corticospinal tract are damaged?

Answer 15

the lower motor neurons are freed from their control and fire excessively in response to reflex stimuli or discharge spontaneously

Question 16

Describe the pathway of the rubrospinal tract

Answer 16

from the red nucleus in the midbrain to lower motor neurons in the cervical part of the spinal cord to supply upper limb muscles

Question 17

Describe the pathway of the vestibulospinal tract

Answer 17

from vestibular nuclei to lower motor neurons in the spinal cord to supply the axial musculature

Question 18

What is the axial musculature?

Answer 18

neck and back muscles responsible for posture

Question 19

Describe the pathway of the reticulospinal tract

Answer 19

from the brainstem reticular formation to lower motor neurons in the spinal cord to supply the axial musculature

Question 20

What is the most common site of injury of lesion to the upper motor neuron?

Answer 20

the cerebral hemisphere before pyramidal decussation

Question 21

What is the most common cause of injury to the upper motor neuron?

Answer 21

an artery becoming occluded which deprives the neurons of their blood supply and they die (cerebrovascular accident/stroke)

Question 22

What happens if damage to the upper motor neuron is above the pyramidal decussation?

Answer 22

the signs and symptoms will be seen in the muscles on the opposite side of the body

Question 23

What happens if damage to the upper motor neuron is below the pyramidal decussation?

Answer 23

the ensuing paralysis will be on the same side as the damage

Question 24

How can a person who is paralysed on one side of the body make crude movements of the trunk?

Answer 24

uncrossed fibres along with some of the crossed ones, supply the muscles of the trunk

Question 25

What does regaining function of muscles after a stroke involve?

Answer 25

plasticity of the cortex; neurons can take on new functions with training

Question 26

How can reflexes still be elicited with an upper motor neuron lesion?

Answer 26

the lower motor neurons are not affected, and the reflex arc is complete

Question 27

What can happen when the lower motor neurons can discharge spontaneously without motor neuron modulation?

Answer 27

the muscles contract strongly (spasticity)

Question 28

What does upper and lower motor neuron paralysis tend to be respectively?

Answer 28

• upper - spastic
• lower - flaccid

Question 29

What is the Babinski reflex?

Answer 29

when the sole of the foot of a healthy person is stroked in a heel-to-toe direction, the toes curl downwards however in a patient with an upper motor neuron lesion, the toes will fan out in the opposite direction

Question 30

When does lower motor neuron lesion occur?

Answer 30

when the cell bodies of the ventral horn are selectively destroyed (e.g. by poliomyelitis) or when a nerve to a muscle is cut

Question 31

What are characteristics of flaccid paralysis?

Answer 31

muscles unable to contract so they become soft, flabby and atrophic

Question 32

What happens if the cell bodies are destroyed in a lower motor neuron lesion?

Answer 32

the axons cannot regenerate, and the paralysis is permanent

Question 33

What happens if the nerve is cut in a lower motor neuron lesion?

Answer 33

the part of the axon attached to the cell body can regenerate and some of the functions may be restored

Question 34

Give examples of muscles commonly weakened by polio (lower motor neuron lesion)

Answer 34

• shoulder muscles
• muscles behind the arm
• hip muscles
• thumb muscles
• muscles that straighten the knee
• muscles that lift the foot