Descending Tract- CST

Question 1

A lesion to lower motor neurons of the CST will result in what type of symptoms?

Answer 1

Flaccid paralysis
Hyporeflexivity
fasciculations (twitching of muscles under your skin).

Example: Polio, ALS

Question 2

What are examples of lower motor neurons of the CST?

Answer 2

Spinal Accessory Nucleus (C1-C6)
Phrenic Nucleus (C3-C5)

Question 3

What are some symptoms that can be observed due to a lesion in upper motor neurons of the CST?

Answer 3

Spasticity
Exaggerated reflexes 
Some hemiplegia -
muscle weakness, not paralysis
No muscle wasting  

Example: ALS, stroke within the Middle Cerebral Artery (wipes out area 4- affects opposite side of the body)

Question 4

Where does the CST begin?

Answer 4

Pre-central gyrus
Area 4
Pyramidal cells
Betz cells

Question 5

Where does the CST terminate?

Answer 5

Ventral/anterior horn cells of the medial and lateral nuclei in the ventral horn of the spinal cord; alpha motor neurons that innervate skeletal muscle

Question 6

What is the function of the CST?

Answer 6

Conscious, fine, detailed motor movements

Question 7

Where does the CST decussate?

Answer 7

90% decussates in the junction between the caudal medulla and the spinal cord, then travel in the lateral funiculus to become the lateral CST. 10% does not decussate, becomes the anterior CST, and later decussates and different segments to synapse on anterior horn cells.

Question 8

Describe how a pyramidal cell looks like anatomically:

Answer 8

1 single triangular soma, large apical dendrite and basal dendrites, single axon. This is a reason why its hard to recover from neuronal cell injury.

Question 9

Where does the CST travel through?

Answer 9

Area 4/Pre-central gyrus axons spread out through the corona radiata and travel through the internal capsule, through the central peduncle/crus cerebra of the midbrain, spread out through the pontine gray of the pons, collect once again on the pyramids of the medulla, where 90% of the fibers will decussate in the junction between the caudal medulla and spinal cord, travel through the lateral funiculus to become the lateral CST and finally terminate on ventral horn cells –> innervate motor neurons that will eventually innervate skeletal muscles.

Question 10

What is a motor unit?

Answer 10

Alpha motor unit plus the skeletal muscle fiber it innervates.
Small motor units (1 nerve/1 muscle) = precise movements
Large motor units (1 nerve/many muscle fibers) = axial/coarser movements

Question 11

What is the somatotopy of the ventral motor horn?

Answer 11

Medial –> Lateral:

Trunk/Legs, shoulder, arm, forearm, hand

Question 12

Why do we see enlargements of the ventral horn at the cervical and lumbar spinal cord levels?

Answer 12

Arms and legs need a lot of innervation, thus higher need of anterior horn cells –> increased white matter in the ventral horn –> enlargements of the cervical and lumbar regions.

Question 13

What are the three possible responses when rubbing the side of the foot or sole with blunt object?

Answer 13

Babinski sign:
Negative- flexor, the toes curve inward and the foot everts.
Indifferent- no response
Positive- extensor, hallux dorsiflexes and other toes fan out; indicates damage to CNS (upper motor lesion) or normal in infants.