03_Descending Motor Pathways of the Spinal Cord

Question 1

What are the tracts associated with lateral motor systems?

Answer 1

Lateral corticospinal tract and Rubrospinal tract

Question 1

The biceps are innervated by which spinal cord levels?

Answer 1

C5-C6

Question 2

What are the tracts associated with medial motor systems?

Answer 2

Anterior corticospinal, Vestibulospinal, Reticulospinal and Tectospinal tracts

Question 2

The triceps are innervated by which spinal cord levels?

Answer 2

C6, C7 and C8

Question 3

What is the function of the Lateral corticospinal tract (one of the lateral motor systems)?

Answer 3

Movement of contralateral limbs

Question 3

The diaphragm is innervated by which spinal cord levels?

Answer 3

C3, C4 and C5

Question 4

What is the function of the Rubrospinal tract (one of the lateral motor systems)?

Answer 4

Movement of contralateral limbs (function is uncertain in humans)

Question 4

If only one or two ventral roots are damaged, partial paralysis or weakness occurs. What is the term for this condition?

Answer 4

Paresis

Question 5

What is the function of the Anterior corticospinal tract (one of the medial motor systems)?

Answer 5

Control of bilateral axial and girdle muscles

Question 5

What events may result in paralysis?

Answer 5

Paralysis results if all roots innervating a particular muscle are destroyed. [Paralysis can also occur when upper motor neurons (e.g., corticospinal tract neurons) are damaged as in stroke].

Question 6

What is the function of the Vestibulospinal tract (one of the medial motor systems)?

Answer 6

Medial VST: positioning of head and neck Lateral VST: balance

Question 6

The bladder and bowels are innervated by which spinal cord levels?

Answer 6

S2, S3 and S4

Question 7

What is the function of the Reticulospinal tract (one of the medial motor systems)?

Answer 7

Automatic posture and gait-related movements

Question 7

Motor neuron and axon classification: Alpha MN (Diameter, Conduction Velocity, Function)

Answer 7

Fiber diameter (microns): 12-20 Conduction velocity (m/sec): 15-120 Function: Innervates extrafusal skeletal muscle fibers (i.e., the motor unit); heavily myelinated

Question 8

What is the function of the Tectospinal tract (one of the medial motor systems)?

Answer 8

Coordination of head and eye movement (uncertain in humans)

Question 8

Motor neuron and axon classification: Gamma MN (Diameter, Conduction Velocity, Function)

Answer 8

Fiber diameter (microns): 2-10 Conduction velocity (m/sec): 10-45 Function: Innervates intrafusal muscle fibers of muscle spindle; myelinated

Question 9

Lateral corticospinal tract (one of the lateral motor systems)-Site of origin

Answer 9

Primary motor cortex, and other frontal and parietal areas

Question 9

Motor neuron and axon classification: Autonomic pre-ganglionic (B motor) (Diameter, Conduction Velocity, Function)

Answer 9

Fiber diameter (microns): about 3 Conduction velocity (m/sec): 3-15 Function: pre-ganglionic autonomic fibers; lightly myelinated

Question 10

Rubrospinal tract (one of the lateral motor systems)-Site of origin

Answer 10

Red nucleus, magnocellular division

Question 10

Motor neuron and axon classification: Autonomic post-ganglionic (C motor) (Diameter, Conduction Velocity, Function)

Answer 10

Fiber diameter (microns): about 1 Conduction velocity (m/sec): 2 Function: post-ganglionic autonomic fibers; unmyelinated

Question 11

Anterior corticospinal tract (one of the medial motor systems)-Site of origin

Answer 11

Primary motor cortex and supplementary motor area

Question 12

Vestibulospinal tract (one of the medial motor systems)-Site of origin

Answer 12

Medial VST: medial and inferior vestibular nuclei Lateral VST: lateral vestibular nucleus

Question 13

Reticulospinal tract (one of the medial motor systems)-Site of origin

Answer 13

Pontine and medullary reticular formation

Question 14

Tectospinal tract (one of the medial motor systems)-Site of origin

Answer 14

Superior colliculus

Question 15

Lateral corticospinal tract-Site of Decussation

Answer 15

Pyramidal decussation, at the cervicomedullary junction

Question 16

Rubrospinal tract-Site of Decussation

Answer 16

Ventral tegmental decussation, in the midbrain

Question 17

Anterior corticospinal tract-Site of Decussation

Answer 17

Not relevant

Question 18

Vestibulospinal tract-Site of Decussation

Answer 18

Not relevant

Question 19

Reticulospinal tract-Site of Decussation

Answer 19

Not relevant

Question 20

Tectospinal tract-Site of Decussation

Answer 20

Dorsal tegmental decussation, in the midbrain

Question 21

Lateral corticospinal tract-Levels of Termination

Answer 21

Entire cord (predominantly at cervical and lumbosacral enlargements)

Question 22

Rubrospinal tract-Levels of Termination

Answer 22

Cervical cord

Question 23

Anterior corticospinal tract-Levels of Termination

Answer 23

Cervical and upper thoracic cord

Question 24

Vestibulospinal tract-Levels of Termination

Answer 24

Medial VST: Cervical and upper thoracic cord Lateral VST: entire cord

Question 25

Reticulospinal tract-Levels of Termination

Answer 25

Entire cord

Question 26

Tectospinal tract-Levels of Termination

Answer 26

Cervical cord

Question 27

The descending motor pathways work directly (monosynaptically, the minority) and indirectly (through interneurons, the majority) on what types of neurons?

Answer 27

alpha and gamma motor neurons of the ventral horn of the spinal cord and cranial nerve motor nuclei

Question 28

What are upper motor neurons?

Answer 28

The descending pathways and the somas they arise from are considered upper motor neurons.

Question 29

What are lower motor neurons?

Answer 29

The alpha and gamma motor neurons (whose axons innervate muscle) are considered lower motor neurons.

Question 30

From where do axons of the corticobulbar tract arise and to where do they descend?

Answer 30

Axons arise from somas in the face region of motor cortex; descend through corona radiata, internal capsule, and cerebral peduncles.

Question 31

Most of the corticobulbar tract projections are crossed. Which projectections are bilateral?

Answer 31

projections to the facial nucleus are bilateral

Question 32

Axons of the corticobulbar tracts project to and synapse with motor nuclei of which cranial nerves in the brainstem?

Answer 32

CN III, IV, V, VI, VII, IX, X, XI and XII

Question 33

What are the two main tracts that make up the reticulospinal tract?

Answer 33

Medullary (lateral) and Pontine (medial) reticulospinal tracts

Question 34

In general, the lateral reticulospinal tract is thought to inhibit which groups of muscles?

Answer 34

lower extremity extensors and facilitate flexors, reducing muscle tone in the lower extremities

Question 35

How does the pontine (medial) reticulospinal tract affect the lower extremities?

Answer 35

This pathway facilitates lower extremity extensor (alpha and gamma) motor neurons, increasing muscle tone, especially to musculature of the lower extremity.