Descending Motor Systems

Question 1

Select the true statement about the anterolateral pathway:
A) Decussates in medulla as internal arcuate fibers
B) Enters cerebellum via the inferior cerebellar peduncle
C) Spinomesencephalic fibers synapse in periaqueductal gray
D) Fibers enter cord in medial division of entry zone
E) Second order neurons located in posterior horn of cord

Answer 1

C

Question 2

Select the true statement about the dorsal column-medial lemniscus pathway:
A) Carries pain and temperature information
B) DRG rootlets enter the lateral division
C) Crosses midline in anterior commissure
D) Cell bodies of crossing fibers are located in anterior horn of spinal cord
E) Injury results in loss of discriminative tactile sensation

Answer 2

E

Question 3

Select the trust statement about the posterior spinocerebellar tract:
A) Enters the cerebellum using the superior cerebellar peduncle
B) Crosses the midline in the anterior commissure of the spinal cord
C) Carries proprioceptive information from the arm
D) Formed by axons from cell bodies in Clarke’s nucleus

Answer 3

D

Question 4

Select the true statement about the somatosensory cortex:
A) It is in the precentral sulcus
B) It is located in the insula
C) It is located in the parietal lobe
D) It is somatotopically organized with foot and leg representation situated laterally

Answer 4

C

Question 5

What is the definition of a lower motor neuron?

Answer 5

-It innervates striated (skeletal) muscle, directly signals the muscle to contract, only way movement can be initiated. It is the last neuron in a chain of neurons

Question 6

What 2 things does a lower motor neon include?

Answer 6

1. Alpha motor neuron: extrafusal muscle fibers
   (tell muscle to contract)
2. Gamma motor neuron: intrfusal muscle fibers
   (regulate spindle/sensitivity)

Question 7

What are some symptoms of a lower motor neuron lesion?

Answer 7

• Atonia: loss of muscle tone
• Areflexia: loss of myotatic (knee jerk) reflex
• Flaccid paralysis
• Fasciculation: spontaneous muscle contractions
• Atrophy: loss of muscle tissue

Question 8

What are some symptoms of an upper motor neuron lesion?

Answer 8

• Spastic paralysis (paresis)
• Hypertonia (increased resting tension)
• Arm flexors, leg extensors
• Hyperreflexia
• Pathologic reflexes:
• Babinski sign/neg. plantar reflex
• Big toe dorsoflexion with fanning of other toes when side of heal is stroked

Question 9

During a routine exam you note that the patient has tongue fasciculations. This raises concern for:
A) Corticospinal tract injury
B) Primary myopathy
C) Lower motor neuron injury
D) Neuromuscular junction disorder
E) Hypoglossal nuclear injury

Answer 9

C

Question 10

Describe the systematic arrangement of motor neurons.

Answer 10

• Neurons controlling axial muscles are medial to those controlling distal muscles
• Neurons controlling flexors are located posterior to the extensor groups

Question 11

What is a motor unit?

Answer 11

• 1 motor neuron plus all of the muscle fibers it innervates

* In areas we have more control over, like eyes, we have less myofibers (~10) per motor unit

Question 12

Role of basal ganglia, cerebellum, and association cortex in motor control?

Answer 12

They are vital in design, choice, and monitoring of movement, but they do NOT have a direct effect on lower motor neurons. Instead, they effect motor and premotor cortex.

Question 13

What do descending motor pathways mostly terminate or synapse on?

Answer 13

• Dorsal horn neurons

- Interneurons

Question 14

Name a couple descending motor pathways

Answer 14

1. Corticospinal tract
2. Corticobulbar tract
3. Corticopontine tract
4. Rubrospinal tract
5. Reticulospinal tract
6. Vestibulospinal tract

Question 15

What 4 areas are involved in the origin of the corticospinal tract?

Answer 15

1. Premotor area (area 6)
2. Primary motor area (area 4)
3. Somatic sensory area (areas 1-3)
4. Superior parietal lobule (areas 5&7)

Question 16

Primary motor area:

1. Brodmann area
2. Function
3. Projects to where
4. Symptoms of lesion

Answer 16

1. Area 4 on medial and lateral aspect
2. Execution of contralateral voluntary movements (control fine digital movements)
3. Projects to brainstem and spinal cord-some monosynaptic termination on spinal cord motor neurons (hand)
4. Paralysis of contralateral musculature

Question 17

Premotor Area:

1. Brodmann area
2. Function
3. Projects to where
4. Symptoms of lesion

Answer 17

1. Area 6 on lateral aspect
2. Plans movements in response to ext. cues (instructions), controls proximal and axial musculature (trunk/shoulder/hip), empathetic facial movements
3. Projects to primary motor area and reticular formation (some fibers to all spinal cord levels)
4. Moderate weakness of contralateral proximal muscles, lose ability to associate learned hand movements to verbal or visual cues

Question 18

Supplementary Motor Area:

1. Brodmann area
2. Function
3. Projects to where

Answer 18

1. Area 6 on medial aspect
2. Plans movements while thinking, assembles (learns) new sequence (playing new music), assembles previously learned sequence, “imagines” movements
3. Projects to premotor & primary motor areas

Question 19

Where does the somatic sensory area and superior parietal lobule project to?

Answer 19

• Primary motor area
• Direct motor patterns in response to sensory input
• Sensory areas of brainstem and spinal cord
• Modulate sensory signals

Question 20

Describe the path of the Corticospinal tract.

Answer 20

-Originates in cerebral cortex, precentral gyrus, and nearby areas (Betz cells; layer V). Then descends thru cerebral peduncle, basis pontis, medullary pyramid, decussates at spinomedullary junction. About 15% of fibers do not decussate and remain ipsilateral.

Question 21

What 2 arteries supply the internal capsule?

Answer 21

• Lateral striate aa.

- Anterior choroidal aa.

Question 22

Select the true response about the corticospinal tract:
A) It has no somatotopic organization
B) It consists of small bundles in the midbrain
C) A left sided spinal cord lesion that involved the CST would cause ipsilateral weakness
D) Lesions are associated with flaccid paralysis
E) Decussates in the pons

Answer 22

C (it has already decussated in the spinomedullary junction)

Question 23

What does the Rubrospinal tract control?

Answer 23

-Control of shoulder and proximal arm musculature

from red nucleus to spinal cord

Question 24

What does the Reticulospinal tract control?

Answer 24

-Control of axial musculature-walking, rhythmic movements

reticular formation

Question 25

What does the Vestibulospinal tract control?

Answer 25

-Control of axial musculature-balance

Question 26

What is the Tectospinal tract believed to be important in?

Answer 26

-Head turning reflexes in response to visual stimuli (unclear function in humans)

Question 27

In the Corticobulbar tract, where do the fibers end?

Answer 27

-Some fibers end directly on motor neurons (XII), but most end on interneurons in the reticular formation.

Question 28

What CN’s receive input from the Corticobulbar tract?

Answer 28

• V,VII, Nucleus ambiguus (part of X), XI, and XII receive bilateral input
• III, IV, VI receive no direct input (all associated with the eye)

Question 29

Where does the Corticobulbar pathway originate?

Answer 29

-In the face/mouth portion of motor cortex and other nearby areas

Question 30

Where does the corticobulbar pathway decussate?

Answer 30

-No decussation exists, it descends with the cortiocspinal tract to level of target nucleus, then splits off.

Question 31

What is the exception to the typical corticobulbar pathway pattern?

Answer 31

Facial motor nucleus

Question 32

Describe the pattern of the Facial motor nucleus.

Answer 32

Motor neurons to lower facial muscles are mainly innervated by contralateral cortex, but upper facial muscles innervated bilaterally.

Question 33

What would be the result of unilateral damage to the Cortiobulbar pathway in cerebral peduncle? (on right side)

Answer 33

-Ability to wrinkle forehead (upper facial muscles) is unaffected. But, there would be an inability to smile or show teeth symmetrically b/c there would be a loss of lower facial muscles on the contralateral side. Lower facial muscles on the damaged side would not be affected.