Descending systems

Question 1

What are the functions of the descending tracts?

Answer 1

1. Control of spinal motor neurons
2. Autonomic functions
3. Modification of sensory transmission

Question 2

What are the two ways in which descending motor tracts influence motor neurons?

Answer 2

1. Directly affecting alpha motor neurons
2. Activating gamma loops
   Both are activated simultaneously with muscle contraction. Gamma loops keep the muscle taut with contraction so that tension can be generated. Resets muscle to a new “set” length

Question 3

Medial motor systems run in the:

Answer 3

ventral/ventrolateral funiculus

Question 4

Lateral motor systems run in the:

Answer 4

lateral funiculus

Question 5

Purpose of the medial motor system

Answer 5

Regulation of interneuron pools to axial and proximal limb muscles

Question 6

Purpose of the lateral motor system

Answer 6

Regulation of interneuron pools going to limb muscles

Question 7

Name the descending tracts which run in the MLF

Answer 7

1. Tectospinal tract
2. Medial vestibulospinal tract
3. Pontine reticulospinal tract

Question 8

The descending tracts that do run in the MLF are primarily involved in:

Answer 8

axial muscle control

Question 9

Where do the tracts in the MLF end?

Answer 9

Upper thoracic regions. Thus, they really only control the head

Question 10

Where do these tracts in the MLF enter the spinal cord?

Answer 10

anterior funiculus

Question 11

What is the tectospinal tract responsible for?

Answer 11

Reflex head and neck movements to visual and auditory stimuli

Question 12

What is the purpose of the medial vestibulospinal tract?

Answer 12

Reflex adjustment of head position to vestibular stimuli

Question 13

What is the purpose of the reticulospinal tract?

Answer 13

Promotes head movement to follow eye movement.

Question 14

What are the other medial motor tracts?

Answer 14

Lateral vestibulospinal tract

Medullary reticulospinal tract

Question 15

What is the function of the lateral vestibulospinal tract?

Answer 15

Powerfully excites extensor motor neurons. Also, vestibular righting reflexes.

Question 16

What does the medullary reticulospinal tract do?

Answer 16

Activates inhibitory interneurons in the spinal cord. Helps control gross movements and muscle tone through gamma motor neuron activity.

Question 17

What are the main descending tracts running in the lateral funiculus? What do they control?

Answer 17

rubrospinal and corticospinal tracts–limb movements

Question 18

Initiation of movement shows activity first in which cortex? And then secondarily in which cortex?

Answer 18

premotor cortex (including supplementary motor area (SMA))
then Primary motor cortex

Question 19

The rubrospinal tract, as part of the lateral motor system, has what function?

Answer 19

Controls movement of proximal limb flexor muscles

Question 20

Where does the rubrospinal tract arise from?

Answer 20

Red nucleus in the superior colliculus

Question 21

Which spinal levels can the rubrospinal tract access?

Answer 21

All spinal levels

Question 22

What is the function of the corticospinal system, as part of the lateral motor system?

Answer 22

Voluntary command for movement

Regulation of sensory transmission

Question 23

What is the function of the corticobulbar system?

Answer 23

Voluntary movement of head and face
Sensory transmission nuclei
Project to pons for relaying information to cerebellum
Activates brainstem nuclei involved in movement

Question 24

The corticospinal tract is the most important of the motor tracts. Describe its pathway.

Answer 24

Arises from the precentral gyrus of the motor cortex (BETZ cells–pyramidal neurons)

• Axons travel along the posterior limb of internal capsule
• Enter the cerebral peduncles of midbrain
• Run through basal pons
• Comprise the pyramids of the medulla

Question 25

What do motor cortex neurons control?

Answer 25

Direction of movement, not a specific muscle

Question 26

If the patient is unable to INITIATE movement, where might he have a brain lesion?

Answer 26

supplementary motor area

Question 27

Which motor neuron fibers are located at the genu of the internal capsule?

Answer 27

Fibers controlling the head muscles

Question 28

Where do the corticospinal tract fibers cross?

Answer 28

90% cross at the caudal medulla | 10% do not cross at all

Question 29

What do the corticospinal tract fibers do if they decussate?

Answer 29

They control interneurons to control finger movements and distal limbs. Some control limb motor neurons. They travel in the lateral funiculis

Question 30

What do the corticospinal tract fibers that DO NOT decussate do?

Answer 30

Descend in the anterior funiculus and innervate crude movements of the trunk/limb girdles. Inhibits reflexes within the spinal cord

Question 31

The corticobulbar tract controls what?

Answer 31

Facial movements

Question 32

What innervates the upper facial muscles? The lower facial muscles?

Answer 32

Upper facial muscles are innervated by corticobulbar tracts bilaterally, from both hemispheres. The lower face is innervated by the contralateral corticobulbar tract. These two tracts join together as the facial nerve when it leaves the facial nucleus of the pons. Note that an exclusive upper facial motor defect (inability to raise the eyebrows) is more serious than full facial paralysis because it is a problem with the contralateral cerebral cortex.

Question 33

Aside from controlling facial muscles, what else do corticobulbar tracts do?

Answer 33

Pontine nuclei--relay to cerebellum Sensory nuclei--modification of sensory info Reticular formation--afferent muscle tone and autonomic responses Brain stem--descending motor tracts

Question 34

What do indirect corticospinal projections do?

Answer 34

Anticipatory maintenance of body posture

Question 35

What will you observe with a corticospinal tract lesion?

Answer 35

Weakness in contralateral movements in distal muscles (assuming lesion is above the pyramidal decussation). This causes hyperactive withdrawal reflexes (Babinski) Also spasticity

Question 36

You observe muscle atrophy, faccidity, decreased reflexes and a normal plantar reflex. Where is the lesion within the corticospinal tract?

Answer 36

It's located within the motor neuron itself, distal to the corticospinal tract

Question 37

You observe little atrophy, spasticity, increased reflexes and a babinski reflex. Where is the lesion within the corticospinal tract?

Answer 37

Upper motor neuron weakness in the corticospinal tract.

Question 38

What is decorticate posturing? What does this mean if it is observed in a comatose patient?

Answer 38

Noxious stimulus causing extension of the legs and flexion of the arms. Indicates a lesion above the red nucleus

Question 39

What is decerebrate posturing? What does it mean if you observe this in a comatose patient?

Answer 39

Extension of arms and legs in response to a noxious stimulus. Indicates a lesion between the red nucleus and vestibular nuclei