Descending tracts

Question 1

What is a lower motor neuron?

Answer 1

It innervates striated muscle and directly signals the muscle to contract. It is the only way movement can be initiated

Question 2

Where is the lower motor neuron found in a chain of neurons?

Answer 2

It is the last one

Question 3

What are the two types of motor neurons included in the lower motor neuron?

Answer 3

Alpha and gamma motor neurons. The alpha is with the extrafusal muscle fibers while the gamma is with the intrafusal muscle fibers

Question 4

What are some possible outcomes of a lower motor neuron lesion?

Answer 4

Atonia- loss of muscle tone
Areflexia- loss of knee jerk relfex
Flaccid paralysis
Fasciculations- spontaneous muscle contractions
Atrophy- loss of muscle tissue

Question 5

Where do upper motor neurons start and end?

Answer 5

They start in the cortex and end on or near the lower motor neuron

Question 6

What are some possible consequences of an upper motor neuron lesion?

Answer 6

Spastic paralysis
Hypertonia (increased resting tension) in arm flexors and leg extensors
Hyperreflexia
Pathologic reflexes like negative plantar reflex or babinski sign
Big toe dorsoflexion with fanning of the other toes when the side of the heal is stroked

Question 7

In the spinal cord, where are the cell bodies of the UMN and LMN found? Where do the axons leave from? Where does each branch end?

Answer 7

In the anterior horn
They leave from the ventral root
Each branch ends at one neuromuscular junction

Question 8

How are motor neurons arranged in the spinal cord?
Where are neurons found for axial muscles and other more distal muscles?
How are the flexor and extensor neurons arranged?

Answer 8

There is a systematic arrangement of motor neurons
Neurons controlling axial muscles are more medial than those controlling distal muscles
Neurons controlling flexors are located posterior to the extensor groups

Question 9

What is a motor unit?

Answer 9

It is one motor neuron plus all the myofibers it innervates

Question 10

What does the size and function of a muscle tell you about the motor unit(s) that control it?

Answer 10

Well, they vary in size. Muscles used for very fine and precise movements (extraocular muscles for example) have 10 myofibers per motor unit whereas a larger muscle (like the gastrocneumius) can have from 100 to 1000 myofibers per motor unit. You want more specificity for finer movements.

Question 11

What are the three types of muscle fibers?

Answer 11

Standing: contract weakly for a long time
Running: contract strongly for a short or long time
Jumping: contract very strongly for a short time

Question 12

Y/N- do the different types of muscle fibers mix?

Answer 12

No- each muscle fiber populates one motor unit, no mixing

Question 13

What is an acronym to help you remember type 1 muscle fibers? Give me a few extra details as well.

Answer 13

“one slow, fat, red, ox”
Type one, slow twitch, fat (lipid-rich), red (appearance), ox (oxidative, mitochondria-rich)
small amounts of glycogen, lots of mitochondria, good for sustained force or weight bearing muscles

Question 14

Describe type 2 muscle fibers.

Answer 14

fast twitch, lean (not much fat/lipid), lots of glycogen, fewer mitochondria, good for sudden movements

Question 15

Motor units are recruited in order of their size. Explain.

Answer 15

Big muscles, “jump” muscles, are almost completely recruited for the required action. The “standing” muscles are only about 25% recruited for the action.

Question 16

The basal ganglia, cerebellum, and association cortex are important for design in, choosing and monitoring movement- what happens if any of these areas are damaged?

Answer 16

Well, they have no direct effect on the LMN so there won’t be weakness associated with the lesion. However, one may experience involuntary movements, lack of coordination, and difficulty initiating movement

Question 17

How do the basal ganglia and cerebellum influence movement?

Answer 17

They are involved in planning and monitoring movements. They don’t have any outputs to the spinal cord, instead they have an effect on the motor and premotor cortex

Question 18

What is the flow of movement planning and execution?

Answer 18

The premotor cortex plans and tells the motor cortex and then the LMN what to do

Question 19

T/F, motor control can be described as both hierarchical and parallel. Why or why not?

Answer 19

True- the cortex decides what movements should occur. That’s hierarchical. The arrangement is parallel because the premotor cortex can talk directly to the LMN. So the LMN and motor cortex are in parallel and the premotor cortex can communicate with both.

Question 20

Where does the corticospinal tract start and end and what is it an example of?

Answer 20

It starts on the cortex and ends on the spinal cord. It is a classic example of an UMN.

Question 21

Where does the corticobulbar tract start and end?

Answer 21

Cortex to brainstem

Question 22

Where does the corticopontine tract start and end?

Answer 22

Cortex to basilar pons

Question 23

Where is the primary motor cortex found?

Answer 23

It is the precentral gyrus, just anterior to the central sulcus

Question 24

Where is the premotor cortex found?

Answer 24

It is found just anterior to the primary motor cortex

Question 25

Where is the somatic sensory cortex found?

Answer 25

The postcentral gyrus, just posterior to the central sulcus

Question 26

What is the primary motor area (brodmann area 4 medial and lateral aspect) responsible for?

Answer 26

Execution of contralateral voluntary movements

Control of fine digital movements

Question 27

What is the result of a lesion in the primary motor area (brodmann area 4 )?

Answer 27

Paralysis of the contralateral musculature (if the lesion is on the left side of the brain, the associated musculature on the right side is affected)

Question 28

T/F the primary motor area (brodmann area 4) has some projections to the brainstem and spinal cord- some monosynaptic terminations on the spinal cord motor neurons that are for the hand?

Answer 28

True.

Question 29

What is the function of the premotor area (area 6 on the lateral aspect)?

Answer 29

Plans movements in response to external cues (like instructions)
Control of proximal and axial musculature (trunk, shoulder, hip)
May assemble empathetic facial movements

Question 30

Does the premotor area (area 6 on the lateral aspect) project to the primary motor area and reticular formation?

Answer 30

Yep. Some fibers project to all spinal cord levels

Question 31

What happens if there is a lesion to the premotor area (area 6 on the lateral aspect)?

Answer 31

Moderate weakness of contralateral proximal muscles results
You also lose the ability to associate learned hand movements to verbal or visual cues. I assume this would be like someone trying to give you a fist bump and you just leaving them hanging..

Question 32

The supplementary motor area (SMA, area 6 on the medial aspect) does what?

Answer 32

It plans movements while thinking
It learns new sequences (like playing new music)
It assembles previously learned sequence (music scale)
It imagines movements

Question 33

True or false, the supplementary motor area (area 6 on the medial aspect) projects to the premotor area (area 6 on the lateral aspect) only?

Answer 33

False. It projects to the premotor (area 6 on the lateral side) as well as the primary motor area (area 4)

Question 34

The somatic sensory area (areas 1-3) is found in which lobe of the brain?

Answer 34

the parietal lobe

Question 35

What is the purpose of the somatic sensory area (areas 3, 1 & 2) and the superior parietal lobule (areas 5 &7)?

Answer 35

Somatic sensory area (areas 3, 1 & 2) and the superior parietal lobule project to the primary motor area to direct motor patterns in response to sensory input and
project to sensory areas of the brainstem and spinal cord to modulate sensory signals

Question 36

The corticospinal tract (CST) has collaterals that project to what?

Answer 36

the basal ganglia, thalamus, reticular formation, various sensory nuclei, and the posterior and intermediate horns of the spinal cord

Question 37

What happens if the SCT is cut in monkeys?

Answer 37

After a period of flaccid paralysis, they move again but fine finger movement has been lost. FOREVER (said a la sandlot)

Question 38

Go draw the path of the CST. Where does it start and end?

Answer 38

It originates in the cerebral cortex, precentral gyrus and nearby areas. It descends through the cerebral peduncle, basis pontis, medullary pyramid, and decussates at the spinomedullary junction

Question 39

The corticospinal tract has a lateral and anterior division. What happens with each division?

Answer 39

Lateral CST- 85% of fibers cross in decussation in the medulla and descend in the lateral funiculus
Anterior CST- 15% of the fibers don’t cross/decussate and descend in the anterior funiculus

Question 40

True or false, the CST is somatotopically organized?

Answer 40

Truth

Question 41

The rubrospinal tract is responsible for what?

Answer 41

Control of the shoulder and proximal arm musculature.

Question 42

The reticulospinal tract is responsible for what?

Answer 42

Control of axial musculature- walking

Question 43

The vestibulospinal tract is responsible for what?

Answer 43

Control of axial musculature- balance

Question 44

The tectospinal tract is responsible for what?

Answer 44

No one really knows…

Question 45

Tell me about the vestibulospinal tract. Origin, what does it receive input from, where do the lateral and medial aspects go to, and what is its function?

Answer 45

It starts in the vestibular nuclei of the pons.
Its lateral tract projects via the lateral funiculus to the ipsilateral spinal cord, which facilitates antigravity muscles
The medial tract projects via the anterior funiculus to the spinal cord cervical levels which controls head movements in response to gravity
Its function is to mediate postural adjustments and head movements, antigravity reflexes and righting reflexes (like in cats

Question 46

Rubrospinal tract. Tell me about it. Origin, where does it receive input from, its course, and its function. Go.

Answer 46

Function: similar to vestibulospinal tract- facilitates upper extremity flexor muscle tone
Origin- red nucleus of the midbrain
Receives input from the primary and premotor areas- shoulder and arm control- as well as the cerebellum
Its course: goes through the ventral tegmental decussation and the lateral funiculus and projects to the contralateral spinal cord

Question 47

Reticulospinal tract. Origin, input, course, function.

Answer 47

Function: supports rhythmic motor actions including walking
Origin: brainstem formation (pons and medulla)
Receives input from the motor and somatic sensory cortex
Its course: brainstem- anterior to medial longitudinal fasiculus; spinal cord- anterior funiculus; projects to the spinal cord bilaterally

Question 48

The corticobulbar pathway ends/acts on what?

Answer 48

well, it leaves the cerebral cortex and descends to the brainstem where it ends on sensory and motor nuclei of cranial nerves and the reticular formation. So, cranial nerves and reticular formation

Question 49

Where at in the cortex does the corticobulbar pathway originate?

Answer 49

It originates in the face/mouth portion of the motor cortex and other nearby areas

Question 50

True or false, corticobulbar decussation exists?

Answer 50

False. It does not. No decussation for you, corticobulbart pathway.

Question 51

How are the muscles of the face controlled by the CBP?

Answer 51

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

Question 52

What happens if there is unilateral damage to the CBP, IE in the cerebral peduncle?

Answer 52

You would have an inability to smile or show teeth symmetrically, but the ability to wrinkle your forehead would be intact. Thus, even though you suffer from neuronal damage, you could still give a menacing glare with those forehead wrinkle