Motor System

Question 1

What are lower motor neurons?

Answer 1

• Neurons within the spinal cord or brainstem that innervate skeletal muscle
• hardwired within the spinal cord
• use acetylcholine onto nicotine receptors
• In the spinal cord, are spatially arranged:
  medial - trunk muscles
  laterally - distal limbs
  dorsal - flexors
  ventral - extensors

Question 2

Explain what is meant by the “size principle”:

Answer 2

Increasing contraction requires the recruitment of more motor units. LMN are recruited in a specific pattern according to size and force:
S units first, then FR then FF.

• allows us to increase force almost directly proportional to the amount of units we are recruiting

Question 3

What is a motor unit?

Answer 3

All the muscle fibers innervated by 1 LMN.

Question 4

S-units:

Answer 4

Type I muscle fibers, Red
Slow twitch
Small force
Fatigue resistant 
Hi mitochondria
Hi capillaries 
Small motor neuron

Question 5

FR-units:

Answer 5

Type IIa muscle fibers, white 
Fast twitch
Moderate force
Fatigue resistant 
Moderate mitochondria
Moderate capillaries

Question 6

FF-units:

Answer 6

Type IIb muscle fibers, white
Fast twitch 
Large force
Fatiguable 
Lo mitochondria
Lo capillaries
Large motor neuron 

• less mitochondria/capillaries means its more dependent on glycogen storage.

Question 7

What is an upper motor neuron?

Answer 7

anything with influence above the LMN.

Question 8

What are the 4 levesl of control on LMN?

Answer 8

1. Reflex and pattern generators in the spinal cord
2. Descending pathways
3. Higher cortical centers
4. Basal ganglia and cerebellum

Question 9

Describe the 5 descending pathways that control the LMN:

Answer 9

1. corticospinal tract- excitatory using glutamate; fine motor movements
2. vestbulospinal tract- postural adjustments/head movements
3. reticulospinal tract- locomotion and postural control
4. tectospinal tract - reflex turing head to visual/auditory stimuli
5. rubrospinal tract- significance in humans??

Question 10

Where in the internal capsule do Corticospinal tract lies?

Answer 10

Posterior limb of the internal capsule.

Corticospinal tract is all excitatory (glutamate is the transmitter)

Question 11

Lesion in the basal ganglia, cerebellum or cerebral cortex will have what effect?

Answer 11

No muscle weakness, issues with coordination.

Question 12

Lesion in lower motor neurons will have what effect?

Answer 12

Muscle weakness, flaccid paralysis, atrophy of muscle, fasciculations

Question 13

What is one test you can use to determine an UMN lesion?

Answer 13

Babinksi:

Positive = fanning of the toes,

Question 14

Describe the corticobulbar tract:

Answer 14

- Cortex to motor nuclei 
(except CN III, IV, and VI).
- sensory relay nuclei 
- reticular formation
- most end on interneurons of the reticular formation, may end on motor neurons

Question 15

What cranial nerves receive no direct corticobulbar innervation?

Answer 15

CN 3, 4 and 6

- controlled by other innervations of the brainstem

Question 16

Where in the internal capsule do the corticobulbar tract lies?

Answer 16

Genu of internal capsule

Question 17

What and how do the descending pathways control spinal cord LMNs?

Answer 17

• Lateral Corticospinal Tract: all excitatory, using glutamate
• Vestibulospinal Tract: postural adjustments/head movements
• Reticulospinal Tract: locomotion and postural control
• Tectospinal tract: reflex turning of head to visual/auditory stimuli
• Rubrospinal tract: more significant for quad-peds?

Question 18

What and how do the higher cortical centers control LMNs?

Answer 18

• Associated cortex: “decide” that a movement is needed
• Supplementary motor area: planning/learning complex movements
• Premotor cortex: devises a “plan” for the movement
• Motor cortex: issues “commands” to motor neurons

Question 19

What and how do the basal ganglia and cerebellum control LMNs?

Answer 19

Planning/monitoring movements.

NO direct control of LMN; control by controlling the other areas that control LMNs.

Question 20

What is unique about CN VII and its innervation of the face?

Answer 20

CN VII, has bilateral innervation of the upper face but uni-lateral innervation of the lower face.

• This is how we can is if its a LMN lesion: if both the brows and lower face is affected.

Question 21

What is unique about CN IX and X, for example, and their innervations?

Answer 21

Almost bilateral innervation, due to the function they perform requiring both sides to do the fxn at the same time. (CN 9 = speaking, CN 10 = swallowing)

Question 22

Which nucleus receives INDIRECT innervation from the corticobulbar tract:

Answer 22

CN nuclei 3,4,6

CBT –> reticular formation –> 3,4,6

Question 23

What two CN have predominantly innervations contralaterally..

Answer 23

CN V and VII