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Flashcards in Motor System Deck (23):

What are lower motor neurons?

- 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


Explain what is meant by the "size principle":

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


What is a motor unit?

All the muscle fibers innervated by 1 LMN.



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



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



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

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


What is an upper motor neuron?

anything with influence above the LMN.


What are the 4 levesl of control on LMN?

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


Describe the 5 descending pathways that control the LMN:

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??


Where in the internal capsule do Corticospinal tract lies?

Posterior limb of the internal capsule.

Corticospinal tract is all excitatory (glutamate is the transmitter)


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

No muscle weakness, issues with coordination.


Lesion in lower motor neurons will have what effect?

Muscle weakness, flaccid paralysis, atrophy of muscle, fasciculations


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

Positive = fanning of the toes,


Describe the corticobulbar tract:

- 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


What cranial nerves receive no direct corticobulbar innervation?

CN 3, 4 and 6
- controlled by other innervations of the brainstem


Where in the internal capsule do the corticobulbar tract lies?

Genu of internal capsule


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

- 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?


What and how do the higher cortical centers control LMNs?

- 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


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

Planning/monitoring movements.
NO direct control of LMN; control by controlling the other areas that control LMNs.


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

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.


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

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)


Which nucleus receives INDIRECT innervation from the corticobulbar tract:

CN nuclei 3,4,6

CBT --> reticular formation --> 3,4,6


What two CN have predominantly innervations contralaterally..

CN V and VII