Motor systems (Chapter 18)

Question 1

What is a motor unit made up of?

Answer 1

One motor unit = one motor neuron and all of the nerve fibers it innervates.

Question 2

What is the size principle?

Answer 2

Basically, motor neurons are recruited in order of size and smaller motor neurons (slow-twitch; S) are more excitable. As S neurons fire faster and faster, FF (fast-twitch fatiguable)

Motor neurons have the same density of channels regardless of size, so a smaller motor neuron will have fewer channels and therefore a higher resistance. This means that a given amount of synaptic current will cause a greater membrane potential change in the smaller neuron (i.e., why it is more easily excitable).

Question 3

What does it mean to say that “motor control systems involve both hierarchical and parallel connections”?

Answer 3

“Hierarchical” refers to the fact that motor system components are organized such that cortical association areas select a movement to execute, premotor areas plan the movement, and motor areas issue commands to motor neurons. “Parallel” refers to the fact that, like sensory pathways, there is parallel control that can occur from every level.

Question 4

Why don’t we see motor symptoms in Alzheimer’s disease until the disease is quite progressed?

Answer 4

Parallel connections

Question 5

Briefly name the respective roles of the spinal cord, brainstem, cortex, and basal ganglia/cerebellum.

Answer 5

Spinal cord regulates reflexes.
Brainstem organizes head and eye movements.
Cortex organizes movements into purposeful sequences.
Basal ganglia and cerebellum plan and modulate movements.

Question 6

Lower motor neurons innervate _________, while upper motor neurons innervate __________.

Answer 6

Skeletal muscle; spinal cord and cranial motor nuclei

Question 7

Damage to lower motor neurons results in _________, while damage to upper motor neurons results in __________.

Answer 7

Flaccid paralysis (muscles die); disorganized/involuntary/uncoordinated movements and problems with initiation (without weakness).

Question 8

How are lower motor neurons organized in the spinal cord?

Answer 8

Lower motor neurons are arranged systematically such that neurons in the more lateral spinal cord innervate distal muscles and neurons in the more medial spinal cord innervate axial muscles.

Question 9

Which type of skeletal muscle fibers contain the most mitochondria, and what do they do?

Answer 9

Slow twitch - generate small amounts of force for longer periods.

Question 10

Which type of skeletal muscle fibers use glycolysis almost exclusively, and what do they do?

Answer 10

Fast twitch, fatiguable - large amounts of force for brief periods.

Question 11

Which neurotransmitter is involved in converting APs in lower motor neurons to force?

Answer 11

Acetylcholine (nicotinic receptors; transmitter-gated ion channels).

Question 12

How is force is related to the rate of firing of lower motor neurons?

Answer 12

Twitches sum temporally, much like post-synaptic potentials do.

Question 13

How are APs converted into force?

Answer 13

An AP in a lower motor neuron axon causes Ach to be released at hundred of active zones, resulting in a single muscle twitch.

Question 14

Name the descending pathways.

Answer 14

Corticobulbar tract
Vestibulospinal tracts
Rubrospinal tract
Reticulospinal tract
Tectospinal tract

Question 15

Where do the fibers of the corticobulbar tract project to?

Answer 15

Fibers of the corticobulbar tract project from the cortex to the motor nuclei of cranial nerves via the IC.

Question 16

Lower facial muscles are innervated by corticobulbar projections __________, while upper facial muscles are innervated __________.

Answer 16

Contralaterally; bilaterally

Question 17

What is the primary function of the corticospinal tract?

Answer 17

The corticospinal tract mediates voluntary movement from the cortex (i.e., primary motor cortex, somatosensory cortex, motor association cortex) and carries sensori.

Question 18

What is the primary function of the vestibulospinal tracts?

Answer 18

The vestibulospinal tracts mediate postural adjustments and movements of the head (via brainstem vestibular nuclei).

Question 19

What pathways are responsible for activity in decorticate animals and why?

Answer 19

Vestibulospinal tracts, because they have direct inputs from brainstem to spinal cord.

Question 20

Which of the vestibulospinal tracts is responsible for smooth movements?

Answer 20

Fibers to cerebellum

Question 21

Which of the vestibulospinal tracts is responsible for adjusting antigravity/axial muscles?

Answer 21

Lateral vestibulospinal tract

Question 22

Which of the vestibulospinal tracts is responsible for stabilizing head movement and coordinating head/eye movement?

Answer 22

Medial vestibulospinal tract

Question 23

Which of the vestibulospinal tracts is responsible for maintaining gaze on moving objects?

Answer 23

Vestibulo-ocular reflex

Question 24

Most fibers of the corticospinal tract cross at the pyramidal decussation, forming the ________ corticospinal tract. However, some fibers stay ipsilateral, forming the ________ corticospinal tract.

Answer 24

Lateral; anterior

Question 25

Where does the reticulospinal tract originate, project to, and what does it do?

Answer 25

Arises in the reticular formation at the level of the pons.
Projects to medial anterior horn.
Axial muscle movement.

Question 26

Where does the rubrospinal tract originate, project to, and what does it do?

Answer 26

Arises in the red nucleus.
Projects to distal/lateral anterior horn.
Distal muscle movement. Alternate route for voluntary control

Question 27

Where does the tectospinal tract originate, project to, and what does it do?

Answer 27

Arises in superior colliculi.

Reflexive head turning.

Question 28

What is the flexor reflex?

Answer 28

The flexor reflex is the patellar tendon reflex.

Question 29

How does the flexor reflex demonstrate crossed and reciprocal components of reflexes?

Answer 29

Reciprocal inhibition: To contract quad, must relax hamstring.
Crossed: Contralateral antagonist muscles of other leg are engaged so that you don’t fall over.

Question 30

If you see a decorticate animal running around that shows complex movements in response to appropriate stimuli (i.e., grooming, copulation), which parts of the brain were likely left intact?

Answer 30

Basal ganglia

Question 31

If you see a decorticate animal running around that shows energized but inappropriate responses to stimuli, which parts of the brain were likely left intact?

Answer 31

Diencephalon (thalamus, hypothalamus)

Question 32

If you see a decorticate animal running around that can walk, right itself, and attack stimuli, which parts of the brain were likely left intact?

Answer 32

Midbrain (tectum, tegmentum)