Week 3 - Spinal Cord and Movement Control: Motor Unit and Spinal Reflexes

Question 1

what are the 4 main components that control movement? what do they control?

Answer 1

1. spinal cord and brainstem (lower motor neurons)
2. descending systems (upper motor neurons)
3. cerebellum (error correction of ongoing movements)
4. basal ganglia (movement initiation and suppression of unwanted movements)

Question 2

what kinds of connections do lower motor neurons (brainstem and spinal cord) have?

Answer 2

simple or complex interconnections between

• primary afferents
• interneurons
• excitatory
• inhibitory
• lower motor neurons

Question 3

how does the spinal cord differ from the spine?

Answer 3

spinal cord has 2 enlargements that innervate things, and cauda equina

• cervical enlargement innervates arms
• lumbar enlargement innervates legs
• cauda equina has all nerves to/from legs and pelvic floor

Question 4

what are the many names for the lower motor neurons and why are they called this?

Answer 4

• lower motor neurons (hierarchy; subservient to brain)
• alpha motoneuron (high speed of axonal conduction)
• spinal motoneuron (physical location)

Question 5

what are lower motor neurons characterized by?

Answer 5

• large cell body (50 um) and axon (5 um), or can be 1+ m in length
• extensive dendritic tree
• myelinated with Schwann cells
• rapid conduction velocity (up to 60 m/s in humans, 120 m/s in cats)

Question 6

which motoneurons go to proximal VS distal muscles?

Answer 6

proximal: medial ventral horn
distal: lateral ventral horn

Question 7

what is a motor neuron pool? what is the lower motor neuron pool typically distributed over?

Answer 7

total of all lower motor neurons innervating a given muscle

-LMNP typically distributed over 2-3 neurologic segments, sometimes 4

Question 8

what is a motor unit made up of?

Answer 8

• cell body
• cell axon (innervates only one muscle)
• muscle unit

Question 9

what is a muscle unit? what are the properties of the muscle fibers? what is the size?

Answer 9

collection of muscle fibers innervated by that one axon of a motor unit

• properties of muscle fibers w/in a muscle unit are ~identical
• all muscle fibers of the unit normally contract simultaneously when parent axon fires
• -exceptions of disease, trauma, anesthetics
• size varies tremendously (6-10 for eye muscles, 1500+ for quads)

Question 10

how many nerves innervate each muscle fiber?

Answer 10

only one nerve axon innervates each muscle fiber

Question 11

where do lower motor neurons synapse? what is their nt? are they reliable?

Answer 11

on muscle fiber NMJs with ACh

-normally extremely reliable, int hat every time an axon fires, the muscle fiber will contract

Question 12

what happens if there’s not enough ATP for force generation?

Answer 12

muscle remains “tight” (rigor mortis)

Question 13

what are the 3 types of motor units? what are their characteristics?

Answer 13

Slow - relatively slow rate of force increase during “twitch”
-little/no force loss with repeated twitches (slow to fatigue)

Fast, fatigue-Resistant (FR) - relatively fast rate of force increase during twitch
-moderate force loss with repeated twitches (eventually stop producing force, but takes many seconds)

Fast, Fatiguable (FF) - fastest rate of force increase during twitch
-rapid force loss w/ repeated twitches (stops producing force w/in seconds of onset)

Question 14

what is a twitch and how does it differ with the 3 types of motor units?

Answer 14

one single stimulus to the motor unit’s axon

• force is highest for FF, lowest for S
• contraction time is fastest for FF (~30 ms), slowest for S (~80 ms)

Question 15

what is tetanus and how does it differ with the 3 types of motor units?

Answer 15

repeated stimuli to the axon, before mechanical increase in force recovers to zero

• S has lowest force, but longest time (doesn’t fatigue)
• FF has highest force, but ends at same time as FR

Question 16

what is the influence of rate on motor unit twitch force?

Answer 16

higher forces come from more rapid rates of stimulation or higher firing rates during movements

Question 17

what happens to the 3 types of motor units during standardized electrical stimulation (2 seconds on, 2 seconds off) VS 10 minutes of continuous stimulation?

Answer 17

standard: S has little to no force drop (although low force), while FF drops quickly, and FR after a longer time

10 min: all had 100% fiber death b/c no blood flow caused ischemia

Question 18

what are 2 ways to modulate force generation? what are these?

Answer 18

1. recruitment of a previously silent motor unit
   - cause a motor unit to begin discharging
   - stronger inputs lead to recruitment of more powerful motor units (Size principle)
2. rate-coding of an already firing motor unit
   - increase firing/discharge rate of an already-recruited motor unit
   - partially-fused force output (most daily living)
   - fully fused (tetanic) force output at highest firing rates (very rarely in daily living, usually only if body builders)

Question 19

with increased motor unit size, what do alpha motoneurons (lower motor neurons) experience that increases or decreases?

Answer 19

increased: cell body size, dendritic complexity, short-term EPSP potentiation with repeated activation, axonal diameter (b/c myelinated), and number of axonal branches
decreased: input resistance, excitability, Ia EPSP amplitude, PSP decay constant, and duration of after-hyperpolarization

Question 20

why would it be bad if motor unit recruitment during movements was random?

Answer 20

• loss of movement precision

- loss of function

Question 21

what does the size principle tell us about motor unit recruitment?

Answer 21

it is very consistent

• standing: only slow
• walk/run: fast fatigue-resistant
• gallop/jump: fast fatiguable

Question 22

what determines motor unit type? (4 things)

Answer 22

1. activity pattern (which are stimulated more in life, but impossible outside lab setting)
2. muscle fibers are poly-innervated at birth
   - pruning of innervation occurs
   - coincident with motoneuron loss w/in spinal cord
   - availability of trophic factors from muscle fibers
3. injury/recovery influences muscle fiber properties
4. genetics