the motor unit

Question 1

what are the 4 components of the control of movement?

Answer 1

spinal cord and brainstem ( lower motor neuron)
descending systems ( upper motor neuron)
cerebellum( error correction of ongoing movements)
basal ganglia (movement initiation and suppression of unwanted movements)

Question 2

primary afferents are in the?

Answer 2

periphery

Question 3

interneurons are in the?

Answer 3

spinal cord

Question 4

what are the 2 enlargements in the spinal cord and what do they innervate?

Answer 4

cervical ( innervates arms)

lumbar ( innervates the legs)

Question 5

what does the cauda equina contain?

Answer 5

nerves to and from the legs and pelvic floor

Question 6

what are 5 characteristics of lower motor neurons?

Answer 6

they have large cell bodies, they have an extensive dendritic tree, large axons, myelinated axons, rapid conduction velocity ( 60 m/s)

Question 7

what is the distribution of lower motor neurons?

Answer 7

lower motor neurons are distributed along the spinal cord in the ventral horn.

Question 8

what is the somatotopy of motorneurons?

Answer 8

proximal muscles are in the medial ventral horn ( e.g biceps), distal muscles are in the lateral ventral horn (e.g. hand muscles)

Question 9

over how many levels is a lower motor neuron typically distributed and what are the implications of this?

Answer 9

2 or 3 sometimes 4. (e.g. T1, C8,C7,C6 for abductor policis brevis)

This is good because if one level doesn’t work well the person won’t be paralyzed because they have input from other levels.

Question 10

what is the motor unit made up of?

Answer 10

cell body, it’s axon, and all the muscle fibers the axon innervates

Question 11

can an axon innervate both the biceps and brachials?

Answer 11

no an axon can only innervate ONE muscle

Question 12

what is the collection of muscle fibers innervated by one axon called?

Answer 12

muscle unit

Question 13

are properties of muscle fibers within a muscle unit the same or different?

Answer 13

they are identical

Question 14

do muscle fibers in a muscle unit contract simultaneously or separately?

Answer 14

simultaneously unless a disease (e.g. mysthenia gravis), trauma, anesthetics

Question 15

what is the typical size of a muscle unit?

Answer 15

it varies from 6-10 ( extraocular eye muscle motor unit to 1500+ fibers for the quadriceps

Question 16

when is a small motor unit needed?

Answer 16

when precise movements need to be made like in the eye or for dexterity in the hands

Question 17

when are large motor units needed?

Answer 17

when you need to generate a lot of force like in the quadriceps

Question 18

what is meant when we say the neuromuscular junction is reliable?

Answer 18

every time an axon fires the muscle fiber will contract

Question 19

where do lower motor neruons synapse?

Answer 19

on a muscle fiber

Question 20

what is the functional unit of a muscle fiber?

Answer 20

sarcomere

Question 21

how is muscle force generated?

Answer 21

when filaments slide past one another

Question 22

force generation in a muscle requires what?

Answer 22

ATP, Ca2+ binding and rotation of myosin cross bridge

Question 23

If there is not enough ATP what happens to the muscle?

Answer 23

it remains in rigor

Question 24

What are the groups which motor units can be grouped into?

Answer 24

slow, fast-fatigue resistant, fast- fatiguable

Question 25

describe slow motor units:

Answer 25

they have a slow rate of force increase during twitch, and they are very slow to fatigue ( really don’t fatigue)

contraction time is 60-120 ms

Question 26

describe fast fatigue resistant motor units:

Answer 26

they fatigue but over a short period of time they won’t fatigue and they are relatively fast.

contraction time is 30-80 ms

Question 27

describe fast fatiguable motor units:

Answer 27

they have the fastest rate of force increase during twitch and rapid force loss with repeated twitches

contraction time is 20-40 ms

Question 28

more rapid rates of stimulation of motor units results in ?

Answer 28

higher force ( tetany)

Question 29

in a fatigue test in which motor units are electrically stimulated for 2 seconds and then relieved for 2 seconds which motor units see a force drop

Answer 29

fast fatiguable see a rapid force drop and fast fatiguable eventually see a force drop whereas slow motor units don’t really see any drop

Question 30

what happens if motor units are constantly stimulated without and termination of stimulation?

Answer 30

after 10 minutes of stimulation 100% of muscle fibers will die.

Question 31

what are 2 ways to modulate force generation?

Answer 31

recruitment of a previously silent motor unit

and rate coding of an already firing motor unit ( partially fused output in humans i.e. not full tenany)

Question 32

what is the size principle?

Answer 32

smallest motor units are recruited before larger motor units

Question 33

what size motor neurons have lower excitability?

Answer 33

bigger motor neurons,

smaller motor neurons are more excitable hence they are recruited earlier

Question 34

bigger motor neuron units have more complex dentrites, more ( name 3 things) :

Answer 34

short term ESPS potentiation w/ repeated activation and axonal muscle fibers

Question 35

what would happen if motor unit recruitment during movements was random?

Answer 35

bad outcomes: loss of movement precision and loss of function

Question 36

when we stand we are using mostly what type of motor unit?

Answer 36

slow

Question 37

when we walk or run we are mainly using what type of motor unit?

Answer 37

fast-fatigue resistant

Question 38

when we jump we are using mainly what type of motor unit?

Answer 38

fast fatiguable

Question 39

what determines motor unit type?

Answer 39

activity pattern ( availability of trophic factors) influences motor unit type

and genetics

Question 40

when are muscle fibers poly-innervated?

Answer 40

at birth

Question 41

when can muscle fiber properties change?

Answer 41

after injury