Motor Units

Question 1

alpha motor neurons innervate what types of fibres

Answer 1

extrafusal

Question 2

gamma motor nuerons innervate what type of fibres

Answer 2

intrafusal

Question 3

Dynamic gamma MN innervate

Answer 3

dynamic bag or Bag 1 fibres

Question 4

static gamma MN innervate

Answer 4

static bag or bag 2 and chain muscle fibres

Question 5

how many fibres per MU in small

Answer 5

<10

Question 6

how many fibres per MU in large

Answer 6

> 1000

Question 7

effects of age on motor units

Answer 7

decrease in number of motor neurons
re-innervation takes places
increase in size of MU’s
increase in innervation ratio

Question 8

muscle fibers types are classified by what three factors

Answer 8

myosin heavy chain gene, fatigue resistance and contraction speed

Question 9

what factors can affect fibre type distribution

Answer 9

training, spaceflight, cross-innervation, immobilization

Question 10

motor neuron qualities Type II alpha

Answer 10

large cell body
large diameter axon
high AP conduction velocity (90-120m/s)
innervate many large muscle fibres

Question 11

Type 1 alpha motor nuerons qualities

Answer 11

smaller cell body
smaller axon diameter
innervate fewer fibers
slower conduction velocities (60-80m/s)

Question 12

muscle force varied by

Answer 12

recruitment (number of active MUs)
Rate coding (changing drift rate of different MUs)
Synchronicitiy

Question 13

how is fatigue avoided in MU firing

Answer 13

asynchronous firing

Question 14

which fibre produces more force at all firing rates

Answer 14

Type II

Question 15

which fibre type produced greater percent of max at all firing rates

Answer 15

Type I

Question 16

Hennemans Size Principle

Answer 16

Regardless of neuron size, same absolute change in V is required to reach threshold

• resistance of smaller neurons is greater than larger neurons
• according to Ohm’s law, to get threshold delta V when same amount of current is given to all nuerons, resistance is higher in smaller neurons, so voltage is higher in smaller nuerons, so they will fire at a lower current than larger neurons will!

Question 17

assuming constant synaptic drive to MU pool, MU will be recruited from (size to size)

Answer 17

smallest to largest

Question 18

What is Rheobase

Answer 18

minimum current required to recruit and maintain during of a MU

Question 19

in human experiments, MU thresholds are usually determined by

Answer 19

force recruitment threshold

Question 20

what three MU differences are there between the two hands

Answer 20

recruitment threshold, firing rate and force fluctuations

Question 21

does D or ND hand have higher firing rate

Answer 21

ND

Question 22

does D or ND hand have higher force

Answer 22

ND

Question 23

does D or ND hand have higher recruitment threshold

Answer 23

ND

Question 24

does untrained D or ND have have higher synchrony

Answer 24

ND

Question 25

violations to the size principle

Answer 25

electrical stimulation - extracellular stimulation reverse recruitment order because the whole nerve trunk is stimulated and it is easier to the the outside current to pass through a bigger axon Cat paw shake -high threshold unit preferentially recruited for maximal velocity repetitive cyclic movements Eccentric contractions - fast, high force eccentric contraction recruit large MU's due to short amount of time needed to withstand the load, prevention of injury

Question 26

4 alterations in control of MU firing rate

Answer 26

MU synchronization MU firing frequency MU doublet firing Fatigue and Muscle wisdom

Question 27

what are the two benefits or asynchronous firing

Answer 27

smooth contraction | less fatigue

Question 28

pros and cons to MU synchronization

Answer 28

PRO: high force and faster contraction CON: fatigue, loss of fine motor control

Question 29

how can you measure the synchrony (2 ways)

Answer 29

single MU records and cross correlograms

Question 30

what do you look for in MU records to find synchrony

Answer 30

look at temporal firing rates (referenced vs correlated)

Question 31

what do you look for in cross-correlation histogram to find synchrony and what do the two types of peaks you find mean

Answer 31

peak at time zero = synchrony narrow peak = less time at synchrony (less force, more control) broad peak = more time at synchrony = more force, less control

Question 32

what is the mechanisms behind short term synchrony

Answer 32

common presynaptic input directly to the motor neuron

Question 33

what is the mechanisms behind broad peak synchrony

Answer 33

common pre-synaptic input to the interneurons before the MN innervating the muscle that will be in synchrony

Question 34

what kind of training increases synchrony

Answer 34

weight training

Question 35

musicians have higher for lower synchrony

Answer 35

lower in both hands

Question 36

untrained people have higher or lower synchrony and what hand is lower between D and ND

Answer 36

lower, and lowest in D hand, bc fine movement is required

Question 37

what is doublet firing

Answer 37

two very close timed firing of neurons

Question 38

what is the increase in doublet discharges after training

Answer 38

5.2% to 32.& %

Question 39

what is different about MU activation in doublet firing

Answer 39

it is earlier

Question 40

what does doublet firing allow MU's to do

Answer 40

enhance maximal firing rate

Question 41

what does synchrony do to the steadiness of force output

Answer 41

decreases it, higher standard deviation

Question 42

what is dystonia

Answer 42

increase muscle co activation that impairs coordination "smearing" in somatosensory cortex increased synchronous discharge of MU

Question 43

what is fatigue

Answer 43

acute impairment of performance due to physical activity

Question 44

what is fatigue quantified as

Answer 44

decrease in maximal force that a muscle can exert

Question 45

what happens to MU discharge rate throughout a fatiguing contraction

Answer 45

reduced

Question 46

fatigue increases what two factors in MU firing

Answer 46

variability and patter

Question 47

want happens to M wave during fatigue and how soon before returning to normal

Answer 47

amplitude decreases, return to normal after 10min

Question 48

four main mechanical changes during muscular fatigue

Answer 48

decline in force, slower force development, slower relaxation, decreases power production

Question 49

four main electrical changes during fatigue

Answer 49

early hyper-polarization with decline in force, late hyperpolarization with slower force development, slowed conduction with flows relaxation, and reduced EMG

Question 50

what is muscle wisdom defined as

Answer 50

teh ability of the muscle to reduce discharge rate of its motor neurons to match the change in reduction of relaxation rate

Question 51

what does muscle wisdom create

Answer 51

increased summation and more economical contractions, essentially a shift in the force frequency relationship, so the same amount of force cane generation with a reduced frequency of contraction

Question 52

three Main factors in muscle wisdom

Answer 52

intrinsic membrane properties (adaptation) - same synaptic input results in fewer APs Increased feedback -from group III and groupIV afferent and disfacillitation of Ia afferent -reduction in central output -descending command to MN is reduced in fatigue