SLE2/MODULE 6- Motor Units ACTIVATION, CONTRACTION SPEED, + NOISE

Question 1

the properties of human motor units are best characterized by measuring what 2 things

Answer 1

-recruitment threshold
-twitch force

Question 2

how do we know an electrode is subcutaneous

Answer 2

we can see the 2 wires exposed

Question 3

differences in innervation number

see slide 4

Answer 3

-reflective of number of fibers and size of motor unit, green smaller.
-motor unit green has lower recruitment threshold
-motor units recruit from smallest to largest

Question 4

recruitment threshold

Answer 4

the force at which a MU begins to discharge APs repetitively
-can distinguish between MU contractile properties

Question 5

how is recruitment threshold quantified

Answer 5

% MVC force
-can indicate the relative position of a MU in a population
-typically distinguished as low/high threshold

Question 6

lower recruitment threshold turns on first

Question 7

how does the brain know which motor neurons (motor units) to recruit?

Answer 7

it doesn’t
-Elwood Henneman
-brain doesn’t know which ones to turn on
-we recognize this BECAUSE of the size principle

Question 8

Henneman size principle experimental protocol

Answer 8

-EMG recorded from a rabbit’s diaphragm show discharges of progressively larger spikes in A to D during a series of increasingly strong inspirations
-the smallest unit discharges spontaneously (A) and are the smallest in amplitude
-recruitment of large MUs, which are larger in amplitude, discharge more rapidly, + are shown in B, C, and D
-during each inspiration in D, the same MUs are recruited in the same orderly sequence
-by noticing the consistent order of the same size and shape spikes, Henneman’s work was validated by a lot of research that followed his initial observation

-protocol shows as intensity increases. Unit with smallest amplitude showed up first, then in increasing size, always in same order
-conistently the same order of turning on, based on size. A always before B then C

-time moving left to right, and we notice increased size of spikes, always same, with each breath

Question 9

depending on the amount of force the brain wants to generate…

Answer 9

it sends the required synaptic current to the muscle

Question 10

motor units in the spinal cord will be recruited based on what

Answer 10

size
-from smallest to largest

Question 11

lower/higher currents will excite small motor neurons

Answer 11

lower
-Ohm’s law

Question 12

Ohm’s law

Answer 12

V = I x R

voltage = current x resistance

Question 13

reciprocal of resistance

Answer 13

conductance (g)

Question 14

conductance (g)

Answer 14

ease by which an electrical current flows

Question 15

R = 1/g

Question 16

V = I x (1/g) = I/g

Question 17

Georg Simon Ohm

Answer 17

-German physicist and mathematician
-as a school teacher, Ohm began his research with the new electrochemical cell, invented by Italian scientist Alessandro Volta
-using equipment/machinery of his own creation, Ohm found that there is a direct proportionality between the potential difference (voltage) applied across a conductor and the resultant electric current- this relationship is known as Ohm’s law

Question 18

small motor neuron has lower/higher input resistance

Answer 18

higher input resistance
-because small surface areas + therfore fewer ion channels + a higher input resistance

Question 19

large motor neuron has lower/higher input resistance

Answer 19

lower
-because large surface areas + therefore more ion channels + a lower input resistance

Question 20

more resistance =

Answer 20

less conductance

Question 21

see slide 11

Question 22

small motor neurons have…

Answer 22

-smaller surface areas
-fewer ion channels

Question 23

2 motor neurons that receive a synaptic current (I syn) from an interneuron

Answer 23

Isyn evokes a larger EPSP in the small motor neuron, which in the figure exceeds voltage threshold and generates an AP

-synaptic current will evoke a larger EPSP in the smaller unit, hits threshold for an AP to occur
-larger unit will take longer to load chargers and make a change

Question 24

how does Ohm’s law affect EPSP

Answer 24

due to V = I/g (still Ohm’s Law), a given synaptic input (Isyn) causes the change in Vm (EPSP) to vary with input conductance (gin) [input resistance (Rin)]

Question 25

what causes the motor neurons in a motor nucleus to be activated in order of increasing size

Answer 25

a gradual increase in synaptic currents

Question 26

which property of the postsynaptic neuron causes the synaptic current to produce a larger EPSP in the smaller motor neuron?

Answer 26

lower conductace (or higher resistance)

Question 27

which property of the postsynaptic neuron causes I syn to produce a larger EPSP in the smaller motor neuron?

Answer 27

lower input conductance (or higher input resistance)

Question 28

Van Cutsem et al. 1997

Answer 28

-a typical example of an intramuscular recording is shown here -increasing from rest to high forces voluntarily at about 10% MVC/s, investigators focused on a MU discharging at the beginning and toward the end of contraction -the expanded EMG shows MUAP ISI differences at beginning of recruitment and near max force -enlarged MUAP denoted by asterisks -they confirmed that it was the same unit because the shape was the same throughout -these kinds of recordings measure DR at recruitment and what peak DR is near max force

Question 29

force-frequency relation

Answer 29

indicates that the force exerted by a motor unit depends on the rate at which its motor neuron discharges APs

Question 30

at low levels of force, what is the primary mechanism for generating + increasing force of voluntary contraction

Answer 30

recruitment

Question 31

as force increases...describe trend

Answer 31

there is an exponential reduction in proportion of MU recruited -more MU recruited at lower forces than high

Question 32

at intermediate + high forces, what is the more important mechanism for force production

Answer 32

discharge rate (DR)

Question 33

rate coding

Answer 33

the discharge rate or spike generation rate of a motor unit -rate coding is the reason why we are able to produce more force

Question 34

how does force increase

Answer 34

by recruiting additional motor units

Question 35

for voluntary contractions, what order are MUs activated

Answer 35

MUs that exert low forces are activated before those that produce high forces

Question 36

interspike interval

Answer 36

space in between each spike

Question 37

# activation of additional motor units as the contraction in MU1 increased, the interspike interval became less than it was at the beginning of the contraction

Question 38

unit of recruitment threshold

Answer 38

% MVC

Question 39

2 ways force is increased/maintained

Answer 39

-motor unit recruitment -increase in discharge rate/rate coding

Question 40

the change in muscle force during a voluntary contraction involves what 2 things

Answer 40

-concurrent recruitment of motor units -variation in discharge rate (rate coding)

Question 41

see slides 21/22

Question 42

slide 23

Question 43

Faraday cage

Answer 43

measures motor unit electrical activity -filtering + amplification helps us to look at electrical activity -formed by conductive material to reduce radio frequency interference by English scientist Michael Faraday in 1836

Question 44

slide 27/28/29