SLE2/MODULE 6- Motor Units ACTIVATION, CONTRACTION SPEED, + NOISE Flashcards
the properties of human motor units are best characterized by measuring what 2 things
-recruitment threshold
-twitch force
how do we know an electrode is subcutaneous
we can see the 2 wires exposed
differences in innervation number
see slide 4
-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
recruitment threshold
the force at which a MU begins to discharge APs repetitively
-can distinguish between MU contractile properties
how is recruitment threshold quantified
% MVC force
-can indicate the relative position of a MU in a population
-typically distinguished as low/high threshold
lower recruitment threshold turns on first
how does the brain know which motor neurons (motor units) to recruit?
it doesn’t
-Elwood Henneman
-brain doesn’t know which ones to turn on
-we recognize this BECAUSE of the size principle
Henneman size principle experimental protocol
-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
depending on the amount of force the brain wants to generate…
it sends the required synaptic current to the muscle
motor units in the spinal cord will be recruited based on what
size
-from smallest to largest
lower/higher currents will excite small motor neurons
lower
-Ohm’s law
Ohm’s law
V = I x R
voltage = current x resistance
reciprocal of resistance
conductance (g)
conductance (g)
ease by which an electrical current flows
R = 1/g
V = I x (1/g) = I/g
Georg Simon Ohm
-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
small motor neuron has lower/higher input resistance
higher input resistance
-because small surface areas + therfore fewer ion channels + a higher input resistance
large motor neuron has lower/higher input resistance
lower
-because large surface areas + therefore more ion channels + a lower input resistance
more resistance =
less conductance
see slide 11
small motor neurons have…
-smaller surface areas
-fewer ion channels
2 motor neurons that receive a synaptic current (I syn) from an interneuron
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
how does Ohm’s law affect EPSP
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)]
what causes the motor neurons in a motor nucleus to be activated in order of increasing size
a gradual increase in synaptic currents
which property of the postsynaptic neuron causes the synaptic current to produce a larger EPSP in the smaller motor neuron?
lower conductace (or higher resistance)
which property of the postsynaptic neuron causes I syn to produce a larger EPSP in the smaller motor neuron?
lower input conductance (or higher input resistance)
Van Cutsem et al.
1997
-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
force-frequency relation
indicates that the force exerted by a motor unit depends on the rate at which its motor neuron discharges APs
at low levels of force, what is the primary mechanism for generating + increasing force of voluntary contraction
recruitment
as force increases…describe trend
there is an exponential reduction in proportion of MU recruited
-more MU recruited at lower forces than high
at intermediate + high forces, what is the more important mechanism for force production
discharge rate (DR)
rate coding
the discharge rate or spike generation rate of a motor unit
-rate coding is the reason why we are able to produce more force
how does force increase
by recruiting additional motor units
for voluntary contractions, what order are MUs activated
MUs that exert low forces are activated before those that produce high forces
interspike interval
space in between each spike
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
unit of recruitment threshold
% MVC
2 ways force is increased/maintained
-motor unit recruitment
-increase in discharge rate/rate coding
the change in muscle force during a voluntary contraction involves what 2 things
-concurrent recruitment of motor units
-variation in discharge rate (rate coding)
see slides 21/22
slide 23
Faraday cage
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
slide 27/28/29
