Motor Units Flashcards
alpha motor neurons innervate what types of fibres
extrafusal
gamma motor nuerons innervate what type of fibres
intrafusal
Dynamic gamma MN innervate
dynamic bag or Bag 1 fibres
static gamma MN innervate
static bag or bag 2 and chain muscle fibres
how many fibres per MU in small
<10
how many fibres per MU in large
> 1000
effects of age on motor units
decrease in number of motor neurons
re-innervation takes places
increase in size of MU’s
increase in innervation ratio
muscle fibers types are classified by what three factors
myosin heavy chain gene, fatigue resistance and contraction speed
what factors can affect fibre type distribution
training, spaceflight, cross-innervation, immobilization
motor neuron qualities Type II alpha
large cell body
large diameter axon
high AP conduction velocity (90-120m/s)
innervate many large muscle fibres
Type 1 alpha motor nuerons qualities
smaller cell body
smaller axon diameter
innervate fewer fibers
slower conduction velocities (60-80m/s)
muscle force varied by
recruitment (number of active MUs)
Rate coding (changing drift rate of different MUs)
Synchronicitiy
how is fatigue avoided in MU firing
asynchronous firing
which fibre produces more force at all firing rates
Type II
which fibre type produced greater percent of max at all firing rates
Type I
Hennemans Size Principle
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!
assuming constant synaptic drive to MU pool, MU will be recruited from (size to size)
smallest to largest
What is Rheobase
minimum current required to recruit and maintain during of a MU
in human experiments, MU thresholds are usually determined by
force recruitment threshold
what three MU differences are there between the two hands
recruitment threshold, firing rate and force fluctuations
does D or ND hand have higher firing rate
ND
does D or ND hand have higher force
ND
does D or ND hand have higher recruitment threshold
ND
does untrained D or ND have have higher synchrony
ND
violations to the size principle
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
4 alterations in control of MU firing rate
MU synchronization
MU firing frequency
MU doublet firing
Fatigue and Muscle wisdom
what are the two benefits or asynchronous firing
smooth contraction
less fatigue
pros and cons to MU synchronization
PRO: high force and faster contraction
CON: fatigue, loss of fine motor control
how can you measure the synchrony (2 ways)
single MU records and cross correlograms
what do you look for in MU records to find synchrony
look at temporal firing rates (referenced vs correlated)
what do you look for in cross-correlation histogram to find synchrony and what do the two types of peaks you find mean
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
what is the mechanisms behind short term synchrony
common presynaptic input directly to the motor neuron
what is the mechanisms behind broad peak synchrony
common pre-synaptic input to the interneurons before the MN innervating the muscle that will be in synchrony
what kind of training increases synchrony
weight training
musicians have higher for lower synchrony
lower in both hands
untrained people have higher or lower synchrony and what hand is lower between D and ND
lower, and lowest in D hand, bc fine movement is required
what is doublet firing
two very close timed firing of neurons
what is the increase in doublet discharges after training
5.2% to 32.& %
what is different about MU activation in doublet firing
it is earlier
what does doublet firing allow MU’s to do
enhance maximal firing rate
what does synchrony do to the steadiness of force output
decreases it, higher standard deviation
what is dystonia
increase muscle co activation that impairs coordination
“smearing” in somatosensory cortex
increased synchronous discharge of MU
what is fatigue
acute impairment of performance due to physical activity
what is fatigue quantified as
decrease in maximal force that a muscle can exert
what happens to MU discharge rate throughout a fatiguing contraction
reduced
fatigue increases what two factors in MU firing
variability and patter
want happens to M wave during fatigue and how soon before returning to normal
amplitude decreases, return to normal after 10min
four main mechanical changes during muscular fatigue
decline in force, slower force development, slower relaxation, decreases power production
four main electrical changes during fatigue
early hyper-polarization with decline in force, late hyperpolarization with slower force development, slowed conduction with flows relaxation, and reduced EMG
what is muscle wisdom defined as
teh ability of the muscle to reduce discharge rate of its motor neurons to match the change in reduction of relaxation rate
what does muscle wisdom create
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
three Main factors in muscle wisdom
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
