Module 4-6 Flashcards
what is a motor unit?
a motor neuron and all of the muscle fibers it innervates
what is the innervation ratio?
the number of muscle fibers innervated by a single motor neuron
what is the neuromuscular junction?
a synapse between a motor axon and a muscle fiber; neurotransmitter is ACh
how does innervation ratio vary with muscle size?
small ratio for small and precise muscles such as hand or eye, large ratio for large muscles such as leg and trunk
what is temporal summation?
when several impulses (action potential) are superimposed before the prior has relaxed, multiple pulses result in higher forces than a single one
what frequency do we see unfused contractions at?
under 20hz we can still distinguish pulses
how do humans generate smooth muscle contractions?
MUs all produce partially fused tetanus, and fire asynchronously with each other to produce a smooth net force
what does EMG measure?
action potentials generated by muscle
what are the two types of electrodes?
surface and indwelling
what is input impedance?
resistance to electrical signals from the amplifiers, should be high so as to not to interfere with EMG signal
what is common-mode rejection?
the amplification of the signal over the dampening of extraneous noise
how does temporal summation show up in EMG?
conduction velocity influence the shape of MU signal
how does spatial summation show up in EMG?
the depth of MUs relative to strength of signal and shape, deeper MUs take longer to get to surface electrode, larger conductive area leads to larger EMG
what are some techniques for surface EMG?
electrode placed on muscle belly
smaller inter-electrode distance leads to higher frequency content
smaller electrode surface area leads to larger EMG signal, smaller duration of MUAP, and less cross-talk
what are some limitations of surface EMG?
crosstalk, movement artifacts, can’t reach deep muscles, can’t record individual MUs
what is high density surface EMG?
using a panel of electrodes to cover an area, can identify where signals are increasing, identify muscle fibers, position of the innervation zone, and the conduction velocity
what can we do with indwelling EMG?
we can eliminate crosstalk, record from deep muscles, and possibly determine recruitment order, rate coding, and the influence of MUs in functional tasks
what are the limitations of indwelling EMG?
records from only a small portion of the muscle, is technically difficult and invasive, and extracting the shape of the MU from the EMG data is time-consuming
what are some general limitations of EMG?
difficult to identify individual MU firing with asynchronous muscle activity (exception being during muscle fatigue or with electrical stimulation)
can’t distinguish between concentric, isometric, and eccentric
does not measure force, but can establish a muscle-force relationship
what is the EMG-force relationship?
during isometric action force stays the same, but EMG amplitude over time during continuous isometric action
can have a good relationship if electrode position is stable with respect to active muscle fibers and MU activation pattern is stable
what are the two parts of the ascending limb of the force-length relationship?
steep: associated with the force required to deform thick filament
shallow: overlap of thin myfilaments, actin from one side overlaps with those on the other side, interferes with cross-bridge formation
what is the plateau region of the force-length relationship?
muscle force remains constant
corresponds to central region of myosin fiber that does not have cross-bridges
even if greater overlap occurs, there is no additional cross-bridge formation
what is the downward limb of the force-length relationship?
sarcomere length increases, overlap between A&M decreases, decreasing available cross-bridges
force continues to decrease until there is no overlap between A&M
how does the force-length relationship change across species?
the plateau and descending limbs are identical, just shifted due to the longer ascending limb in humans because of longer thin filaments
how does muscle fiber length affect the force-length relationship? (assuming identical PCSA)
increased active range, identical maximal tension is reached at a longer muscle length with longer fibers
how does PCSA affect the force-length relationship? (assuming identical muscle fiber length)
increased maximal tension, but muscle active range remains the same
describe the position and role of titin in the sarcomere
spans from the Z-line to the M-line, it stabilizes myosin in the middle of the sarcomere
what adaptations would we expect to see in the rectus femoris muscles between runners and cyclists?
runners would be stronger at long lengths, and weak at shorter lengths (more sarcomeres in series)
cyclists would be weaker at long lengths, and strong at shorter lengths (more sarcomeres in parallel)
how does velocity impact absolute tension of eccentric action?
it doesn’t, it is independent of velocity
