L9: muscle mechanics Flashcards
isometric contraction =
constant length (fixed ends)
isotonic contraction =
fixed tension (constant rate of shortening)
roughly how long does a twitch last
from 20 to 100 ms depending on fiber type and temperature
what determines the length of a twitch?
density of Ca2+ channels on SR
perhaps fiber type
perhaps temperature
what is the latent period of a twitch?
time interval required to depolarize cell membrane and activate contractile apparatus in response to a stimulus
mechanical summation
muscle contraction tensions can be summed by successive stimuli
how does unfused tetanus contraction tension compare to fused tetanus contraction tension?
unfused tension is lower (can only overcome so much elastic force)
why is tetanic tension so much greater than single twitch tension?
elastic component must be stretched before full tension is possible, this requires multiple stimuli and more time to reach full contractile tension
-may also involve the rate of A-M cross bridge mobilization)
if there were no elastic elements (completely rigid) in the muscle, how would twitch tension compare to tetanic tension?
they would be equal
no elastic components to stretch so full tetanic tension possible in one twitch
(this is called a theoretical “active state” curve)
the maximum force that a contractile unit is capable of generating is called
tetanic tension
what is a theoretical “active state” curve
elastic components replaced by an inextensible element, so full tetanic tension possible in one twitch
the series elastic component accounts for __ % of entire muscle length
2-3% (pretty small, exaggerated in examples)
the series elastic component is due to
active elasticity from contraction
e.g. two-way stretch of myosin from middle, stretch of actin filaments and anchors
the parallel elastic component is due to
passive elasticity in titin, SR, t-tubules, sarcolemma, etc
between series elastic component and parallel elastic component, which contributes to active tension and which contributes to passive tension
SEC - active tension
PEC - passive tension
what is the most significant structure that contributes to the parallel elastic component?
titin (anchors myosin to z-line)
during isometric contraction, initial tension response decreases exponentially with time to a lower stable value. why?
stress-relaxation
SEC elements are visco-elastic, they lose some elasticity and settle into a lower elasticity when stretched
T/F the series elastic tension is equal and opposite to active tension
true
maximum active isometric force is dependent on
length of the muscle
the muscle length at which maximum active tension is possible is often called…
rest length
because muscles usually rest at this length
lo or lmax is…
the length at which the muscle can produce maximum active tension
(also called “rest length” because muscles usually rest at this length)
T/F the strength of a myofibril is the sum of the strength of the individual sarcomeres
false
myofibril strength = strength of 1 sarcomere
(chain links in series only as strong as weakest link)
why does lmax or “rest length” occur?
this is where actin/myosin overlap is optimal for cross-bridging… at shorter lengths, actin fibrils overlap and can even pass into the wrong half of the sarcomere to interact with the wrong myosin heads, and crumpled titin and thick filaments can crash into z-lines… at longer lengths, myosin heads lose contact with actin
which can operate over a wider window of muscle lengths, smooth or striated muscle?
smooth
- no z-lines
- side-polarized filaments
- must aid large luminal volume changes
how do skeletal and cardiac passive length-tension curves compare?
cardiac develops passive tension at lower lengths (intercalated disks, mono/binucleated cells)
skeletal muscle can stretch more (syncytium)
is passive tension more significant below rest length for cardiac or skeletal muscle?
cardiac develops passive tension at lower lengths (intercalated disks, mono/binucleated cells)
skeletal muscle can stretch more (syncytium)
how do skeletal and smooth active length-tension curves compare?
smooth is wider
skeletal muscle loses contractility at ~60% Lrest where smooth muscle still retains about half of its contractility, and can contract down to shorter lengths
when muscle shortens at constant velocity, active tension =
force of the load
in muscle contraction, “preload” =
passive tension due to parallel elastic components (titin, membranes, etc)
in muscle contraction, “afterload” =
active tension due to contraction / lifting
during a contraction, series elastic elements are stretched until
tension = weight of load
then shortening and isotonic contraction occurs
why does it take a certain time interval (latent period) before contraction begins to lift a load?
it takes time to build up isometric tension equal to weight of the load before you can begin to lift and contract isotonically
how does increasing the load affect the latent period before isotonic contraction?
increases the latent period
because it takes time to build up isometric tension equal to weight of the load
how does decreasing the load affect the latent period before isotonic contraction?
decreases the latent period
because it takes less time to build up isometric tension equal to weight of the lesser load
how is the extent of isotonic shortening affected by increasing the load?
less shortening
because the active tension possible decreases as muscle shortens less than Lmax/Lrest
how is the extent of isotonic shortening affected by decreasing the load?
more shortening
because the active tension possible decreases as muscle shortens less than Lmax/Lrest, and less load can shorten further
how does maximum speed of shortening change with increasing load?
lower maximal speed of shortening
what is the load at maximum velocity of shortening?
load = 0 at Vmax for shortening
with loads greater than ~160% of the maximum load that a muscle can hold, what happens?
muscle lengthening is very rapid due to muscle tearing
with loads greater than the maximum load that a muscle can hold, what happens?
the muscle lenghtens
T/F smooth muscle can generate forces comparable to those of skeletal muscle even though its myosin concentration is only one fifth that of skeletal muscle
true
it just shortens more slowly
(molecular basis for this is unknown)
which generally have higher velocities of shortening, skeletal or smooth muscle?
skeletal
which generally generate a greater force, skeletal or smooth muscle?
can be comparable
smooth muscle just takes longer
what molecule buffers muscular ATP stores?
PCr creatine phosphate PCr
what enzyme catalyzes the equilibrium between PCr and ATP?
CPK creatine phosphokinase
what is the equilibrium catalyzed by creatine phosphokinase?
CPK
PCr + ADP < - > ATP + Cr
what is the equilibrium constant for this reaction and what does that mean?
CPK
PCr + ADP –> ATP + Cr
Keq ~ 20
so products favored
so PCr will be used up until its concentration falls to a very low level if ATP is being used
how is ATP generated aerobically vs anaerobically?
aerobic = krebs / TCA cycle anaerobic = glycolysis
2 things that are responsible for the breakdown of ATP during muscle activity
actomyosin ATPase
SR Ca++ ATPase pumps
give two examples of muscles that adequately match ATP & PCr synthesis to their rate of breakdown
cardiac
smooth
give an example of a muscle that breaks down ATP & PCr at a much higher rate than it can synthesize it
fast, white, skeletal muscles
what is oxygen debt
the oxygen consumed during recovery to replenish depleted ATP, PCr and glycogen stores
the oxygen consumed during recovery to replenish depleted ATP, PCr and glycogen stores
what is oxygen debt
what is a motor unit
motor neuron together with all of the individual muscle fibers it innervates
(motor neuron axon splits into multiple projections that innervate a number of fibers
how many motor neurons can a skeletal muscle fiber be innervated by?
1
how many skeletal muscle fibers can a motor neuron innervate?
multiple (axon splits into multiple projections
how does a threshold stimulus applied to a motor neuron affect the innervated skeletal muscle fibers?
produces a maximal twitch contraction in all skeletal muscle fibers of that motor unit
what is a twitch?
1 contraction of a motor unit
T/F in the case of individual muscle fibers, twitch contraction is always maximal
true
threshold stimulus always produces max twitch of that fiber
how can the strength of a single contraction of muscle be increased?
motor unit summation
when is maximum muscle twitch achieved?
when all possible motor units are activated
what is the “all or none” law of skeletal muscle contraciton
a single threshold stimulus produces the same twitch contraction in a motor unit, no matter how much greater than threshold the stimulus may be
(greater contraction must be achieved by summing multiple motor units with a larger stimulus or by temporally summing with wave summation)
white muscle fibers size mitochondrial content blood supply glycogen content contraction speed / fatigue motor unit size
largest fibers few mitochondria meager blood supply lots of glycogen -fast contractions but easily fatigued large motor unit
red muscle fibers size mitochondrial content blood supply glycogen content contraction speed / fatigue motor unit size
smaller fibers lots of mitochondria good blood supply little glycogen slow contractions but good endurance small motor unit
intermediate muscle fibers size mitochondrial content blood supply glycogen content contraction speed / fatigue
intermediate in all cases
between red and white
T/F the smallest increment that can be added to the force exerted by a skeletal muscle becomes greater as the total force of contraction itself increases
true
because small motor units (red fibers) are more excitable and usually activated first, or even constantly
then intermediate fibers are activated
then large motor units (white fibers) are activated, and they are the largest and most powerful
what is the “size principle” in motor units
the size principle refers to the relationships between skeletal muscle on the spectrum from white to intermediate to red
what is an alpha motor neuron
the neuron from which many processes project to multiple motor end plates
