L9: muscle mechanics

Question 1

isometric contraction =

Answer 1

constant length (fixed ends)

Question 2

isotonic contraction =

Answer 2

fixed tension (constant rate of shortening)

Question 3

roughly how long does a twitch last

Answer 3

from 20 to 100 ms depending on fiber type and temperature

Question 4

what determines the length of a twitch?

Answer 4

density of Ca2+ channels on SR
perhaps fiber type
perhaps temperature

Question 5

what is the latent period of a twitch?

Answer 5

time interval required to depolarize cell membrane and activate contractile apparatus in response to a stimulus

Question 6

mechanical summation

Answer 6

muscle contraction tensions can be summed by successive stimuli

Question 7

how does unfused tetanus contraction tension compare to fused tetanus contraction tension?

Answer 7

unfused tension is lower (can only overcome so much elastic force)

Question 8

why is tetanic tension so much greater than single twitch tension?

Answer 8

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)

Question 9

if there were no elastic elements (completely rigid) in the muscle, how would twitch tension compare to tetanic tension?

Answer 9

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)

Question 10

the maximum force that a contractile unit is capable of generating is called

Answer 10

tetanic tension

Question 11

what is a theoretical “active state” curve

Answer 11

elastic components replaced by an inextensible element, so full tetanic tension possible in one twitch

Question 12

the series elastic component accounts for __ % of entire muscle length

Answer 12

2-3% (pretty small, exaggerated in examples)

Question 13

the series elastic component is due to

Answer 13

active elasticity from contraction

e.g. two-way stretch of myosin from middle, stretch of actin filaments and anchors

Question 14

the parallel elastic component is due to

Answer 14

passive elasticity in titin, SR, t-tubules, sarcolemma, etc

Question 15

between series elastic component and parallel elastic component, which contributes to active tension and which contributes to passive tension

Answer 15

SEC - active tension

PEC - passive tension

Question 16

what is the most significant structure that contributes to the parallel elastic component?

Answer 16

titin (anchors myosin to z-line)

Question 17

during isometric contraction, initial tension response decreases exponentially with time to a lower stable value. why?

Answer 17

stress-relaxation

SEC elements are visco-elastic, they lose some elasticity and settle into a lower elasticity when stretched

Question 18

T/F the series elastic tension is equal and opposite to active tension

Answer 18

true

Question 19

maximum active isometric force is dependent on

Answer 19

length of the muscle

Question 20

the muscle length at which maximum active tension is possible is often called…

Answer 20

rest length

because muscles usually rest at this length

Question 21

lo or lmax is…

Answer 21

the length at which the muscle can produce maximum active tension
(also called “rest length” because muscles usually rest at this length)

Question 22

T/F the strength of a myofibril is the sum of the strength of the individual sarcomeres

Answer 22

false
myofibril strength = strength of 1 sarcomere
(chain links in series only as strong as weakest link)

Question 23

why does lmax or “rest length” occur?

Answer 23

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

Question 24

which can operate over a wider window of muscle lengths, smooth or striated muscle?

Answer 24

smooth

• no z-lines
• side-polarized filaments
• must aid large luminal volume changes

Question 25

how do skeletal and cardiac passive length-tension curves compare?

Answer 25

cardiac develops passive tension at lower lengths (intercalated disks, mono/binucleated cells) skeletal muscle can stretch more (syncytium)

Question 26

is passive tension more significant below rest length for cardiac or skeletal muscle?

Answer 26

cardiac develops passive tension at lower lengths (intercalated disks, mono/binucleated cells) skeletal muscle can stretch more (syncytium)

Question 27

how do skeletal and smooth active length-tension curves compare?

Answer 27

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

Question 28

when muscle shortens at constant velocity, active tension =

Answer 28

force of the load

Question 29

in muscle contraction, "preload" =

Answer 29

passive tension due to parallel elastic components (titin, membranes, etc)

Question 30

in muscle contraction, "afterload" =

Answer 30

active tension due to contraction / lifting

Question 31

during a contraction, series elastic elements are stretched until

Answer 31

tension = weight of load | then shortening and isotonic contraction occurs

Question 32

why does it take a certain time interval (latent period) before contraction begins to lift a load?

Answer 32

it takes time to build up isometric tension equal to weight of the load before you can begin to lift and contract isotonically

Question 33

how does increasing the load affect the latent period before isotonic contraction?

Answer 33

increases the latent period | because it takes time to build up isometric tension equal to weight of the load

Question 34

how does decreasing the load affect the latent period before isotonic contraction?

Answer 34

decreases the latent period | because it takes less time to build up isometric tension equal to weight of the lesser load

Question 35

how is the extent of isotonic shortening affected by increasing the load?

Answer 35

less shortening | because the active tension possible decreases as muscle shortens less than Lmax/Lrest

Question 36

how is the extent of isotonic shortening affected by decreasing the load?

Answer 36

more shortening | because the active tension possible decreases as muscle shortens less than Lmax/Lrest, and less load can shorten further

Question 37

how does maximum speed of shortening change with increasing load?

Answer 37

lower maximal speed of shortening

Question 38

what is the load at maximum velocity of shortening?

Answer 38

load = 0 at Vmax for shortening

Question 39

with loads greater than ~160% of the maximum load that a muscle can hold, what happens?

Answer 39

muscle lengthening is very rapid due to muscle tearing

Question 40

with loads greater than the maximum load that a muscle can hold, what happens?

Answer 40

the muscle lenghtens

Question 41

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

Answer 41

true it just shortens more slowly (molecular basis for this is unknown)

Question 42

which generally have higher velocities of shortening, skeletal or smooth muscle?

Answer 42

skeletal

Question 43

which generally generate a greater force, skeletal or smooth muscle?

Answer 43

can be comparable | smooth muscle just takes longer

Question 44

what molecule buffers muscular ATP stores?

Answer 44

PCr creatine phosphate PCr

Question 45

what enzyme catalyzes the equilibrium between PCr and ATP?

Answer 45

CPK creatine phosphokinase

Question 46

what is the equilibrium catalyzed by creatine phosphokinase?

Answer 46

CPK | PCr + ADP < - > ATP + Cr

Question 47

what is the equilibrium constant for this reaction and what does that mean? CPK PCr + ADP --> ATP + Cr

Answer 47

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

Question 48

how is ATP generated aerobically vs anaerobically?

Answer 48

``` aerobic = krebs / TCA cycle anaerobic = glycolysis ```

Question 49

2 things that are responsible for the breakdown of ATP during muscle activity

Answer 49

actomyosin ATPase | SR Ca++ ATPase pumps

Question 50

give two examples of muscles that adequately match ATP & PCr synthesis to their rate of breakdown

Answer 50

cardiac | smooth

Question 51

give an example of a muscle that breaks down ATP & PCr at a much higher rate than it can synthesize it

Answer 51

fast, white, skeletal muscles

Question 52

what is oxygen debt

Answer 52

the oxygen consumed during recovery to replenish depleted ATP, PCr and glycogen stores

Question 53

the oxygen consumed during recovery to replenish depleted ATP, PCr and glycogen stores

Answer 53

what is oxygen debt

Question 54

what is a motor unit

Answer 54

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

Question 55

how many motor neurons can a skeletal muscle fiber be innervated by?

Answer 55

1

Question 56

how many skeletal muscle fibers can a motor neuron innervate?

Answer 56

multiple (axon splits into multiple projections

Question 57

how does a threshold stimulus applied to a motor neuron affect the innervated skeletal muscle fibers?

Answer 57

produces a maximal twitch contraction in all skeletal muscle fibers of that motor unit

Question 58

what is a twitch?

Answer 58

1 contraction of a motor unit

Question 59

T/F in the case of individual muscle fibers, twitch contraction is always maximal

Answer 59

true | threshold stimulus always produces max twitch of that fiber

Question 60

how can the strength of a single contraction of muscle be increased?

Answer 60

motor unit summation

Question 61

when is maximum muscle twitch achieved?

Answer 61

when all possible motor units are activated

Question 62

what is the "all or none" law of skeletal muscle contraciton

Answer 62

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)

Question 63

``` white muscle fibers size mitochondrial content blood supply glycogen content contraction speed / fatigue motor unit size ```

Answer 63

``` largest fibers few mitochondria meager blood supply lots of glycogen -fast contractions but easily fatigued large motor unit ```

Question 64

``` red muscle fibers size mitochondrial content blood supply glycogen content contraction speed / fatigue motor unit size ```

Answer 64

``` smaller fibers lots of mitochondria good blood supply little glycogen slow contractions but good endurance small motor unit ```

Question 65

``` intermediate muscle fibers size mitochondrial content blood supply glycogen content contraction speed / fatigue ```

Answer 65

intermediate in all cases | between red and white

Question 66

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

Answer 66

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

Question 67

what is the "size principle" in motor units

Answer 67

the size principle refers to the relationships between skeletal muscle on the spectrum from white to intermediate to red

Question 68

what is an alpha motor neuron

Answer 68

the neuron from which many processes project to multiple motor end plates