Pre-Lab 6: Mechanisms of Muscle Contraction

twitch

isolated contraction that comes from a single action potential to produce a single contraction in the fibers that motor neuron controls

latent period

first phase of the twitch - action potential is being propagated along sarcolemma

• when excitation and contraction are coupled

contraction phase

Ca2+ in sarcoplasm bound to troponin, tropomyosin shift away from actin-binding sites, cross-bridges formed, sarcomeres actively shortening

relaxation phase

tension decreases, contraction stops, Ca2+ pumped into SR, cross-bridge cycling stops

a single twitch does/does not produce any significant muscle activity

does not

graded muscle potential

a series of action potentials

wave summation

fibers stimulated while a previous twitch is still occurring — second twitch will be stronger

if frequency of motor neuron signaling ____, summation and subsequent muscle tension in the motor unit continue to ___ until it reaches a ____

increases, rise, peak point

incomplete tetatanus

tension is 3-4x greater than tension of single twitch

• quick cycles of contraction with short relaxation phase for each

tetanus

stimulus frequency so high that relaxation phase disappears completely, contractions continuous, maximum recruitment of muscle fibers

What is necessary first step important for the start of muscle contraction?

a Arrival of action potential from motor neurons

b Application of stimulus at threshold

c Both A and B

both A and B

motor unit

single alpha motor neuron and its associated muscle fibers

two ways to increase force of muscle contraction

recruitment and frequency modulation

T/F: muscle units produce the same force regardless of their size

false

force increases with summation

…

T/F: it is possible to achieve full tetanus without summation

false

muscle fatigue

any decline in the ability of a muscle to produce the desired level of force

muscle fatigue causes

• conduction failure (neural impulses no long act sufficiently due to shortage/build-up of ions)

• lactic acid buildup (when oxygen limited, glucose broken down into pyruvate which turns to lactate)

• build-up of metabolites (inhibits cross-bridge cycling)

The production and build-up of lactic acid is the result of:

a the breakdown of muscle tissue

b byproduct of glycolysis

c byproduct of sweat production

byproduct of glycolysis

length-tension relationship

cross-bridges can only form where thick/thin filaments already overlap — length of sarcomere has direct influence on the force generated when the sarcomere shortens

optimal sarcomere length

80-120% of its resting length

What is the event that causes gliding motion of filaments to halt during cross-bridge formation?

a ATP binding to myosin heads

b ADP binding to myosin heads

c ADP released from the myosin heads

ATP binding to myosin heads

What is the crucial event for exposure of actin binding sites and binding of energized myosin heads?

a An increase in ATP concentration

b An increase in Ca2+ concentration

c Decrease in Ca2+ concentration

d Decrease in ATP concentration

increase in Ca2+ concentration

muscle contraction - troponin and tropomyosin

tropomyosin - blocks myosin binding site

troponin - holds tropomyosin in place

ca2+ rises —> binds troponin —> tropomyosin moves off actin —> myosin binds to actin

experimental setup

…

parts of setup + parts of frog

femur clamp — part of femur within gastrocnemius muscles

force transducer - attachment for belly of the tendon

first step

do calibration of force transducer — raw output be calibrated to give us force unit

second step

identify minimum stimulus to elicit response and then supramaximal excitation voltage needed for maximal recruitment

supermaximal stimulus is ___ times the maximal stimulus

1.5

purpose of experiment

to determine the relationship between the length of the muscle and the tension it is capable of producing

when you raise the force transducer, you may

mark the ring stand

during experiment, use supermaximal stimulus to determine the relationship between

the stimulus frequency and the tension it is capable of producing

if unable to detect signal when muscle is twitching, the problem is most likely

muscle contracts away from stimulation and can’t see full tetanus

remember things

• stimulating electrode set to deliver a voltage

• muscle is still alive — wet the muscle continuously and regularly so muscle doesn’t die

• complete all experiments before analyzing data

• add comments regularly

In this lab we will be looking at-------------------- in the frog muscle?

a Isotonic contractions

b Isometric contractions

c Eccentric contractions

d Concentric contractions

isometric contractions

Calibration of the force transducer is necessary to

a Zero force transducer

b Remove any tension that is not a result of muscle contraction

c Gain precise measurements

d Convert mV into mN units

e All the above

all of the above

Why is a stimulator used in this experiment?

The stimulator delivers controlled electrical voltage to the frog muscle, mimicking the action potentials that a motor neuron would normally send. It lets you control the stimulus strength and frequency precisely — which is how you can generate a single twitch, then produce summation and tetanus by increasing frequency, and fatigue by prolonged stimulation.

Twitch

a single, isolated contraction of a muscle fiber (its motor unit) in response to one action potential/stimulus. Phases: latent → contraction → relaxation.

Summation (wave summation)

when a second stimulus arrives before the previous twitch has fully relaxed, the contractions add together, producing greater tension than a single twitch

Tetanus

sustained contraction from high-frequency stimulation.

• Incomplete = rapid contraction/relaxation cycles with brief relaxation (tension ~3–4× a twitch)

• complete = relaxation phase disappears, continuous maximal contraction.

Fatigue

a decline in the muscle's ability to produce the desired force despite ongoing stimulation.

Purpose of the supramaximal stimulus (in terms of muscle force)

It's a voltage above threshold that activates all motor units (maximal recruitment). This ensures every muscle fiber contracts, producing maximal force — so any subsequent changes in tension are due to frequency or fatigue, not incomplete recruitment.

Which curve do we replicate by stretching the frog muscle?

The length-tension relationship curve. By stretching the muscle to different sarcomere lengths, you vary the overlap between actin and myosin filaments, which changes the number of cross-bridges that can form and therefore the tension produced. Maximal tension occurs at the optimal length (80–120% of resting length).