Lab 6- Isometric Muscle Contraction

Refractory Period

A period of time after repolarization where the cell is incapable of producing another action potential

How can we measure the activity of muscles?

1. Mechanical

2. Electrical

What is transduction

conversion of one from of energy to another

Why is a force transducer needed

to convert mechanical force into an electrical signal that can be interpreted by the software

What is the stimulator analogy in this experiment?

motor neuron

Where is the stimulator placed?

on the muscle (frog's gastrocnemius) to record activity (similar to a motor neuron)

In this set up, the stimulator and stimulus are analogous to _____ and ____ in the human body.

1. motor neuron

2. action potential

Calibrate the force transducer before starting the experiment, and then remove what?

Remove weight after calibrated

Before stimulating, we need to make sure

1. both electrodes are in contact with the muscle

2. the electrodes do not touch each other

What should we be doing frequently with the frog muscle

moistening frog muscle with ringer solution

What is a typical threshold stimulus

0.5V

Summation

providing additional stimulation while a previous twitch is occuring leads to a larger muscle response

Incomplete (unfused) tetanus

still observing some relaxation between contractions

Complete (fused) tetanus

no relaxation periods shows up as a smooth peak

True or false: A motor unit is made up of a single motor neuron and all of the muscle fibers it innervates

True

When an increased stimulus beyond the threshold is applied to the muscle the phenomena called___________________occurs:

Recruitment

Assuming the stimulus is sufficient to induce an action potential, the time between the beginning of an action potential and the initial increase in tension is called the

Latent phase

During crossbridge formation...

A) Calcium (Ca2+) binds to troponin

B) Z lines more toward the center of the sarcomere

C) Myosin heads are energized by ATP and

Phosphate

D) All of the above

Which of the following factors will influence muscle tension?

A . Amplitude of motor neuron action potential (NO because all or none)

B. Overlap of thin/thick filaments (yes)

C. Frequency of motor neuron action potential (yes)

D. B and C

E. All of the above

In a state of rigor mortis, the muscles are highly contracted and difficult to manipulate. Why does this phenomenon occur?

A. Excess stimulation from motor neuron

B. No stimulation from motor neuron

C. Lack of ATP

D. Lack of Ca2+

(Rigor mortis happens after death

A - no MN firing while dead

B - Sure, but doesn't explain phenomenon

C- ATP is required to break apart cross bridges from A-M overlap; no ATP, contractions get "stuck"

D - lack of Ca2+ would make A-M)

Astronauts living and working in space face a lot of health challenges, both physically and mentally. Studies have shown that astronauts lose up to a 20 % muscle mass (resulting in loss of muscle strength) on spaceflights lasting five to 11 days. The muscles most affected are the calf muscles, quadriceps, muscles in the back and neck.

What is a possible way to prevent this condition?

-exercise

-Selective Androgen Receptor Modulators (SARMs) are a class of drugs developed to treat conditions like muscle wasting (for example, in HIV) or osteoporosis. Many have structure similar to testosterone, but more selectively activate androgen receptors on muscle and bones, resulting in higher anabolic effects and minimizing potential side-effects like hypogonadism (testicular shrinkage and reduction in sperm production) in men or development of male secondary sex characteristics (e.g. facial hair) in women. They are being used (or abused) as "designer anabolic steroids" by athletes.

What was the smallest voltage required to produce a contraction (the threshold voltage)? What proportion of the fibers in the muscle do you think were contracting to produce this small response?

The smallest voltage required to produce a contraction was 0.35 V. There was a small proportion of fibers in the muscle contracting to produce the small response.

What was the smallest voltage required to produce the maximum (largest) contraction? What proportion of the fibers in the muscle do you think were contracting to produce this maximal response?

The smallest voltage required to produce the maximum contraction was 1.00V. There was a large proportion of fibers in the muscle contracting to produce the small response.

What happened to the number of fibers contracting as you increased the voltage?

As you increase the voltage, more fibers are contracted due to a stronger voltage

The muscle recruits fibers until there are no more fibers to recruit at maximum.

(more and more motor units were recruited as the stimulus amplitude increased)

Describe how the isolated muscle behaved as the interpulse interval was decreased progressively. Explain physiology underlaying this event.

As the stimulus interval between the two pulses is reduced, the two contractions gradually merge into each other. As the two contractions merge (summation), there is a resulting increase in the force of contraction. This occurs due to myosin heads overlapping creating an additive force due to induced stimulus contraction.

Can you describe what happens to the frequency of stimulation when you decrease the interval between the pulses?

As the interpulse interval decrease, the frequency fo stimulation increases.

How does varying the frequency affect contractile force? Which interval caused the greatest contraction?

Varying frequency affects the strength of stimulus. If there is less time between each stimulus, then there is greater contraction as the stimuli do not have enough time to return to baseline.

The interval that caused the greatest contraction is 20 ms which caused a contraction force of 8.19mN.

Tetanus Supramaximal Excitation Voltage (V)

1.50 V

Describe how the isolated muscle behaved as the stimulus interval was further decreased.

With longer stimulus intervals, the individual contractions can still be seen. As the stimulus interval is further decreased, these contractions merge into a continuous contraction known as tetanus. This produces an increase in the force of contraction

In light of the "all or none" law of muscle contraction, how can you explain the graded response?

When multiple muscle fibers are recruited, the summation of their contractile force results in a graded potential. The contraction force from the muscle fibers must meet threshold, otherwise no responses will occur, which is why the contraction would be labeled "all or none".

Define tetanus (0.5 pt). At which stimulus interval did you observe tetanus? (0.5 pt) Explain the mechanism behind this phenomenon. (1 pt)

Tetanus can be seen when stimuli occur at a fast rate, leading to a muscle contraction that is being sustained by multiple stimuli. The stimuli are occurring so fast there is no time for the stimuli to return to baseline. The mechanism behind this phenomenon are the myosin heads in the muscle overlapping.

Provide a possible mechanism for why the muscle was unable to maintain a prolonged contraction in the fatigue experiment.

While the muscle sustains muscle contraction, the stimuli are occurring so rapidly that the stimuli never return to baseline. Muscle fatigue occurs because of a weakening in strength as the stimuli continues for a prolonged period of time. Furthermore, during this prolonged contraction with rapid firing action potentials, there is a lack of ATP produced, resulting in the lack of energy to contract. Therefore, muscles are not able to maintain a prolonged contraction.

Does your trace roughly match with a typical length tension curve? If not, could you provide some explanations?

Yes, a muscle contraction resulting in fatigue roughly matches the typical length tension curve. There are 3 notable points on the curve.

A.) too relaxed

B.) optimal physiological length

C.) Too stretched

During fatigue, the muscle becomes too stretched, illustrating the similar rising, plateau, and falling, as seen in the length tension curve.

What stretch resulted in the highest contraction force? What happens to the muscle at the highest stretch levels?

The stretch that resulted in the highest contraction force was the twitch force of 61.33 mN at a length of 10 mm.

As the highest stretch levels are seen, the net force decreases. At 10 mm, the net force is 4.36 mm. This is opposed to the net force at 5 mm with a force of 14.26.

Therefore, as the muscle reaches the highest stretch levels, the muscle contraction force decreases.

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

Arival of action potential from motor neurons

Application of stimulus at threshold

Muscle units produce the same force regardless of their size.

False

It is possible to achieve full tetanus without summation.

False

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

byproduct of glycolysis

The range of the optimal muscle alignment is ___ of resting length.

80-120

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

ATP binding to myosin heads

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

An increase in Ca2+ concentration

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

Isometric contractions

Calibration of the force transducer is necessary to

-Zero force transducer

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

-Gain precise measurements

-Convert mV into mN units