4th, 2+

What three things determine the duration of a muscle contraction?

The duration of the stimulus at the neuromuscular junction, the presence of calcium (Ca²⁺) in the sarcoplasm, and the availability of ATP.

What three things are required for muscle relaxation?

Stop the stimulus at the neuromuscular junction, remove calcium by transporting it back into the sarcoplasmic reticulum (SR), and have ATP available.

What removes acetylcholine (ACh) from the neuromuscular junction?

Acetylcholinesterase.

Where is calcium transported during muscle relaxation?

Back into the sarcoplasmic reticulum (SR).

Why is ATP needed during muscle relaxation?

ATP actively transports calcium back into the SR.

Why does muscle contraction require large amounts of ATP?

ATP is needed for myosin activation/deactivation during contraction and for pumping calcium back into the SR during relaxation.

What are the three major sources of energy for muscle contraction?

ATP, creatine phosphate (CP), and glycogen.

Where is ATP made?

In the mitochondria.

Where is ATP located in the muscle fiber?

Free in the sarcoplasm.

How long does stored ATP last during sustained activity?

About 2 seconds.

Why is creatine phosphate (CP) important?

It stores energy that can quickly regenerate ATP.

What happens when ATP transfers its energy to creatine?

ATP becomes ADP, and creatine becomes creatine phosphate (CP).

What is glycogen?

The storage form of glucose.

What are the two pathways glucose can use to produce ATP?

Aerobic respiration (36 ATP) and anaerobic respiration (2 ATP).

What happens during resting activity?

Mitochondria produce ATP using fatty acids or glucose, excess ATP is converted to creatine phosphate, and excess glucose is stored as glycogen.

What happens during moderate activity?

ATP demand is met by the mitochondria because enough oxygen is available, and no excess ATP is produced.

What happens during intense activity?

Oxygen cannot meet the mitochondria’s demand, so ATP is mainly produced by anaerobic respiration.

Why is anaerobic respiration used during intense exercise?

Because the mitochondria do not receive enough oxygen to meet ATP demands.

What is the sequence of muscle shortening during contraction?

Sarcomere shortens → Muscle fiber shortens → Whole muscle shortens → Tendon pulls on the bone.

What must happen before an object can be moved?

Enough tension must be produced to overcome the object’s resistance.

What determines the amount of muscle tension produced?

The number of pivoting myosin cross-bridges.

What is the length-tension relationship?

The relationship between the length of a sarcomere and the tension it produces.

What part of the sarcomere is most important for producing tension?

The overlap between actin and myosin.

What happens if there is no overlap between actin and myosin?

No cross-bridges can form, so no tension is produced.

What happens if the Z-lines are too close together?

The filaments cannot move properly, so less tension is produced.

What is the bouton (axon terminal)?

The end of the motor neuron where voltage-gated calcium channels open, allowing calcium to enter.

What is the synaptic cleft?

The small gap between the neuron and the muscle cell where acetylcholine (ACh) is released.

What is acetylcholine (ACh)?

A neurotransmitter that carries signals from a neuron to another cell.

How can the duration of a muscle contraction be increased?

By repeated stimulation.

What effect does repeated stimulation have on muscle tension?

It increases muscle tension.

What is a twitch?

A single stimulus-contraction-relaxation cycle in one muscle fiber.

Does a twitch produce tension?

Yes, but only a small amount.

What are the three phases of a twitch?

Resting phase, contraction phase, and relaxation phase.

What happens during the resting phase of a twitch?

Calcium is released after stimulation.

What happens during the contraction phase of a twitch?

Cross-bridge cycling occurs, producing the highest tension.

What happens during the relaxation phase of a twitch?

Calcium is pumped back into the SR, actin is covered again, and cross-bridges detach.

What is treppe?

Repeated stimulation immediately after the relaxation phase.

Is treppe caused by a single stimulus?

No, it is caused by repeated stimuli.

Why does tension increase during treppe?

Because calcium is not completely removed before the next stimulus.

Does tension continue increasing forever during treppe?

No, it rises only to a certain point.

What is summation?

When another stimulus arrives before the muscle has completely relaxed.

Why does summation produce greater tension?

Because more calcium remains in the sarcoplasm.

Why are more cross-bridges formed during summation?

Because calcium is still available.

What is incomplete tetanus?

Repeated stimuli during the relaxation phase that produce maximum but shaky contractions.

Does the muscle completely relax during incomplete tetanus?

No.

What type of contraction occurs during incomplete tetanus?

A shaky (unfused) contraction.

What is complete tetanus?

A continuous, smooth contraction caused by very rapid repeated stimuli.

Why does complete tetanus occur?

Because the sarcoplasmic reticulum cannot reclaim calcium between stimuli.

What happens to calcium during complete tetanus?

Calcium remains in the sarcoplasm continuously.

What type of tension is produced during complete tetanus?

Continuous, smooth, maximum tension.

Why do muscles contract?

Because they receive signals from neurons.

What is a motor unit?

A motor neuron and all the muscle fibers it controls.

What does a small motor unit provide?

Fine motor control.

How many muscle fibers are in most motor units?

Thousands of muscle fibers per neuron.

Are all muscle fibers activated at the same time?

No. Only the number needed for the movement are activated.

What is recruitment?

The activation of additional motor units as more force or tension is needed.