lec 14 gg way

During skeletal muscle shortening, what happens to the lengths of actin and myosin filaments?

Both remain the same length; only their overlap changes.

During a power stroke, in what direction do myosin heads pull the thin filaments?

Inward toward the H zone.

As a sarcomere contracts, what happens to the distance between its Z-discs?

It decreases as the Z-discs move closer together.

What carries the muscle action potential from the sarcolemma deep into the muscle fiber?

T-tubules.

Which receptors in the T-tubule membrane sense voltage changes during excitation-contraction coupling?

Dihydropyridine (DHP) receptors.

What happens to SR calcium-release channels when the T-tubule DHP receptors respond to the action potential?

The SR calcium-release channels open.

When SR calcium-release channels open, where does stored Ca2+ go?

From the sarcoplasmic reticulum into the sarcoplasm.

Which thin-filament regulatory protein directly binds the Ca2+ released during excitation-contraction coupling?

Troponin.

What change follows Ca2+ binding to troponin?

Tropomyosin shifts to expose myosin-binding sites on actin.

If tropomyosin remains over actin's myosin-binding sites, which event is blocked?

Myosin cross-bridge attachment to actin.

What is the immediate direct energy source for skeletal muscle contraction?

ATP.

Which part of the myosin molecule contains the ATPase that supplies energy for cross-bridge cycling?

The myosin head (cross-bridge).

What products form when myosin ATPase hydrolyzes ATP?

ADP and inorganic phosphate (Pi).

What does the energy released by myosin ATP hydrolysis do to the cross-bridge?

Energizes the myosin head.

Which cross-bridge movement directly pulls thin filaments during contraction?

The myosin power stroke.

What directly causes a myosin head to detach from actin after the power stroke?

Binding of a new ATP molecule to myosin.

If no ATP is available after a power stroke, what happens to the actin-myosin connection?

The cross-bridge cannot detach.

According to Lecture 14 slide 9, how many ATP molecules does each cycle require?

Two ATP molecules, as stated on the slide.

Which enzyme terminates the acetylcholine signal at the neuromuscular junction?

Acetylcholinesterase (AChE).

Toward which compartment does acetylcholinesterase face at the neuromuscular junction?

The extracellular synaptic cleft.

How is Ca2+ removed from the sarcoplasm during muscle relaxation?

Active transport pumps return it to the sarcoplasmic reticulum.

Which SR protein binds and stores Ca2+ after it is pumped back during relaxation?

Calsequestrin.

In relaxed skeletal muscle, how does sarcoplasmic Ca2+ concentration compare with SR Ca2+ concentration?

It is 10,000 times lower in the sarcoplasm.

What missing molecule prevents actin-myosin detachment in rigor mortis?

ATP.

According to Lecture 14, how many hours after death does rigor mortis begin?

Approximately 3 to 4 hours.

According to Lecture 14, about how long does rigor mortis last?

Approximately 24 hours.

What ultimately ends rigor mortis according to the lecture notes?

Disintegration of the muscle tissues.

When blood Ca2+ becomes abnormally low, what happens to neuronal Na+ permeability?

It increases.

What effect does hypocalcemia have on nervous-system excitability?

It causes hyperexcitability.

Why do neuronal Na+ channels open more easily during hypocalcemia?

Fewer surface-bound Ca2+ ions reduce the membrane surface-charge difference.

In severe hypocalcemia, sustained contraction of which muscles can interfere with breathing?

Respiratory muscles.

Under the all-or-none principle, how do sarcomeres within one activated muscle fiber respond?

All sarcomeres shorten simultaneously.

Why can an entire skeletal muscle generate graded force despite all-or-none individual fibers?

Different numbers of muscle fibers can be activated.

How many muscle action potentials trigger a single muscle twitch?

One.

What does a myogram record during a skeletal muscle twitch?

Muscle tension over time.

What is the approximate duration of the latent period in the professor's twitch myogram?

2 milliseconds.

What key Ca2+ event occurs during the latent period before measurable tension develops?

Ca2+ is released from the sarcoplasmic reticulum.

What duration range is given for the contraction period of a twitch?

10 to 100 milliseconds.

What duration range is given for the relaxation period of a twitch?

10 to 100 milliseconds.

During the relaxation phase of a twitch, where is Ca2+ transported?

Back into the sarcoplasmic reticulum.

Why can a second stimulus fail to initiate another action potential during the refractory period?

Voltage-gated Na+ channels are inactivated.

Which has the longer refractory period: skeletal muscle or cardiac muscle?

Cardiac muscle.

What physiological signal does an electromyogram detect from contracting skeletal muscle?

Small electrical potentials produced by muscle activity.

Where are surface EMG electrodes placed to detect skeletal muscle electrical activity?

On the skin over the muscle.

How can an EMG activity index reveal the effect of a drug or fatigue?

Compare the same muscle's activity index under different conditions.

According to the Lecture 14 twitch slide, what muscle characteristics help explain different twitch durations?

Fiber type, enzymes, and metabolism.

At what filament arrangement does a muscle fiber develop its greatest tension?

Optimal overlap of thick and thin filaments.

Why does optimal thick-thin filament overlap produce high muscle tension?

It permits the greatest effective number of cross-bridges to form.

A muscle's first few contractions grow progressively stronger even though stimuli are far apart. What pattern is this?

Treppe (staircase effect).

In treppe, how does the spacing of successive stimuli compare with wave summation?

Stimuli are too far apart to produce wave summation.

What happens to contraction strength when another stimulus arrives before a muscle fiber has completely relaxed?

It increases through wave (temporal) summation.

When must a second stimulus arrive to produce wave summation?

Before the preceding contraction has completely relaxed.

Why can repeated stimulation produce tetanic contraction?

Additional Ca2+ is released while Ca2+ from earlier stimuli remains.

In which form of tetanus does the muscle partially relax between stimuli?

Incomplete (unfused) tetanus.

In which form of tetanus is there no partial relaxation between stimuli?

Complete (fused) tetanus.

Which stimulation pattern characterizes most voluntary muscle contractions according to the lecture?

Tetanic contractions.

What is the process called when a muscle activates additional motor units to increase force?

Recruitment (multiple motor unit summation).

How does recruitment affect the smoothness of muscle contraction?

It makes contraction smoother rather than jerky.

According to the professor, what does recruitment help prevent?

Muscle fatigue.

What energy-related depletion contributes to muscle fatigue?

Depletion of metabolic reserves, especially ATP.

What change in acidity contributes to muscle fatigue in the lecture notes?

A decrease in pH, associated with excess lactic acid.

If only a few muscle fibers are recruited, how does whole-muscle force compare with recruiting many fibers?

It is lower.

During an isotonic contraction, what happens to muscle tension as the muscle changes length?

Tension remains constant.

Which feature of muscle length distinguishes isotonic from isometric contraction?

Muscle length changes during isotonic contraction.

What happens to muscle length during a concentric isotonic contraction?

It shortens.

What happens to muscle length during an eccentric isotonic contraction?

It lengthens while contracting.

What happens to muscle length during an isometric contraction?

It remains constant while tension develops.

What happens to joint movement during an isometric contraction?

There is no movement at the affected joint.

What variable is held at a constant preset value during isokinetic exercise?

Speed of contraction.

An isokinetic bicycle is set to 90 revolutions per minute. What is the maximum permitted pedaling speed?

90 revolutions per minute.

What is the pattern of muscle-fiber activity during normal resting muscle tone?

A few fibers contract while most remain relaxed.

Which everyday function depends on sustained partial contraction from muscle tone?

Maintaining posture.

What term describes decreased or absent muscle tone with flaccidity?

Hypotonia.

How do stretch reflexes change in spastic hypertonia?

They increase.

How do stretch reflexes change in rigid hypertonia?

They remain unaffected.

Which kind of atrophy results when muscles are not used?

Disuse atrophy.

Which kind of atrophy results from severed or damaged nerve supply?

Denervation atrophy.

During muscular hypertrophy, which muscle-fiber dimension increases rather than fiber number?

Diameter of existing muscle fibers.

Which energy-producing organelles become more numerous during muscle hypertrophy?

Mitochondria.