a&p unit 3 exam

The physiological properties that all muscle types have in common:

Excitability, Conductivity, Contractility, Extensibility, and Elasticity.

Excitability

The ability to receive and respond to stimuli. When stimulated by chemical signals or stretch, muscle cells respond with electrical changes across the plasma membrane.

Conductivity

Local electrical excitation sets off a wave that travels rapidly along the muscle fiber, initiating contraction.

Contractability

The ability to shorten forcibly. Muscle cells uniquely shorten substantially when stimulated, enabling them to pull on bones and organs to create movement.

Extensibility

The ability to be stretched or extended. Skeletal muscle cells can stretch up to three times their contracted length between contractions.

Elasticity

The ability to recoil and resume original resting length. Without this recoil, resting muscles would be too slack and wouldn’t contract strongly when stimulated.

What does striated mean in skeletal muscle?

The fibers have a striped appearance caused by actin and myosin filaments arranged in overlapping patterns, creating light and dark bands.

Why do skeletal muscle fibers look striated?

Because of the alternating arrangement of internal contractile proteins (actin and myosin).

What does voluntary mean in skeletal muscle?

It is controlled by conscious effort (you choose when to move it).

How is skeletal muscle different from smooth and cardiac muscle in terms of control?

Skeletal muscle is voluntary, while smooth and cardiac muscles are involuntary.

What does it mean that skeletal muscle cells are elongated and multinucleated?

The cells are long (called muscle fibers) and contain multiple nuclei.

Where are the nuclei located in skeletal muscle fibers?

Around the periphery of the cell near the plasma membrane.

How long can a single skeletal muscle fiber be?

It can extend from one end of the muscle to the other.

How does skeletal muscle contract compared to other muscle types?

It contracts rapidly but gets tired easily.

Where is skeletal muscle typically attached?

To one or more bones.

What are the main functions of skeletal muscle?

Movement, maintaining posture, stabilizing joints, and producing heat.

What is the role of connective tissue in muscle structure?

It wraps the muscle at every level and helps organize and support it.

What does the epimysium surround?

The entire muscle.

What does the perimysium surround?

Groups of muscle fibers called fascicles.

What does the endomysium surround?

Each individual muscle fiber.

How are connective tissue layers related to tendons?

They merge together to form tendons, which connect muscle to bone.

How does connective tissue help protect muscles from injury?

Collagen resists overstretching when the muscle lengthens, preventing damage.

Is collagen excitable or contractile?

No, it does not respond to signals or contract.

How does connective tissue help muscles return to normal length?

Elastic recoil of collagen pulls the muscle back to its resting length after it relaxes.

What happens if muscles didn’t have elastic recoil?

They would be too loose (flaccid) and wouldn’t contract effectively.

How does connective tissue contribute to power output?

Tendons and collagen store and release energy, helping muscles produce stronger, more efficient movements.

What is an example of connective tissue helping with movement?

The calcaneal (Achilles) tendon recoils to help lift the heel when running.

What is the debate about connective tissue’s role in power?

Some scientists believe its contribution is small and that most power comes from proteins inside muscle fibers.

What is the sarcolemma?

The muscle cell membrane. It has T tubules that carry signals into the cell to tell the sarcoplasmic reticulum (SR) when to release calcium.

What is the sarcoplasm?

The cytoplasm of a muscle cell. It contains glycogen (energy) and myoglobin (oxygen storage).

What are myofibrils?

Long, rod-like structures that fill most of the muscle cell. They are made of bundles of myofilaments and run the entire length of the cell.

What is special about nuclei in skeletal muscle?

There are many nuclei (30–80 per mm), located at the edge of the cell, formed by fused cells, and they help repair muscle damage.

What is the sarcoplasmic reticulum (SR)?

A network around myofibrils that stores calcium and releases it to trigger contraction.

What are T tubules?

Tubes from the sarcolemma that go deep into the cell and signal the SR to release calcium.

What is the role of mitochondria in muscle cells?

They produce ATP and are packed between myofibrils to supply energy for contraction.

What is myosin (thick filament)?

A contractile protein with heads that break down ATP and form cross-bridges with actin to pull and shorten muscle.

What happens when a cross-bridge forms?

A myosin head binds to an active site on actin to create movement.

What is actin (thin filament)?

A protein made of G actin units with active sites where myosin binds to cause contraction.

What is the function of tropomyosin?

It blocks actin’s active sites in a relaxed muscle, preventing contraction.

What is the function of troponin?

It binds calcium, causing tropomyosin to move and expose actin’s active sites so contraction can happen.

What is titin?

A spring-like protein that anchors myosin, prevents overstretching, and helps the muscle recoil.

What is dystrophin?

A protein that connects actin to the sarcolemma and helps transfer force to the whole muscle; defects cause muscular dystrophy.

What is the A band?

The dark band with thick filaments; it does not change length during contraction.

What is the I band?

The light band with only thin filaments; it gets shorter during contraction.

What is the H zone?

The lighter center of the A band with only thick filaments; it shrinks or disappears during contraction.

What is the M line?

The center line that holds thick filaments together.

What is the Z disc?

A structure that anchors thin filaments; when Z discs move closer, the muscle shortens.

What is a sarcomere?

The smallest contractile unit of muscle, from Z disc to Z disc, where contraction happens.

How does a muscle fiber shorten?

Sarcomeres shorten, pulling Z discs closer together.

What is the sliding filament model?

Actin and myosin do not change length; instead, actin slides over myosin, shortening the sarcomere.

What happens to bands during contraction?

A band = stays the same

I band = shortens

H zone = shrinks/disappears

What is a motor unit?

One motor neuron and all the muscle fibers it controls.

Why do muscle fibers in a motor unit act together?

Because one neuron sends signals to all of them, so they contract at the same time.

Can you activate only some fibers in a motor unit?

No, all fibers in a motor unit contract together (all-or-none within that unit).

How does a nerve signal cause contraction in a motor unit?

The signal travels down the neuron and spreads to all branches, causing all connected muscle fibers to contract together.

Why are multiple motor units needed for strong contraction?

Because one motor unit alone isn’t enough — many must be activated to produce strong force.

What is a small motor unit?

A motor unit with 3–5 muscle fibers, used for fine control (like eye or finger movement).

What is a large motor unit?

A motor unit with 500–1,000 fibers, used for strength (like leg muscles).

How do motor units prevent muscle fatigue?

They take turns working (work in shifts), so some fibers rest while others contract.

What is the neuromuscular junction (NMJ)?

The connection between a nerve ending and a muscle fiber.

What are the main parts of the NMJ?

• Axon terminal (nerve ending)

• Synaptic cleft (gap)

• Motor end plate (muscle membrane with receptors)

What is the synaptic cleft?

The small gap (60–100 nm) between the neuron and muscle cell.

What happens at the axon terminal?

It releases acetylcholine (ACh) by exocytosis into the synaptic cleft.

Why can’t the electrical signal cross the synaptic cleft directly?

Because there is a gap, so it must be converted into a chemical signal (ACh).

What does ACh do?

It binds to receptors on the muscle cell and starts an electrical signal.

What are ACh receptors and postsynaptic folds?

Receptors on the motor end plate (about 50 million) in folded membrane that increases surface area.

What is acetylcholinesterase (AChE)?

An enzyme that breaks down ACh to stop the signal and allow muscle relaxation.

Why is AChE important?

It prevents continuous contraction by turning off the signal.

What is the basal lamina?

A layer of collagen that surrounds the NMJ and fills the synaptic cleft.

What is the resting membrane potential?

The inside of the cell is negative (~ -90 mV) compared to the outside.

Why is the inside of the cell negative?

Because of negative proteins and more K+ inside and Na+ outside.

What maintains ion balance in the cell?

The sodium-potassium pump.

What is depolarization?

Na+ enters the cell, making the inside more positive.

What is repolarization?

K+ leaves the cell, making the inside negative again.

What is an action potential?

A rapid electrical signal that travels along the membrane.

What does “all-or-none” mean?

The signal either happens fully or not at all (no partial signals).

How does an action potential move?

It triggers the next one along the membrane, creating a wave.

What is excitation-contraction coupling?

The process linking the nerve signal to muscle contraction.

What are the steps of excitation-contraction coupling?

1. Signal travels along sarcolemma

2. Goes down T tubules

3. SR releases Ca²⁺

4. Ca²⁺ binds to troponin

5. Tropomyosin moves

6. Myosin binds actin → contraction

What triggers muscle contraction directly?

Calcium (Ca²⁺) binding to troponin.

How does muscle relaxation occur?

Ca²⁺ is pumped back into the SR, tropomyosin blocks actin, and contraction stops.

What is the length–tension relationship?

The amount of force a muscle produces depends on how stretched or shortened it is before contracting.

What happens if a muscle is too shortened before contraction?

Weak contraction because thick filaments push against Z discs.

What happens if a muscle is too stretched before contraction?

Weak contraction because there is less overlap between actin and myosin.

When does a muscle produce the greatest force?

At its optimal resting length.

What is muscle tone?

A constant, partial contraction that keeps muscles ready to act.

Why is muscle tone important?

It keeps muscles near optimal length for strong contractions.

What is rigor mortis?

Stiffening of muscles starting 3–4 hours after death.

What causes rigor mortis?

Ca²⁺ leaks into cells, causing contraction, but no ATP is available for relaxation.

Why can’t muscles relax during rigor mortis?

Because ATP is no longer produced.

When does rigor mortis peak and fade?

Peaks at ~12 hours, fades over 48–60 hours.

What is a muscle twitch?

A single contraction from one stimulus.

What are the three phases of a muscle twitch?

Lag, contraction, and relaxation.

How does a single muscle fiber respond to stimuli?

It contracts with the same force every time.

How does a whole muscle increase force?

By recruiting more motor units.

What is multiple motor unit summation?

Increasing force by activating more motor units.

What is multiple-wave summation?

Increasing force by increasing the frequency of stimulation.

Why does higher frequency increase force?

More Ca²⁺ stays in the muscle → stronger contraction

What is incomplete tetanus?

Partial relaxation between contractions.

What is complete tetanus?

No relaxation between contractions (continuous contraction).