Muscle Types, Functions, and Anatomy: A Comprehensive Review

What are the three basic types of muscle found in the body?

Skeletal muscle, cardiac muscle, and smooth muscle.

What is the primary function of muscles?

Muscles are responsible for all types of body movement.

What do the prefixes 'myo-' and 'mys-' refer to?

'Myo-' and 'mys-' refer to 'muscle.'

What does the prefix 'sarco-' refer to?

'Sarco-' refers to 'flesh.'

What shape are skeletal muscle cells?

Skeletal muscle cells are large, cigar-shaped, and multinucleate.

Why are skeletal muscles also known as striated muscles?

They are called striated muscles because of their obvious stripes.

What type of control do skeletal muscles have?

Skeletal muscles are voluntary muscles, subject to conscious control.

What is the function of the endomysium?

The endomysium encloses a single muscle fiber.

What does the perimysium wrap around?

The perimysium wraps around a fascicle (bundle) of muscle fibers.

What is the role of the epimysium?

The epimysium covers the entire skeletal muscle.

What are tendons primarily made of?

Tendons are mostly made of collagen fibers.

What is the difference between tendons and aponeuroses?

Tendons are cordlike structures, while aponeuroses are sheetlike structures that attach muscles indirectly to bones or connective tissue.

What are the characteristics of smooth muscle?

Smooth muscle has no striations, is involuntary, and is found mainly in the walls of hollow visceral organs.

What shape are smooth muscle fibers?

Smooth muscle fibers are spindle-shaped and uninucleate.

What are the characteristics of cardiac muscle?

Cardiac muscle has striations, is involuntary, and is found only in the walls of the heart.

What are intercalated discs?

Intercalated discs are gap junctions that join branching cells in cardiac muscle.

What are the three additional roles of skeletal muscle besides producing movement?

Skeletal muscle helps maintain posture and body position, stabilize joints, and generate heat.

Sarcolemma

specialized plasma membrane

Myofibrils

long organelles inside muscle cell

I band

light band that contains only thin filaments

Z disc

midline interruption in the I band

A band

dark band that contains the entire length of the thick filaments

H zone

lighter central area within the A band that lacks actin filaments

M line

center of the H zone

Sarcomere

contractile unit of a muscle fiber

Thick filaments

myosin filaments composed mostly of the protein myosin

Thin filaments

actin filaments composed mostly of the contractile protein actin

Sarcoplasmic reticulum (S R)

specialized smooth endoplasmic reticulum that surrounds the myofibril and stores/releases calcium

Irritability

ability to receive and respond to a stimulus

Contractility

ability to forcibly shorten when an adequate stimulus is received

Extensibility

ability of muscle cells to be stretched

Elasticity

ability to recoil and resume resting length after stretching

Motor unit

one motor neuron and all the skeletal muscle cells stimulated by that neuron

Neuromuscular junction

association site of axon terminal of the motor neuron and sarcolemma of a muscle

Neurotransmitter

chemical released by nerve upon arrival of nerve impulse in the axon terminal

Acetylcholine (A C h)

the neurotransmitter that stimulates skeletal muscle

Synaptic cleft

gap between nerve and muscle filled with interstitial fluid

Acetylcholinesterase (A C h E)

enzyme that breaks down acetylcholine into acetic acid and choline, ending muscle contraction

What initiates the events at the neuromuscular junction?

A nerve impulse reaches the axon terminal of the motor neuron.

What happens when calcium channels open at the axon terminal?

Calcium ions enter the axon terminal.

What is released from synaptic vesicles in response to calcium ion entry?

Acetylcholine (ACh) is released.

What occurs after acetylcholine diffuses across the synaptic cleft?

It attaches to receptors on the sarcolemma of the muscle cell.

What effect does acetylcholine have on the sarcolemma if enough is released?

The sarcolemma becomes temporarily more permeable to sodium ions.

What imbalance is created when sodium ions enter the muscle cell?

The interior of the cell becomes more positive, leading to depolarization.

What happens after depolarization at the neuromuscular junction?

More sodium channels open, allowing sodium ions to enter and creating an action potential.

What is the result of the action potential generated in the muscle cell?

It conducts the electrical impulse from one end of the cell to the other.

What role does acetylcholinesterase (AChE) play in muscle contraction?

AChE breaks down acetylcholine, ending muscle contraction.

What happens to the muscle cell after a single nerve impulse?

It produces only one contraction and returns to its resting state.

How does the muscle cell return to its resting state?

Potassium ions diffuse out of the cell, and the sodium-potassium pump restores original ion positions.

What role do calcium ions play in muscle contraction?

Calcium ions bind to regulatory proteins on thin filaments, exposing myosin-binding sites.

What is the function of ATP in muscle contraction?

ATP provides the energy for the sliding process of filaments during contraction.

What happens to the sarcomere during muscle contraction?

Thin filaments slide toward the center of the sarcomere, causing the cell to shorten.

What is a cross bridge in muscle contraction?

A cross bridge is the attachment of myosin heads to actin filaments during contraction.

What occurs when an action potential sweeps along the sarcolemma?

Calcium ions are released from the sarcoplasmic reticulum, triggering muscle contraction.

What is the H zone in a sarcomere?

The H zone is the area in the center of the A band that widens during relaxation and disappears during contraction.

What are graded responses in muscle contraction?

Graded responses are different degrees of skeletal muscle shortening based on stimulation frequency and number of fibers activated.

What is meant by 'all-or-none' contraction in muscle fibers?

A muscle fiber will contract to its fullest extent when adequately stimulated, but not partially.

What is a muscle twitch?

A muscle twitch is a single, brief, jerky contraction that is not a normal muscle function.

What is unfused (incomplete) tetanus?

Unfused tetanus occurs when muscle contractions are summed together, resulting in stronger and smoother contractions.

What is fused (complete) tetanus?

Fused tetanus is achieved when muscle stimulation is so rapid that no relaxation occurs, resulting in smooth, sustained contractions.

How does muscle force depend on the number of fibers stimulated?

Greater muscle tension results from the contraction of more fibers.

What happens when all motor units are active and stimulated?

The muscle contraction reaches its maximum strength.

What is the role of myosin heads during contraction?

Myosin heads attach to actin, pivot toward the center of the sarcomere, and pull thin filaments during contraction.

How does the muscle respond to increasingly rapid stimulation?

Muscle contractions become stronger and smoother as stimulation frequency increases.

What initiates the exposure of myosin-binding sites on actin?

Calcium binding to regulatory proteins on actin filaments changes their shape and position.

What is the significance of the sarcoplasmic reticulum in muscle contraction?

The sarcoplasmic reticulum stores calcium ions, which are released to trigger contraction.

What is the relationship between actin and myosin during relaxation?

In relaxation, regulatory proteins prevent myosin from binding to actin.

What does the term 'myosin attachment' refer to?

Myosin attachment refers to the binding of myosin heads to exposed sites on actin during contraction.

What is the effect of muscle fiber contraction on the whole muscle?

Not all muscle fibers may contract simultaneously, leading to varying muscle responses.

What is the only energy source that can directly power muscle contraction?

ATP (adenosine triphosphate)

What happens to ATP stored in muscle fibers?

It is stored in small amounts that are quickly used up.

What are the three ways to regenerate ATP?

1. Direct phosphorylation of ADP by creatine phosphate, 2. Aerobic pathway, 3. Anaerobic glycolysis and lactic acid formation.

What is the fastest method to regenerate ATP?

Direct phosphorylation of ADP by creatine phosphate (CP).

How long do creatine phosphate supplies last?

Less than 15 seconds.

How much ATP is produced per creatine phosphate molecule?

1 ATP.

What is ADP and its role in metabolism?

ADP (adenosine diphosphate) is crucial for energy transfer, converting to ATP during metabolic reactions.

What is aerobic respiration and when does it occur?

It supplies ATP at rest and during light/moderate exercise through oxidative phosphorylation in the mitochondria.

How much ATP is released during aerobic respiration?

About 32 ATP.

What is anaerobic glycolysis?

A reaction that breaks down glucose without oxygen to produce about 2 ATP.

What happens to pyruvic acid in anaerobic glycolysis?

It is converted to lactic acid.

What are some suspected factors contributing to muscle fatigue?

Ion imbalances, oxygen deficit, lactic acid accumulation, and decrease in ATP supply.

What is an isotonic contraction?

A contraction where myofilaments slide past each other, shortening the muscle and causing movement.

Give an example of isotonic contraction.

Bending the knee, lifting weights, or smiling.

What is an isometric contraction?

A contraction where muscle filaments try to slide but are pitted against an immovable object, increasing tension without shortening.

Give an example of isometric contraction.

Pushing your palms together in front of you.

What is muscle tone?

A state of continuous partial contractions that keeps muscles firm and ready for action.

How does exercise affect muscle size and strength?

Exercise increases muscle size, strength, and endurance.

What type of exercise results in stronger, more flexible muscles?

Aerobic (endurance) exercise, such as biking or jogging.

What is the effect of resistance (isometric) exercise?

It increases muscle size and strength.

What happens to individual muscle fibers during resistance exercise?

They enlarge.