Learn: Muscle Anatomy and Physiology: Skeletal Muscle Functions, Structure, and Contraction Mechanisms

What are the general functions of skeletal muscles?

Permit movement; produce heat; stabilize joints; and maintain posture.

What percentage of body weight is made up of muscle?

Muscles make up approximately 40-50% of body weight.

How many muscles does the human body have?

Over 600 muscles.

What characteristic allows muscles to respond to nervous and endocrine signals?

Excitability.

What does excitability mean in muscle tissue?

The ability of muscle to respond to stimulation from nervous and endocrine signals.

What do muscles do to produce movement?

Muscles create tension by pulling on structures; they do not push.

Are most skeletal muscle contractions voluntary?

Yes, although some contractions occur through reflexes.

What does it mean that muscles are extensible?

Muscles can be stretched, strengthened, lengthened, or shortened.

Why is a muscle considered an organ?

A muscle is an organ because it contains multiple tissue types.

What is a muscle fiber?

A muscle fiber is an individual muscle cell.

What is the basic organization of skeletal muscle?

Muscle fibers are grouped into fascicles, and fascicles are bundled together to form the whole muscle.

What is endomysium?

A fine connective tissue layer that surrounds individual muscle fibers.

What is a fascicle?

A bundle of muscle fibers.

What is perimysium?

A thick connective tissue layer that surrounds fascicles.

What is epimysium?

A coarse connective tissue sheath that surrounds the entire muscle.

What is the order of skeletal muscle organization from smallest to largest?

Muscle fiber → fascicle → whole muscle.

What connects muscle to bone?

A tendon.

How can the connective tissue layers of a muscle contribute to a tendon?

The epimysium, perimysium, and endomysium can join with fibrous tissue extending from the muscle to form a tendon.

What is an aponeurosis?

A broad, flat sheet of connective tissue.

What is fascia?

Fibrous connective tissue that surrounds and supports muscle and lies outside the epimysium and tendon.

What is the plantar fascia?

An aponeurosis of the foot.

How are skeletal muscle fibers formed?

Individual muscle cells fuse together during development to form long, thread-like multinucleated cells.

What is the sarcolemma?

The cell membrane of a muscle fiber; it is equivalent to the plasma membrane of a typical cell.

What is the function of the sarcolemma?

It surrounds the muscle fiber and allows electrical signals to travel along the cell.

What is sarcoplasm?

The cytoplasm of a muscle cell.

What is the sarcoplasmic reticulum?

A network of tubules and sacs that temporarily stores calcium ions used in muscle contraction.

What is the function of the sarcoplasmic reticulum during contraction?

It releases calcium ions that help initiate muscle contraction.

What are T-tubules?

Inward extensions of the sarcolemma that carry electrical signals deep into the muscle fiber.

Why are T-tubules important?

They allow electrical signals to travel deep into the muscle fiber and trigger calcium release.

What are myofibrils?

Long structures that extend along the length of a skeletal muscle fiber and contain the machinery responsible for contraction.

What are myofilaments?

The protein filaments involved in muscle contraction; they are classified as thick or thin filaments.

What is the basic contractile unit of a muscle cell?

The sarcomere.

What are the two major types of myofilaments?

Thick filaments and thin filaments.

What protein makes up the main component of thick filaments?

Myosin.

What protein makes up the main component of thin filaments?

Actin.

What is the function of the myosin heads?

Myosin heads bind to actin and pull on the thin filaments during contraction.

What does actin contain?

Myosin-binding sites.

What is tropomyosin?

A regulatory protein that lies along the actin filament and covers myosin-binding sites when the muscle is at rest.

What is troponin?

A regulatory protein associated with the thin filament that binds calcium ions.

What happens when calcium binds to troponin?

Troponin changes shape, causing tropomyosin to shift and expose the myosin-binding sites on actin.

What is a neuromuscular junction?

The site where a motor neuron communicates with a skeletal muscle fiber at the motor end plate.

What neurotransmitter is released at the neuromuscular junction?

Acetylcholine (ACh).

What do vesicles at the end of a motor neuron contain?

Neurotransmitters, including acetylcholine.

What happens when acetylcholine reaches the sarcolemma?

ACh binds to receptors on the sarcolemma and produces an electrical impulse.

What happens when acetylcholine binds to its receptors?

Sodium ion gates open and an excitation/depolarization wave is produced.

What is depolarization?

The electrical activation of the muscle cell membrane following stimulation.

Where does the depolarization wave travel?

Across the sarcolemma and down into the T-tubules.

What does the impulse traveling through the T-tubules trigger?

The release of calcium ions from the adjacent sarcoplasmic reticulum.

What does calcium bind to during muscle contraction?

Troponin on the thin filament.

What happens to tropomyosin when calcium binds to troponin?

Tropomyosin shifts away from the myosin-binding sites on actin.

What happens after the myosin-binding sites on actin are exposed?

Myosin heads bind to actin.

What is the sliding filament mechanism?

The mechanism in which thin actin filaments slide toward the center of each sarcomere as myosin pulls on them, shortening the muscle fiber.

Do actin and myosin themselves become shorter during contraction?

No. The filaments slide relative to one another; their individual lengths do not change.

What causes the sarcomere to shorten?

Myosin heads pull the thin actin filaments toward the center of the sarcomere.

What are Z-discs?

Structures that form the boundaries between adjacent sarcomeres.

What is the power stroke?

The movement in which a myosin head pivots and pulls the actin filament toward the center of the sarcomere.

What is required for the myosin head to perform the power stroke?

Energy supplied by ATP.

What is the immediate energy source for muscle contraction?

ATP.

What happens to ATP to release energy for muscle contraction?

ATP is broken down into ADP and phosphate.

How long does the body's stored ATP supply support muscle contraction?

Only for a few seconds.

How is ATP regenerated after it is used?

ADP is phosphorylated by adding phosphate back to it.

What is phosphorylation?

The process of adding phosphate to ADP to regenerate ATP.

What is creatine phosphate?

A rapid energy source that helps regenerate ATP and provides energy for a few additional seconds of contraction.

What is glycolysis?

An anaerobic pathway in which glucose is broken down into pyruvate and produces ATP.

How much ATP does glycolysis produce according to the slides?

About 2 ATP.

What happens to pyruvate during aerobic respiration?

It enters the aerobic pathway involving oxidative phosphorylation to produce large amounts of ATP.

What is oxidative phosphorylation?

The aerobic pathway that produces a large amount of ATP from the products of glucose breakdown.

How much ATP does oxidative phosphorylation produce according to the slides?

Approximately 36 ATP.

What happens to pyruvate when oxygen is unavailable?

Pyruvate is converted to lactate, the dissolved form of lactic acid.

What is myoglobin?

An oxygen-storage molecule found in muscle fibers that has a red color.

What is the function of myoglobin?

It stores oxygen within muscle fibers.

In what form is glucose stored in muscle?

Glycogen.

What happens to glycogen when energy is needed?

Glycogen can be converted back to glucose, which can then undergo glycolysis to produce ATP.

What is the aerobic pathway of respiration?

A pathway that uses oxygen and produces the maximal amount of energy available from each glucose molecule.

What is the anaerobic pathway of respiration?

A pathway that produces ATP without relying on oxygen and produces a relatively small amount of ATP.

How does anaerobic respiration compare with aerobic respiration in ATP production?

Anaerobic respiration produces much less ATP.

What is an advantage of anaerobic respiration?

It produces ATP more quickly and supports powerful, short-lived muscle contractions.

When can anaerobic processes supplement aerobic processes?

When oxygen availability becomes insufficient during activity.

What happens when anaerobic processes supplement aerobic respiration?

Pyruvate is converted to lactate.

What is oxygen debt?

A lack of oxygen in some tissues following heavy exercise that is associated with excessive lactic acid production and the need for additional oxygen afterward.

What can happen when oxygen delivery to muscle tissue is reduced?

Myoglobin oxygen stores can become depleted and anaerobic metabolism can increase, leading to lactate production.

What is a motor neuron?

A nerve cell that controls muscle fibers.

What is a motor unit?

One motor neuron plus all of the muscle fibers to which it attaches.

How does the number of muscle fibers in a motor unit affect movement?

Fewer muscle fibers per motor unit allow more finely coordinated movement.

What type of motor unit provides finer control?

A motor unit containing fewer muscle fibers.

Where is cardiac muscle found?

Only in the heart.

How does cardiac muscle contract?

It contracts rhythmically and continuously.

Is cardiac muscle normally under voluntary control?

No, cardiac muscle is normally involuntary.

What are intercalated discs?

Strong junctions between cardiac muscle cells that also allow electrical coupling between cells.

Why are intercalated discs important?

They allow electrical impulses to travel from one cardiac muscle cell to another and help the cells contract together.

What is unique about cardiac muscle excitation?

Cardiac muscle is self-exciting and can generate a continual rhythm of excitation and contraction.

How are cardiac muscle cells connected?

They branch and connect with one another through specialized junctions including intercalated discs.

Does cardiac muscle require a motor neuron to initiate every contraction?

No. Cardiac muscle is self-exciting and can generate its own rhythm.

What is smooth muscle?

Muscle composed of small, tapered cells with large, single nuclei.

Does smooth muscle have striations?

No. Smooth muscle lacks the striations seen in skeletal and cardiac muscle.

What is the major function of smooth muscle?

It controls movement of substances through hollow organs and regulates the diameter of tubes such as blood vessels and airways.

Where is smooth muscle found?

In structures such as airways, blood vessels, bladder, intestines, and uterus.

What are the two major types of smooth muscle described in the slides?

Single-unit (visceral) smooth muscle and multiunit smooth muscle.

What is single-unit or visceral smooth muscle?

The most common type of smooth muscle; it is organized into sheets and can contract rhythmically as a unit.

What is an example of the function of visceral smooth muscle?

Peristalsis, which moves substances through hollow organs.