Chapter 7 Muscle System

What is the etymological origin of the word "muscle" and how many skeletal muscles are there in the human body?

The word "muscle" comes from the Latin word "mus," meaning "little mouse." There are approximately 700 skeletal muscles in the human body.

What are the five primary functions of muscles?

1. Movement of skeleton
2. Maintain posture and body position
3. Support soft tissues
4. Guard entrances and exits
5. Maintain body temperature

Name the three types of muscle tissue.

1. Skeletal
2. Smooth
3. Cardiac

Describe the key characteristics of skeletal muscle tissue.

• Attach to skeleton
• Voluntary
• Long cylindrical shape
• Multinucleated (nuclei near edge)
• Striated
• Can contract rapidly but need rest
• Largest of the three types (up to 24 inches long)

What are skeletal muscle cells called, and what do the terms 'mys' and 'sarkos' refer to?

Skeletal muscle cells are called muscle fibers. 'Mys' refers to muscle, and 'sarkos' refers to flesh.

What components make up a skeletal muscle (organ)?

A skeletal muscle organ contains thousands of muscle cells (skeletal muscle tissue), nervous tissue, blood, and three layers of connective tissue.

List and describe the three connective tissue layers that bundle skeletal muscle fibers.

1. Endomysium: Wraps around individual muscle fiber.
2. Perimysium: A coarser membrane that encloses a bundle of muscle fibers, called a fascicle. It contains collagen and elastic fibers, nerves, and blood vessels.
3. Epimysium: Surrounds the entire muscle, separating it from surrounding tissues and organs. It is made of collagen fibers.

What is fascia in relation to muscle structure?

Fascia encloses the entire muscle and epimysium. It separates individual skeletal muscles and holds them in place.

Differentiate between a tendon and an aponeurosis.

• Tendon: A bundle of collagen fibers where muscle meets bone. All three connective tissue layers taper and blend to form it, intertwining with the periosteum of the bone.
• Aponeurosis: A broad sheet of collagen fibers that attach skeletal muscles to other skeletal muscles, not to bone coverings.

Identify the key components of a skeletal muscle fiber (cell).

• Sarcolemma: The cell membrane.
• Sarcoplasm: The cytoplasm.
• Transverse tubules (T tubules): Form passageways, encircle myofibrils, and help with contractions.
• Sarcoplasmic reticulum (SR): Specialized smooth endoplasmic reticulum that surrounds each myofibril and plays a major role in storing and releasing calcium, essential for muscle contraction.

What are myofibrils and what are they composed of?

Myofibrils are responsible for muscle fiber contraction and make up muscle fibers. They are made of bundles of myofilaments and each contains approximately 10,000 sarcomeres.

Define a sarcomere.

A sarcomere is the smallest functional unit of a muscle cell. Each can contract and they are arranged end-to-end, forming a segment from one Z line to another Z line.

What are the two types of protein myofilaments found in sarcomeres?

1. Actin (thin filaments)
2. Myosin (thick filaments)

Describe actin filaments and their associated regulatory proteins.

Actin filaments are thin filaments located at the ends of the sarcomere. They are made up of the protein actin and also contain the regulatory proteins troponin and tropomyosin. They form the light bands (I bands) and slide toward each other during contraction.

Describe myosin filaments and their unique projections.

Myosin filaments are larger, thicker filaments located in the middle of the sarcomere. They are made up of the protein myosin and have projections called myosin heads or cross-bridges, which link thick and thin filaments during contraction.

Explain how striations (I bands and A bands) are formed in skeletal muscle.

• I bands (light): Contain only thin actin filaments attached to the Z line (midline boundary).
• A bands (dark): Contain thick myosin filaments overlapping with actin filaments. They include the H zone (central region) and the M line (thickening).

Summarize the Sliding Filament Model of muscle contraction.

The Sliding Filament Model explains sarcomere contraction: Sarcomeres shorten when myosin cross-bridges pull on thin (actin) filaments at their binding sites. This cycle requires calcium to initiate and ATP to continue, allowing myosin cross-bridges to attach, pull, release, and reattach to actin. The A-band does not change size during this process.

What neurotransmitter is essential for skeletal muscle contraction?

The neurotransmitter acetylcholine is necessary to stimulate skeletal muscles to contract.

What are the two functional properties of skeletal muscle activity?

1. Irritability: The ability to receive and respond to stimuli.
2. Contractility: The ability to shorten.

Define a motor neuron and a motor unit.

• Motor neuron: A nerve cell that interacts with muscle cells, stimulating one or thousands of muscle fibers.
• Motor unit: Consists of one neuron and all the skeletal muscle cells it stimulates (controls).

Describe the Neuromuscular Junction (NMJ).

The Neuromuscular Junction (NMJ) is the site where a muscle cell and a nerve cell meet. It includes:

• Synaptic cleft: The gap between the neuron and the muscle.
• Motor end plate: The area on the muscle fiber where it meets the neuron, characterized by abundant nuclei and mitochondria and extensive sarcolemma folds.
• Neurotransmitters: Contained in vesicles in the synaptic terminals of the motor neuron's axons, such as Acetylcholine (ACh), which stimulate muscle fibers by changing the sarcolemma's permeability.

What is the "all or none" law as it applies to muscle cells?

The "all or none" law states that a muscle cell contracts totally or not at all; it does not contract partially.

Explain what a muscle twitch is and its three phases.

A muscle twitch is a single, very short contraction resulting from a single stimulus. It consists of:

1. Latent period: A delay between the stimulus application and the muscle's response, with no muscle tension.
2. Contraction phase: Muscle tension rises to a peak as cross-bridges interact with active sites.
3. Relaxation phase: Muscle tension falls back to resting levels.

Differentiate between summation and recruitment in muscle contraction.

• Summation: Occurs when individual twitches combine because a second stimulus arrives before the relaxation phase of the first twitch has ended, adding one twitch to another.
• Recruitment: An increase in the number of motor units being activated, allowing the muscle to contract with maximum tension if all units are used.

Define tetanus in the context of muscle stimulation and its two types.

Tetanus occurs when the frequency of stimulation is increased, causing the muscle to contract without relaxation.

1. Incomplete tetanus: Muscle produces almost peak tension (typical for normal muscular contractions).
2. Complete tetanus: The rate of stimulation is so high that the muscle never relaxes.

What is muscle tone and why is it important?

Muscle tone is caused by the sustained contraction of certain fibers, where some motor units within a muscle are always active even when the entire muscle is not contracting. It is important for maintaining posture.

What is atrophy and what can cause it?

Atrophy is when muscle fibers become small and weak, caused by a lack of regular stimulation. It is initially reversible, but dying muscle fibers are not replaced, potentially leading to permanent damage.

Distinguish between isotonic and isometric contractions.

• Isotonic contraction: Tension rises, and the muscle length changes (e.g., walking, lifting an object).
• Isometric contraction: Tension never exceeds resistance, so the overall muscle length does not change (e.g., pushing against a closed door, holding body upright against gravity).

Explain the phenomenon of rigor mortis.

Rigor mortis occurs a few hours after death when muscles run out of ATP. Calcium ions trigger a sustained contraction, but without ATP, the cross-bridges cannot detach, causing muscles to lock in position. It lasts approximately 15-25 hours until lysosomal enzymes break down myofilaments.

What are the three pathways for muscle cells to generate ATP?

1. Aerobic Respiration: Requires oxygen, makes 95\% of ATP in mitochondria, slow, yields 36 ATP per glucose, occurs during light exercise.
2. Anaerobic Respiration (Glycolysis): Occurs in the absence of oxygen, yields 2 ATP, produces lactic acid which causes fatigue during intense exercise.
3. Phosphorylation: Converts ADP back to ATP using creatine phosphate (CP), found only in muscle cells, exhausted in 20 seconds.

What causes muscle fatigue?

Muscle fatigue is caused by the exhaustion of energy reserves or the buildup of lactic acid, which lowers tissue pH and impairs muscle function, particularly during intense exercise.

What is oxygen debt?

Oxygen debt is the amount of oxygen required to restore normal pre-exertion levels, specifically the oxygen needed to convert lactic acid back to glucose. Breathing depth and rate increase until this debt is repaid.

Compare and contrast fast-twitch and slow-twitch skeletal muscle fibers.

• Fast-twitch fibers: Large diameter, few mitochondria, lots of glycogen, densely packed myofibrils. Contract quickly (<0.01 sec), powerful contractions, but fatigue quickly. Most fibers are fast-twitch.
• Slow-twitch fibers: Half the diameter of fast-twitch, contract 3 times slower, but are slow to fatigue. Have more capillaries (more oxygen) and mitochondria, and contain myoglobin (stores oxygen temporarily in the muscle).

How do white fibers and red fibers differ in appearance and composition?

• White fibers: Pale appearance, dominated by fast fibers (e.g., chicken breasts).
• Red fibers: Dark appearance, dominated by slow fibers, due to myoglobin and excessive blood vessels (e.g., chicken legs).

What is anaerobic endurance and what effect does it have on muscles?

Anaerobic endurance refers to contractions supported by glycolysis and existing energy reserves, typically quick and intense activities. Muscles experience hypertrophy (enlargement in diameter) but the number of fibers does not change.

What is aerobic endurance and what is the preferred energy source?

Aerobic endurance refers to contractions supported by mitochondrial activity, typically low levels of activity. Glucose is the preferred source of energy for aerobic endurance, making

List the key characteristics of cardiac muscle.

• Striated

• Branched

• One nucleus

• Found only in the heart

• Cells connected by intercalated discs

• Contractions controlled by pacemaker cells and last 10x longer than skeletal muscle

• Cannot undergo tetanus

• Relies on aerobic metabolism for energy

Describe the two types of smooth muscle tissue.

1. Multiunit smooth muscle: Cells are separate and do not contract rhythmically (e.g., in walls of blood vessels).

2. Visceral (single-unit) smooth muscle: Made of sheets of cells in close contact that stimulate each other, leading to rhythmic contractions (e.g., stomach, intestines, hollow organs).

What is peristalsis and what type of muscle is responsible for it?

Peristalsis is a wavelike motion that occurs in tubular organs (e.g., esophagus, intestines) caused by the transmission of impulses and rhythmicity of visceral smooth muscle.

What are the two points of attachment for muscles?

1. Origin: The point or end of the muscle attached to the immovable or less movable bone.

2. Insertion: The point or end of the muscle attached to the movable bone.

Explain how muscles move the body and define antagonistic muscles.

Muscles can only move by contracting (pulling), not pushing. Antagonistic muscles are pairs of muscles that oppose each other's motion; one contracts while the other relaxes.

Differentiate between a prime mover (agonist), antagonist, and synergist in muscle interaction.

• Prime mover (agonist): Provides most of the movement (e.g., deltoid for arm abduction).

• Antagonist: Resists the prime mover's actions and must relax for the movement to occur.

• Synergist: Assists the prime mover by stabilizing or adding to the pull.

What is the genetic cause of muscular dystrophy?

Muscular dystrophy is caused by a missing gene that fails to produce proteins needed for muscles. Each form is caused by a mutated gene.