The Muscular System: Structure, Function, and Disorders

The Muscular System

6.1 Muscles: Movement and Tension

Muscles are specialized tissues responsible for producing movement and generating tension in the human body. They play essential roles in voluntary and involuntary actions, maintaining posture, and generating heat.

• Voluntary muscles are under conscious control (e.g., skeletal muscles moving limbs).

• Involuntary muscles function without conscious control (e.g., cardiac muscle pumping blood).

• Muscles can resist movement by generating force equal and opposite to external forces, aiding in posture maintenance.

• Muscle contractions generate heat, accounting for over 75% of body heat production.

Fundamental Activity: Muscle contraction is triggered by chemical or electrical signals, causing the muscle to shorten. When relaxed, muscles return to their original length.

6.1.1 Skeletal Muscle Structure and Function

• Most skeletal muscles attach to bones via tendons and cause movement by pulling on bones.

• Synergistic muscles work together for the same movement; antagonistic muscles oppose each other.

• Each skeletal muscle has an origin (stationary attachment, closer to the body's midline) and an insertion (attachment to the moving bone across a joint).

• Contraction pulls the insertion toward the origin.

6.1.2 Muscle Cell Organization

• Muscle is composed of bundles called fascicles, each surrounded by fascia (connective tissue).

• Each fascicle contains multiple muscle fibers (cells), which are large, cylindrical, and multinucleated.

• Muscle fibers are packed with myofibrils, which contain the contractile proteins actin and myosin.

6.1.3 The Sarcomere: Contractile Unit

• A sarcomere is the segment between two Z-lines in a myofibril and is the basic unit of muscle contraction.

• Each myofibril contains thousands of sarcomeres.

• Sarcomeres contain thick filaments (myosin) and thin filaments (actin).

• The sliding filament mechanism describes how myosin and actin slide past each other to shorten the sarcomere and contract the muscle.

6.2 Muscle Cell Contraction and Relaxation

Skeletal muscle contraction is initiated by nerve impulses and regulated by calcium ions.

• Motor neurons release acetylcholine at the neuromuscular junction, exciting the muscle cell.

• Sequence of events:

  1. Electrical impulse arrives at neuromuscular junction; acetylcholine is released.

  2. Acetylcholine binds to receptors, generating an electrical impulse in the muscle cell membrane.

  3. T-tubules transmit the impulse deep into the cell.

  4. Impulse triggers calcium release from the sarcoplasmic reticulum.

  5. Calcium diffuses to myofibrils, initiating contraction.

• Calcium binds to troponin, allowing myosin heads to form cross-bridges with actin (requires ATP).

• When nerve activation ends, calcium is actively transported back, and the muscle relaxes.

• Myasthenia gravis is a disorder where the body attacks acetylcholine receptors, impairing muscle contraction.

6.2.1 Energy for Muscle Contraction

• ATP is required for both contraction and relaxation.

• Myosin acts as an enzyme, hydrolyzing ATP to ADP and phosphate, releasing energy for cross-bridge cycling.

• ATP is also needed to transport calcium back into the sarcoplasmic reticulum.

Sources of ATP:

• Creatine phosphate: Rapidly transfers phosphate to ADP to form ATP (short-term, reversible).

• Anaerobic glycolysis: Breaks down glucose to produce ATP and lactic acid (short-term, high-intensity).

• Aerobic respiration: Uses glucose and fatty acids in mitochondria for efficient, long-term ATP production.

• Glycogen is stored in muscles as a reserve of glucose.

• Muscle fatigue is a decline in performance due to prolonged activity.

6.3 Types of Muscle Contractions and Fiber Types

6.3.1 Isotonic vs. Isometric Contractions

• Isotonic contractions: Muscle shortens while maintaining constant force, producing movement (e.g., lifting an object).

• Isometric contractions: Muscle generates tension without shortening; no movement occurs (e.g., holding a posture).

6.3.2 Motor Units and Force Generation

• A motor unit consists of a motor neuron and all the muscle fibers it controls.

• Force depends on:

  1. Number of muscle cells per motor unit

  2. Number of active motor units

  3. Frequency of stimulation

• Larger motor units generate more force but less precise control.

• All-or-none principle: Muscle fibers contract fully when stimulated above threshold.

• Muscle tone: Baseline level of tension in resting muscles.

• Recruitment: Increasing force by activating more motor units.

• Stimulation frequency affects force:

  • Latent period: Delay between stimulation and contraction onset.

  • Summation: Additional stimuli increase force if delivered before relaxation.

  • Tetanus: Sustained contraction due to high-frequency stimulation; can lead to fatigue.

6.3.3 Slow Twitch vs. Fast Twitch Muscle Fibers

Feature	Slow Twitch (Type I)	Fast Twitch (Type II)
ATP Breakdown	Slow	Fast
Mitochondria & Blood Supply	Many mitochondria, rich blood supply	Fewer mitochondria, less blood supply
Myoglobin Content	High (red muscle)	Low (white muscle)
Glycogen Storage	Low	High
Fatigue Resistance	High	Low
Primary Energy Source	Aerobic respiration	Creatine phosphate, anaerobic glycolysis
Lactic Acid Accumulation	Low	High

• Proportion of fiber types varies among individuals.

• Exercise training enhances muscle mass, strength, and endurance.

6.4 Cardiac and Smooth Muscle

6.4.1 Skeletal Muscle

• Attached to bones; responsible for voluntary movement.

• Large, cylindrical, multinucleated cells arranged in parallel bundles.

• Initiated only by nerve cells; no gap junctions; striated appearance.

• Fatigue varies by fiber type; not highly sustainable.

6.4.2 Cardiac Muscle

• Found only in the heart; pumps blood.

• Short, branched cells joined by intercalated discs and gap junctions.

• Initiated by pacemaker cells; modifiable by nerves; not voluntary.

• Striated; low fatigue rate; moderate contraction speed.

6.4.3 Smooth Muscle

• Located in walls of blood vessels and hollow organs (digestive, respiratory, urinary, reproductive tracts).

• Small, spindle-shaped cells with gap junctions; not striated.

• Some contractions are always maintained; modifiable by nerves; not voluntary.

• Sustainable indefinitely; does not fatigue; very slow contraction speed.

6.4.4 Activation and Control

• Cardiac and smooth muscles are activated by the autonomic nervous system (no conscious control).

• Intercalated discs in cardiac muscle allow electrical stimulation to spread rapidly.

• Pacemaker cells set the contraction rate for the heart.

• Smooth muscle contraction is regulated by the arrangement of actin and myosin (not organized in sarcomeres).

• Nerve activity can be stimulatory or inhibitory for both cardiac and smooth muscle.

6.4.5 Speed and Sustainability

• Skeletal muscle: fastest contraction, variable fatigue.

• Cardiac muscle: moderate speed, rhythmic contraction, low fatigue.

• Smooth muscle: slowest contraction, sustained indefinitely, does not fatigue.

• Smooth muscle helps regulate blood pressure by maintaining vessel diameter.

6.5 Muscle Diseases and Disorders

• Muscular dystrophy: Genetic disorder causing lack of dystrophin protein, leading to excessive calcium influx and muscle cell death.

• Tetanus: Bacterial infection producing toxins that overstimulate nerves, causing sustained muscle contraction ("lockjaw").

• Muscle cramps: Caused by shifts in potassium ions between intracellular and extracellular fluids.

• Pulled muscles: Result from torn muscle fibers.

• Fasciitis: Inflammation of the connective tissue sheath (fascia) surrounding a muscle.

Example: During a sprint, fast twitch fibers provide rapid, powerful contractions but fatigue quickly due to lactic acid buildup. In contrast, slow twitch fibers are more active during endurance activities like marathon running, relying on aerobic metabolism and resisting fatigue.

Additional info: The sliding filament mechanism is fundamental to all muscle contraction and is regulated by the presence of calcium ions and ATP. Disorders affecting neuromuscular transmission, such as myasthenia gravis, highlight the importance of proper nerve-muscle communication for movement and posture.