The Muscular System: Structure, Function, and Physiology

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The Muscular System

Overview of Muscle Tissue

The muscular system is one of the major systems of the human body, responsible for movement, posture, and heat production. Muscle tissue is specialized for contraction and is classified into three types: skeletal, cardiac, and smooth muscle.

Skeletal muscle: Voluntary, striated muscle attached to bones for movement.
Cardiac muscle: Involuntary, striated muscle found only in the heart.
Smooth muscle: Involuntary, non-striated muscle found in walls of organs.

Anterior view of the muscular system

Skeletal Muscle Structure and Organization

Components of Skeletal Muscle

Skeletal muscles are complex organs composed of muscle tissue, connective tissues, blood vessels, and nerves. Each muscle fiber (cell) is elongated and multinucleate.

Muscle fiber: The basic cellular unit of muscle tissue.
Connective tissue layers:
- Epimysium: Surrounds the entire muscle.
- Perimysium: Divides muscle into fascicles (bundles of fibers).
- Endomysium: Surrounds individual muscle fibers.
Tendon: Bundle of collagen fibers attaching muscle to bone.
Aponeurosis: Broad sheet of connective tissue connecting muscles to each other.

Organization of skeletal muscle tissue Connective tissue layers in skeletal muscle Tendons, aponeuroses, and skeletal muscles

Microscopic Structure of Skeletal Muscle

Skeletal muscle fibers are striated due to the arrangement of myofibrils, which are composed of repeating units called sarcomeres. The sarcolemma is the plasma membrane, and the sarcoplasm is the cytoplasm of the muscle fiber.

Myofibrils: Cylindrical structures containing myofilaments (actin and myosin).
Sarcomere: The functional unit of contraction, defined by Z lines.
Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum storing calcium ions.
Transverse (T) tubules: Invaginations of the sarcolemma that transmit action potentials.

Striated appearance of skeletal muscle fibers Structure of a skeletal muscle fiber

Sarcomere Structure

The sarcomere contains thick (myosin) and thin (actin) filaments arranged in a precise pattern, producing the striated appearance of skeletal muscle.

Z line: Boundary of each sarcomere.
M line: Center of the sarcomere, holding thick filaments together.
A band: Dark region containing thick filaments.
I band: Light region containing only thin filaments.
H band: Central region with only thick filaments (relaxed state).

Organization of a sarcomere Sarcomere lines: Z line and M line Sarcomere bands: A band, I band, H band

Myofilaments: Thin and Thick Filaments

Thin filaments are primarily composed of actin, with regulatory proteins tropomyosin and troponin. Thick filaments are composed of myosin molecules with heads that interact with actin during contraction.

Actin: Contains active sites for myosin binding.
Tropomyosin: Covers active sites on actin at rest.
Troponin: Binds calcium and moves tropomyosin to expose active sites.
Myosin: Has a tail and globular head; heads form cross-bridges with actin.

Structure of a thin filament Structure of a thick filament

Muscle Contraction

Sliding Filament Theory

Muscle contraction occurs when thin filaments slide past thick filaments, shortening the sarcomere without changing the length of the filaments themselves.

Myosin heads bind to actin, forming cross-bridges.
Heads pivot, pulling thin filaments toward the center of the sarcomere.
ATP is required for cross-bridge detachment and re-cocking of the myosin head.

Sliding filament theory: relaxed and contracted myofibril Mechanism for sliding filaments

Neuromuscular Junction and Excitation-Contraction Coupling

The neuromuscular junction (NMJ) is the site where a motor neuron communicates with a skeletal muscle fiber. Acetylcholine (ACh) is released, triggering an action potential in the muscle fiber, leading to calcium release from the SR and muscle contraction.

Motor end plate: Region of sarcolemma with ACh receptors.
Synaptic cleft: Gap between neuron and muscle fiber.
Excitation-contraction coupling: Sequence linking action potential to contraction.

Neuromuscular junction structure Events at the neuromuscular junction

The Contraction Cycle

The contraction cycle involves repeated formation and breaking of cross-bridges between actin and myosin, powered by ATP hydrolysis.

The contraction cycle Steps that initiate a muscle contraction Steps that end a muscle contraction

Muscle Tension and Control

Muscle Twitch and Tension Development

A muscle twitch is a single contraction-relaxation cycle in a muscle fiber. The amount of tension produced depends on the number of cross-bridges formed and the frequency of stimulation.

Latent period: Time between stimulus and contraction onset.
Contraction phase: Tension rises to peak.
Relaxation phase: Tension falls to resting level.

Development of tension in a twitch Phases of a muscle twitch

Summation and Tetanus

Summation occurs when stimuli arrive before the muscle has completely relaxed, increasing tension. Tetanus is a sustained contraction resulting from high-frequency stimulation.

Incomplete tetanus: Partial relaxation between stimuli.
Complete tetanus: No relaxation; maximum tension achieved.

Summation of twitches Complete tetanus Incomplete tetanus

Motor Units and Recruitment

A motor unit consists of a single motor neuron and all the muscle fibers it controls. Recruitment is the process of increasing the number of active motor units to produce greater tension.

Small motor units: Fine control (e.g., eye muscles).
Large motor units: Gross movements (e.g., leg muscles).

Arrangement of motor units in a skeletal muscle Motor unit structure Motor unit size and control

Muscle Tone and Atrophy

Muscle tone is the resting tension in skeletal muscle, important for posture and joint stability. Atrophy is the loss of muscle mass due to lack of stimulation.

Isotonic contraction: Muscle changes length (e.g., lifting a book).
Isometric contraction: Muscle length remains the same (e.g., pushing against a wall).

Muscle Metabolism and Fatigue

ATP and Energy Storage

Muscle contraction requires ATP, which is generated by aerobic metabolism, anaerobic glycolysis, and stored as creatine phosphate (CP).

Aerobic metabolism: Requires oxygen, produces most ATP at rest.
Anaerobic glycolysis: Occurs without oxygen, produces less ATP but is faster.
Creatine phosphate: Stores energy to rapidly regenerate ATP.

Muscle metabolism at rest Muscle metabolism during moderate activity Muscle metabolism at peak activity

Muscle Fatigue and Recovery

Muscle fatigue occurs when a muscle can no longer contract efficiently, often due to depletion of energy reserves or accumulation of metabolic byproducts. Recovery involves restoring energy reserves, removing lactic acid, and repaying oxygen debt.

Types of Muscle Fibers

Fast-Twitch vs. Slow-Twitch Fibers

Fast-twitch fibers: Large, powerful, fatigue quickly, rely on glycolysis.
Slow-twitch fibers: Smaller, contract slowly, fatigue-resistant, rely on aerobic metabolism and myoglobin for oxygen storage.
Muscles vary in the proportion of fiber types, affecting their color and function.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle

Cardiac muscle is found only in the heart, is striated, and contracts involuntarily. Cells are connected by intercalated discs, allowing coordinated contraction.

Cardiac muscle tissue

Smooth Muscle

Smooth muscle is found in the walls of hollow organs, is non-striated, and contracts involuntarily. It can contract over a wide range of lengths and is regulated by neural, hormonal, or automatic mechanisms.

Smooth muscle tissue

Comparison of Muscle Tissue Types

Property	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Yes	Yes	No
Control	Voluntary	Involuntary	Involuntary
Location	Attached to bones	Heart	Walls of organs
Cell shape	Long, cylindrical	Branched	Spindle-shaped
Nuclei	Multiple, peripheral	Single, central	Single, central

Comparison table of muscle tissue types

Muscle Actions and Naming

Origin, Insertion, and Action

Muscles are described by their origin (stationary attachment), insertion (moving attachment), and action (movement produced). Actions are classified as flexion, extension, abduction, adduction, etc.

Muscle Roles

Prime mover (agonist): Main muscle responsible for movement.
Antagonist: Opposes the prime mover.
Synergist: Assists the prime mover.
Fixator: Stabilizes the origin of the prime mover.

Muscle Naming

Muscles are named based on location, shape, size, direction of fibers, number of origins, and action.

Muscle terminology table

Major Skeletal Muscles

Axial Muscles

Muscles of the head and neck: Control facial expressions, mastication, and head movement.
Muscles of the spine: Maintain posture and move the vertebral column.
Muscles of the trunk: Involved in breathing, supporting abdominal organs, and moving the trunk.
Muscles of the pelvic floor: Support pelvic organs and control openings.

Muscles of the head and neck Muscles associated with the mouth Epicranium and platysma Muscles of the neck

Appendicular Muscles

Muscles of the shoulder and upper limbs: Move the pectoral girdle, arm, forearm, wrist, hand, and fingers.
Muscles of the pelvic girdle and lower limbs: Move the thigh, leg, foot, and toes.

Effects of Aging and System Integration

Aging Effects

Muscle fibers decrease in size and number.
Muscles become less elastic and more fibrous.
Exercise tolerance and thermoregulation decrease.
Recovery from injury is slower.

System Integration

The muscular system works closely with the skeletal, cardiovascular, respiratory, integumentary, nervous, and endocrine systems to produce movement, maintain posture, and regulate body temperature.