Chapter 6 The Muscular System: Structure, Function, and Disorders

The Muscular System

Introduction to Muscles

Muscles are essential for all activities involving movement and constitute a significant portion of body weight. There are three primary types of muscle tissue in the human body, each with distinct locations and functions.

• Skeletal muscle: Attaches to the skeleton, providing strength and mobility.

• Cardiac muscle: Found exclusively in the heart.

• Smooth muscle: Located in the walls of the digestive tract, blood vessels, uterus, and ureters.

• Body weight: Muscle makes up about 40% of body weight in males and 32% in females.

Functions of Muscles

Muscles serve multiple functions beyond movement, including maintaining posture, regulating blood vessel diameter, and generating heat.

• Movement: Can be voluntary (conscious control) or involuntary (unconscious control).

• Resistance: Some muscles resist movement to maintain posture and stabilize joints.

• Heat generation: Muscles account for approximately 75% of the body's heat production.

The Fundamental Activity of Muscle: Contraction

Muscle cells are excitable and contract in response to electrical or chemical stimuli. All muscle cells share a common mechanism: they contract (shorten) and then relax (lengthen).

• Excitability: Ability to respond to stimuli.

• Contraction: Shortening of muscle fibers.

• Relaxation: Lengthening of muscle fibers after contraction.

Skeletal Muscles Cause Bones to Move

Skeletal muscles are responsible for moving bones and are organized into synergistic and antagonistic groups. They attach to bones via tendons, with specific points of origin and insertion.

• Synergistic muscles: Work together to produce the same movement.

• Antagonistic muscles: Oppose each other's actions.

• Tendons: Connect muscles to bones.

• Origin: Attachment to stationary bone.

• Insertion: Attachment to moving bone across a joint.

Muscle Structure

A muscle is composed of many muscle cells grouped into fascicles, which are bundles wrapped in connective tissue. Muscle fibers are long, multinucleated cells packed with myofibrils containing actin and myosin proteins.

• Fascicles: Bundles of muscle fibers.

• Muscle fibers: Long, tube-shaped, multinucleated cells.

• Myofibrils: Cylindrical structures containing actin and myosin.

The Sarcomere: Muscle Contractile Unit

The sarcomere is the fundamental contractile unit of muscle, composed of thick (myosin) and thin (actin) filaments. The arrangement of these filaments gives skeletal muscle its striated appearance.

• Myosin: Forms thick filaments.

• Actin: Forms thin filaments.

• Z-lines: Define the boundaries of each sarcomere.

• Striations: Result from the regular arrangement of actin and myosin.

Muscle Contraction and Relaxation

Muscle contraction involves the shortening of sarcomeres, triggered by nerve activation and increased calcium ion concentration. Relaxation occurs when nerve stimulation ceases and calcium is removed.

• Nerve activation: Required for skeletal muscle contraction.

• Calcium ions: Enable contraction by interacting with contractile proteins.

• Relaxation: Occurs when calcium is pumped back into the sarcoplasmic reticulum.

Nerve Activation of Skeletal Muscles

Motor neurons release acetylcholine at the neuromuscular junction, generating an electrical impulse in the muscle cell and triggering calcium release.

• Acetylcholine: Neurotransmitter released by motor neurons.

• Neuromuscular junction: Site of communication between nerve and muscle.

• T tubules: Transmit electrical impulses within the muscle cell.

Sliding Filament Mechanism

Muscle contraction occurs when thick and thin filaments slide past each other, shortening the sarcomere. Myosin heads form cross-bridges with actin, pulling the filaments toward the center.

• Resting muscle: Myosin heads do not contact actin.

• Contraction: Myosin heads bind to actin, bend, and pull actin filaments.

• Repeated cycles: Shorten the sarcomere and muscle.

Role of Calcium in Contraction

Calcium binds to troponin, causing the troponin–tropomyosin complex to shift and expose myosin binding sites on actin. This enables cross-bridge formation and muscle contraction.

• Troponin–tropomyosin complex: Regulates access to myosin binding sites.

• Calcium: Initiates contraction by binding to troponin.

Energy Requirements for Muscle Activity

Muscles require ATP for both contraction and relaxation. ATP is replenished through creatine phosphate, glycogen breakdown, and aerobic metabolism.

• ATP: Primary energy source for muscle activity.

• Creatine phosphate: Rapidly regenerates ATP.

• Glycogen: Stored in muscle, metabolized anaerobically.

• Aerobic metabolism: Utilizes glucose and fatty acids.

Types of Muscle Contractions

Muscle contractions can be isotonic (muscle shortens, movement occurs) or isometric (muscle tension increases, no movement).

• Isotonic: Muscle shortens, constant force, movement occurs.

• Isometric: Muscle tension increases, muscle does not shorten, no movement.

Motor Units and Muscle Force

Motor units consist of a motor neuron and all the muscle cells it controls. The size and number of active motor units, as well as the frequency of stimulation, influence muscle force.

• Motor unit: Motor neuron plus controlled muscle cells.

• Muscle tension: Mechanical force generated during contraction.

• All-or-none principle: Individual muscle cells fully contract or relax.

• Muscle tone: Intermediate level of force maintained by whole muscles.

• Recruitment: Activation of additional motor units increases muscle tone.

Muscle Twitch, Summation, and Tetanus

A muscle twitch is a complete cycle of contraction and relaxation. Summation occurs when stimuli are frequent enough to prevent complete relaxation, increasing force. Tetanus is a state of maximum contraction.

• Latent period: Delay between stimulation and contraction.

• Summation: Increased force due to rapid stimulation.

• Tetanus: Maximum sustained contraction.

Slow-Twitch vs. Fast-Twitch Muscle Fibers

Skeletal muscle fibers are classified as slow-twitch or fast-twitch based on their contraction speed and metabolic pathways. Most muscles contain a mix of both types, influencing endurance and strength.

• Slow-twitch fibers: Contract slowly, use aerobic metabolism, rich in mitochondria and blood supply, suited for endurance.

• Fast-twitch fibers: Contract quickly, use anaerobic metabolism, fewer mitochondria, suited for brief, high-intensity activities.

Exercise Training and Muscle Adaptation

Strength training increases muscle mass and strength by building more myofibrils, especially in fast-twitch fibers. Aerobic training enhances endurance by increasing blood supply, mitochondria, and myoglobin in muscle cells.

• Strength training: Builds muscle mass and strength.

• Aerobic training: Improves endurance and oxygen utilization.

Cardiac and Smooth Muscle Activation

Cardiac and smooth muscles contract involuntarily and can do so without nerve stimulation. Cardiac muscle cells are joined by intercalated discs and gap junctions, while smooth muscle cells coordinate contraction via gap junctions.

• Cardiac muscle: Striated, sarcomere arrangement, pacemaker cells set contraction pace.

• Smooth muscle: No striations, filaments arranged in bundles, slow and sustained contraction.

Speed and Sustainability of Contraction

• Skeletal muscle: Fastest contraction.

• Cardiac muscle: Moderate speed.

• Smooth muscle: Very slow, almost never fatigues.

Diseases and Disorders of the Muscular System

Several diseases and disorders affect the muscular system, including muscular dystrophy, tetanus, muscle cramps, pulled muscles, and fasciitis.

• Muscular dystrophy: Genetic disease causing muscle weakening and wasting.

• Tetanus: Bacterial infection causing forceful muscle contractions; preventable by vaccine.

• Muscle cramps: Often due to dehydration and ion imbalances.

• Pulled muscles: Result from overstretching and tearing of muscle fibers.

• Fasciitis: Inflammation of fascia; plantar fasciitis affects the sole of the foot.

Summary Table: Defining Characteristics of Muscle Types

Muscle Type	Location	Appearance	Control	Contraction Speed	Fatigue Resistance
Skeletal	Attached to bones	Striated	Voluntary	Fast	Variable
Cardiac	Heart	Striated	Involuntary	Moderate	High
Smooth	Walls of organs	Non-striated	Involuntary	Slow	Very high

Key Equations

• ATP Hydrolysis: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Creatine Phosphate Reaction: $\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$