The Muscular System: Structure, Function, and Physiology

The Muscular System

Introduction to Muscle Tissue

Muscle tissue is essential for movement and is present in every organ of the human body. It accounts for a significant portion of body weight and is involved in both voluntary and involuntary activities.

• Muscle tissue percentage: Approximately 40% of body weight in males and 32% in females.

• Three types of muscle:

  • 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.

Functions of Muscles

Muscles are responsible for producing movement, generating tension, maintaining posture, regulating blood vessel diameter, and generating heat.

• Voluntary movement: Under conscious control (e.g., picking up a pen).

• Involuntary movement: Not under conscious control (e.g., heartbeat).

• Heat generation: Muscles account for a large proportion of body heat production.

Muscle Structure and Organization

Skeletal Muscle Anatomy

Skeletal muscles are composed of bundles of muscle cells, connective tissue, and are attached to bones via tendons.

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

• Antagonistic muscles: Oppose each other's actions.

• Tendons: Connect muscle to bone.

• Origin: Attachment to stationary bone.

• Insertion: Attachment to moving bone across a joint.

Muscle Cell Structure

• Muscle (organ): Group of muscle cells with common origin, insertion, and function.

• Fascicles: Bundles of muscle fibers wrapped in connective tissue (fascia).

• Muscle fibers (cells): Long, multinucleate, packed with myofibrils.

• Myofibrils: Cylindrical structures containing actin and myosin proteins.

The Sarcomere: Contractile Unit

The sarcomere is the fundamental contractile unit of muscle, responsible for the striated appearance of skeletal muscle.

• Myosin: Forms thick filaments.

• Actin: Forms thin filaments, attached to Z-lines.

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

• One myofibril may contain up to 100,000 sarcomeres in series.

Muscle Contraction Physiology

Excitability and Contraction

Muscle cells are excitable and contract in response to electrical or chemical stimuli. The basic mechanism involves contraction (shortening) and relaxation (lengthening).

Neuromuscular Activation

• Motor neurons: Stimulate muscle contraction by releasing acetylcholine at the neuromuscular junction.

• Acetylcholine: Binds to muscle cell receptors, generating an electrical impulse transmitted along T tubules.

Calcium Release and Role

• Electrical impulse triggers release of calcium ions from the sarcoplasmic reticulum.

• Calcium initiates the contraction process by interacting with contractile proteins.

Sliding Filament Mechanism

Muscle contraction occurs when thick and thin filaments slide past each other, shortening the sarcomere.

• Resting muscle: Myosin heads do not contact actin.

• Contraction: Myosin heads form cross-bridges with actin, bend, and pull actin filaments toward the center.

Role of Troponin–Tropomyosin Complex

• In the absence of calcium, the troponin–tropomyosin complex blocks myosin binding sites on actin.

• Calcium binding to troponin shifts the complex, exposing binding sites and enabling contraction.

Summary of Contraction Steps

1. Calcium is released from the sarcoplasmic reticulum.

2. Calcium binds to troponin.

3. Troponin–tropomyosin complex shifts, exposing myosin binding sites.

4. Myosin heads form cross-bridges with actin and bend, pulling actin filaments.

5. Sarcomere shortens, leading to muscle contraction.

Muscle Relaxation

• Nerve activation ends; calcium is pumped back into the sarcoplasmic reticulum (requires ATP).

• Troponin–tropomyosin returns to its original position, blocking myosin binding sites.

• ATP is required for myosin heads to detach from actin.

• Without calcium, cross-bridges cannot form, and the muscle relaxes.

Muscle Energy Metabolism

ATP and Muscle Activity

• ATP: The primary energy source for both contraction and relaxation.

• ATP is needed to energize myosin heads, form cross-bridges, and detach myosin from actin.

• ATP is also required to pump calcium back into the sarcoplasmic reticulum.

Sources of ATP

• Creatine phosphate: Rapidly transfers a phosphate to ADP to replenish ATP.

• Stored glycogen: Hydrolyzed to glucose, metabolized anaerobically.

• Aerobic metabolism: Utilizes glucose, fatty acids, and other molecules for sustained ATP production.

Energy Source	Speed	Duration	Oxygen Required?
Creatine Phosphate	Very Fast	Seconds	No
Anaerobic Glycolysis	Fast	Short-term	No
Aerobic Respiration	Slower	Long-term	Yes

Types of Muscle Contractions

Isotonic vs. Isometric Contractions

• Isotonic contractions: Muscle shortens while maintaining constant force; movement occurs.

• Isometric contractions: Muscle generates force without shortening; no movement occurs.

Control of Muscle Force

Motor Units and Muscle Tension

• Motor unit: A motor neuron and all the muscle cells it controls; the smallest functional unit of contraction.

• Muscle tension: The mechanical force generated during contraction.

• All-or-none principle: Individual muscle cells contract fully or not at all.

• Muscle tone: Intermediate level of sustained contraction in whole muscles.

• Recruitment: Increasing the number of active motor units increases muscle force.

Motor Unit Size and Control

• Large motor units: More force, less fine control (e.g., thigh muscles).

• Small motor units: Less force, more fine control (e.g., eye muscles).

Muscle Twitch, Summation, and Tetanus

• Muscle twitch: Single contraction-relaxation cycle in response to a stimulus.

• Summation: Increased force due to rapid, repeated stimulation without complete relaxation.

• Tetanus: Sustained, maximal contraction due to high-frequency stimulation.

Term	Definition
Latent Period	Delay between stimulus and contraction onset
Contraction	Muscle shortens
Relaxation	Muscle returns to original length
Summation	Increased force from repeated stimuli
Tetanus	Maximum, sustained contraction

Muscle Fiber Types

Slow-Twitch vs. Fast-Twitch Fibers

• Slow-twitch fibers:

  • Contract slowly, use aerobic metabolism, many mitochondria, well-supplied with blood vessels, little glycogen, red color.

  • Used for endurance activities (e.g., jogging, swimming, posture).

• Fast-twitch fibers:

  • Contract quickly, use anaerobic metabolism, fewer mitochondria and blood vessels, store more glycogen, white color.

  • Used for brief, high-intensity activities (e.g., sprinting, lifting weights).

Note: Most muscles contain a mix of both fiber types; proportions are genetically determined and influence athletic ability.

Effects of Exercise on Muscle

Strength vs. Aerobic Training

• Strength (resistance) training: Short, intense activity; increases myofibrils (especially in fast-twitch fibers), muscle mass, and strength.

• Aerobic training: Prolonged activity; increases endurance, blood supply, mitochondria, and myoglobin in muscle cells.

Cardiac and Smooth Muscle

Activation and Structure

• Involuntary control: Both cardiac and smooth muscle contract without conscious control.

• Cardiac muscle: Cells joined by intercalated discs with gap junctions; pacemaker cells set contraction rhythm; striated appearance due to sarcomeres.

• Smooth muscle: Filaments arranged in criss-cross bundles (no sarcomeres); no striations; gap junctions coordinate contraction.

Contraction Speed and Fatigue

• Skeletal muscle: Fastest contraction speed.

• Cardiac muscle: Moderate speed.

• Smooth muscle: Slowest; partially contracted most of the time and rarely fatigues.

Muscle Type	Location	Control	Striations	Fatigue Resistance
Skeletal	Skeleton	Voluntary	Yes	Variable
Cardiac	Heart	Involuntary	Yes	High
Smooth	Viscera, vessels	Involuntary	No	Very high

Diseases and Disorders of the Muscular System

Muscular Dystrophy

• Duchenne muscular dystrophy: Genetic disorder; abnormal dystrophin protein causes calcium leakage, activating enzymes that destroy muscle proteins.

• Results in muscle weakening, wasting, and replacement by fibrous tissue; life expectancy ~30 years.

Tetanus

• Cause: Infection by Clostridium tetani bacteria.

• Bacteria produce toxin causing forceful muscle contractions; can lead to respiratory failure.

• Preventable by vaccination.

Other Disorders

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

• Pulled muscles: Overstretching leads to torn fibers.

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

Key Equations and Concepts

• ATP hydrolysis: Energy for muscle contraction is released by the breakdown of ATP: $\mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

• Creatine phosphate reaction: Rapid ATP regeneration: $\mathrm{Creatine\ phosphate} + \mathrm{ADP} \rightarrow \mathrm{Creatine} + \mathrm{ATP}$

Example: During a sprint, fast-twitch fibers use stored glycogen and anaerobic glycolysis to rapidly generate ATP, while during a marathon, slow-twitch fibers rely on aerobic metabolism for sustained energy.

Additional info: The sliding filament theory is central to understanding muscle contraction and is supported by the arrangement of actin and myosin within the sarcomere. Disorders such as muscular dystrophy highlight the importance of structural proteins in muscle function.