The Muscular System

Overview of Muscle Types

The muscular system is responsible for all types of body movement and consists of three basic muscle types found in the body: skeletal muscle, cardiac muscle, and smooth muscle. Each type has distinct structural and functional characteristics.

• Skeletal Muscle: Attached to bones by tendons, voluntary, striated, multinucleate, responsible for conscious movement.

• Cardiac Muscle: Found only in the heart, involuntary, striated, uninucleate, branching cells joined by intercalated discs.

• Smooth Muscle: Found in walls of hollow organs, involuntary, non-striated, spindle-shaped, uninucleate, slow and sustained contractions.

Prefixes: myo- and mys- refer to muscle; sarco- refers to flesh.

Comparison of Muscle Types

The following table summarizes the main differences between skeletal, cardiac, and smooth muscle:

Functions of Skeletal Muscle

While all muscle types produce movement, skeletal muscle has additional roles:

• Maintain posture and body position

• Stabilize joints

• Generate heat

Microscopic Anatomy of Skeletal Muscle

Muscle Cell Structure

Skeletal muscle cells (fibers) are large, elongated, and multinucleate. They are surrounded by connective tissue layers:

• Endomysium: Encloses a single muscle fiber

• Perimysium: Wraps around a fascicle (bundle) of muscle fibers

• Epimysium: Covers the entire skeletal muscle

• Fascia: On the outside of the epimysium

Epimysium blends into connective tissue attachments:

• Tendons: Cordlike, mostly collagen, attach muscle to bone

• Aponeuroses: Sheetlike, attach muscles indirectly to bones, cartilage, or connective tissue coverings

Myofibrils and Sarcomeres

Muscle fibers contain myofibrils, which are long organelles responsible for contraction. Myofibrils display alternating light (I) and dark (A) bands, giving the muscle its striated appearance.

• Sarcolemma: Specialized plasma membrane of muscle cell

• Sarcomere: Contractile unit of a muscle fiber; extends from one Z disc to the next

• Myofilaments: Thick filaments (myosin) and thin filaments (actin) produce the banding pattern

Organization of the Sarcomere

• I band: Light band, contains only thin (actin) filaments, interrupted by Z disc

• A band: Dark band, contains entire length of thick (myosin) filaments, includes H zone (lighter central area) and M line (center of H zone)

Thick and Thin Filaments

• Thick Filaments (Myosin): Composed of myosin protein, contain ATPase enzymes, possess myosin heads that form cross bridges during contraction

• Thin Filaments (Actin): Composed of actin protein, anchored to Z disc, overlap with myosin during contraction

Sarcoplasmic Reticulum (SR)

The SR is a specialized smooth endoplasmic reticulum that surrounds myofibrils, storing and releasing calcium ions, which are essential for muscle contraction.

Stimulation and Contraction of Skeletal Muscle Cells

Functional Properties

• Irritability (Responsiveness): Ability to receive and respond to a stimulus

• Contractility: Ability to forcibly shorten when adequately stimulated

• Extensibility: Ability to be stretched

• Elasticity: Ability to recoil and resume resting length after stretching

Nerve Stimulus and Action Potential

Skeletal muscles must be stimulated by a motor neuron to contract. The motor unit consists of one motor neuron and all the muscle fibers it stimulates.

• Neuromuscular Junction: Site where the axon terminal of the motor neuron meets the sarcolemma of a muscle fiber

• Neurotransmitter: Chemical released by nerve upon arrival of nerve impulse; acetylcholine (ACh) is the neurotransmitter for skeletal muscle

• Synaptic Cleft: Gap between nerve and muscle filled with interstitial fluid

Steps of Muscle Cell Stimulation

1. Nerve impulse reaches axon terminal of motor neuron

2. Calcium channels open; calcium ions enter axon terminal

3. Calcium entry causes synaptic vesicles to release ACh

4. ACh diffuses across synaptic cleft and binds to receptors on sarcolemma

5. Sarcolemma becomes more permeable to sodium (Na+); depolarization occurs

6. Action potential is generated and spreads along the muscle fiber

7. Acetylcholinesterase (AChE) breaks down ACh, ending muscle contraction

Muscle returns to resting state when potassium ions (K+) diffuse out and the sodium-potassium pump restores ion balance.

Sliding Filament Theory of Contraction

Muscle contraction occurs when calcium ions bind to regulatory proteins on actin, exposing myosin-binding sites. Myosin heads attach to actin, forming cross bridges, and pivot to pull actin filaments toward the center of the sarcomere. ATP provides the energy for this process.

• Cross bridges attach and detach multiple times during contraction

• Contraction continues as long as calcium ions and ATP are present

Contraction of a Skeletal Muscle as a Whole

Graded Responses

Graded responses are different degrees of skeletal muscle shortening. While individual muscle fibers contract in an "all-or-none" fashion, whole muscles can vary their contraction strength by:

• Changing the frequency of stimulation

• Changing the number of muscle cells stimulated at one time

Types of Muscle Responses

• Muscle Twitch: Single, brief, jerky contraction; not typical in normal muscle function

• Summing of Contractions: Rapid nerve impulses cause contractions to be added together

• Unfused (Incomplete) Tetanus: Frequent stimulation leads to stronger, smoother contractions with partial relaxation

• Fused (Complete) Tetanus: Very rapid stimulation; no relaxation; smooth, sustained contraction

Muscle Force and Motor Units

• Muscle force depends on the number of fibers stimulated

• More active motor units result in greater muscle tension

• Maximum contraction occurs when all motor units are active

Providing Energy for Muscle Contraction

ATP and Energy Pathways

ATP is the only direct energy source for muscle contraction, but is stored in small amounts. Muscles regenerate ATP by three main pathways:

• Direct Phosphorylation by Creatine Phosphate (CP): Fastest method; CP transfers a phosphate to ADP to form ATP. Supplies last less than 15 seconds.

• Aerobic Respiration: Occurs in mitochondria, uses oxygen, breaks down glucose to CO2 and H2O, produces about 32 ATP per glucose. Slower, but efficient and used during rest or light/moderate exercise.

• Anaerobic Glycolysis and Lactic Acid Formation: Occurs without oxygen, breaks down glucose to pyruvic acid (producing 2 ATP), which is converted to lactic acid. Used during intense activity.

Equation for Aerobic Respiration:

Muscle Fatigue and Oxygen Deficit

Muscle fatigue occurs after strenuous, prolonged activity due to:

• Ion imbalances (Ca2+, K+)

• Oxygen deficit and lactic acid accumulation

• Decreased ATP supply

After exercise, rapid, deep breathing helps repay the oxygen deficit.

Types of Muscle Contractions

Isotonic and Isometric Contractions

• Isotonic Contractions: Muscle shortens and movement occurs (e.g., bending the knee, lifting weights, smiling)

• Isometric Contractions: Muscle does not shorten; tension increases against an immovable object (e.g., pushing palms together)

Muscle Tone

Muscle tone is a state of continuous partial contraction, resulting from alternating motor units being stimulated systematically. It keeps muscles firm, healthy, and ready for action.

Effect of Exercise on Muscles

Exercise Adaptations

• Aerobic (Endurance) Exercise: Increases muscle strength, flexibility, and resistance to fatigue; improves metabolism, digestion, and coordination

• Resistance (Isometric) Exercise: Increases muscle size and strength; individual muscle fibers enlarge (hypertrophy)