The Muscular System: Structure, Function, and Physiology

The Muscular System

Overview of Muscle Types

The muscular system is responsible for all types of body movement and is composed of three basic muscle types: skeletal muscle, cardiac muscle, and smooth muscle. Each type has distinct structural and functional characteristics that enable the body to perform a wide range of activities.

• Skeletal muscle: Voluntary, striated, and attached to bones by tendons.

• Cardiac muscle: Involuntary, striated, found only in the heart.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

Muscle Terminology

Muscle cells are elongated and are often referred to as muscle fibers. The prefixes myo- and mys- refer to muscle, while sarco- refers to flesh. Contraction and shortening of muscles are due to the movement of microfilaments within these fibers.

Structure of Skeletal Muscle

Connective Tissue Components

Skeletal muscle fibers are organized and supported by several layers of connective tissue:

• Endomysium: Encloses a single muscle fiber.

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

• Epimysium: Covers the entire skeletal muscle.

• Fascia: Located on the outside of the epimysium.

Muscle Attachments

The epimysium blends into connective tissue attachments:

• Tendons: Cordlike structures, mostly collagen fibers, often cross joints due to their toughness and small size.

• Aponeuroses: Sheetlike structures that attach muscles indirectly to bones, cartilages, or connective tissue coverings.

Types of Muscle Tissue

Skeletal Muscle

Skeletal muscle fibers are large, cigar-shaped, multinucleate, and striated. They are the only muscle tissue under conscious control (voluntary).

Smooth Muscle

Smooth muscle fibers are spindle-shaped, uninucleate, and lack striations. They are involuntary and found mainly in the walls of hollow visceral organs such as the stomach, urinary bladder, and respiratory passages. Contractions are slow and sustained.

Cardiac Muscle

Cardiac muscle fibers are striated, uninucleate, and involuntary. They are found only in the heart and are joined by gap junctions called intercalated discs. Cardiac muscle contracts at a steady rate set by the heart's pacemaker.

Functions of Skeletal Muscle

In addition to producing movement, skeletal muscle has three other important roles:

• Maintaining posture and body position

• Stabilizing joints

• Generating heat

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

Each muscle fiber contains:

• Sarcolemma: Specialized plasma membrane

• Myofibrils: Long organelles inside the muscle cell, responsible for the striated appearance due to alternating light (I) and dark (A) bands

Banding Pattern and Sarcomere Organization

The banding pattern of myofibrils is due to the arrangement of myofilaments:

• I band (light band): Contains only thin filaments (actin), interrupted by the Z disc

• A band (dark band): Contains the entire length of thick filaments (myosin), with the H zone in the center and the M line in the middle of the H zone

Sarcomere: The Contractile Unit

The sarcomere is the structural and functional unit of skeletal muscle. It is defined as the segment between two Z discs and contains organized arrays of thick (myosin) and thin (actin) filaments, which produce the muscle's banded appearance.

Thick and Thin Filaments

• Thick filaments (myosin): Composed mainly of myosin, contain ATPase enzymes, and have myosin heads that form cross bridges during contraction.

• Thin filaments (actin): Composed mainly of actin, anchored to the Z disc. At rest, the H zone lacks actin filaments, but during contraction, actin and myosin overlap, causing the H zone to disappear.

Sarcoplasmic Reticulum (SR)

The SR is a specialized smooth endoplasmic reticulum that surrounds myofibrils and stores/releases 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

The Nerve Stimulus and Action Potential

Skeletal muscles must be stimulated by a motor neuron to contract. A motor unit consists of one motor neuron and all the muscle fibers it stimulates. The neuromuscular junction is the association site between the axon terminal of the motor neuron and the sarcolemma of a muscle fiber.

• Neurotransmitter: Acetylcholine (ACh) is released upon nerve impulse arrival and stimulates the muscle cell.

• Synaptic cleft: The gap between nerve and muscle, filled with interstitial fluid.

Events at the Neuromuscular Junction

1. Nerve impulse reaches the axon terminal of the motor neuron.

2. Calcium channels open, and calcium ions enter the axon terminal.

3. Calcium entry causes synaptic vesicles to release ACh.

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

5. Sarcolemma becomes more permeable to sodium ions, leading to depolarization and the creation of an action potential.

6. Action potential travels along the muscle fiber, triggering contraction.

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

Mechanism of Muscle Contraction: The Sliding Filament Theory

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

Contraction of a Skeletal Muscle as a Whole

Graded Responses

Muscle fibers contract in an "all-or-none" fashion, but whole muscles can produce graded responses by varying the frequency of stimulation and the number of fibers activated.

• Muscle twitch: Single, brief contraction (not typical in normal muscle function).

• Summed contractions: Rapid nerve impulses produce stronger, smoother contractions.

• Tetanus: Incomplete (unfused) tetanus occurs with rapid stimulation; complete (fused) tetanus is a smooth, sustained contraction.

Providing Energy for Muscle Contraction

ATP is the only direct energy source for muscle contraction. Muscle fibers store limited ATP, so additional ATP must be generated by three pathways:

1. Direct phosphorylation of ADP by creatine phosphate (CP): Fastest, but limited to about 15 seconds of activity.

2. Aerobic respiration: Occurs in mitochondria, uses oxygen, and produces about 32 ATP per glucose molecule. Supplies ATP during rest and light/moderate exercise.

3. Anaerobic glycolysis and lactic acid formation: Occurs without oxygen, produces 2 ATP per glucose, and leads to lactic acid buildup.

Muscle Fatigue and Oxygen Deficit

Muscle fatigue results from prolonged activity and is associated with ion imbalances, oxygen deficit, lactic acid accumulation, and decreased ATP supply. After exercise, rapid, deep breathing helps repay the oxygen deficit.

Types of Muscle Contractions

• Isotonic contractions: Muscle shortens and movement occurs (e.g., lifting weights).

• Isometric contractions: Muscle tension increases but does not shorten (e.g., pushing against an immovable object).

Muscle Tone and Effects of Exercise

Muscle tone is the state of continuous partial contraction, keeping muscles firm and ready for action. Exercise increases muscle size, strength, and endurance. Aerobic exercise enhances flexibility and fatigue resistance, while resistance exercise leads to muscle hypertrophy (enlargement of individual fibers).