Muscle Tissue and Physiology

Introduction to Muscle Tissue

Muscle tissue is specialized for contraction, enabling movement by converting chemical energy into mechanical energy. Muscles also generate heat, contributing to homeostasis.

• Contractility: Ability to forcibly shorten.

• Extensibility: Ability to stretch without being damaged.

• Elasticity: Ability to return to original shape after stretching.

• Excitability: Ability to respond to stimuli, typically electrical signals.

Example: The property most directly related to converting chemical energy to mechanical energy is contractility.

Types of Muscle Tissue

There are three types of muscle tissue in the human body, each with distinct locations, control mechanisms, and structural features.

Example: Skeletal muscle is voluntary and striated, cardiac muscle is involuntary and striated, and smooth muscle is involuntary and non-striated.

Organization of Muscle Tissue

Muscles are composed of muscle fibers, nerves, blood vessels, and connective tissue. The connective tissue layers organize muscle fibers into bundles and connect muscles to bones via tendons and aponeuroses.

• Muscle Fiber: Single, long, multinucleated cell surrounded by endomysium.

• Fascicle: Bundle of muscle fibers surrounded by perimysium.

• Muscle: Bundle of fascicles surrounded by epimysium.

Example: Marbling in beef is due to fat in the connective tissue layers, primarily in the perimysium and endomysium.

The Muscle Fiber: Structure and Components

Each muscle fiber contains specialized structures for contraction:

• Sarcolemma: Plasma membrane wrapping myofibrils.

• T-Tubules: Invaginations of the sarcolemma that reach deep into the fiber.

• Myofibrils: Long, rod-shaped organelles containing myofilaments (actin and myosin).

• Sarcoplasmic Reticulum: Endoplasmic reticulum of muscle cells, stores and releases Ca2+ ions.

Sliding Filament Theory and the Sarcomere

The sarcomere is the smallest contractile unit of muscle. Muscle contraction occurs when myosin (thick filament) pulls on actin (thin filament), shortening the sarcomere.

• Myosin: Anchored to the center (M line) of the sarcomere.

• Actin: Anchored to the ends (Z disc) of the sarcomere.

• During contraction, actin filaments move toward the center, increasing overlap with myosin.

Proteins of the Sarcomere

The sarcomere contains contractile, regulatory, and structural proteins:

• Contractile Proteins:

  • Myosin: Thick filament, many-headed structure.

  • Actin: Thin filament.

• Regulatory Proteins:

  • Tropomyosin: Rope-like protein, blocks myosin binding sites on actin.

  • Troponin: Binds Ca2+, moves tropomyosin to expose binding sites.

• Structural Proteins:

  • Titin: Elastic filament, helps sarcomere retain shape.

Structure of the Sarcomere: Bands, Zones, Discs & Lines

The sarcomere is divided into regions based on appearance under a microscope:

• I Band: Area with only actin (thin filaments).

• A Band: Area with both actin and myosin.

• H Zone: Center region with only myosin.

• Z Disc: End of the sarcomere.

• M Line: Center of the sarcomere, anchors myosin.

Steps of Muscle Contraction

Overview of Muscle Contraction

Muscle contraction involves the transmission of a nervous signal and the contraction of the sarcomere. The process is divided into three main stages:

1. Events at the Neuromuscular Junction: Motor neuron stimulates muscle cell, initiating an action potential.

2. Excitation-Contraction Coupling: Action potential propagates along the sarcolemma and T-tubules, sarcoplasmic reticulum releases Ca2+, exposing myosin binding sites.

3. Cross Bridge Cycle: Myosin binds to actin, performs power stroke, releases, and recocks with ATP.

Neurotransmitters & Action Potentials

Neurotransmitters are chemical messengers used at synapses. Acetylcholine is the neurotransmitter at the neuromuscular junction. Action potentials are waves of electric signal that move along membranes, caused by movement of Na+ and K+ ions.

• Muscle fibers are polarized: negatively charged inside, positively charged outside.

• Depolarization: Na+ moves inside, making the cell more positive.

• Repolarization: K+ moves outside, restoring charge.

Events at the Neuromuscular Junction

1. Action potential arrives at the axon terminal.

2. Voltage gated Ca2+ channels open.

3. Ca2+ enters, axon releases acetylcholine into the synapse.

4. Acetylcholine diffuses across the synaptic cleft and binds to receptors in the motor end plate.

5. Na+ ion channels open in the sarcolemma, starting an action potential in the muscle fiber.

6. Acetylcholine is broken down by acetylcholinesterase, ending the signal.

Excitation-Contraction Coupling

Excitation-contraction coupling refers to the events that turn the action potential into a muscle contraction:

1. Action potential travels down the sarcolemma and T-tubules.

2. Voltage gated channels of the sarcoplasmic reticulum release Ca2+ into the sarcomere.

3. Calcium binds to troponin, moving tropomyosin.

4. Myosin binding sites on actin are exposed.

5. Myosin head binds to actin, creating a cross bridge.

Cross Bridge Cycle

The cross bridge cycle is the interaction of actin and myosin that leads to sarcomere shortening:

1. Myosin head binds to actin (cross bridge formation).

2. Power stroke: Myosin pulls actin, releasing ADP and Pi.

3. ATP binds to myosin head, releasing actin.

4. ATP is hydrolyzed, myosin head moves to cocked position.

Summary Table: Muscle Contraction Steps

Key Equations

ATP Hydrolysis:

Resting Membrane Potential:

Additional info:

• Muscle contraction is essential for movement, posture, and heat production.

• ATP is required for both contraction and relaxation of muscle fibers.

• Disruption of any step in the contraction process can lead to muscle dysfunction.