Muscles and Muscle Tissue

Introduction

Muscle tissue is a specialized tissue found throughout the human body, responsible for movement, posture, and vital functions such as heartbeats and digestion. There are three main types of muscle tissue: skeletal, cardiac, and smooth, each with unique structures and functions.

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs; controls movement of substances within these organs.

Example: Skeletal muscles move limbs, cardiac muscle contracts the heart, and smooth muscle controls digestion.

Properties of Muscle Tissue

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to recoil to resting length after stretching.

Muscle Attachments: Origin and Insertion

• Origin: The fixed attachment, usually proximal.

• Insertion: The movable attachment, usually distal. (Flanagin's Rule: insertion is always closest to the bone that moves.)

Microscopic Anatomy of Skeletal Muscle

• Muscle Fiber: Individual muscle cell.

• Myofibril: Rod-like unit within muscle fiber, composed of sarcomeres.

• Sarcomere: Functional contractile unit of muscle, defined by Z discs.

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores calcium ions.

• T Tubules: Invaginations of sarcolemma that help transmit action potentials.

Muscle Contraction: Sliding Filament Model

The sliding filament model explains how muscles contract by the sliding of actin (thin) filaments past myosin (thick) filaments, shortening the sarcomere.

• Steps:

  1. Action potential travels along sarcolemma and T tubules.

  2. Calcium ions released from sarcoplasmic reticulum.

  3. Calcium binds to troponin, moving tropomyosin and exposing binding sites on actin.

  4. Myosin heads bind to actin, forming cross-bridges.

  5. Myosin heads pivot, pulling actin filaments toward center of sarcomere.

  6. ATP binds to myosin, causing it to release actin and reset for another cycle.

Equation:

Neuromuscular Junction and Excitation-Contraction Coupling

• Neuromuscular Junction: Synapse between motor neuron and muscle fiber.

• Excitation-Contraction Coupling: Sequence of events from nerve signal to muscle contraction.

Example: Acetylcholine released from neuron binds to receptors on sarcolemma, initiating action potential.

Phases of Muscle Contraction

• Latent Period: Time between stimulus and contraction onset.

• Contraction Period: Muscle fibers shorten.

• Relaxation Period: Muscle fibers return to resting length.

Motor Units and Graded Muscle Responses

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Graded Response: Variation in muscle contraction strength due to frequency and number of motor units activated.

Muscle Metabolism and Energy Sources

• ATP: Primary energy source for muscle contraction.

• Creatine Phosphate: Rapidly regenerates ATP.

• Anaerobic Glycolysis: Produces ATP without oxygen, yields lactic acid.

• Aerobic Respiration: Produces ATP with oxygen, more efficient.

Equation:

Muscle Fatigue

• Definition: Inability of muscle to contract despite stimulation, often due to ATP depletion, ionic imbalances, or lactic acid accumulation.

Types of Muscle Contractions

• Isotonic Contraction: Muscle changes length (shortens or lengthens) while tension remains constant.

• Isometric Contraction: Muscle tension increases but length remains unchanged.

Effects of Exercise on Muscles

• Aerobic (Endurance) Exercise: Increases capillaries, mitochondria, and myoglobin content; improves stamina.

• Resistance Exercise: Increases muscle size (hypertrophy) and strength.

Comparison of Skeletal, Cardiac, and Smooth Muscle

Unitary vs. Multiunit Smooth Muscle

• Unitary (Visceral) Smooth Muscle: Cells contract as a single unit due to gap junctions; found in walls of hollow organs.

• Multiunit Smooth Muscle: Cells contract independently; found in large airways, arteries, and eye muscles.

Example: Unitary smooth muscle in intestines, multiunit smooth muscle in iris of the eye.

Key Terms and Definitions

• Myoglobin: Oxygen-binding protein in muscle cells.

• Troponin and Tropomyosin: Regulatory proteins in muscle contraction.

• Acetylcholine: Neurotransmitter at neuromuscular junction.

• Motor End Plate: Region of sarcolemma with acetylcholine receptors.

Summary Table: Muscle Tissue Comparison

Additional info: Some definitions and explanations have been expanded for clarity and completeness.