Chapter 12: Muscles

Overview

This chapter provides an integrated approach to the anatomy and physiology of muscles, focusing on their structure, types, and mechanisms of contraction. It covers skeletal, cardiac, and smooth muscle, with emphasis on skeletal muscle anatomy and the molecular basis of muscle contraction.

Types of Muscle

Three Major Types

• Skeletal Muscle: Voluntary muscles attached to bones, responsible for body movement.

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

• Smooth Muscle: Involuntary muscle found in internal organs and blood vessels, responsible for regulating movement of substances within the body.

Example: Skeletal muscles move limbs, cardiac muscle contracts the heart, and smooth muscle controls the diameter of blood vessels.

Skeletal Muscle Structure

Attachment and Function

• Usually attached to bones by tendons.

• Origin: End of the muscle attached to the stationary bone.

• Insertion: End attached to the bone that moves during contraction.

• Flexor muscles bring bones together; extensor muscles move bones away.

• Muscles often work in antagonistic pairs (e.g., biceps and triceps).

Muscle Fiber Anatomy

• Muscle cells are called muscle fibers.

• Long, cylindrical, multinucleated cells.

• Fibers bundled into fascicles surrounded by connective tissue.

• Connective tissue forms tendons at the ends.

Muscle Fiber Contractile Structures

• Myofibrils: Contractile elements within muscle fibers.

• Composed of repeating units called sarcomeres.

• Contain thick filaments (myosin) and thin filaments (actin).

• Regulatory proteins: Troponin and tropomyosin.

• Accessory proteins: Titin and nebulin (provide structural support).

Muscle Terminology

Sarcomere Structure

Organization

• Sarcomeres are the contractile units of myofibrils.

• Bounded by Z-disks where thin filaments attach.

• I band: Region with only thin filaments.

• A band: Length of thick filaments; includes overlap with thin filaments.

• H zone: Central region of A band with only thick filaments.

• M line: Center of sarcomere where thick filaments attach.

Accessory Proteins

• Titin: Provides elasticity and stabilizes myosin.

• Nebulin: Aligns actin filaments.

Muscle Contraction Mechanism

Sliding Filament Theory

• Muscle contraction occurs when actin and myosin filaments slide past each other.

• Requires ATP and is regulated by calcium ions.

Crossbridge Cycle

• Myosin heads bind to actin, forming crossbridges.

• ATP binding and hydrolysis cause myosin to detach and reattach, pulling actin filaments.

• Cycle repeats as long as calcium and ATP are present.

Role of Calcium

• Calcium binds to troponin, causing tropomyosin to move and expose binding sites on actin.

• Initiates contraction by allowing myosin to bind actin.

Excitation-Contraction Coupling

• Action potential from motor neuron releases acetylcholine at neuromuscular junction.

• Triggers depolarization of sarcolemma and release of calcium from sarcoplasmic reticulum.

• Calcium initiates contraction; relaxation occurs when calcium is pumped back into the sarcoplasmic reticulum.

Summary Table: Muscle Fiber Components

Key Equations

• Force of Muscle Contraction:

• ATP Hydrolysis:

Summary of Muscle Contraction Steps

1. Action potential arrives at neuromuscular junction.

2. Acetylcholine released, depolarizes sarcolemma.

3. Calcium released from sarcoplasmic reticulum.

4. Calcium binds troponin, moves tropomyosin, exposes actin binding sites.

5. Myosin binds actin, crossbridge cycle begins.

6. ATP hydrolysis powers movement.

7. Calcium reuptake leads to relaxation.

Additional info:

• Cardiac and smooth muscle contraction mechanisms share similarities with skeletal muscle but differ in regulation and structure.

• Muscle contraction is essential for movement, posture, and many physiological processes.