Muscle Overview

Types of Muscle Tissue

Muscle tissue is specialized for contraction, enabling movement and force generation in the body. The most distinguishing functional characteristic of muscle is its ability to convert chemical energy into mechanical energy. Muscle tissue makes up nearly half the mass of the human body and is classified into three main types:

• Skeletal Muscle

  • Striated (has a banded appearance under the microscope)

  • Voluntary (under conscious control)

  • Contraction speed: slow to fast; tires easily

  • Wide range of force production

• Cardiac Muscle

  • Striated

  • Involuntary (not under conscious control)

  • Contraction speed: slow to moderate

• Smooth Muscle

  • Non-striated (lacks visible bands)

  • Involuntary

  • Visceral (found in walls of hollow organs); slow, sustained contractions

Muscle Terminology

Key Terms and Structures

• Muscle fibers: Elongated cells of skeletal and smooth muscle

• Muscle contraction depends on two main myofilaments:

  • Actin (thin filament)

  • Myosin (thick filament)

• Prefixes myo-, mys-, and sarco- all refer to muscle

• Sarcolemma: The plasma membrane of a muscle cell

• Sarcoplasm: The cytoplasm of a muscle cell

Characteristics and Functions of Muscle

Functional Characteristics

• Excitability/Irritability: 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 and resume resting length after stretching

Functions of Muscle Tissue

• Produce movement of the body and its parts

• Maintain posture and body position

• Stabilize joints

• Generate heat during contraction (important for maintaining body temperature)

Skeletal Muscle Structure

Components of a Skeletal Muscle

A skeletal muscle is a discrete organ composed of:

• Muscle tissue (muscle fibers)

• Blood vessels (supply oxygen and nutrients, remove wastes)

• Nerve fibers (each muscle fiber is innervated for precise control)

• Connective tissue (covers and supports muscle parts)

Connective Tissue Sheaths

Connective tissue sheaths organize and protect muscle tissue at various levels:

• Endomysium: Surrounds each individual muscle fiber

• Perimysium: Surrounds a bundle of muscle fibers (fascicle)

• Epimysium: Surrounds the entire muscle

This organization is often described as a "bundle within a bundle" structure: fibers are grouped into fascicles, which are grouped to form the whole muscle.

Muscle Attachments

• Origin: Attachment to the immovable (or less movable) bone

• Insertion: Attachment to the movable bone (in limbs, origin is usually proximal to insertion)

• Muscles attach to bones in two ways:

  • Directly: Epimysium fused to the periosteum of bone

  • Indirectly: Connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

Microscopic Anatomy of a Skeletal Muscle Fiber

Structure of Muscle Fibers

• Each muscle fiber is a long, cylindrical cell with multiple nuclei located just beneath the sarcolemma (plasma membrane)

• Muscle fibers are syncytia (multinucleated cells formed by fusion of precursor cells)

• Sarcoplasm (cytoplasm) contains:

  • Numerous glycosomes (store glycogen for energy)

  • Unique oxygen-binding protein called myoglobin

  • Usual organelles, including many mitochondria (for ATP production)

• Specialized structures:

  • Myofibrils: Densely packed, rodlike contractile elements

  • Sarcoplasmic reticulum: Specialized endoplasmic reticulum for calcium storage

  • T tubules: Invaginations of the sarcolemma that help transmit action potentials

Example: Organization of Skeletal Muscle

• The epimysium surrounds the whole muscle, which is composed of bundles called fascicles. Each fascicle is surrounded by perimysium and contains multiple muscle fibers, each wrapped in endomysium.

Additional info: The hierarchical organization of muscle tissue allows for efficient force transmission and coordination during contraction. The presence of multiple nuclei and abundant mitochondria in muscle fibers supports their high metabolic and contractile demands.