Muscular System Overview

Key Terms and Introduction

The muscular system comprises nearly half of the body's mass and is essential for movement, posture, and heat production. Muscles transform chemical energy (ATP) into mechanical energy, enabling forceful contractions. Understanding muscle tissue involves learning about its types, characteristics, and functions.

• Muscle tissue: Specialized tissue capable of contraction.

• ATP (Adenosine Triphosphate): The energy currency used for muscle contraction.

• Myo-, mys-, sarco-: Prefixes commonly used in muscle terminology.

Types of Muscle Tissue

Classification and Characteristics

There are three main types of muscle tissue in the human body, each with distinct structure and function.

• Skeletal muscle: Attached to bones and skin; responsible for voluntary movements.

• Cardiac muscle: Found only in the heart; responsible for pumping blood.

• Smooth muscle: Located in walls of hollow organs; controls involuntary movements such as digestion.

Sarcoplasm refers to the cytoplasm of muscle cells. Muscle fibers are elongated cells specialized for contraction.

Comparison Table: Skeletal, Cardiac, and Smooth Muscle

Characteristics and Functions of Muscles

Primary Characteristics

Muscle tissue shares four main characteristics:

• Excitability (responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length after being stretched.

Functions of Muscle Tissue

• Movement: Responsible for locomotion and manipulation (e.g., walking, digesting, pumping blood).

• Posture and body position: Maintains posture.

• Stabilizing joints: Supports and stabilizes joints.

• Heat production: Generates heat as muscles contract.

Structure of Skeletal Muscle

Parts and Attachments

A skeletal muscle is an organ composed of muscle fibers, connective tissue, blood vessels, and nerves. Key features include:

• Rich blood supply: Delivers oxygen and nutrients, removes waste.

• Connective tissue sheaths: Surround and support muscle fibers.

• Nerves: Control muscle activity.

Each muscle receives a nerve, artery, and veins, ensuring proper function and responsiveness.

Connective Tissue Sheaths

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Endomysium: Surrounds individual muscle fibers.

Muscle Attachments

• Tendon: Connects muscle to bone; transmits force.

• Origin: Attachment site that does not move during contraction.

• Insertion: Attachment site that moves during contraction.

Attachments can be direct (to bone or cartilage) or indirect (via tendons or aponeuroses).

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

Skeletal muscle fibers are long, cylindrical cells containing multiple nuclei. Key components include:

• Sarcolemma: Muscle fiber plasma membrane.

• Sarcoplasm: Cytoplasm containing glycosomes (glycogen storage) and myoglobin (oxygen storage).

• Myofibrils: Densely packed, rodlike elements responsible for contraction; make up ~80% of cell volume.

• Sarcoplasmic Reticulum (SR): Specialized smooth endoplasmic reticulum that regulates calcium levels.

• Mitochondria: Provide energy for contraction.

Striations and Sarcomeres

• Striations are formed by repeating dark and light bands along the length of each myofibril.

• Sarcomere: The contractile unit of muscle fiber; sarcomeres align end to end within myofibrils.

• Myofilaments: Thread-like structures within sarcomeres, including actin (thin filaments) and myosin (thick filaments).

Muscle Contraction: Sliding Filament Model

Mechanism of Contraction

Muscle contraction occurs through the sliding filament model, where actin and myosin filaments slide past each other, shortening the sarcomere without changing the length of the filaments themselves.

• Activation of cross bridges generates force.

• Contraction begins when myosin heads bind to actin, forming cross bridges.

• ATP is required for cross bridge cycling and detachment.

Equation:

Stimulation of Muscle Fibers

Neuromuscular Junction and Action Potential

Muscle fibers are stimulated by motor neurons at the neuromuscular junction. The process involves:

• Release of neurotransmitter acetylcholine (ACh) from neuron.

• ACh binds to receptors on the sarcolemma, opening ion channels.

• Influx of Na+ ions generates an end plate potential.

• If threshold is reached, an action potential is generated, leading to contraction.

Equation:

Types of Skeletal Muscle Contractions

Isometric and Isotonic Contractions

Muscle contractions can be classified based on changes in muscle length and tension:

• Isometric contraction: Muscle tension increases, but muscle does not shorten.

• Isotonic contraction: Muscle changes length; includes concentric (shortening) and eccentric (lengthening) contractions.

Example: Biceps contract to lift a book (concentric); lower a book (eccentric).

Energy for Muscle Contraction

Sources and Utilization

ATP is the immediate source of energy for muscle contraction, but stores are depleted quickly. Muscles regenerate ATP through:

• Direct phosphorylation of ADP by creatine phosphate.

• Anaerobic glycolysis (breakdown of glucose/glycogen).

• Aerobic respiration (oxidative phosphorylation).

Equation:

Factors Influencing Muscle Contraction

Force and Duration

The force and duration of muscle contraction depend on several factors:

• Number of muscle fibers stimulated (recruitment): More fibers = greater force.

• Relative size of fibers: Larger fibers = more tension.

• Frequency of stimulation: Higher frequency = greater force.

• Degree of muscle stretch: Optimal sarcomere length = maximal force.

Effects of Exercise on Skeletal Muscles

Endurance vs. Resistance Exercise

Exercise induces changes in muscle structure and function:

• Endurance exercise (e.g., jogging, swimming): Increases capillaries, mitochondria, and myoglobin; enhances endurance and resistance to fatigue.

• Resistance exercise (e.g., weight lifting): Increases fiber size (hypertrophy), myofilaments, glycogen stores, and connective tissue; enhances strength and size.

• Inactivity leads to muscle atrophy (loss of mass and strength).

Characteristics of Smooth Muscle Tissue

Structure and Function

Smooth muscle is found in the walls of hollow organs (digestive, urinary, reproductive, circulatory systems) except the heart. It consists of two layers of fibers arranged at angles to each other, allowing coordinated contractions that propel substances through organs.

• No striations; cells are spindle-shaped.

• Involuntary control by autonomic nervous system, hormones, and local chemicals.

• Contractions are slow, sustained, and can be self-excitatory.

• Cells are connected by gap junctions, allowing coordinated activity.

Example: Peristalsis in the digestive tract.

Regulation of Contraction

• Neurotransmitter binding can cause either stimulation or inhibition, depending on the organ.

• Contraction is regulated by nerves, hormones, or local chemical changes.