Muscular System: Structure, Function, and Major Muscles

Muscular System Overview

Functions of the Muscular System

The muscular system is essential for movement, posture, and various physiological processes. Muscles generate force and enable locomotion, stability, and heat production.

• Movement: Muscles contract to produce movement of body parts.

• Posture: Muscles maintain body position and posture.

• Heat Production: Muscle activity generates heat, helping regulate body temperature.

Muscle Tissue Types and Properties

Types of Muscle Tissue

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

• Skeletal muscle: Voluntary, striated muscle attached to bones for movement.

• Cardiac muscle: Involuntary, striated muscle found only in the heart.

• Smooth muscle: Involuntary, non-striated muscle found in walls of internal organs.

Properties of Muscle Tissue

Muscle tissue exhibits several key physiological properties:

• Contractility: Ability to draw proteins together, producing force but not necessarily shortening.

• Excitability: Responsiveness to stimuli (chemical, mechanical, or electrical).

• Conductivity: Ability to transmit electrical changes across the plasma membrane.

• Extensibility: Ability to be stretched beyond resting length without damage.

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

Muscle Cell Structure

Muscle cells (also called muscle fibers) have specialized structures for contraction.

• Myofibrils: Bundles of contractile proteins within muscle cells.

• Sarcoplasm: Cytoplasm of muscle cells.

• Sarcoplasmic reticulum: Specialized endoplasmic reticulum for calcium storage.

Skeletal Muscle Structure

Skeletal Muscle Tissue/Fibers

Skeletal muscle fibers are long, multinucleated cells specialized for contraction.

• Much longer than wide

• Multiple nuclei

• Myofibrils: Composed of bundles called myofilaments

• Myofilaments: Include contractile and regulatory proteins

Myofilaments

Myofilaments are the protein filaments responsible for muscle contraction.

• Thick filaments: Made of contractile proteins (primarily myosin)

• Thin filaments: Made of actin, regulatory proteins (troponin, tropomyosin)

• Elastic filaments: Composed of titin, help maintain structure

Sarcomere

The sarcomere is the basic contractile unit of muscle fiber, defined by the arrangement of thick and thin filaments.

• Striations: Created by overlap of thick and thin filaments

• I band: Light region (thin filaments only)

• A band: Dark region (thick filaments and overlap)

Muscle Contraction Mechanisms

Sliding Filament Mechanism

Muscle contraction occurs via the sliding filament mechanism, where actin and myosin filaments slide past each other.

• Latent period: Delay after stimulus before contraction begins

• Contraction period: Tension increases as crossbridges form

• Relaxation period: Tension decreases as muscle fibers relax

Muscle Tension

Muscle tension is determined by the frequency and strength of stimulation and the length of the muscle fiber.

• Increased frequency: Increases signal strength, more Ca2+ in the muscle cell

• Refractory period: Short in skeletal muscle, allows rapid repeated stimulation

Muscle Contractions

Muscle contractions can be classified based on the relationship between muscle force and external load.

• Isotonic concentric: Muscle force is greater than external load, muscle shortens

• Isotonic eccentric: External load is greater, muscle lengthens

• Isometric: Muscle force equals external load, muscle length does not change

Skeletal Muscle Fiber Types

Types of Skeletal Muscle Fibers

Skeletal muscle fibers differ in contraction speed, energy usage, and fatigue resistance.

• Slow-twitch fibers (Type I): Low myosin ATPase activity, slower contractions, high endurance, rely on oxidative metabolism, rich in myoglobin ("red muscle")

• Fast-twitch fibers (Type II): High myosin ATPase activity, faster contractions, lower endurance, rely more on glycolysis, less myoglobin ("white muscle")

• Type I fibers: Small to intermediate diameter, contract for long periods, fatigue slowly

• Type II fibers: Large diameter, contract quickly, fatigue rapidly

Muscle Organization and Naming

Muscle Components

Muscles are organized into bundles and covered by connective tissue.

• Fascia: Connective tissue covering muscles

• Fascicles: Bundles of muscle fibers within a muscle

• Tendons: Connective tissue attaching muscle to bone

Muscle Shapes

Muscles are classified by shape, which affects their function.

• Parallel

• Convergent

• Pennate (unipennate, multipennate)

• Circular/sphincters

• Spiral

• Fusiform

Muscle Movements

Muscles contract to move bones by shortening the distance between origin and insertion.

• Origin: Attachment to stationary bone

• Insertion: Attachment to moving bone

• Action: The movement produced by muscle contraction

Muscle Levers

Muscles and bones act as levers to create movement. There are three classes of levers:

• 1st class levers: Provide speed or mechanical advantage

• 2nd class levers: Provide mechanical advantage

• 3rd class levers: Provide speed advantage

Muscle Roles in Movement

• Agonist (prime mover): Main muscle responsible for movement

• Antagonist: Opposes the action of the agonist

• Synergist: Assists the agonist, adds force or stabilizes movement

• Fixator: Stabilizes the origin of the agonist

Muscle Naming

Muscles are named based on various characteristics:

• Origin, insertion, or both

• Shape

• Size

• Position

• Number of heads

• Action

• Region

• Function

Major Muscles of the Body

Muscles of the Head and Neck

• Orbicularis oris

• Orbicularis oculi

• Buccinator

• Masseter

• Digastric

• Platysma

• Temporalis

• Sternocleidomastoid

• Mylohyoid

• Sternohyoid

• Stylohyoid

• Splenius capitis

• Splenius cervicis

• Rhomboid minor

• Rhomboid major

• Levator scapulae

• Scalene

Muscles of the Eye

• Superior oblique

• Superior rectus

• Medial rectus

• Lateral rectus

• Inferior oblique

• Inferior rectus

Muscles of the Abdomen and Back

• External abdominal oblique

• Internal abdominal oblique

• Transversus abdominis

• Rectus abdominis

• Longissimus muscle column (capitus cervicis and thoracis)

• Iliopsoas

Muscles of the Chest and Shoulder

• Pectoralis major

• Pectoralis minor

• Trapezius

• Serratus anterior

• External and internal intercostal muscles

• Deltoid

• Trapezius

• Subscapularis

• Supraspinatus*

• Infraspinatus*

• Subscapularis*

• Teres major

• Teres minor*

• Latissimus dorsi

Muscles of the Arm

• Triceps brachii long head

• Triceps lateral head

• Triceps medial head

• Brachialis

• Brachioradialis

• Extensor carpi radialis longus

• Extensor carpi ulnaris

• Extensor digitorum

• Flexor carpi ulnaris

• Flexor digitorum superficialis

• Biceps brachii

• Pronator teres

• Supinator

• Palmaris longus

Muscles of the Leg

• Gluteus maximus

• Biceps femoris

• Sartorius

• Gracilis

• Rectus femoris

• Vastus lateralis

• Vastus intermedius

• Vastus medialis

• Tibialis cranialis (anterior)

• Gastrocnemius

Summary Table: Muscle Fiber Types

The following table summarizes the main differences between slow-twitch and fast-twitch muscle fibers:

Fiber Type	Contraction Speed	Energy Source	Myoglobin Content	Fatigue Resistance
Type I (Slow-twitch)	Slow	Oxidative (aerobic)	High	High
Type II (Fast-twitch)	Fast	Glycolytic (anaerobic)	Low	Low

Key Equations

• Muscle Force Equation:

Muscle force is proportional to the number of crossbridges formed:

$F_{muscle} \propto \text{number of crossbridges}$

• Lever Mechanical Advantage:

$\text{Mechanical Advantage} = \frac{\text{Length of Effort Arm}}{\text{Length of Resistance Arm}}$

Additional info:

• Some muscle names and actions are referenced for exam preparation; students should know origin, insertion, and action for listed muscles.

• Rotator cuff muscles are marked with an asterisk (*).