Muscular System: Structure, Function, and Anatomy Study Notes

Muscle Tissue: Structure and Function

Functions of Muscle

Muscle tissue is essential for movement, posture, and many physiological processes. There are three main types of muscle: skeletal, cardiac, and smooth, each with distinct functions.

• Five Functions of Muscle:

  1. Producing movement

  2. Maintaining posture

  3. Stabilizing joints

  4. Generating heat

  5. Supporting soft tissues

• Types of Muscle:

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

  • Cardiac muscle: Involuntary, found in the heart, pumps blood.

  • Smooth muscle: Involuntary, found in walls of hollow organs, moves substances through the body.

• Characteristics of Muscle Tissue: Excitability, contractility, extensibility, elasticity.

• Response to Nervous Stimulation: Muscle tissue contracts in response to signals from the nervous system.

• Voluntary vs. Involuntary Muscles: Skeletal muscle is voluntary; cardiac and smooth muscles are involuntary.

Structural Organization of Muscle

Muscle tissue is organized into hierarchical structures that support its function.

• Connective Tissue Layers:

  • Epimysium: Surrounds entire muscle

  • Perimysium: Surrounds bundles of muscle fibers (fascicles)

  • Endomysium: Surrounds individual muscle fibers

• Tendon: Connects muscle to bone

• Fascicle: Bundle of muscle fibers

Anatomy of Skeletal Muscle

Connective Tissue Organization

Skeletal muscle is surrounded and supported by connective tissue layers that provide structure and transmit force.

• Three Layers: Epimysium (outer), Perimysium (middle), Endomysium (inner)

• Attachment to Bone: Muscles attach to bones via tendons; bones serve as levers for movement.

• Muscle Structures: Muscle > Fascicle > Muscle fiber > Myofibril > Sarcomere

Sarcomere and Muscle Fiber Organization

The sarcomere is the functional unit of muscle contraction, composed of thick and thin filaments.

• Sarcomere: Repeating unit within myofibrils; contains actin (thin) and myosin (thick) filaments.

• Neuromuscular Junction (NMJ): Site where motor neuron communicates with muscle fiber to initiate contraction.

Muscle Contraction

Role of Ions in Muscle Contraction

Muscle contraction depends on the movement of ions across cell membranes.

• Calcium (Ca2+): Triggers contraction by binding to troponin, allowing actin-myosin interaction.

• Sodium (Na+): Initiates action potential in muscle fiber.

• Potassium (K+): Restores resting membrane potential after contraction.

• Acetylcholine (ACh): Neurotransmitter that stimulates muscle contraction at the NMJ.

Events of Muscle Contraction

• Action potential arrives at NMJ

• ACh released, binds to receptors on muscle fiber

• Na+ influx generates action potential in muscle

• Ca2+ released from sarcoplasmic reticulum

• Ca2+ binds to troponin, exposing binding sites on actin

• Myosin binds to actin, power stroke occurs

• Muscle contracts

Motor Units and Muscle Control

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Motor units allow for graded control of muscle force.

• Smallest Functional Unit: Muscle fiber

• Motor Unit: One motor neuron and its associated muscle fibers

Types of Muscle Fibers

Muscle fibers vary in their contraction speed and fatigue resistance.

• Type I (Slow-twitch): Fatigue-resistant, used for endurance

• Type II (Fast-twitch): Rapid contraction, used for power

Microanatomy and Organization of Muscle Fibers

Muscle Shapes and Names

Muscles are named based on their shape, location, and function.

• Examples: Deltoid (triangle-shaped), biceps (two heads), rectus (straight)

Sarcomere Organization

• Thick Filaments: Myosin

• Thin Filaments: Actin, troponin, tropomyosin

Muscle Terminology

Origin and Insertion

Muscles attach to bones at two points: the origin (fixed) and insertion (movable).

• Origin: Proximal attachment, less movable

• Insertion: Distal attachment, more movable

Agonist, Antagonist, and Muscle Tone

• Agonist: Primary muscle responsible for movement

• Antagonist: Opposes the agonist

• Muscle Tone: Continuous, passive partial contraction

• Tension: Force produced by muscle contraction

• Peak Tension: Maximum force generated

• RMP (Resting Membrane Potential): $V_{rest}$, the electrical potential across the cell membrane at rest

Axial Muscles

Four Groups of Axial Muscles

Axial muscles support and move the head, neck, and trunk.

• Muscles of Mastication: Chewing muscles (e.g., masseter, temporalis)

• Muscles of the Head and Neck: Facial expression, movement of the head

• Muscles of the Skull: Protect and move the skull

• Oblique and Rectus Muscles: Abdominal wall muscles

• Diaphragm: Main muscle of respiration

Appendicular Muscles

Pectoral Girdle and Upper Arm

Appendicular muscles move the limbs and stabilize the shoulder and hip joints.

• Rotator Cuff: Group of muscles stabilizing the shoulder (supraspinatus, infraspinatus, teres minor, subscapularis)

• Forearm Nerves: Median, ulnar, radial nerves; control hand and forearm movement

• Hamstrings: Posterior thigh muscles (biceps femoris, semitendinosus, semimembranosus)

• Thigh Muscles: Quadriceps, adductors

• Chest Muscles: Pectoralis major, minor

• Arm Muscles: Biceps brachii, triceps brachii

• Calcaneal Tendon (Achilles): Connects calf muscles to heel bone

Surface Anatomy and Regional Approach

Posterior Cervical Triangle

The posterior cervical triangle is an anatomical region important for clinical assessment and palpation.

• Purpose: Used to assess lymph nodes, vessels, and nerves

• Palpation: Technique to feel anatomical structures

Blood Sample Sites and Bone Landmarks

• Blood Sample Site: Commonly the median cubital vein in the antecubital fossa

• Styloid Processes: Bony projections on the radius and ulna; serve as attachment points

• Medial and Lateral Malleolus: Bony prominences at the ankle (tibia and fibula)

• Palpation Areas: Used to assess pulses (e.g., radial, carotid, dorsalis pedis)

Table: Muscle Types and Key Features

Muscle Type	Location	Control	Main Function
Skeletal	Attached to bones	Voluntary	Movement of body parts
Cardiac	Heart	Involuntary	Pumping blood
Smooth	Walls of hollow organs	Involuntary	Movement of substances

Table: Connective Tissue Layers of Muscle

Layer	Location	Function
Epimysium	Surrounds entire muscle	Protects and supports muscle
Perimysium	Surrounds fascicles	Bundles muscle fibers
Endomysium	Surrounds individual fibers	Provides support and insulation

Key Equations

• Resting Membrane Potential: $V_{rest} = -70\,\text{mV}$ (typical value for muscle cells)

• Force of Muscle Contraction: $F = m \cdot a$ where $F$ is force, $m$ is mass, $a$ is acceleration

Additional info: Some content was expanded for clarity and completeness, including definitions, examples, and tables to support exam preparation.