Muscular System Overview

Functions of Muscle

The muscular system is essential for movement, posture, and various bodily functions. Muscles are specialized tissues that contract to produce force and motion.

• Five Functions of Muscle:

  • Producing movement

  • Maintaining posture

  • Stabilizing joints

  • Generating heat

  • Supporting soft tissues

• Types of Muscle Tissue: Skeletal, cardiac, and smooth muscle, each with distinct functions and characteristics.

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

• Voluntary vs. Involuntary Muscles: Voluntary muscles (skeletal) are consciously controlled; involuntary muscles (cardiac and smooth) function automatically.

Structural Organization of Muscle

Connective Tissue Components

Muscles are organized into bundles surrounded by connective tissue layers that provide support and transmit force.

• Tendon: Connects muscle to bone.

• Epimysium: Surrounds the entire muscle.

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

• Endomysium: Surrounds individual muscle fibers.

Muscle Tissue Characteristics

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten and generate force.

• Extensibility: Ability to be stretched.

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

• Note: Fat and areolar tissue are not characteristics of muscle tissue.

Anatomy of Skeletal Muscle

Connective Tissue Layers

Skeletal muscle is surrounded by three layers of connective tissue, from outermost to innermost: epimysium, perimysium, and endomysium.

• Attachment: Muscles attach to bones via tendons; the origin is the fixed attachment, and the insertion moves with contraction.

• Muscle Structure Hierarchy: Muscle → Fascicle → Muscle fiber → Myofibril → Sarcomere

Sarcomere and Muscle Contraction

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

• Structures: Z-line, M-line, A-band, I-band, H-zone

• Neuromuscular Junction (NMJ): Site where a motor neuron stimulates a muscle fiber.

Muscle Contraction: Role of Ions and Neurotransmitters

• Calcium (Ca2+): Triggers contraction by binding to troponin, exposing binding sites on actin.

• Sodium (Na+): Involved in action potential propagation along the muscle membrane.

• Potassium (K+): Helps repolarize the membrane after contraction.

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

Motor Units and Muscle Control

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Fine control muscles have small motor units; powerful muscles have large motor units.

Types of Muscle Fibers

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

• Fast-twitch (Type II): Fatigue quickly, used for power and speed.

Organization of Skeletal Muscle Fibers

Muscle Fiber Shapes and Names

• Shapes: Parallel, pennate, convergent, circular

• Examples: Biceps brachii (parallel), deltoid (multipennate), orbicularis oris (circular)

Sarcomere Organization

• Thick Filaments: Myosin

• Thin Filaments: Actin, troponin, tropomyosin

Events of Muscle Contraction

• Action potential arrives at NMJ

• ACh released, binds to receptors

• Depolarization of muscle fiber

• Ca2+ released from sarcoplasmic reticulum

• Cross-bridge cycling and contraction

Muscle Terminology

• Origin: Fixed attachment point of a muscle

• Insertion: Movable attachment point

• Agonist: Main muscle responsible for movement

• Antagonist: Muscle that opposes the agonist

• Synergist: Assists the agonist

• Tension: Force produced by muscle contraction

• Muscle Tone: Continuous, passive partial contraction

Axial Muscles

Four Groups of Axial Muscles

• Muscles of the head and neck

• Muscles of the vertebral column

• Muscles of the trunk

• Muscles of the pelvic floor

Axial muscles support and move the head, neck, and trunk, and assist in breathing and swallowing.

Muscles of Mastication and Facial Expression

• Mastication: Masseter, temporalis, pterygoids

• Facial Expression: Orbicularis oris, zygomaticus, buccinator

Muscles of the Skull and Neck

• Examples: Sternocleidomastoid, trapezius

Oblique and Rectus Muscles

• Abdominal Muscles: External oblique, internal oblique, rectus abdominis, transversus abdominis

• Function: Support abdominal organs, flex and rotate the trunk

Diaphragm

• Function: Main muscle of respiration, separates thoracic and abdominal cavities

Appendicular Muscles

Pectoral Girdle and Upper Arm

• Rotator Cuff Muscles: Supraspinatus, infraspinatus, teres minor, subscapularis

• Forearm Nerves: Median, ulnar, radial, and musculocutaneous nerves

• Major Muscle Groups: Dorsal side (triceps), hamstrings, thigh (quadriceps), chest (pectoralis major), arm (biceps brachii)

• Calcaneal Tendon: Also known as the Achilles tendon

Surface Anatomy and Regional Approach

Posterior Cervical Triangle

• Purpose: Important anatomical landmark for assessing lymph nodes, vessels, and nerves in the neck

Palpation and Clinical Assessment

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

• Styloid Processes: Bony projections on the distal ends of the radius and ulna, important for wrist movement

• Medial and Lateral Malleolus: Bony prominences on the tibia and fibula at the ankle, important for ankle stability

• Pulse Palpation Areas: Radial artery (wrist), carotid artery (neck), dorsalis pedis (foot), etc.

Key Table: Muscle Tissue Types Comparison

Key Equations and Concepts

• Sliding Filament Theory: Muscle contraction occurs as myosin heads bind to actin and pull, shortening the sarcomere.

• Muscle Force Equation:

• Resting Membrane Potential (RMP):

• Action Potential Propagation: Involves Na+ influx and K+ efflux across the muscle cell membrane.

Additional info: Some content and explanations have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.