Support & Movement: Articulations and Axial Muscles

2.3 Support & Movement

Overview

This section covers the anatomical and physiological aspects of articulations (joints) and axial muscles, focusing on their types, functions, and structural features. Understanding these concepts is essential for comprehending how the human body achieves movement and stability.

Ch 14: Articulations (Joints)

Definition and Function of Articulations

• Articulation: A point of contact between bones, or between bone and cartilage.

• Joints allow for complex, coordinated, and purposeful movements.

• They also provide stability and support to the skeletal system.

Classification of Joints

Structural Classification

• Joints are classified by the material connecting the bones and the presence or absence of a joint cavity.

• Fibrous joints: Bones joined by dense connective tissue; no joint cavity.

• Cartilaginous joints: Bones joined by cartilage; no joint cavity.

• Synovial joints: Bones separated by a fluid-filled joint cavity; most movable type.

Functional Classification

• Synarthroses: Immovable joints (e.g., skull sutures).

• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs).

• Diarthroses: Freely movable joints (e.g., shoulder, knee).

Table: Joint Types and Range of Motion

Joint	Range of Motion and Classification
Skull Sutures	Immovable fibrous joint (synarthrosis)
Knee	Full movement synovial joint (diarthrosis)
Vertebrae	Some movement cartilaginous joint (amphiarthrosis)

Types of Joints

Fibrous Joints (Synarthroses)

• Sutures: Found only in the skull; interlocking, immovable.

• Syndesmoses: Ligaments connect bones (e.g., distal tibiofibular joint); slight movement possible.

• Gomphoses: Peg-in-socket joints (e.g., teeth in alveolar sockets).

Cartilaginous Joints (Amphiarthroses)

• Synchondroses: Bones united by hyaline cartilage (e.g., epiphyseal plates in children).

• Symphyses: Bones united by fibrocartilage (e.g., pubic symphysis, intervertebral discs).

Synovial Joints (Diarthroses)

• Most common and movable type of joint.

• Key features:

  • Joint capsule

  • Synovial membrane

  • Articular cartilage

  • Joint cavity

  • Menisci (articular disks)

  • Ligaments

Types of Synovial Joints

• Uniaxial joints: Movement in one plane (e.g., hinge joints like the elbow).

• Biaxial joints: Movement in two perpendicular planes (e.g., saddle joint of the thumb).

• Multiaxial joints: Movement in three or more planes (e.g., ball-and-socket joints like the shoulder and hip).

Examples of Major Joints

• Shoulder (Humeroscapular Joint): Most mobile joint; stabilized by muscles, tendons, and bursae.

• Elbow Joint: Hinge joint; stabilized by ligaments and a joint capsule.

• Hip Joint: Ball-and-socket; strong ligaments and deep socket provide stability.

• Knee Joint: Largest and most complex; vulnerable to injury, especially in sports.

• Ankle Joint: Hinge joint; supported by ligaments, susceptible to sprains.

• Vertebral Joints: Cartilaginous joints with intervertebral discs for shock absorption.

Special Structures

• Bursa: Closed, fluid-filled sacs that reduce friction between tissues (e.g., in shoulder, elbow).

• Meniscus: Crescent-shaped cartilage in certain joints (e.g., knee) for shock absorption and stability.

Movements at Synovial Joints

• Flexion/Extension: Decreasing/increasing the angle between bones.

• Hyperextension: Extension beyond anatomical position.

• Abduction/Adduction: Movement away from/toward the midline.

• Rotation: Movement around a longitudinal axis.

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Gliding: Flat bone surfaces slide past each other.

• Special Movements: Dorsiflexion, plantar flexion, pronation, supination, inversion, eversion, opposition.

Measuring Range of Motion (ROM)

• ROM assessment determines the extent of movement at a joint.

• Measured actively (by the patient) or passively (by the examiner).

• Instrument used: Goniometer.

Ch 15: Axial Muscles

Introduction to Axial Muscles

• Axial muscles make up 40-50% of total body weight.

• They are responsible for movements of the head, neck, trunk, and support of the vertebral column.

Structure of Skeletal Muscle

• Composed of muscle fibers grouped into fascicles, surrounded by connective tissue sheaths.

• Muscles vary in size, shape, and fiber arrangement (e.g., parallel, convergent, pennate, bipennate, circular).

Muscle Attachments

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

• Insertion: Attachment site that moves when the muscle contracts.

• Muscles are often attached to bones via tough connective tissue cords called tendons.

Muscle Actions and Coordination

• Movements are produced by the coordinated action of several muscles.

• Prime mover (agonist): Main muscle responsible for movement.

• Antagonist: Muscle that opposes the prime mover.

• Synergist: Assists the prime mover.

• Fixator: Stabilizes the origin of the prime mover.

Levers in the Musculoskeletal System

• The body uses bones as levers, joints as fulcrums, and muscles as the force to produce movement.

• Three classes of levers exist, depending on the relative positions of the fulcrum, load, and effort.

Muscle Naming Conventions

• Muscles are named based on location, function, shape, direction of fibers, number of heads, points of attachment, and size.

• Examples: sternocleidomastoid (attachments), rectus abdominis (fiber direction), deltoid (shape).

Major Axial Muscle Groups

• Muscles of facial expression: e.g., frontalis, orbicularis oculi, orbicularis oris, buccinator, zygomaticus.

• Muscles of mastication: e.g., masseter, temporalis, pterygoids.

• Muscles that move the head: e.g., sternocleidomastoid, semispinalis capitis, splenius capitis.

• Muscles of the thorax: e.g., intercostal muscles, diaphragm.

• Muscles of the abdominal wall: e.g., external oblique, internal oblique, transversus abdominis, rectus abdominis.

• Muscles of the back: e.g., erector spinae group, latissimus dorsi.

• Muscles of the pelvic floor: e.g., levator ani, coccygeus.

Muscles of Respiration

• Primary inspiratory muscles: Diaphragm and external intercostals.

• These muscles expand the thoracic cavity, allowing inhalation.

• Muscles of forced expiration: Internal intercostals, transversus thoracis, subcostals, abdominal muscles.

• Expiration is usually passive but can be active during forceful breathing.

Summary Table: Major Axial Muscle Groups and Functions

Muscle Group	Main Function	Example Muscles
Facial Expression	Move facial skin, express emotions	Frontalis, orbicularis oculi
Mastication	Chewing movements	Masseter, temporalis
Neck	Move head and neck	Sternocleidomastoid
Thorax	Breathing, move rib cage	Diaphragm, intercostals
Abdominal Wall	Compress abdomen, flex trunk	Rectus abdominis
Back	Extend and stabilize vertebral column	Erector spinae
Pelvic Floor	Support pelvic organs	Levator ani

Key Terms

• Origin, Insertion, Agonist, Antagonist, Synergist, Fixator, Tendon, Ligament, Bursa, Meniscus

Example: Lever System in the Body

• The elbow joint acts as a fulcrum, the forearm as the lever, and the biceps brachii as the force to lift the hand (load).

Additional info: For more detailed study, refer to diagrams of joint types, muscle fiber arrangements, and lever classes in standard anatomy textbooks.