Overview of the Skeleton, Joints, and Muscular System

Overview of the Skeleton

Axial vs. Appendicular Skeleton

The human skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton. Each serves distinct structural and functional roles in the body.

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage. It forms the central supporting axis of the body.

• Appendicular Skeleton: Includes the bones of the limbs and girdles (shoulder and pelvic) that attach them to the axial skeleton.

Example: The femur is part of the appendicular skeleton, while the vertebrae are part of the axial skeleton.

Classification of Bones

Types of Bones

Bones are classified by their shapes, which relate to their functions.

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, ribs, skull bones).

• Long Bones: Longer than they are wide; mostly found in limbs (e.g., femur, humerus).

• Short Bones: Cube-shaped; found in the wrist and ankle (e.g., carpals, tarsals).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

• Sesamoid Bones: Shaped like a sesame seed; form within tendons (e.g., patella).

• Sutural Bones: Small bones located within sutures of the skull.

Bone Markings

Types of Bone Markings

Bone markings are surface features that serve as sites for muscle, ligament, and tendon attachment, or as passages for nerves and blood vessels.

• Tuberosity: Large, rounded projection; may be roughened.

• Crest: Narrow ridge of bone; usually prominent.

• Trochanter: Very large, blunt, irregularly shaped process (only on femur).

• Tubercle: Small, rounded projection or process.

• Epicondyle: Raised area on or above a condyle.

• Spine: Sharp, slender, often pointed projection.

• Process: Any bony prominence.

• Head: Bony expansion carried on a narrow neck.

• Facet: Smooth, nearly flat articular surface.

• Condyle: Rounded articular projection.

• Fissure: Narrow, slitlike opening.

• Foramen: Round or oval opening through a bone.

• Sinus: Cavity within a bone, filled with air and lined with mucous membrane.

• Fossa: Shallow, basinlike depression in a bone.

Gross Anatomy of a Long Bone

Main Structures

Long bones have a complex structure that supports their function in movement and support.

• Diaphysis: The shaft or central part of a long bone.

• Proximal Epiphysis: The end of the bone closest to the body's center.

• Distal Epiphysis: The end of the bone farthest from the body's center.

• Epiphyseal Plate: Hyaline cartilage plate where bone growth occurs in children and adolescents.

• Epiphyseal Line: Remnant of the epiphyseal plate in adults.

• Articular Cartilage: Covers the surfaces of bones where they come together to form joints.

• Medullary Cavity: Central cavity of bone shafts where marrow is stored.

• Compact Bone: Dense outer layer that provides strength.

• Spongy Bone: Internal network of bone with spaces filled with marrow.

Appendicular Skeleton

Upper Limb Bones

• Clavicle (collarbone)

• Scapula (shoulder blade)

• Humerus (upper arm bone)

• Ulna and Radius (forearm bones)

• Carpal Bones (wrist)

• Metacarpal Bones (palm)

• Phalanges (fingers)

Lower Limb Bones

• Coxal (hip bone)

• Femur (thigh bone)

• Patella (kneecap)

• Tibia and Fibula (leg bones)

• Tarsal Bones (ankle)

• Metatarsal Bones (foot)

• Phalanges (toes)

Articulations and Body Movements

Structural Classification of Joints

Joints, or articulations, are classified based on their structure and the material binding the bones together.

• Fibrous Joints: Bones joined by dense fibrous connective tissue; no joint cavity (e.g., sutures of the skull).

• Cartilaginous Joints: Bones united by cartilage; no joint cavity (e.g., intervertebral discs).

• Synovial Joints: Bones separated by a fluid-filled joint cavity; most movable type of joint (e.g., shoulder, knee).

Functional Classification of Synovial Joints

Synovial joints are further classified by the type of movement they allow.

• Plane: Gliding or sliding movements (e.g., intercarpal joints).

• Hinge: Flexion and extension (e.g., elbow, knee).

• Pivot: Rotation around a single axis (e.g., proximal radioulnar joint).

• Condylar: Movement in two planes (e.g., knuckle joints).

• Saddle: Both concave and convex areas; allows greater movement (e.g., thumb joint).

• Ball-and-Socket: Multiaxial movement (e.g., shoulder, hip).

Synovial Joint Movements

Movements at synovial joints are described by specific terms.

• Origin: Attachment of a muscle to the immovable bone.

• Insertion: Attachment to the movable bone.

• Flexion: Decreases the angle of the joint.

• Extension: Increases the angle of the joint.

• Hyperextension: Extension beyond the anatomical position.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

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

• Rotation: Turning of a bone around its own long axis.

• Supination: Rotating the forearm so the palm faces up.

• Pronation: Rotating the forearm so the palm faces down.

• Dorsiflexion: Lifting the foot so its superior surface approaches the shin.

• Plantar Flexion: Pointing the toes downward.

Gross Anatomy of the Muscular System

Classification of Skeletal Muscles by Fascicle Arrangement

Skeletal muscles are classified based on the arrangement of their fascicles (bundles of muscle fibers).

• Circular: Fascicles arranged in concentric rings (e.g., orbicularis oris).

• Convergent: Fascicles converge toward a single tendon (e.g., pectoralis major).

• Parallel: Fascicles run parallel to the long axis of the muscle (e.g., sartorius).

• Unipennate: Fascicles insert into one side of the tendon (e.g., extensor digitorum longus).

• Bipennate: Fascicles insert into the tendon from opposite sides (e.g., rectus femoris).

• Multipennate: Fascicles attach from many directions to several tendons (e.g., deltoid).

Types of Muscles Based on Function

• Agonist (Prime Mover): Muscle that provides the major force for producing a specific movement.

• Antagonist: Muscle that opposes or reverses a particular movement.

• Synergist: Muscle that aids the action of an agonist by assisting with the same movement or by reducing undesirable movement.

• Fixator: Muscle that stabilizes the origin of the agonist so that it can act more efficiently.

Muscle Type	Function	Example
Agonist	Primary mover of a joint	Biceps brachii (elbow flexion)
Antagonist	Opposes the agonist	Triceps brachii (elbow extension)
Synergist	Assists the agonist	Brachialis (assists biceps in elbow flexion)
Fixator	Stabilizes origin	Muscles of the scapula during arm movement

Additional info: Understanding the structure and classification of bones, joints, and muscles is fundamental for comprehending human movement and the musculoskeletal system's role in health and disease.