Anatomical planes, position and terms Bones and cartilage Joints Muscles basics
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Anatomy Basics: Planes, Bones & Cartilage, Joints, and Muscles
1. Anatomical Planes, Position, and Terms
Standard Anatomical Position
The body is standing erect, arms by the sides, palms facing forward. All anatomical descriptions are made relative to this position. The skull has its own standard: the Frankfurt horizontal, where the lower margin of the orbit and upper margin of the external acoustic meatus lie in the same horizontal plane, parallel to the ground. - Forensic Anthropology: A Comprehensive Introduction
The Three Reference Planes
| Plane | Orientation | Divides Body Into |
|---|---|---|
| Sagittal | Vertical | Left and right sections; the mid-sagittal (midsagittal/median) plane creates equal halves |
| Coronal (Frontal) | Vertical, perpendicular to sagittal | Anterior (front) and posterior (back) portions |
| Transverse (Axial/Horizontal) | Horizontal | Superior (upper) and inferior (lower) sections |
Directional Terms
| Term | Meaning |
|---|---|
| Anterior / Ventral | Toward the front / belly side |
| Posterior / Dorsal | Toward the back |
| Superior / Cranial | Toward the head / above |
| Inferior / Caudal | Away from the head / below |
| Medial | Toward the midline of the body |
| Lateral | Away from the midline |
| Proximal | Nearer to the trunk (used for limbs) |
| Distal | Farther from the trunk (used for limbs) |
| Superficial | Toward the body surface |
| Deep | Away from the body surface |
| Ipsilateral | On the same side |
| Contralateral | On the opposite side |
Example: The humerus is proximal to the radius and ulna. The tibia is distal to the femur.
- Forensic Anthropology: A Comprehensive Introduction
Movements at Joints
| Term | Definition |
|---|---|
| Flexion | Decreasing the angle between body parts |
| Extension | Increasing the angle between body parts |
| Abduction | Moving away from the midline |
| Adduction | Moving toward the midline |
| Rotation | Turning around a long axis |
| Circumduction | Combining flexion, extension, abduction, and adduction in a circular motion |
| Pronation / Supination | Rotating forearm so palm faces down / up |
| Inversion / Eversion | Turning sole inward / outward |
| Dorsiflexion / Plantarflexion | Foot upward / foot downward |
2. Bones and Cartilage
Bone - Overview
Bone is a living connective tissue that provides structural support, protects organs, enables movement (via muscle attachment), stores minerals (especially calcium and phosphate), and houses bone marrow for hematopoiesis.
Classification of Bones by Shape
| Type | Description | Examples |
|---|---|---|
| Long bones | Greater length than width; shaft (diaphysis) + two ends (epiphyses) | Femur, humerus, tibia |
| Short bones | Roughly equal dimensions | Carpals, tarsals |
| Flat bones | Thin, broad plates | Skull bones, scapula, sternum |
| Irregular bones | Complex shapes, don't fit other categories | Vertebrae, facial bones |
| Sesamoid bones | Embedded in tendons | Patella |
Bone Structure (Macroscopic)
- Periosteum - tough fibrous outer covering; contains osteoprogenitor cells
- Compact (cortical) bone - dense outer layer, arranged in osteons (Haversian systems)
- Cancellous (spongy/trabecular) bone - inner meshwork with marrow spaces
- Endosteum - thin membrane lining the inner surfaces
- Medullary cavity - hollow center in long bones; contains yellow (fat) or red (hematopoietic) marrow
Ossification (Bone Formation)
- Intramembranous ossification - bone forms directly from mesenchymal cells (flat bones of the skull, clavicle)
- Endochondral ossification - bone replaces a cartilage model (most of the skeleton). The epiphyseal growth plate (hyaline cartilage) is responsible for longitudinal growth.
Cartilage - Three Types
| Type | Composition | Key Features | Location |
|---|---|---|---|
| Hyaline cartilage | Type II collagen + ground substance; chondrocytes in lacunae | Smooth, glassy; most common; no blood/nerve supply | Articular surfaces, costal cartilages, tracheal rings, fetal skeleton, growth plates |
| Fibrocartilage | Dense type I collagen bundles | Strongest; highest tensile strength | Intervertebral discs, pubic symphysis, menisci, tendon insertions |
| Elastic cartilage | Elastin fibers + type II collagen | Flexible and resilient | Auricle (pinna), epiglottis, Eustachian tube |
Key fact: Cartilage is avascular - it receives nutrients by diffusion through the matrix. This is why cartilage heals slowly after injury.
3. Joints (Articular System)
Joints exist wherever two or more bones meet. They display a broad spectrum of shapes, sizes, tissue composition, and organization, each uniquely fitted to the specific type and range of motion needed.
Classification by Histological Composition
| Joint Type | Tissue Bridge | Movement | Also Called | Examples |
|---|---|---|---|---|
| Fibrous | Dense connective tissue | Minimal to none | Synarthroses | Skull sutures, gomphoses (teeth), syndesmoses (tibia-fibula) |
| Cartilaginous | Hyaline cartilage or fibrocartilage | Slight | Amphiarthroses | Pubic symphysis, intervertebral discs, costal cartilage-rib1 junction, epiphyseal plates |
| Synovial | Synovial cavity + fluid | Freely moveable | Diarthroses | Hip, knee, shoulder, elbow, wrist, ankle |
- Firestein & Kelley's Textbook of Rheumatology
Synovial Joints in Detail
Synovial joints are the major functional joints of the skeleton. Their features:
- Articular cartilage covers opposing bone ends - resilient, zonal, provides nearly frictionless motion
- Synovial cavity filled with synovial fluid (water, plasma filtrate, hyaluronic acid, lubricin, phospholipids)
- Synovial lining (synovium) - produces fluid and lines the cavity
- Fibrous capsule - encloses and protects the joint
Synovial Joint Sub-types by Shape
| Shape | Axes | Motion | Example |
|---|---|---|---|
| Hinge | Uniaxial | Flexion/Extension | Elbow (humeroulnar) |
| Pivot | Uniaxial | Rotation | Atlantoaxial (C1-C2), proximal radio-ulnar |
| Condyloid (Ellipsoid) | Biaxial | Flex/Ext + Abd/Add | Wrist (radiocarpal) |
| Saddle | Biaxial | Flex/Ext + Abd/Add | First carpometacarpal |
| Ball and Socket | Multiaxial | All planes | Shoulder (glenohumeral), hip |
| Plane (Gliding) | Multiaxial (gliding) | Gliding | Patellofemoral, intercarpal |
4. Muscles - Basics
Three Types of Muscle Tissue
| Type | Control | Striations | Location |
|---|---|---|---|
| Skeletal | Voluntary / reflex | Yes | Attached to bones; movement, posture |
| Cardiac | Involuntary | Yes | Heart wall |
| Smooth | Involuntary | No | Hollow organs (gut, blood vessels, bladder) |
Skeletal Muscle Structure (Hierarchical Organization)
From largest to smallest:
| Level | Structure | Covering Sheath |
|---|---|---|
| Whole muscle | Made of fascicle bundles | Epimysium |
| Fascicle | Bundle of muscle fibers | Perimysium |
| Muscle fiber (cell/myofiber) | Multinucleated elongated cell | Endomysium |
| Myofibril | Cylindrical element inside fiber | Sarcolemma (cell membrane) |
| Sarcomere | Functional/contractile unit | - |
| Myofilaments | Actin (thin) + Myosin (thick) | - |
- Medical Physiology (Boron & Boulpaep)
The Sarcomere
The sarcomere is the basic contractile unit, bounded by Z-lines. Key bands:
- A band - full length of thick (myosin) filaments
- I band - only thin (actin) filaments; bisected by Z-line
- H zone - central zone of A band, only myosin
- M line - center of H zone
Contraction works by the sliding filament theory: actin thin filaments slide over myosin thick filaments, shortening the sarcomere without changing filament length.
Neuromuscular Control
- All skeletal muscle is under somatic (voluntary) motor control
- Motor neuron axon terminals release acetylcholine (ACh) at the neuromuscular junction (NMJ)
- ACh binds nicotinic receptors on the motor end plate, generating an end-plate potential
- A motor unit = one motor neuron + all the muscle fibers it innervates
- Small innervation ratio (few fibers per neuron) = fine, precise movements (e.g., extraocular muscles ~3 fibers/neuron)
- Large innervation ratio = powerful, coarse movements (e.g., gastrocnemius ~100-1000 fibers/neuron)
Connective Tissue Associated with Muscles
| Structure | Description |
|---|---|
| Tendon | Collagen-rich cord connecting muscle to bone |
| Aponeurosis | Flat, sheet-like tendon |
| Ligament | Connects bone to bone (stabilizes joints) |
| Fascia | Connective tissue sheath surrounding muscles and compartments |
Muscle Actions (Classification by Role)
| Role | Description | Example |
|---|---|---|
| Agonist (prime mover) | Main muscle producing a movement | Biceps brachii in elbow flexion |
| Antagonist | Opposes the agonist; relaxes to allow movement | Triceps brachii during elbow flexion |
| Synergist | Assists the agonist; stabilizes intermediate joints | Brachialis assisting biceps |
| Fixator | Stabilizes the origin of the agonist | Rotator cuff during deltoid action |
Sources:
- Forensic Anthropology: A Comprehensive Introduction, 2nd Edition - Anatomical position and planes
- Firestein & Kelley's Textbook of Rheumatology - Classification of joints
- Medical Physiology (Boron & Boulpaep) - Skeletal muscle structure and neuromuscular control
- Imaging Anatomy: Bones, Joints, Vessels and Nerves, Vol. 3 - Bones, muscles, tendons, joints, and cartilage
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