Skeletal System, Joints, and Muscular System: Comprehensive Study Notes

Skeletal System

Thoracic Cage

The thoracic cage protects vital organs in the thorax and supports the shoulder girdles and upper limbs. It consists of the sternum, ribs, and thoracic vertebrae.

• Sternum: Flat bone at the anterior midline of the thorax, composed of the manubrium, body, and xiphoid process.

• Manubrium: Superior portion of the sternum; articulates with clavicles and first two pairs of ribs.

• Sternal angle: Junction between manubrium and body; important anatomical landmark.

• Xiphoid process: Inferior, cartilaginous tip of the sternum; ossifies with age.

• Clavicular notch: Articulation site for the clavicle on the manubrium.

• Jugular notch: Central indentation at the superior border of the manubrium.

• True ribs (1-7): Attach directly to the sternum via their own costal cartilages.

• False ribs (8-12): Attach indirectly or not at all to the sternum.

• Floating ribs (11-12): Do not attach to the sternum.

• Costal cartilage: Hyaline cartilage connecting ribs to sternum, providing flexibility.

Vertebral Column

The vertebral column supports the body, protects the spinal cord, and provides attachment points for ribs and muscles.

• Total number of vertebrae: 33 in early development; 26 in adults (after fusion).

• Cervical vertebrae (7): Smallest, located in the neck.

• Thoracic vertebrae (12): Articulate with ribs.

• Lumbar vertebrae (5): Largest, support lower back.

• Sacrum (5 fused): Forms posterior wall of pelvis.

• Coccyx (4 fused): Tailbone.

• Atlas (C1): Supports the skull; allows nodding motion.

• Axis (C2): Has the dens (odontoid process) for rotation of the head.

• Curvatures: Cervical and lumbar (concave posteriorly); thoracic and sacral (convex posteriorly).

General Vertebra Anatomy

• Vertebral body: Weight-bearing anterior portion.

• Vertebral arch: Posterior part forming the vertebral foramen.

• Spinous process: Posterior projection for muscle/ligament attachment.

• Transverse processes: Lateral projections for muscle/ligament attachment.

• Articular processes (superior/inferior): Form joints with adjacent vertebrae.

• Pedicle and lamina: Parts of the vertebral arch.

• Facets: Smooth articular surfaces on processes.

• Vertebral foramen: Canal for spinal cord.

• Intervertebral foramen: Openings for spinal nerves.

• Intervertebral disc: Fibrocartilage pad between vertebrae for cushioning.

Cervical Vertebrae

• Identifying features: Small body, bifid spinous process (except C7), transverse foramina.

• Transverse foramina: Passage for vertebral arteries/veins.

• Comparison: Cervical vertebrae are smaller and have unique foramina compared to thoracic/lumbar vertebrae.

Thoracic Vertebrae

• Costal facets: Articulate with ribs.

• Superior and transverse costal facets: For rib head and tubercle articulation.

• Spinous process: Long, angled downward.

• Identification: Presence of costal facets and downward spinous process.

Lumbar Vertebrae

• Thick vertebral body: Supports more weight.

• Short, blunt spinous process: For muscle attachment.

• Functional significance: Robust structure for weight-bearing.

Sacrum and Coccyx

• Sacrum: Five fused vertebrae; forms posterior pelvic wall.

• Coccyx: Four fused vertebrae; vestigial tailbone.

• Sacral promontory: Anterior projecting edge of the sacrum.

• Median sacral crest: Fused spinous processes.

• Sacral foramina: Openings for sacral nerves.

• Auricular surface: Articulates with ilium at sacroiliac joint.

Pectoral Girdle

The pectoral girdle attaches the upper limb to the trunk and consists of the clavicle and scapula.

• Clavicle: S-shaped bone; articulates medially with sternum (sternal end) and laterally with scapula (acromial end).

• Scapula: Triangular bone; features include acromion, coracoid process, spine, glenoid cavity, and fossae (supraspinous, infraspinous, subscapular).

• Articulations: Clavicle with sternum and scapula; scapula with humerus at glenoid cavity.

Upper Limb

• Humerus: Proximal bone of the arm; articulates with scapula (shoulder) and radius/ulna (elbow).

• Radius and Ulna: Forearm bones; radius lateral (thumb side), ulna medial (pinky side).

• Carpals: Eight wrist bones.

• Metacarpals: Five bones of the palm.

• Phalanges: Fourteen finger bones.

• Proximal and distal radioulnar joints: Allow rotation of the forearm.

Humerus Landmarks

• Head: Articulates with glenoid cavity.

• Anatomical/surgical necks: Sites of attachment and common fracture.

• Greater/lesser tubercles: Muscle attachment.

• Intertubercular sulcus: Groove for tendon of biceps brachii.

• Deltoid tuberosity: Attachment for deltoid muscle.

• Capitulum/trochlea: Articulate with radius/ulna.

• Epicondyles: Muscle attachment.

• Coronoid/olecranon fossae: Accommodate ulna during flexion/extension.

Radius and Ulna Landmarks

• Head of radius: Articulates with capitulum of humerus.

• Radial tuberosity: Biceps brachii attachment.

• Olecranon: Prominent process of ulna; forms elbow point.

• Trochlear notch: Articulates with trochlea of humerus.

• Coronoid process: Anterior projection of ulna.

• Styloid processes: Distal projections of radius and ulna.

• Interosseous membrane: Connects radius and ulna along their length.

Hand

• Carpals: Scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate.

• Metacarpals: Numbered I-V from thumb to pinky.

• Phalanges: Proximal, middle, distal (thumb has only proximal and distal).

Hip Bone / Pelvic Girdle

• Hip bone: Fusion of ilium, ischium, and pubis.

• Acetabulum: Socket for femur head.

• Obturator foramen: Large opening for nerves/vessels.

• Ischial spine/tuberosity: Landmarks for muscle/ligament attachment.

• Pelvic brim: Edge of pelvic inlet.

• Greater (false) pelvis: Superior to pelvic brim.

• Lesser (true) pelvis: Inferior to pelvic brim.

• Pelvic inlet/outlet: Openings for passage of fetus during birth.

Hip Bone Articulations

• Sacroiliac joint: Sacrum and ilium articulation.

• Pubic symphysis: Cartilaginous joint between pubic bones.

• Hip joint: Acetabulum and femur head.

• Labrum: Fibrocartilage rim deepening acetabulum.

• Joint cavity/capsule: Encloses synovial joint.

Femur

• Proximal features: Head, neck, greater/lesser trochanters.

• Distal features: Medial/lateral condyles, epicondyles, intercondylar fossa.

• Identification: Largest bone; articulates with hip and knee.

Tibia and Fibula

• Tibia: Medial, weight-bearing bone of the leg.

• Fibula: Lateral, non-weight-bearing; stabilizes ankle.

• Landmarks: Tibial tuberosity, medial/lateral malleolus.

• Functional differences: Tibia supports weight; fibula provides muscle attachment.

Foot

• Tarsals: Seven bones (e.g., calcaneus, talus).

• Metatarsals: Five bones.

• Phalanges: Fourteen toe bones.

Joints

Structural / Functional Joint Categories

Joints are classified by structure and function, determining their movement capabilities.

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

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

• Diarthroses: Freely movable joints (e.g., synovial joints of limbs).

Synovial Joints

Synovial joints are diarthroses with a joint cavity filled with synovial fluid, allowing a wide range of motion.

• Six major types:

  • Plane (gliding): Intercarpal joints

  • Hinge: Elbow, knee

  • Pivot: Proximal radioulnar joint

  • Metacarpophalangeal joints

  • Saddle: Thumb (carpometacarpal joint)

  • Ball-and-socket: Shoulder, hip

• Joint shape and range of motion: The shape of articulating surfaces determines possible movements (e.g., ball-and-socket allows multiaxial movement).

Skeletal Muscle Physiology

Contractile Proteins and Regulation

Muscle contraction is regulated by the interaction of contractile proteins and calcium ions.

• Actin: Thin filament; contains binding sites for myosin.

• Myosin: Thick filament; forms cross-bridges with actin.

• Troponin: Regulatory protein; binds calcium to initiate contraction.

• Tropomyosin: Blocks actin binding sites at rest.

• Calcium regulation: Calcium binds troponin, causing tropomyosin to move and expose actin sites.

• Cross-bridge formation: Myosin heads bind to actin, pulling filaments past each other.

• ATP binding: ATP binds myosin, causing detachment from actin and re-cocking of the head.

• Relaxation: Calcium is pumped back into the sarcoplasmic reticulum, tropomyosin covers actin sites.

Sarcomere / Banding Concepts

• I-band: Light band; contains only actin.

• A-band: Dark band; contains myosin (and overlapping actin).

• M-line: Center of A-band; holds myosin in place.

• Contraction: I-band and H-zone shorten; A-band remains constant.

• Relaxation: I-band expands as filaments slide back.

Myogram

A myogram records muscle tension over time in response to stimulation.

• Single stimulus/twitch: Brief contraction and relaxation cycle.

• Latent period: Time between stimulus and contraction onset.

• Contraction period: Muscle tension rises.

• Relaxation period: Tension falls as muscle returns to rest.

Repeated Stimulation

• Wave summation: Increased tension from repeated stimuli before relaxation completes.

• Incomplete tetanus: Sustained, quivering contraction.

• Complete tetanus: Smooth, sustained contraction with no relaxation.

• Stimulus frequency: Higher frequency increases muscle tension.

Types of Contractions

• Concentric: Muscle shortens (e.g., lifting a weight).

• Eccentric: Muscle lengthens (e.g., lowering a weight).

• Isometric: Muscle length unchanged; tension increases (e.g., holding a weight steady).

• Isotonic: Muscle changes length; tension constant (includes concentric and eccentric).

• Examples: Biceps curl (concentric up, eccentric down), plank (isometric).

Muscle Memorization Framework

• Origin (O): Fixed attachment point.

• Insertion (I): Movable attachment point.

• Function/Action (F/A): Movement produced by muscle contraction.

Muscles (Part 1)

Muscles of Breathing

• External intercostals: Elevate ribs; inspiration.

• Internal intercostals: Depress ribs; forced expiration.

• Diaphragm: Main muscle of inspiration; flattens to increase thoracic volume.

• Central tendon: Insertion for diaphragm muscle fibers.

• Relationship: Diaphragm attaches to ribs, xiphoid process, and lumbar vertebrae.

Muscles of the Chest / Anterior Trunk

• Trapezius: Elevates, retracts, and rotates scapula.

• Deltoid: Abducts arm.

• Pectoralis major: Flexes, adducts, and medially rotates arm.

• Pectoralis minor: Depresses and protracts scapula.

• Serratus anterior: Protracts scapula; holds it against thoracic wall.

• External oblique: Compresses abdomen; flexes vertebral column.

• Rectus abdominis: Flexes lumbar spine.

Muscles of the Back

• Trapezius: (see above)

• Deltoid: (see above)

• Rhomboid minor/major: Retract scapula.

• Latissimus dorsi: Extends, adducts, and medially rotates arm.

• Erector spinae: Extends vertebral column.

• Serratus posterior inferior: Depresses lower ribs.

Muscles of the Neck

• Sternocleidomastoid: Flexes and rotates head.

• Semispinalis capitis: Extends head.

• Splenius capitis: Extends and rotates head.

• Suprahyoid muscles: Elevate hyoid bone.

• Infrahyoid muscles: Depress hyoid bone.

Muscles of the Face

• Zygomaticus: Elevates corners of mouth (smiling).

• Orbicularis oculi: Closes eyelids.

• Orbicularis oris: Closes and protrudes lips.

• Frontalis: Raises eyebrows.

• Depressor anguli oris: Depresses corners of mouth (frowning).

• Buccinator: Compresses cheek.

Muscles of Mastication / TMD

• Symptoms of TMD: Jaw pain, clicking, limited movement.

• Strained muscles: Masseter, temporalis, pterygoids.

• Primary muscles: Masseter (elevates mandible), temporalis (elevates/retracts mandible), lateral pterygoid (protracts/depresses mandible), medial pterygoid (elevates/protracts mandible).

• Origin, insertion, function: Each muscle has specific attachments and actions for chewing.

Joint Type	Movement Allowed	Example
Synarthrosis	Immovable	Sutures of skull
Amphiarthrosis	Slightly movable	Intervertebral discs
Diarthrosis (Synovial)	Freely movable	Shoulder, hip

*Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard anatomy and physiology content.*