Skeletal System

Main components:

• Bones (206 in the adult): form the body’s framework, protect vital organs, and act as levers for movement.

• Cartilage: a flexible connective tissue that covers articular surfaces, reducing friction and absorbing shock.

• Joints: connections between bones that permit varying degrees of movement.

• Ligaments: fibrous bands that link bones together, stabilizing joints.

• Bursae: fluid‑filled sacs that reduce friction between tissues during movement.

Primary functions:

• Structural support and protection of internal organs

• Storage of minerals such as calcium and phosphorus

• Blood cell production in the bone marrow (hematopoiesis)

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💪 Muscular System

Types of muscle:

• Skeletal muscle: under voluntary control, attached to bones via tendons, enables movement.

• Smooth muscle: involuntary, found in walls of internal organs and blood vessels.

• Cardiac muscle: involuntary, specific to the heart, responsible for cardiac contraction.

Muscle structure:

• Muscle fibers: elongated cells containing myofibrils, which are made up of sarcomeres (the contractile units).

• Tendons: fibrous connective tissue that attaches muscle to bone, transmitting the force of contraction.

• Aponeuroses: flat, sheet‑like tendinous expansions that connect muscles to each other or to other structures.

Primary functions:

• Generation of bodily movement

• Maintenance of posture and joint stability

• Heat production through muscle contraction

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🔗 Interaction Between Muscles and Bones

Skeletal muscles attach to bones via tendons. When a muscle contracts, it pulls on its bony attachment, producing movement. Joints serve as fulcrums, allowing rotation or linear motion of the bones.

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🧠 Neurological Control

The somatic nervous system controls skeletal muscles via motor neurons. Nerve impulses travel from motor centers in the brain down the spinal cord to the muscles, triggering contraction.

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🏥 Clinical Relevance

A detailed understanding of the musculoskeletal system is essential for diagnosing and treating conditions such as:

• Osteoporosis: decreased bone density that increases fracture risk

• Arthritis: joint inflammation causing pain and stiffness

• Sarcopenia: age‑related loss of muscle mass and strength

• Muscular dystrophies: a group of genetic diseases leading to progressive muscle weakness.

Musculoskeletal System – Upper Limb Overview (Shoulder to Hand)

General Description

• Shoulder: Area where the upper limb attaches to the trunk; suspended by muscles and the sternoclavicular joint (between clavicle and sternum).

• Upper Limb Divisions:

• Shoulder – from neck to arm.

• Arm – between shoulder and elbow.

• Forearm – between elbow and wrist.

• Hand – distal to the wrist.

Key Transitional Areas:

• Axilla: Pyramidal area between shoulder and thorax; major pathway for nerves and vessels between neck and arm.

• Cubital Fossa: Triangular depression anterior to the elbow; contains the brachial artery and median nerve.

• Carpal Tunnel: Passageway into the palm; formed by carpal bones and flexor retinaculum; contains median nerve and long flexor tendons.

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Functions of the Upper Limb

1. Positioning the Hand

• Mobility: Unlike lower limbs, upper limbs are designed for movement and positioning.

• Shoulder Movement:

• Scapular movements (protraction/retraction, rotation) alter the position of the glenohumeral (shoulder) joint.

• Shoulder joint allows:

• Flexion / Extension

• Abduction / Adduction

• Medial (Internal) / Lateral (External) Rotation

• Circumduction

• Elbow Joint:

• Flexion and Extension of the forearm.

• Forearm Rotation:

• Pronation: Radius crosses over ulna (palm faces down).

• Supination: Radius returns to original position (palm up/anatomical position).

• Wrist Joint:

• Allows: Abduction, Adduction, Flexion, Extension, and Circumduction.

• These movements, combined with shoulder and elbow, provide extensive hand positioning.

2. Hand as a Mechanical Tool

• Main Function: Grasping and manipulating objects.

• Grip Mechanics:

• Involves finger flexion against the thumb.

• Intrinsic hand muscles:

• Modify actions of long tendons from forearm.

• Enable fine motor control and complex joint movements not possible with extrinsic muscles alone.

Anatomy of the Upper Limb: 

1. Shoulder Muscles

• Scapulothoracic (connecting scapula/clavicle to trunk):

• Trapezius

• Levator scapulae

• Rhomboid major & minor

• Axiohumeral & Axioappendicular (connecting trunk/pectoral girdle to humerus):

• Pectoralis major

• Pectoralis minor

• Latissimus dorsi

• Teres major

• Deltoid

• Rotator Cuff (scapula → humerus; stabilize glenohumeral joint):

• Subscapularis (anterior)

• Supraspinatus (superior)

• Infraspinatus (posterior)

• Teres minor (posterior)

2. Compartments of the Arm

• Dividing structures:

• Humerus

• Medial & lateral intermuscular septa (continuations of deep fascia)

• Anterior (flexor) compartment:

• Location: front of humerus

• Main muscles:

• Biceps brachii

• Brachialis

• Coracobrachialis

• Function: flexion at elbow; supination of forearm

• Posterior (extensor) compartment:

• Location: back of humerus

• Main muscles:

• Triceps brachii

• Anconeus

• Function: extension at elbow

3. Compartments of the Forearm

• Dividing structures:

• Radius & ulna shafts

• Interosseous membrane

• Lateral intermuscular septum

• Anterior compartment (flexor–pronator):

• Superficial layer: pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris

• Intermediate layer: flexor digitorum superficialis

• Deep layer: flexor digitorum profundus, flexor pollicis longus, pronator quadratus

• Functions: wrist/finger flexion, forearm pronation

• Posterior compartment (extensor–supinator):

• Superficial layer: extensor digitorum, extensor carpi radialis longus & brevis, extensor carpi ulnaris, extensor digiti minimi

• Deep layer: supinator, abductor pollicis longus, extensor pollicis longus & brevis, extensor indicis

• Functions: wrist/finger extension, forearm supination

4. Intrinsic Muscles of the Hand

• Located entirely within the hand; refine movements and augment tendon forces.

• Thenar eminence (thumb side):

• Abductor pollicis brevis

• Flexor pollicis brevis

• Opponens pollicis

• Hypothenar eminence (little‑finger side):

• Abductor digiti minimi

• Flexor digiti minimi brevis

• Opponens digiti minimi

• Central & interosseous groups: lumbricals, palmar & dorsal interossei, adductor pollicis.

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5. Relationship to the Neck

• Axillary inlet (superior thoracic aperture):

• Bounded by rib I (laterally), clavicle (anteriorly), superior scapular margin & coracoid process (posteriorly)

• Transmits: subclavian → axillary vessels, brachial plexus roots/trunks into axilla

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6. Relationship to the Back & Thoracic Wall

• Back muscles attaching shoulder to trunk: trapezius, levator scapulae, rhomboids, latissimus dorsi.

• Thoracic wall & breast:

• Pectoralis major forms anterior axillary wall; lies deep to breast tissue.

• Axillary tail (process) of breast extends into axilla.

• Lymphatic drainage (lateral/superior breast) → axillary lymph nodes; vascular supply/drainage from axillary vessels.

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Key Functions Summary

• Shoulder girdle muscles: position & stabilize scapula/clavicle.

• Rotator cuff: dynamic stabilization of glenohumeral joint.

• Arm muscles: primarily move forearm at elbow.

• Forearm muscles: primarily move wrist, fingers, thumb.

• Intrinsic hand muscles: fine motor control of digits; opposition and precision grip.

1. Spinal Cord Levels & Key Functions

• C4: innervates the diaphragm (above all upper‑limb levels)

• C5: abduction of the arm at the glenohumeral joint

• C6: flexion of the forearm at the elbow joint

• C7: extension of the forearm at the elbow joint

• C8: flexion of the fingers

• T1: abduction/adduction of the index, middle, and ring fingers

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2. Dermatomes (Minimal Overlap Areas)

• C5: upper lateral arm

• C6: palmar pad of the thumb

• C7: palmar pad of the index finger

• C8: palmar pad of the little finger

• T1: skin on the medial aspect of the elbow

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3. Tendon (Deep) Reflex Testing

• Biceps reflex (tap tendon in cubital fossa) → predominantly C6

• Triceps reflex (tap tendon posterior to the elbow) → predominantly C7

* Testing dermatomes, myotomes, and reflexes helps localize lesions in C5–T1 and may signal problems just below C4 that could affect breathing.

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4. Major Peripheral Nerves & Muscle Compartments

5. Cutaneous (Somatic Sensory) Territories

• Musculocutaneous nerve: anterolateral forearm

• Median nerve: palmar surface of lateral three‑and‑a‑half digits

• Ulnar nerve: palmar surface of medial one‑and‑a‑half digits

• Radial nerve: posterior surface of forearm; dorsolateral surface of hand

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Key Point:

Mapping spinal levels (dermatomes, myotomes, reflexes) and peripheral‑nerve territories is essential for diagnosing upper‐limb neuromuscular injuries and anticipating complications (e.g., respiratory compromise from lesions near C4).

Nerves Related to the Humerus

• Axillary nerve

• Courses around the surgical neck on the posterior aspect of the proximal humerus

• Innervates the deltoid (primary abductor of the humerus at the glenohumeral joint)

• Fractures at the surgical neck risk injury to this nerve

• Radial nerve

• Winds obliquely in the radial (spiral) groove on the posterior mid‑shaft of the humerus

• Supplies all extensor muscles of arm and forearm

• Mid‑shaft fractures can damage the radial nerve

• Ulnar nerve

• Passes posterior to the medial epicondyle at the distal humerus

• Travels into the forearm toward the hand

• “Funny bone” sensations occur when this nerve is struck at the medial epicondyle; fractures here endanger the nerve

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Superficial Veins of the Upper Limb

• Dorsal venous network on the back of the hand gives rise to the major superficial veins

1. Cephalic vein

• Origin: dorsal venous network over the anatomical snuff box (base of thumb)

• Course: runs laterally up the forearm → anterolateral arm → crosses elbow → deltopectoral (clavipectoral) triangle → penetrates deep fascia to enter axilla

2. Basilic vein

• Origin: medial side of the dorsal venous network of the hand

• Course: ascends posteromedially up the forearm → crosses to anterior arm just below the elbow → penetrates deep fascia midway up the arm → continues toward axillary vessels

3. Median cubital vein

• Connects cephalic and basilic veins across the roof of the cubital fossa

• Common site for venipuncture and IV access

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Orientation and Movement of the Thumb

• Anatomical orientation:

• Metacarpal I is rotated ~90° relative to metacarpals II–V

• Thumb lies in a plane at right angles to the other fingers

• Movements:

• Flexion: brings the thumb pad across the palm (at right angles to finger flexion)

• Abduction: moves the thumb away from the plane of the palm

• Opposition: combines flexion, abduction, and slight medial rotation of metacarpal I to face the thumb pad directly toward the pads of the other digits—crucial for precision grip.