The Axilla, Compartments, and Brachial Plexus of the Upper Limb

Comprehensive practice flashcards covering anatomical terminology, regional boundaries, osteology, joints, fascial compartments, blood vessels, and brachial plexus innervation of the human upper limb.

Sternoclavicular Joint

The articulation between the clavicle and the manubrium of the sternum that attaches the upper limb skeleton to the axial skeleton.

Shoulder Joint

The articulation between the humerus and the scapula proximally, demarcating the shoulder from the arm.

Elbow Joint

The articulation where the humerus articulates distally with the radius and ulna, distinguishing the arm from the forearm.

Wrist Joint

The articulation formed where the carpal bones occupy the space between the radius and ulna proximally and the metacarpals distally, separating the forearm from the hand.

Neurovascular Funnels of the Upper Limb

Three specialized transition spaces that transmit neurovascular structures distally through the upper limb: the axilla (the largest), the cubital fossa, and the carpal tunnel.

Axilla

A pyramid-shaped, fat-filled space in the armpit bounded by muscles, which begins superiorly at the first rib and ends inferiorly at the teres major muscle.

Anterior Wall of the Axilla

The muscular boundary of the axillary space formed by the pectoralis major and pectoralis minor muscles.

Posterior Wall of the Axilla

The boundary of the axilla formed by the subscapularis, teres major, and latissimus dorsi muscles.

Medial Wall of the Axilla

The boundary of the axilla formed by the serratus anterior muscle situated along the thoracic wall.

Lateral Wall of the Axilla

The narrow boundary of the axilla formed by the intertubercular groove of the humerus.

Axillary Sheath

A fibrous connective tissue sleeve within the axilla that encloses the axillary artery, axillary vein, and cords of the brachial plexus.

Fascial Compartment

A group of muscles bound by thick, stretch-resistant fascia that typically act synergistically on shared joints and share a common neurovascular bundle.

Anterior Compartment of the Arm

The muscular compartment responsible for flexion of the arm/forearm, innervated by the musculocutaneous nerve.

Posterior Compartment of the Arm

The muscular compartment responsible for extension of the arm/forearm, innervated by the radial nerve.

Anterior Compartment of the Forearm

The muscular compartment responsible for flexion and pronation, innervated in its middle region by the median nerve and along its medial aspect by the ulnar nerve.

Posterior Compartment of the Forearm

The muscular compartment responsible for extension and supination, innervated by the radial nerve.

Compartment Syndrome

A clinical condition where bleeding, trauma, or swelling increases pressure within an unyielding fascial compartment, compressing nerves and blood vessels to cause severe pain; most common in the leg and forearm.

Fasciotomy

A surgical incision through the deep, inextensible fascia of a compartment performed to relieve elevated pressure and prevent tissue necrosis in acute compartment syndrome.

Axillary Artery

The continuation of the subclavian artery that begins once it crosses the first rib and transitions into the brachial artery after passing the inferior border of the teres major muscle.

Brachial Artery

The major artery of the arm that extends from the lower border of the teres major muscle to the cubital fossa, where it bifurcates into the radial and ulnar arteries.

Palmar Arches

Superficial and deep arterial arches in the hand formed by anastomosing terminal divisions of the radial and ulnar arteries to supply the palm and digits.

Allen's Test

A physical exam technique evaluating collateral circulation to the hand where the radial and ulnar arteries are compressed during fist clenching; releasing one artery should return normal colour within approximately $7\text{ seconds}$.

Cephalic Vein

A major superficial vein that ascends laterally along the forearm and arm, travels through the deltopectoral groove/triangle, and drains directly into the axillary vein.

Basilic Vein

A major superficial vein that ascends medially along the forearm and arm, penetrates deep fascia to join the brachial vein, and travels with the brachial artery toward the axillary vein.

Median Cubital Vein

A superficial vein situated in the cubital fossa connecting the cephalic and basilic veins, commonly used as a site for clinical venipuncture and blood draws.

Dermatome

A circumscribed area of cutaneous skin innervated by sensory fibers originating from a single spinal nerve and its ganglion.

Roots of the Brachial Plexus

The anterior rami of spinal nerves $C5$, $C6$, $C7$, $C8$, and $T1$, which emerge in the neck between the anterior scalene and middle scalene muscles.

Trunks of the Brachial Plexus

The three primary nerve trunks formed by combining roots: the Superior trunk ($C5\text{--}C6$), Middle trunk ($C7$), and Inferior trunk ($C8\text{--}T1$).

Divisions of the Brachial Plexus

Six total nerve pathways (three anterior and three posterior) situated deep to the clavicle, formed as each trunk splits to supply flexor or extensor muscle groups.

Cords of the Brachial Plexus

The three nerve bundles (lateral, posterior, and medial) located in the axilla, named specifically according to their spatial orientation relative to the axillary artery.

Posterior Cord

The cord formed by the convergence of all three posterior divisions from the brachial plexus trunks, giving off branches that innervate the extensor compartments.

The 'M' of the Brachial Plexus

A prominent anterior anatomical landmark formed by the musculocutaneous nerve, the median nerve (with roots from both lateral and medial cords), and the ulnar nerve.

Musculocutaneous Nerve

A terminal branch of the lateral cord ($C5\text{--}C7$) that supplies motor innervation to the muscles of the anterior compartment of the arm.

Median Nerve

A major terminal nerve derived from contributions of both the lateral and medial cords ($C5\text{--}T1$), innervating most flexor muscles of the anterior forearm and select intrinsic hand muscles.

Ulnar Nerve

A terminal branch continuing from the medial cord ($C8\text{--}T1$), responsible for innervating the medial anterior forearm flexors and the majority of intrinsic hand muscles.

Radial Nerve

The largest terminal branch of the posterior cord receiving fibers from all roots ($C5\text{--}T1$), innervating all muscles in the posterior compartments of both the arm and forearm.

Axillary Nerve

A branch originating from the posterior cord ($C5\text{--}C6$) that innervates the deltoid and teres minor muscles.

Long Thoracic Nerve

A direct root branch arising from $C5$, $C6$, and $C7$ ('C5, C6, C7, fly to heaven') descending on the medial wall of the axilla to innervate the serratus anterior muscle.

Winged Scapula

A physical finding where the medial border of the scapula lifts away from the thoracic wall due to paralysis or lesion of the long thoracic nerve innervating the serratus anterior muscle.

Dorsal Scapular Nerve

A nerve emerging directly from the $C5$ root that innervates the levator scapulae, rhomboid major, and rhomboid minor muscles.

Suprascapular Nerve

A branch arising from the superior trunk ($C5\text{--}C6$) that passes through the scapular region to innervate the supraspinatus and infraspinatus muscles.

Thoracodorsal Nerve

A branch from the posterior cord ($C6\text{--}C8$) that runs inferiorly along the posterior wall of the axilla to innervate the latissimus dorsi muscle.

Subscapular Nerves

Upper and lower branches emerging from the posterior cord ($C5\text{--}C6$); the upper innervates subscapularis, while the lower innervates both subscapularis and teres major muscles.

Pectoral Nerves

The lateral pectoral nerve ($C5\text{--}C7$ from the lateral cord) and medial pectoral nerve ($C8\text{--}T1$ from the medial cord) that supply the pectoralis major and pectoralis minor muscles of the anterior axillary wall.

Myotome

A muscle or group of muscles innervated by motor fibers of a single spinal level, evaluated clinically via arm abduction ($C5$), forearm flexion ($C6$), forearm extension ($C7$), finger flexion ($C8$), and finger abduction/adduction ($T1$).

Erb-Duchenne Palsy

An upper trunk ($C5\text{--}C6$) brachial plexus lesion caused by excessive lateral neck traction, resulting in adducted shoulder, medially rotated arm, and extended elbow ('waiter's tip position').

Dejerine-Klumpke Palsy

A lower trunk ($C8\text{--}T1$) brachial plexus lesion caused by forceful arm hyperabduction, resulting in paralysis and atrophy of intrinsic hand muscles and long digital flexors, producing a 'clawed hand.'

Saturday Night Palsy

A temporary compressive neuropraxia of the radial nerve within the axilla or arm resulting in wrist drop due to paralysis of the wrist and finger extensors, along with sensory loss over the posterior upper limb.