Brachial Plexus and Axillary Fossa Anatomy

The brachial plexus is a network of nerves that control movement and sensation in the upper limb. It arises from the ventral rami of spinal nerves C5-T1.

The brachial plexus is divided into supraclavicular and infraclavicular structures. Supraclavicular structures include roots and trunks, while infraclavicular structures include divisions and cords.

The axillary fossa is a space under the arm, shaped like a pyramid. It contains important structures like blood vessels, nerves, and lymph nodes.

The anterior wall of the axillary fossa is formed by the pectoralis major and minor muscles. The medial wall is formed by the serratus anterior and the lateral ribcage. The lateral wall is the intertubercular sulcus, and the posterior wall includes the subscapularis, teres major, and latissimus dorsi tendon.

The brachial plexus roots derive from five spinal nerves: C5, C6, C7, C8, and T1. These roots combine to form the trunks of the plexus.

The upper limbs develop as buds from the body wall during the fourth week of embryonic development. These buds grow and differentiate, forming the arms and hands.

Dermatomes are areas of skin innervated by a specific spinal nerve. They are aligned with the embryonic origin of the upper limb.

C5 innervates the lateral aspect of the arm, C6 innervates the lateral forearm, hand, and radial two digits, C7 innervates the middle finger and middle of the hand, C8 innervates the ulnar two digits and medial hand/wrist, and T1 innervates the medial aspect of the forearm, elbow, and lower arm. C4's upper limb sensation is not specified.

The brachial plexus has a distal innervation gradient. C5-C6 fibers innervate proximal muscles, C7 fibers innervate mid-region muscles, and C8-T1 fibers innervate distal muscles.

The dorsal scapular nerve arises from C5 and innervates the rhomboids and levator scapulae muscles, which help control scapular movement.

Damage to the long thoracic nerve, which arises from C5, C6, and C7, can weaken the serratus anterior muscle. This can lead to a condition called "winged scapula."

The scapula (shoulder blade) can move in various directions. Elevation/depression is controlled by the upper trapezius and pectoralis minor muscles. Protraction/retraction is controlled by the serratus anterior for protraction and the rhomboids for retraction.

The three trunks of the brachial plexus are formed by the roots: the superior trunk is formed by C5 and C6 fibers, the middle trunk is formed by C7 fibers, and the inferior trunk is formed by C8 and T1 fibers.

Each trunk divides into an anterior and posterior division, focusing on different muscle groups. The anterior divisions innervate anterior muscles of the arm and forearm, while the posterior divisions innervate posterior muscles.

The cords of the brachial plexus are named based on their location relative to the axillary artery, which runs through the armpit. The cords are formed by the joining of the anterior and posterior divisions.

The axillary artery is a continuation of the subclavian artery after it passes the first rib. It supplies blood to the upper limb.

The roots and trunks of the brachial plexus pass between the anterior and middle scalene muscles, which are located in the neck. Compression in this area can lead to problems with the brachial plexus.

Scalene interval syndrome occurs when the brachial plexus is compressed within the scalene muscles. Symptoms include a cool, pale upper limb and diminished pulses.

The posterior cord of the brachial plexus gives rise to several nerves that innervate muscles in the shoulder and back. These include the upper subscapular, thoracodorsal, and lower subscapular nerves.

The axillary nerve is vulnerable to injury during shoulder trauma, such as a dislocation. Injury to the axillary nerve can impair movement of the shoulder joint.

The radial nerve is also susceptible to compression injuries, which can affect movement and sensation in the arm and hand. Common causes of radial nerve compression include prolonged pressure on the arm or a fracture of the humerus, leading to 'crutch palsy'.

The rotator cuff is a group of four muscles that surround the shoulder joint. These muscles work together to stabilize the joint and allow for a wide range of motion.

Avulsion fractures are breaks in a bone that occur when a tendon or ligament pulls a piece of bone away from the main part of the bone.

Avulsion fractures are common in the greater tubercle of the humerus (the upper arm bone). This is often caused by a fall onto an outstretched arm.

The supraspinatus muscle is the primary muscle responsible for initiating arm abduction (lifting the arm away from the body) at the shoulder joint. The deltoid muscle assists with this movement once the arm is raised.

The brachial plexus is a complex network of nerves that innervates the shoulder and upper limb. Understanding its anatomy and function is critical for diagnosing and treating injuries or conditions affecting this region.