The term “arm” in common language often refers to the entire upper limb, encompassing the region from the shoulder to the hand. However, in anatomical terms, the arm specifically denotes the segment between the shoulder and the elbow. This article explores the key anatomical components that facilitate the arm’s diverse capabilities.
Bones That Form the Arm
The humerus is the single bone of the upper arm, extending from the shoulder to the elbow. It provides attachment points for numerous muscles. The humerus connects with the shoulder joint and with the forearm bones to form the elbow joint.
The forearm, located between the elbow and the wrist, consists of two bones: the radius and the ulna. The radius is positioned on the thumb side of the forearm, while the ulna is on the pinky finger side. The radius is shorter than the ulna and is important for wrist and hand rotation, while the ulna provides stability at the elbow joint.
Muscles Powering Arm Movement
In the upper arm, the biceps brachii and triceps brachii are primary movers. The biceps brachii, located on the front of the upper arm, is a two-headed muscle that primarily flexes the elbow and supinates (rotates outward) the forearm. Its antagonist, the triceps brachii, is situated on the back of the upper arm and is responsible for extending the elbow joint.
The forearm contains numerous muscles, broadly categorized into anterior and posterior compartments. Muscles in the anterior compartment primarily act as flexors, bending the wrist and fingers. Muscles in the posterior compartment are mainly extensors, straightening the wrist and fingers.
Nerves and Blood Vessels of the Arm
The arm and forearm receive their neural supply from the brachial plexus, a network of nerves originating from the spinal cord in the neck and shoulder region. Three major nerves branch from this plexus: the radial, ulnar, and median nerves. The radial nerve controls the extensor muscles in the arm and forearm, enabling elbow, wrist, and finger extension, and provides sensation to parts of the arm, forearm, wrist, and hand.
The ulnar nerve controls most of the small muscles in the hand, aiding gripping and fine motor skills, and provides sensation to the pinky and ring fingers. The median nerve supplies most of the flexor muscles in the forearm and hand, facilitating movements like wrist, thumb, and finger flexion, and provides sensation to the palm and certain fingers.
Blood supply to the arm is primarily carried by the brachial artery, which is a continuation of the axillary artery. This vessel runs along the front of the upper arm and supplies blood to the muscles of the upper arm and elbow joint. Near the elbow, the brachial artery divides into two main arteries of the forearm: the radial artery and the ulnar artery. The radial artery supplies the lateral forearm and hand, while the ulnar artery supplies the medial forearm and hand.
Key Joints of the Arm
The shoulder joint, anatomically known as the glenohumeral joint, connects the humerus to the shoulder blade (scapula). This ball-and-socket joint allows for an extensive range of motion, including flexion, extension, abduction (raising the arm away from the body), adduction (lowering the arm towards the body), and rotation. The shoulder joint’s wide mobility supports the arm’s function.
The elbow joint is a hinge joint formed by the articulation of the humerus with the ulna and radius. This joint primarily allows for flexion and extension of the forearm. In addition to the hinge movement, the proximal radioulnar joint, formed between the radius and ulna at the elbow, enables the forearm to rotate, allowing movements like turning the palm up (supination) and down (pronation).
The wrist joint, or radiocarpal joint, connects the forearm bones (radius and ulna) to the carpal bones of the hand. This joint permits movements such as wrist flexion (bending the hand forward), extension (bending the hand backward), and side-to-side movements. These interconnected joints provide the upper limb with dexterity and adaptability.