The nerves that serve your upper limbs arise from the spinal cord at five specific levels in the neck and upper back: the fifth, sixth, seventh, and eighth cervical vertebrae (C5 through C8) plus the first thoracic vertebra (T1). These five spinal nerve roots merge together to form a complex network called the brachial plexus, which is responsible for all motor control and sensation in the shoulder, arm, forearm, and hand.
The Five Spinal Roots
Each spinal nerve root exits the vertebral column through small openings between adjacent vertebrae. The roots that supply the upper limb emerge from the front-facing (ventral) side of the spinal cord, specifically from a bundle of fibers called the ventral ramus at each level. These ventral rami from C5 through T1 are the starting point of every nerve that will eventually reach your fingertips.
Not everyone’s wiring is identical. About 11% of people have a “prefixed” brachial plexus, meaning C4 (one level higher in the neck) also contributes nerve fibers. Roughly 1% have a “postfixed” arrangement, where T2 sends fibers into the network. Fewer than 0.1% receive contributions from both C4 and T2. These variations rarely cause problems, but they matter during surgery or nerve block procedures in the neck and shoulder region.
Where the Roots Travel First
After exiting the spine, the nerve roots pass through a tight anatomical corridor in the side of the neck called the interscalene triangle. This triangle is formed by two neck muscles (the scalenus anterior on one side and the scalenus medius on the other), with the first rib forming the base. The brachial plexus sits at the bottom of this triangle before continuing its path toward the armpit and arm. This space is a common site for nerve compression and is also where anesthesiologists place nerve blocks before shoulder or arm surgery.
From Roots to Branches
The brachial plexus doesn’t simply send five separate nerves down the arm. Instead, the fibers from all five roots mix and reorganize in a specific hierarchy: roots merge into trunks, trunks split into divisions, divisions regroup into cords, and cords finally produce the terminal branches that reach specific muscles and skin areas. This layered reorganization is why a single muscle in your forearm can receive nerve fibers originating from two or three different spinal levels, and why damage at one level doesn’t necessarily paralyze an entire limb.
The three trunks form in the neck. The upper trunk combines C5 and C6, the middle trunk continues from C7 alone, and the lower trunk joins C8 and T1. Each trunk then splits into a front and back division as it passes behind the collarbone. These divisions regroup into three cords (named lateral, posterior, and medial based on their position around the axillary artery in the armpit), which finally give off the five major nerves of the upper limb.
The Five Major Nerves
The brachial plexus produces five terminal nerves, each responsible for specific movements and areas of sensation:
- Musculocutaneous nerve: Powers the muscles that bend your elbow (primarily the biceps) and provides sensation to the outer forearm.
- Axillary nerve: Controls the deltoid muscle that lifts your arm out to the side and provides sensation over the outer shoulder.
- Radial nerve: The largest branch, responsible for straightening the elbow, wrist, and fingers. It supplies sensation to the back of the hand.
- Median nerve: Controls most forearm muscles that bend the wrist and fingers, and provides sensation to the thumb, index, middle, and half of the ring finger. Compression of this nerve at the wrist causes carpal tunnel syndrome.
- Ulnar nerve: Powers the small muscles of the hand that spread and close the fingers, and provides sensation to the little finger and the adjacent half of the ring finger. This is the nerve you irritate when you hit your “funny bone.”
How Each Spinal Level Maps to Your Skin
Because each spinal root carries sensory fibers from a specific strip of skin (called a dermatome), doctors can trace numbness or tingling back to the spinal level causing the problem. The key landmarks for the upper limb are straightforward: C6 supplies the thumb, C7 the middle finger, C8 the little finger, and T1 covers the inner forearm and upper arm. C5 supplies the outer arm near the shoulder. If you feel numbness in your thumb, for instance, that points toward an issue at the C6 level.
Blood Supply to the Nerve Roots
The nerves themselves need their own blood supply to function. At the spinal cord level, the vertebral artery feeds the nerve rootlets through small spinal branches. Once the plexus moves into the neck and toward the arm, the subclavian artery and its branches take over as the primary blood source. This is clinically relevant because conditions that compress or narrow the subclavian artery (such as thoracic outlet syndrome) can starve the nerves of oxygen and cause pain, weakness, or numbness in the arm.
What Happens When These Roots Are Injured
Injuries to the brachial plexus produce different patterns depending on which roots are damaged. Erb’s palsy affects the upper roots (C5 and C6), causing numbness and paralysis mainly in the shoulder and upper arm. The arm typically hangs limply with the forearm rotated inward, sometimes called the “waiter’s tip” position. This pattern is most commonly seen in newborns after a difficult delivery or in adults after motorcycle accidents or falls that force the head and shoulder apart.
Klumpke’s palsy affects the lower roots (C8 and T1), resulting in loss of sensation and paralysis in the wrist and hand. Grip strength disappears, and the small muscles of the hand waste away. This type of injury can occur when the arm is yanked forcefully overhead. Both types range from mild stretching injuries that heal over weeks to complete nerve tears that require surgical repair.