Formation and Components of the Brachial Plexus
The brachial plexus is a network of nerves formed by the anterior primary rami (ventral rami) of the lower four cervical and first thoracic spinal nerves (C5, C6, C7, C8, T1), with occasional contributions from C4 and T2. It is the nerve plexus that innervates the entire upper limb.
Mnemonic: Roots Trunks Divisions Cords Nerves = “Really Tired Drink Coffee Now”
Overview
| Component | Number | Location |
|---|---|---|
| Roots | 5 | Neck — deep to scalenus anterior |
| Trunks | 3 | Neck — between scalenus anterior and medius (posterior triangle) |
| Divisions | 6 (3 anterior + 3 posterior) | Behind the clavicle |
| Cords | 3 | Axilla — around the axillary artery |
| Branches (terminal) | 5 major + collateral | Axilla and arm |
Roots
- Constituted by the anterior primary rami of C5, C6, C7, C8, and T1
- Located in the neck, deep to the scalenus anterior muscle
- C4 and T2 make minor contributions in some individuals
Prefixed and Postfixed Plexus (BD Chaurasia)
| Variant | Description |
|---|---|
| Prefixed plexus | Contribution from C4 is large; T2 is often absent; plexus shifted one segment upward |
| Postfixed plexus | Contribution from T1 is large; T2 always present; C4 absent; C5 reduced in size; plexus shifted one segment downward |
Trunks
Three trunks, formed in the posterior triangle of the neck, occupy the cleft between scalenus medius (behind) and scalenus anterior (in front):
| Trunk | Formed By | Common Name |
|---|---|---|
| Upper trunk | C5 + C6 roots | — |
| Middle trunk | C7 root alone | — |
| Lower trunk | C8 + T1 roots | — |
Memory: “C5C6 upper, C7 middle, C8T1 lower”
Divisions
Each of the three trunks divides into an anterior division and a posterior division = 6 divisions total.
- Lie behind the clavicle
- Anterior divisions → supply the anterior (flexor) compartment of the limb
- Posterior divisions → supply the posterior (extensor) compartment of the limb
Cords
Three cords, named by their relationship to the second part of the axillary artery:
| Cord | Formed By | Position to Axillary Artery |
|---|---|---|
| Lateral cord | Anterior divisions of upper + middle trunks (2 divisions) | Lateral |
| Medial cord | Anterior division of lower trunk (1 division) | Medial |
| Posterior cord | Posterior divisions of all three trunks (3 divisions) | Posterior |
Memory: “Lateral loves 2, Medial has 1, Posterior gets all 3”
Topography
Supraclavicular Part (Roots + Trunks)
- Located in the posterior triangle of the neck
- Roots: between anterior and middle scalene muscles
- Trunks: in the gap between scalenus anterior and scalenus medius
- Exits the neck by passing over the 1st rib
Infraclavicular Part (Divisions + Cords + Branches)
- Located in the axilla
- Enclosed in the axillary sheath — derived from the prevertebral fascia
- Cords surround the axillary artery
Relations of the Cords to the Axillary Artery (2nd Part)
The cords are named for their relation to the second part of the axillary artery (deep to pectoralis minor):
- Lateral cord → lateral side
- Medial cord → medial side
- Posterior cord → posterior aspect
Sympathetic Contribution
Sympathetic fibres for the upper limb are derived from spinal segments T2–T6:
- Preganglionic fibres → white rami communicantes → sympathetic chain → ascend to cervical and upper thoracic ganglia
- Postganglionic fibres → grey rami → join brachial plexus roots
- Vasomotor (constricts skin arterioles), sudomotor (sweat), pilomotor (hair)
Viva Pearls
- Brachial plexus = anterior primary rami of C5–T1
- 5 Roots → 3 Trunks → 6 Divisions → 3 Cords → Branches
- Upper trunk = C5 + C6; Middle = C7; Lower = C8 + T1
- Lateral cord = anterior div of upper + middle trunks
- Medial cord = anterior div of lower trunk
- Posterior cord = all 3 posterior divisions
- Cords named relative to 2nd part of axillary artery
- Axillary sheath derived from prevertebral fascia
- Prefixed plexus: large C4 contribution; Postfixed: T2 present, C4 absent