The 31 pairs of spinal nerves connect the spinal cord to the body, carrying sensory information from the periphery and motor commands to muscles and glands.
General Organization
Spinal Nerve Numbers
| Region | Pairs | Spinal Segments |
|---|---|---|
| Cervical | 8 | C1-C8 |
| Thoracic | 12 | T1-T12 |
| Lumbar | 5 | L1-L5 |
| Sacral | 5 | S1-S5 |
| Coccygeal | 1 | Co1 |
Nerve Root Anatomy
Each spinal nerve is formed by the union of:
| Root | Direction | Fibers | Cell Bodies |
|---|---|---|---|
| Dorsal (posterior) root | Sensory (afferent) | GSA, GVA | Dorsal root ganglion |
| Ventral (anterior) root | Motor (efferent) | GSE, GVE | Anterior horn (GSE), lateral horn (GVE) |
Spinal nerve: Mixed nerve (sensory + motor) formed by union of dorsal and ventral roots
Meningeal branch (recurrent/sinuvertebral nerve): Returns through intervertebral foramen to innervate meninges, ligaments, discs
Rami
After emerging from the intervertebral foramen, the spinal nerve divides:
| Ramus | Distribution |
|---|---|
| Dorsal ramus | Deep back muscles, skin of back |
| Ventral ramus | Anterior and lateral trunk, limbs (forms plexuses) |
Dermatomes and Myotomes
Dermatomes
A dermatome is the area of skin supplied by a single spinal nerve and its spinal segment.
| Segment | Key Landmark |
|---|---|
| C2 | Posterior scalp |
| C3 | Upper neck |
| C4 | Lower neck, upper shoulder |
| C5 | Lateral shoulder (deltoid) |
| C6 | Lateral forearm, thumb, index finger |
| C7 | Middle finger, palm |
| C8 | Ring finger, little finger, medial forearm |
| T1 | Medial forearm (above elbow) |
| T4 | Nipple line |
| T7 | Xiphoid process |
| T10 | Umbilicus |
| L1 | Inguinal region |
| L2 | Anterior upper thigh |
| L3 | Anterior lower thigh, knee |
| L4 | Medial leg, great toe |
| L5 | Lateral leg, middle toes, dorsum of foot |
| S1 | Lateral foot, little toe, posterior calf |
| S2 | Posterior thigh, popliteal fossa |
| S3-S4 | Perineum, buttocks |
Myotomes
A myotome is the group of muscles innervated by a single spinal nerve.
| Segment | Muscle | Action |
|---|---|---|
| C1-C2 | Upper neck flexors/extensors | Head flexion/extension |
| C3-C4 | Diaphragm (C3-C5) | Inspiration |
| C5 | Deltoid, biceps | Shoulder abduction, elbow flexion |
| C6 | Extensor carpi radialis, biceps | Wrist extension, elbow flexion |
| C7 | Triceps, flexor carpi radialis | Elbow extension, wrist flexion |
| C8 | Flexor digitorum profundus | Finger flexion |
| T1 | Interossei | Finger abduction/adduction |
| L2 | Iliopsoas | Hip flexion |
| L3 | Quadriceps | Knee extension |
| L4 | Tibialis anterior | Ankle dorsiflexion |
| L5 | Extensor hallucis longus | Great toe extension |
| S1 | Gastrocnemius | Ankle plantarflexion |
| S2 | Hamstrings | Knee flexion |
Plexuses
Spinal nerves C1-T1 and L1-S4 form plexuses (networks) that redistribute nerve fibers before forming peripheral nerves.
Cervical Plexus (C1-C4)
| Nerve | Roots | Motor | Sensory |
|---|---|---|---|
| Phrenic | C3-C5 | Diaphragm | Mediastinal pleura, pericardium |
| Ansa cervicalis (superior/inferior roots) | C1-C3 | Infrahyoid muscles | — |
| Lesser occipital | C2 | — | Posterior scalp |
| Great auricular | C2-C3 | — | Ear, parotid |
| Transverse cervical | C2-C3 | — | Anterior neck |
| Supraclavicular | C3-C4 | — | Upper chest, shoulder |
Brachial Plexus (C5-T1)
The brachial plexus has a complex structure formed by roots, trunks, divisions, cords, and terminal branches.
Structure:
- Roots: C5, C6, C7, C8, T1
- Trunks: Upper (C5-C6), Middle (C7), Lower (C8-T1)
- Divisions: Anterior (flexor) and posterior (extensor) for each trunk
- Cords: Lateral (anterior divisions of upper/middle), Medial (anterior division of lower), Posterior (all three posterior divisions)
Terminal branches:
| Nerve | Cord | Roots | Motor | Sensory |
|---|---|---|---|---|
| Musculocutaneous | Lateral | C5-C7 | Elbow flexors (biceps, brachialis, coracobrachialis) | Lateral forearm |
| Median | Lateral + Medial | C6-T1 | Forearm flexors (except FCU), thenar muscles, lumbricals 1-2 | Palmar hand (lateral 3.5 digits) |
| Ulnar | Medial | C8-T1 | FCU, hypothenar, interossei, lumbricals 3-4, adductor pollicis, deep finger flexors | Medial 1.5 digits |
| Radial | Posterior | C5-C8, T1 | Triceps, wrist/finger extensors, brachioradialis | Posterior arm, forearm, dorsum of hand |
| Axillary | Posterior | C5-C6 | Deltoid, teres minor | Shoulder (regimental badge area) |
Branches from roots:
- Dorsal scapular (C5): Rhomboids, levator scapulae
- Long thoracic (C5-C7): Serratus anterior
Branches from trunks:
- Suprascapular (C5-C6): Supraspinatus, infraspinatus
- Nerve to subclavius (C5-C6)
Branches from cords:
- Lateral pectoral (C5-C7): Pectoralis major
- Medial pectoral (C8-T1): Pectoralis minor, major
- Medial cutaneous nerves of arm/forearm (C8-T1)
- Upper/lower subscapular (C5-C6): Subscapularis, teres major
- Thoracodorsal (C6-C8): Latissimus dorsi
Lumbar Plexus (L1-L4)
| Nerve | Roots | Motor | Sensory |
|---|---|---|---|
| Iliohypogastric | T12-L1 | Abdominal wall | Suprapubic, lateral gluteal |
| Ilioinguinal | L1 | — | Inguinal, scrotal/labial |
| Genitofemoral | L1-L2 | Cremaster | Anterior thigh, genital |
| Lateral femoral cutaneous | L2-L3 | — | Lateral thigh |
| Femoral | L2-L4 | Quadriceps, sartorius, pectineus, iliacus | Anterior thigh, medial leg |
| Obturator | L2-L4 | Adductors (longus, brevis, magnus), gracilis, obturator externus | Medial thigh |
Sacral Plexus (L4-S4)
| Nerve | Roots | Motor | Sensory |
|---|---|---|---|
| Sciatic (common peroneal + tibial) | L4-S3 | Hamstrings, adductor magnus (hamstring part) | Posterior thigh |
| Superior gluteal | L4-S1 | Gluteus medius, minimus, tensor fasciae latae | — |
| Inferior gluteal | L5-S2 | Gluteus maximus | — |
| Posterior femoral cutaneous | S1-S3 | — | Posterior thigh, perineum |
| Pudendal | S2-S4 | Perineal muscles, external sphincters | Perineum, genitals |
Common peroneal (fibular) nerve:
- Motor: Anterior/lateral leg muscles (dorsiflexion, eversion)
- Sensory: Lateral leg, dorsum of foot
- Lesion: Foot drop (loss of dorsiflexion), loss of sensation on dorsum of foot
Tibial nerve:
- Motor: Posterior leg muscles (plantarflexion, toe flexion)
- Sensory: Heel, sole of foot
- Lesion: Loss of plantarflexion, loss of sensation on sole
Peripheral Neuropathies
Common Entrapment Neuropathies
| Nerve | Entrapment Site | Syndrome |
|---|---|---|
| Median (above wrist) | Carpal tunnel | Carpal tunnel syndrome |
| Ulnar | Cubital tunnel (elbow) | Cubital tunnel syndrome |
| Ulnar | Guyon canal (wrist) | Ulnar tunnel syndrome |
| Radial | Spiral groove (humerus) | Saturday night palsy |
| Common peroneal | Fibular neck | Foot drop |
| Tibial (distal) | Tarsal tunnel | Tarsal tunnel syndrome |
| Lateral femoral cutaneous | Inguinal ligament | Meralgia paresthetica |
Nerve Injury Classification (Seddon)
| Grade | Type | Prognosis |
|---|---|---|
| I | Neurapraxia | Conduction block, myelin damage, excellent recovery |
| II | Axonotmesis | Axon disruption, intact endoneurium, good recovery (1 mm/day) |
| III | Neurotmesis | Complete nerve transection, poor recovery without surgery |