Spinal nerve architecture, the four major plexuses and the nerves they spin off, intercostal nerves, dermatomes, and the classic peripheral nerve injury patterns.
The peripheral nervous system is everything outside the brain and cord: the receptors that pick up the world, the nerves that carry signals, the ganglia where cell bodies cluster, and the motor endings that move us. Before tracing individual nerves, get the framework.
The PNS includes all neural structures outside the brain and spinal cord.
The same structures use different names depending on whether they live in the CNS or PNS.
| Cell Bodies | Axons | |
|---|---|---|
| CNS | Nuclei | Tracts |
| PNS | Ganglia | Nerves |
Receptors respond to changes (stimuli) in the internal or external environment.
Receptors are classified three ways: by location, by stimulus type, and by structure.
A nerve is a cordlike organ of the PNS, made of parallel bundles of peripheral axons (some myelinated, some not), supported by connective tissue.
The PNS is functionally divided into the somatic system (voluntary, skeletal muscle) and the autonomic system (involuntary, viscera). The autonomic system splits further into sympathetic, parasympathetic, and enteric divisions.
The autonomic divisions get their own dedicated note set. This file focuses on the somatic side: spinal nerves and the named peripheral nerves they form.
Every spinal nerve is a mixed nerve. Sensory axons enter through the back, motor axons leave through the front, and they merge just outside the cord into a single nerve trunk. The connective tissue wrapping is the same idea as muscle wrapping, just with different names.
Spinal nerves are paths of communication between the cord and specific regions of the body.
| Region | Pairs |
|---|---|
| Cervical | 8 (C1 to C8) |
| Thoracic | 12 (T1 to T12) |
| Lumbar | 5 (L1 to L5) |
| Sacral | 5 (S1 to S5) |
| Coccygeal | 1 (Co1) |
All 31 are mixed nerves: they carry both sensory and motor axons.
Roots are bundles of axons that connect each spinal nerve to a segment of the cord. Rootlets are the even smaller bundles that converge to form a root.
Three layers of connective tissue organize the axons of a peripheral nerve, parallel to the wrapping pattern in skeletal muscle.
After passing through the intervertebral foramen, a spinal nerve divides into branches called rami. Each ramus carries both sensory and motor fibers to its target territory.
Recall from the cord notes: because the cord ends at L1 to L2 but the column continues, lumbar, sacral, and coccygeal roots travel inferiorly inside the canal as the cauda equina before reaching their respective intervertebral foramina.
Anterior rami above and below the thoracic region do not run straight to a target. They form networks called plexuses, then send out named nerves. The cervical plexus drives the diaphragm; the brachial plexus drives the entire upper limb.
A plexus is a network of axons from the anterior rami of multiple spinal nerves. Axons from adjacent levels intermingle, then emerge as named nerves that supply specific regions.
The named nerves emerging from a plexus are typically named for the region they serve.
The anterior rami of T2 through T12 do not form plexuses. They run directly to their targets and are called intercostal nerves (or thoracic nerves).
T1 contributes to the brachial plexus instead.
Formed by the anterior rami of C1 through C4, with some contributions from C5. One plexus on each side of the neck, alongside the first four cervical vertebrae.
Clinical Significance
Complete cord transection above the origin of the phrenic nerves (above C3) causes respiratory arrest. The phrenic nerves can no longer send impulses to the diaphragm. Pressure from malignant tracheal or esophageal tumors in the mediastinum can also damage the phrenic nerves.
Formed by the anterior rami of C5, C6, C7, C8, and T1. Passes above the first rib, posterior to the clavicle, and enters the axilla.
The branches of the brachial plexus provide almost the entire nerve supply of the shoulders and upper limbs.
| Nerve | Cord of Origin | Supplies |
|---|---|---|
| Axillary | Posterior | Deltoid and teres minor muscles |
| Musculocutaneous | Lateral | Anterior muscles of the arm (biceps brachii, brachialis, coracobrachialis) |
| Radial | Posterior | Posterior aspect of the arm and forearm (extensors) |
| Median | Lateral and Medial | Most muscles of the anterior forearm and some hand muscles |
| Ulnar | Medial | Anteromedial muscles of the forearm and most hand muscles |
Six classic brachial plexus injury patterns appear repeatedly on exams and in clinic. Each one has a signature posture, a signature mechanism, and a single nerve or root to blame. Learn the picture, learn the cause, learn the nerve.
Erb-Duchenne Palsy ("Waiter's Tip" Posture)
Injury location: superior roots of the brachial plexus (C5 to C6).
Mechanism: forceful pulling away of the head from the shoulder. Common scenarios include a heavy fall on the shoulder, an object falling onto the shoulder, or excessive stretching of an infant's neck during childbirth.
Presentation: the affected upper limb hangs in a characteristic posture. Shoulder is adducted, arm is medially rotated, elbow is extended, forearm is pronated, and wrist is flexed. The hand looks as if it is reaching back for a tip.
Sensory loss: along the lateral side of the arm.
| Syndrome | Nerve / Root | Hallmark |
|---|---|---|
| Erb-Duchenne palsy | C5, C6 roots | Waiter's tip posture |
| Wrist drop | Radial nerve | Cannot extend wrist or fingers |
| Ape hand / Benediction | Median nerve | Cannot oppose thumb; cannot make a fist |
| Claw hand | Ulnar nerve | Hyperextended MCPs, flexed IPs in digits 4 and 5 |
| Winged scapula | Long thoracic nerve | Medial scapular border protrudes |
| Thoracic outlet syndrome | Brachial plexus compression | Diffuse upper limb pain and numbness |
The lower-body plexuses follow the same logic as the upper. Anterior rami merge, then send named nerves to specific regions. Two stand out: the femoral nerve (powerhouse of the anterior thigh) and the sciatic nerve (the largest nerve in the body).
Formed by the anterior rami of L1 through L4. Located alongside the four lumbar vertebrae, between the two heads of the psoas major muscle. Minimal intermingling of fibers compared with the brachial plexus.
Formed by the anterior rami of L4 and L5 together with S1 through S4. Located just anterior to the sacrum.
The smallest plexus. Formed by the anterior rami of S4 and S5 and the coccygeal nerves.
Clinical: Femoral Nerve Injury
Mechanism: most commonly stab or gunshot wounds. Pelvic fractures, hip surgery, and prolonged retraction during abdominal surgery are also causes.
Presentation:
Clinical: Obturator Nerve Injury
Mechanism: often pressure on the nerve from the fetal head during pregnancy. Pelvic surgery and pelvic tumors can also injure the obturator.
Presentation:
Clinical: Sciatica
Definition: pain along the distribution of the sciatic nerve, typically extending from the buttock down the posterior and lateral aspect of the leg and foot.
Causes:
Why gluteal IM injections must avoid the medial buttock: the sciatic nerve runs through the gluteal region, and a misplaced injection can damage it directly. Injections are placed in the upper outer quadrant of the buttock to stay clear of the nerve.
| Plexus | Roots | Major Nerves | Classic Injury |
|---|---|---|---|
| Lumbar | L1 to L4 | Femoral, obturator, lateral femoral cutaneous | Femoral (cannot extend leg); obturator (cannot adduct thigh) |
| Sacral | L4 to S4 | Sciatic (tibial + common fibular), gluteal nerves, pudendal | Sciatica (radiating posterior leg pain) |
| Coccygeal | S4 to Co1 | Anococcygeal nerves | Coccydynia (rare) |
Each spinal nerve carries sensory information from a specific patch of skin. Map those patches across the body and you have a dermatome map. A few landmark dermatomes are worth knowing cold; they let you localize a cord or root injury at the bedside.
The skin over the entire body is supplied by somatic sensory neurons that carry impulses from the skin to the cord. Each spinal nerve contains sensory neurons that serve a specific region of skin. That region is called a dermatome.
Adjacent dermatomes overlap, which is why injury to a single spinal nerve rarely produces complete sensory loss in any one area. The exception is the trigeminal nerve territory of the face, which is served by cranial nerve V rather than spinal nerves.
| Level | Skin Region |
|---|---|
| C2 | Posterior scalp and angle of jaw |
| C3 | Neck and posterior cheek |
| C4 | Top of shoulder, clavicle area |
| C5 | Lateral arm (deltoid area) |
| C6 | Lateral forearm and thumb |
| C7 | Middle finger |
| C8 | Medial hand and little finger |
| T1 | Medial forearm |
| Level | Skin Region |
|---|---|
| T2 | Axilla |
| T4 | Nipple line |
| T7 | Inferior to the costal margin |
| T10 | Umbilicus |
| L1 | Inguinal region |
| Level | Skin Region |
|---|---|
| L2 | Anterior mid-thigh |
| L3 | Medial thigh and knee |
| L4 | Medial leg and medial foot (big toe) |
| L5 | Lateral leg and dorsum of foot |
| S1 | Lateral foot and posterior leg |
| S2 to S5 | Posterior thigh and perineum (concentric "saddle" pattern around the anus at S4 to S5) |