6.2 Peripheral and Autonomic Nervous Systems
Key Takeaways
- Each mixed spinal nerve is dorsal root (sensory, cell bodies in the dorsal-root ganglion) plus ventral root (motor); it immediately gives a dorsal ramus, a ventral ramus, a recurrent meningeal (sinuvertebral) nerve, and rami communicantes.
- The brachial plexus is C5–T1 arranged as roots, trunks, divisions, cords, and terminal branches; long thoracic (C5–C7) and dorsal scapular arise from the roots, before trunks.
- White rami communicantes (preganglionic sympathetics) exist only at T1–L2; gray rami (postganglionic) join spinal nerves at all levels.
- CN VII and VIII traverse the internal acoustic meatus; CN IX, X, and XI the jugular foramen; CN XII the hypoglossal canal; CN III, IV, V1, and VI the superior orbital fissure.
- Parasympathetic preganglionic outflow is CN III, VII, IX, and X plus S2–S4 pelvic splanchnics; sympathetic preganglionic outflow is the intermediolateral column at T1–L2.
Why PNS and ANS anatomy is the heaviest Spinal Anatomy topic
Anatomy of the peripheral and autonomic nervous systems is 19% of Spinal Anatomy, the largest official bullet group in the 22% domain. The test plan lists spinal nerves, plexuses, and branches; cranial nerves; visceral innervation; cutaneous innervation; development; and histology. Expect matching items: which ventral rami form the phrenic nerve, which foramen transmits CN IX–XI, which splanchnic nerve reaches the celiac ganglion, which dermatome covers the umbilicus.
Spinal nerves: roots, rami, and the canal
There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. C1–C7 exit above the same-numbered vertebra; C8 exits between C7 and T1; T1 and below exit below the same-numbered vertebra.
Each nerve is built the same way:
- Dorsal root — sensory (GSA and GVA) axons. Cell bodies sit in the dorsal-root (spinal) ganglion in the intervertebral foramen, a pseudounipolar neural-crest derivative. There is no synapse in the DRG.
- Ventral root — motor axons from ventral-horn cells (GSE) plus, at T1–L2 and S2–S4, preganglionic autonomic axons (GVE) that leave via the ventral root.
- Roots unite distal to the DRG as a short mixed spinal nerve in the IVF.
- The spinal nerve immediately branches.
| Branch | Fiber types | Territory |
|---|---|---|
| Dorsal (posterior) ramus | Mixed | Epaxial muscles (true back muscles) and overlying skin in a segmental strip; C1 dorsal ramus is the suboccipital nerve (motor to suboccipitals, typically no cutaneous patch); C2 dorsal ramus' medial branch is the greater occipital nerve (semispinalis capitis, scalp to the vertex) |
| Ventral (anterior) ramus | Mixed | Hypaxial body wall and limbs; cervical, brachial, lumbar, and sacral plexuses; thoracic ventral rami remain segmental as intercostal nerves (T12 = subcostal) |
| Recurrent meningeal (sinuvertebral) nerve | Mixed, with a sympathetic contribution | Re-enters the canal; supplies posterior longitudinal ligament, outer anulus fibrosus, ventral dura, and vessels. This is the anatomic substrate of discogenic and PLL pain, not a technique name |
| White ramus communicans | Myelinated preganglionic sympathetic | T1–L2 only |
| Gray ramus communicans | Unmyelinated postganglionic sympathetic | All spinal levels, returning vasomotor, sudomotor, and pilomotor fibers to the spinal nerve |
A typical thoracic nerve then splits into lateral and anterior cutaneous branches. The meningeal branch is easily forgotten on items that ask what innervates the outer disc and PLL from within the canal.
Nerve-root versus peripheral-nerve lesions are distinguished anatomically: a root lesion follows a dermatome/myotome/sclerotome; a peripheral-nerve lesion follows the named nerve's cutaneous map after plexus mixing. That distinction is still anatomy, not a full electrodiagnostic protocol.
Plexuses and key branches
Ventral rami mix in four named plexuses. Thoracic ventral rami T2–T11 do not form a limb plexus (T1 contributes to brachial; T12 is subcostal and may contribute to lumbar).
Cervical plexus (C1–C4)
Forms in the posterior triangle on levator scapulae and scalenus medius, deep to SCM. Motor and cutaneous limbs:
| Nerve | Roots | Function / course |
|---|---|---|
| Ansa cervicalis | C1–C3 (superior root hitchhikes on CN XII) | Infrahyoid muscles except thyrohyoid (thyrohyoid is C1 fibers traveling with CN XII) |
| Phrenic | C3–C5, mainly C4 | Sole motor to the diaphragm; sensory to mediastinal and diaphragmatic pleura and peritoneum; runs on anterior scalene |
| Lesser occipital | C2 | Scalp behind the ear |
| Great auricular | C2–C3 | Angle of mandible, parotid fascia, auricle |
| Transverse cervical | C2–C3 | Anterior neck skin |
| Supraclavicular | C3–C4 | Skin over clavicle and upper thorax |
Cutaneous nerves of the plexus radiate from nerve point (Erb's point) at the midpoint of the posterior border of SCM. Phrenic is the motor item that is missed when a candidate only memorizes cutaneous names.
Brachial plexus (C5–T1)
Lies between anterior and middle scalenes (roots and trunks), then in the posterior triangle, then under the clavicle (divisions), then in the axilla around the axillary artery (cords named for their position relative to that artery). Prefixed (C4–C8) and postfixed (C6–T2) variants exist; the exam default is C5–T1.
Organization mnemonic: Randy Travis Drinks Cold Beer — Roots, Trunks, Divisions, Cords, Branches.
| Stage | Composition |
|---|---|
| Roots | Ventral rami C5–T1, between scalenes |
| Trunks | Upper (C5–C6), middle (C7), lower (C8–T1) |
| Divisions | Each trunk splits into anterior and posterior (supraclavicular; no named nerves arise from divisions) |
| Cords | Lateral = anterior divisions of upper + middle; medial = anterior division of lower; posterior = all three posterior divisions |
| Terminal branches | Musculocutaneous, median, ulnar, axillary, radial |
Root-level branches (tested because they are spared in some infraclavicular lesions and hit in scalene/root lesions):
| Nerve | Roots | Motor target |
|---|---|---|
| Dorsal scapular | C5 (often C4) | Rhomboids and levator scapulae |
| Long thoracic | C5–C7 | Serratus anterior (winged scapula if lost) |
| Nerve to subclavius | C5–C6 | Subclavius |
Upper-trunk branches: suprascapular (C5–C6) to supraspinatus and infraspinatus, through the scapular notch under the transverse scapular ligament (artery goes over). Lateral cord: lateral pectoral (pectoralis major clavicular), musculocutaneous (C5–C7: coracobrachialis, biceps, brachialis; continues as lateral cutaneous of forearm), lateral root of median. Medial cord: medial pectoral (pectoralis minor and major), medial cutaneous of arm and forearm, ulnar (C8–T1, often C7 to flexor carpi ulnaris/flexor digitorum profundus), medial root of median. Posterior cord: upper subscapular (subscapularis), thoracodorsal / middle subscapular (latissimus dorsi), lower subscapular (subscapularis and teres major), axillary (C5–C6: deltoid and teres minor; upper lateral cutaneous of arm; quadrangular space with posterior humeral circumflex), radial (C5–T1: posterior compartments of arm and forearm; triangular interval with profunda brachii).
Median nerve (C6–T1, sometimes C5) forms from lateral and medial cords, lateral to the axillary/brachial artery, then crosses to medial. No motor branches in the arm. In the forearm it supplies pronator teres, FCR, palmaris longus, FDS; anterior interosseous supplies FDP (lateral two), FPL, pronator quadratus. Thenar motor and lumbricals 1–2 in the hand. Ulnar supplies FCU, medial FDP, hypothenar, interossei, adductor pollicis, lumbricals 3–4. Radial supplies triceps, anconeus, brachioradialis, extensors; posterior interosseous (deep radial) is the motor continuation after the arcade of Frohse / supinator.
Lumbar plexus (L1–L4, often with T12)
Forms in psoas major. Branches emerge relative to psoas:
| Nerve | Roots | Course / function |
|---|---|---|
| Iliohypogastric | L1 | Internal oblique and transversus; skin of upper lateral gluteal and suprapubic |
| Ilioinguinal | L1 | Inguinal canal; skin of medial thigh, root of penis/mons, anterior scrotum/labium |
| Genitofemoral | L1–L2 | Pierces psoas; genital branch to cremaster; femoral branch to skin of femoral triangle |
| Lateral femoral cutaneous | L2–L3 | Under inguinal ligament near ASIS; skin of lateral thigh (meralgia anatomy) |
| Femoral | L2–L4 | Lateral to psoas, under inguinal ligament in the femoral triangle; iliacus, pectineus, sartorius, quadriceps; saphenous nerve is the terminal cutaneous |
| Obturator | L2–L4 | Medial to psoas, through obturator canal; adductor compartment (adductor magnus also tibial); skin of medial thigh |
| Lumbosacral trunk | L4–L5 | Joins sacral plexus |
Sacral plexus (L4–S4)
Forms on piriformis, exits via the greater sciatic foramen.
| Nerve | Roots | Function |
|---|---|---|
| Superior gluteal | L4–S1 | Gluteus medius, minimus, tensor fasciae latae; exits above piriformis |
| Inferior gluteal | L5–S2 | Gluteus maximus; exits below piriformis |
| Sciatic | L4–S3 | Tibial division: hamstrings except short head of biceps, then posterior leg and plantar foot. Common fibular division: short head of biceps, then anterior and lateral leg and dorsum of foot. Usually splits at the apex of the popliteal fossa |
| Posterior femoral cutaneous | S1–S3 | Skin of posterior thigh |
| Pudendal | S2–S4 | Exits greater sciatic foramen, around the ischial spine/sacrospinous ligament, re-enters through lesser sciatic foramen into Alcock's canal; perineal motor and sensory, external anal and urethral sphincters |
| Nerve to quadratus femoris / inferior gemellus | L4–S1 | Named small lateral rotators |
| Nerve to obturator internus / superior gemellus | L5–S2 | Named small lateral rotators |
| Nerve to piriformis | S1–S2 | Piriformis |
Tibial continues behind the medial malleolus in the tarsal tunnel. Common fibular winds around the fibular neck—the most vulnerable lower-limb nerve to compression against bone. Deep fibular: anterior compartment and first web space skin. Superficial fibular: lateral compartment and most of the dorsum of the foot.
Coccygeal plexus (S4–Co) is small: anococcygeal nerves to skin over the coccyx.
Cranial nerves I–XII
Treat each nerve as a row: nuclei, cranial exit, skull foramen, fiber types, function. CN I and II are CNS tracts (oligodendrocyte myelin); CN III–XII are true peripheral nerves (Schwann myelin) after they leave the brainstem.
| CN | Name | Brainstem attachment / nuclei | Skull exit | Primary functions |
|---|---|---|---|---|
| I | Olfactory | Forebrain; no brainstem nucleus | Cribriform plate of ethmoid | Special sense smell (SVA) |
| II | Optic | Diencephalic tract to LGN, pretectum, superior colliculus | Optic canal with ophthalmic artery | Vision (SSA) |
| III | Oculomotor | Midbrain: oculomotor nucleus, Edinger–Westphal | Superior orbital fissure | SR, IR, MR, IO, levator palpebrae; parasympathetic to sphincter pupillae and ciliary muscle via ciliary ganglion |
| IV | Trochlear | Midbrain (dorsal, crossed) | Superior orbital fissure | Superior oblique |
| V | Trigeminal | Midbrain mesencephalic; pontine chief sensory and motor; spinal trigeminal to upper cord | V1 superior orbital fissure; V2 foramen rotundum; V3 foramen ovale | Face/sinus/cornea sensation; muscles of mastication, mylohyoid, anterior digastric, tensor tympani, tensor veli palatini |
| VI | Abducens | Pons | Superior orbital fissure | Lateral rectus |
| VII | Facial | Pons: facial motor, superior salivatory, solitary (rostral) | Internal acoustic meatus → facial canal → stylomastoid foramen | Muscles of facial expression, stapedius, posterior digastric, stylohyoid; taste anterior two-thirds (chorda tympani); lacrimal and nasal glands via pterygopalatine ganglion; submandibular and sublingual via submandibular ganglion |
| VIII | Vestibulocochlear | Pontomedullary | Internal acoustic meatus | Hearing and equilibrium (SSA) |
| IX | Glossopharyngeal | Medulla: ambiguus, inferior salivatory, solitary, spinal trigeminal | Jugular foramen | Stylopharyngeus; taste and sensation posterior one-third of tongue; carotid sinus and body; parotid via otic ganglion; middle-ear sensation |
| X | Vagus | Medulla: ambiguus, dorsal motor, solitary | Jugular foramen | Palate, pharynx, larynx (except stylopharyngeus and tensor veli palatini); thoracic and abdominal viscera to the midgut/hindgut junction; taste from epiglottis; aortic baro- and chemoreceptors |
| XI | Accessory | Spinal nucleus C1–C5; cranial root often grouped with vagus | Spinal root up through foramen magnum, then jugular foramen | Sternocleidomastoid and trapezius |
| XII | Hypoglossal | Medulla, pre-olivary | Hypoglossal canal | Tongue muscles except palatoglossus (vagus) |
Parasympathetic ganglia (all in the head except vagal intramural):
| Ganglion | Preganglionic nerve | Postganglionic target |
|---|---|---|
| Ciliary | CN III (Edinger–Westphal) | Sphincter pupillae, ciliary muscle |
| Pterygopalatine | CN VII (greater petrosal → nerve of pterygoid canal) | Lacrimal, nasal, palatine glands |
| Submandibular | CN VII (chorda tympani hitchhiking on lingual nerve) | Submandibular and sublingual glands |
| Otic | CN IX (lesser petrosal) | Parotid gland |
Sympathetic postganglionics to the head hitchhike on the internal carotid plexus after synapsing in the superior cervical ganglion. Interruption (T1–T2 ciliospinal outflow, cervical chain, or carotid plexus) is the anatomic basis of Horner signs: ptosis (Müller's muscle), miosis, facial anhidrosis.
Visceral innervation
Sympathetic preganglionic cell bodies occupy the intermediolateral column of T1–L2 (sometimes L3). Myelinated axons exit via ventral roots, join the spinal nerve, and reach the sympathetic chain (paravertebral ganglia) via white rami. Four options: synapse at the same level; ascend (cervical ganglia: superior, middle, inferior/stellate); descend (lumbar and sacral chain); or pass through without synapsing as splanchnic nerves to prevertebral ganglia.
| Splanchnic pathway | Preganglionic levels | Synapse | Territory |
|---|---|---|---|
| Cervical and upper thoracic (cardiopulmonary) | T1–T4 (T5) | Cervical and upper thoracic chain | Heart, lungs, esophagus; head and neck via carotid plexuses |
| Greater splanchnic | T5–T9 | Celiac ganglion | Foregut derivatives (stomach, liver, pancreas, spleen, proximal duodenum) |
| Lesser splanchnic | T10–T11 | Aorticorenal / superior mesenteric | Midgut, kidney (partial) |
| Least splanchnic | T12 | Renal ganglion | Kidney, proximal ureter |
| Lumbar splanchnics | L1–L2 | Inferior mesenteric and hypogastric | Hindgut, pelvic viscera (sympathetic) |
| Sacral splanchnics | Sacral chain (sympathetic) | Inferior hypogastric plexus | Pelvic viscera (sympathetic) |
Parasympathetic outflow is craniosacral: CN III, VII, IX, X and pelvic splanchnic nerves (nervi erigentes) from S2–S4. Pelvic splanchnics are not chain branches. Vagus supplies viscera into the midgut; S2–S4 take over at about the left colic (splenic) flexure for hindgut, bladder detrusor, and erectile tissue. Enteric neurons in the gut wall are a third division; they receive both autonomic inputs.
Referred pain follows visceral afferents that travel with autonomics and then synapse in the same cord segments as somatic afferents: heart to T1–T5 (often left arm and jaw), gallbladder/diaphragmatic peritoneum to C3–C5 (shoulder), appendix to T10 (umbilicus). That is segmental anatomy.
Cutaneous innervation
Two maps must not be mixed. Dermatomes are dorsal-root territories. Named cutaneous nerves are post-plexus.
High-yield dermatomes:
| Dermatome | Landmark skin |
|---|---|
| C2 | Occipital scalp (greater occipital) |
| C4 | Cape of the shoulder |
| C5 | Lateral arm |
| C6 | Thumb and lateral forearm |
| C7 | Middle finger |
| C8 | Little finger |
| T1 | Medial arm |
| T4 | Nipple line |
| T10 | Umbilicus |
| T12 | Suprapubic / lower abdomen |
| L1 | Inguinal region |
| L4 | Medial leg and medial malleolus |
| L5 | Dorsum of foot and great toe |
| S1 | Lateral foot and small toe |
| S2 | Posterior thigh |
| S3–S5 | Perineum and saddle |
Named-nerve examples that diverge from a single dermatome: median (palmar digits 1–3 and half of 4), ulnar (digit 5 and half of 4), superficial radial (dorsal first web), saphenous (medial leg, from femoral), sural (lateral foot, from tibial and fibular contributions), lateral femoral cutaneous (L2–L3, not a root lesion of L3 alone). Autonomic cutaneous supply is sympathetic only to skin (sweat, piloerection, vasomotor); there is no parasympathetic innervation of cutaneous effectors.
Development
Neural crest is the source of DRG, sympathetic and parasympathetic ganglia (except some sacral/cranial preganglionic neurons, which are basal-plate CNS cells), Schwann cells, satellite cells, and adrenal medulla. Motor axons grow out from basal-plate neuroepithelium. Limb plexuses form as ventral rami follow somitic and lateral-plate mesoderm into the limb buds; anterior divisions generally supply flexor compartments, posterior divisions extensor compartments—the reason cords and terminal nerves have predictable compartment identities.
Cranial-nerve motor nuclei follow the same alar/basal logic, then migrate: somatic motor nuclei stay near midline (III, IV, VI, XII); branchiomotor nuclei (V motor, VII, IX/X ambiguus, XI) shift ventrolaterally with the pharyngeal arches. Arch map: 1 → V3 muscles of mastication; 2 → VII facial expression; 3 → IX stylopharyngeus; 4 and 6 → X larynx/pharynx.
Histology of peripheral nerve
A peripheral nerve is axons plus connective tissue. Endoneurium surrounds each axon–Schwann unit. Perineurium (epithelioid, tight-junctioned) wraps a fascicle and is the blood–nerve barrier. Epineurium is the outer collagenous sheath continuous with dura at the root sleeves. Schwann cells myelinate a single internodal segment of one axon, or envelope several unmyelinated C fibers in a Remak bundle. Nodes of Ranvier sit between internodes. Satellite cells surround DRG somata.
Fiber-size language used on anatomy items: large myelinated A-alpha (motor, proprioception), A-beta (touch), thinly myelinated A-delta (fast pain, cold), unmyelinated C (slow pain, warmth, postganglionic autonomic). Preganglionic autonomic fibers are thinly myelinated B fibers (white rami); postganglionic are C (gray rami).
Wallerian degeneration distal to a PNS axon cut, and Schwann-cell bands of Büngner, are histologic facts; regeneration is far more successful in PNS than in CNS because of those Schwann cells—not because the axon is a different cell type.
The phrenic nerve provides the motor innervation of the diaphragm. It is formed primarily from which ventral rami?
Which pair of cranial nerves leaves the posterior cranial fossa by traveling together through the internal acoustic meatus?
White rami communicantes carry myelinated preganglionic sympathetic fibers from spinal nerve to sympathetic chain. They are present at which spinal levels?