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Peripheral nervous system

Week one — the wiring chart every region is written in

Before any region makes sense you need the wiring key: every nerve fiber in the body is one of five functional components, each with a fixed home for its cell body, a fixed set of targets and a fixed job. Programs hand this out as a one-page chart in the first week and test it early, then assume it for the rest of the year — every plexus, every "what does this nerve carry", every autonomic question is this table applied. Learn it as two rules (afferent somata outside the CNS, efferent inside; sympathetic thoracolumbar, parasympathetic craniosacral) plus one short table of branches, and the chart rebuilds itself from memory.

On the PANCE this region feeds:Neurology · 7%
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Afferent somata live outside the CNS (dorsal root ganglion), efferent somata inside it (ventral horn for GSE, lateral horn for GVE); sympathetic comes from T1–L2 and reaches every ramus through gray rami, parasympathetic comes from S2–S4 and the brain and never touches the body wall.

Practice it
Active recall over the structure list below — flip and claim, match the pairs, label from a bank, or fill in the diagram, rebuild the chart, then run a quiz and a timed exam on the unit.

Every structure on the practical list, one card at a time — the name up front, the identifying relationship on the flip. Claim each card honestly and the deck learns what to lead with next time. Progress lives in this browser only.

What the written exam asks
Relationships, supply, levels, and what happens when each fails.
  • The grid: two questions, one letter each. Every component is named by two choices. Afferent (A) or efferent (E): sensation in, or a command out. Somatic (S) or visceral (V): the body wall — skin, skeletal muscle, tendon, bone — or the cavity contents — viscera, glands, vessels, smooth and cardiac muscle. Read GSA as General Somatic Afferent and the row writes itself: sensation from the body wall. The only row outside the grid is the SPECIAL afferents (SA): hearing, equilibrium, vision, smell and taste from the inner ear, retina, nose and tongue.
  • The soma rule — afferent outside, efferent inside. Sensory (afferent) somata sit OUTSIDE the central nervous system, in the dorsal root ganglia (spinal ganglia) — GSA and GVA alike. Motor (efferent) somata sit INSIDE the grey matter of the cord: GSE in the ventral horn, the presynaptic GVE neuron in the lateral horn. The chart writes "Brain &" in front of each because the cranial nerves carry the same components from the brainstem. When a question asks where a cell body lives, decide afferent-or-efferent first and the answer is nearly automatic.
  • GVE is a two-neuron chain with a segment restriction. Every visceral efferent pathway is two neurons: a presynaptic soma in the lateral horn, a synapse in a ganglion, and a postsynaptic neuron to smooth muscle, cardiac muscle or a gland. Sympathetic (GVEs) presynaptic somata are in the lateral horn of T1–L2 only — thoracolumbar. Parasympathetic (GVEp) presynaptic somata are in S2–S4 and the brain — craniosacral. Same functions on the chart (control of smooth and cardiac muscle contraction, control of glandular secretions); opposite tone.
  • Targets and functions, row by row. GSA: skin, skeletal muscle, tendon, bone — LOCALISED pain, pressure, temperature, touch and proprioception. GSE: skeletal muscle — contraction of skeletal muscle fibers. GVA: viscera, glands, blood vessels — DIFFUSE sensations of distension or stretching, ischemia, chemical irritation, smooth-muscle spasm, O2/CO2 levels and blood pressure. GVE: smooth muscle, cardiac muscle, glands — control of contraction and of secretion. SA: inner ear, retina, nose, tongue — hearing, equilibrium, vision, smell, taste. "Localised" versus "diffuse" is the word pair the written exam tests.
  • The branches table. Dorsal rami and ventral rami both carry GSA, GSE and GVEs, and both exist on every spinal nerve from C1 to Co1. White rami communicantes carry presynaptic GVEs and GVA, and branch from the T1–L2 ventral rami only. Pelvic splanchnic nerves carry presynaptic GVEp and GVA, and branch from the S2–S4 ventral rami. Notice what never appears: GVEp on a dorsal or ventral ramus, and a white ramus outside T1–L2.
  • Why GVEs rides every ramus but white rami stop at L2. Presynaptic sympathetic fibers can only ENTER the sympathetic chain where their somata are — T1–L2 — and they enter through white rami communicantes (white because presynaptic fibers are myelinated). Inside the chain they ascend or descend to every level and synapse, and the postsynaptic fibers rejoin EVERY spinal nerve through a gray ramus communicans (unmyelinated, hence gray). That is how a C5 dermatome sweats and its vessels constrict: postsynaptic GVEs on both rami of all 31 pairs, fed by white rami at only 14 levels.
  • Parasympathetic never reaches the body wall. Pelvic splanchnic nerves leave the S2–S4 ventral rami and run straight to the pelvic plexuses and viscera; they never re-enter a spinal nerve. Cranial parasympathetics travel with cranial nerves to the head and, via the vagus, to the thoracic and most abdominal viscera. So no dorsal or ventral ramus carries GVEp, and "parasympathetic to skin, sweat glands or limb vessels" is always the wrong option.
What the practical tags · 22 structures
In the order a dissection meets them — each with the one relationship that identifies it. Your lab's study list is the authority; use this to check yours against.
  1. 1General somatic afferent (GSA) — localised sensation from skin, skeletal muscle, tendon and bone; somata in the dorsal root ganglia and brain
  2. 2General somatic efferent (GSE) — contraction of skeletal muscle; somata in the ventral horn of the cord's grey matter and brain
  3. 3General visceral afferent (GVA) — diffuse sensation from viscera, glands and vessels; somata in the dorsal root ganglia of T1–L2 and S2–S4, and brain
  4. 4General visceral efferent (GVE) — the two-neuron motor pathway to smooth muscle, cardiac muscle and glands; presynaptic somata in the lateral horn
  5. 5Sympathetic outflow (GVEs) — presynaptic somata in the lateral horn of T1–L2; thoracolumbar
  6. 6Parasympathetic outflow (GVEp) — presynaptic somata in S2–S4 and the brain; craniosacral
  7. 7Special afferent (SA) — hearing, equilibrium, vision, smell and taste from the inner ear, retina, nose and tongue; somata charted as brain
  8. 8Dorsal rami — GSA, GSE and GVEs to the intrinsic back muscles and the skin over them, from every spinal nerve C1–Co1
  9. 9Ventral rami — GSA, GSE and GVEs to everything else, including the plexuses, from every spinal nerve C1–Co1
  10. 10White rami communicantes — presynaptic GVEs and GVA between the T1–L2 ventral rami and the sympathetic chain; myelinated, hence white
  11. 11Gray rami communicantes — postsynaptic GVEs from the chain back to every spinal nerve; unmyelinated, hence gray
  12. 12Pelvic splanchnic nerves — presynaptic GVEp and GVA off the S2–S4 ventral rami to the pelvic viscera
  13. 13Dorsal root ganglion — a.k.a. the spinal ganglion; home of every GSA and GVA soma of that spinal nerve
  14. 14Ventral horn — grey matter housing the GSE somata whose axons leave in the ventral root
  15. 15Lateral horn — grey matter housing presynaptic GVE somata; present at T1–L2 (sympathetic) and S2–S4 (parasympathetic)
  16. 16Sympathetic chain ganglion — where presynaptic GVEs fibers synapse after a white ramus, before postsynaptic fibers leave by a gray ramus
  17. 17Thoracolumbar outflow (T1–L2) — the only segments with presynaptic sympathetic somata and white rami communicantes
  18. 18Sacral outflow (S2–S4) — the spinal parasympathetic segments; source of the pelvic splanchnic nerves
  19. 19C1–Co1 — the full run of 31 spinal nerves, each with a dorsal and a ventral ramus
  20. 20Presynaptic vs postsynaptic neuron — the first GVE neuron (soma in the CNS) and the second (soma in a ganglion); only the first is on the chart
  21. 21Localised vs diffuse sensation — the GSA signature versus the GVA signature; the word pair the written exam tests
  22. 22Somatic vs visceral — body wall (skin, skeletal muscle, tendon, bone) versus cavity contents (viscera, glands, vessels, smooth and cardiac muscle)
What everyone misses
The distinctions that lose points on the practical, year after year.
  • GVEs and GVEp are one row with two addresses. Sympathetic and parasympathetic share targets and functions on the chart; they differ in where the presynaptic soma sits (T1–L2 versus S2–S4 and brain) and in which branch carries them out (white rami versus pelvic splanchnics). An answer that puts GVEp in the lateral horn of T1–L2 has swapped the addresses.
  • White rami versus gray rami. White = presynaptic, myelinated, T1–L2 only, spinal nerve TO chain, and carries GVA back. Gray = postsynaptic, unmyelinated, all 31 levels, chain TO spinal nerve. The chart lists only the white rami — the gray rami are how GVEs shows up on every dorsal and ventral ramus.
  • Dorsal ROOT versus dorsal RAMUS. Roots are purely afferent or purely efferent and sit inside the dura; rami are mixed branches outside the foramen. The dorsal root carries only sensation (with its ganglion); the dorsal ramus carries GSA, GSE and GVEs to the back.
  • "Parasympathetic to the body wall" is always wrong. Sweat glands, arrector pili and the vessels of skin and muscle get sympathetic fibers only. GVEp never travels in a dorsal or ventral ramus — it leaves the CNS in cranial nerves and pelvic splanchnics and goes to viscera.
  • Special afferent somata are charted as "Brain". The chart files SA under brain, and that is the answer it grades. The fuller story sits in the FAQ; on the exam, do not move SA somata into a dorsal root ganglion.
  • Thirty-one pairs, C1 to Co1. Eight cervical nerves over seven cervical vertebrae, then 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. C1–Co1 on the branches table means every level — not a sub-range like T1–L2.
Clinical correlations
Where this region shows up again — in clinical medicine, on rotations, and on the boards.
  • Referred pain. GVA fibers from an inflamed viscus enter the cord through the white rami at T1–L2 and share dorsal root ganglia and dorsal-horn neurons with GSA fibers from the T1–L2 dermatomes, so the brain reads a heart, gallbladder or appendix as chest, right shoulder-blade or periumbilical pain. Pelvic viscera refer through S2–S4 to the perineum and sacrum.
  • Horner syndrome. Ptosis, miosis and anhidrosis on one side mean the sympathetic pathway has been interrupted somewhere between the T1 lateral horn and the eye — an apical lung tumor, a neck dissection, a carotid dissection. It is a GVEs lesion, so the somatic (GSE) muscles of the lid and eye are intact.
  • Autonomic dysreflexia. After a cord injury above T6, the T1–L2 lateral horns still fire but have lost descending control; a full bladder or bowel triggers a massive sympathetic discharge below the lesion — severe hypertension, pounding headache, flushing above the level. First step: sit the patient up and empty the bladder.
  • What a spinal or epidural block removes, in order. Small autonomic fibers go first (GVEs — hypotension, warm feet), then pain and temperature, then touch and proprioception (GSA), then motor (GSE). The sympathetic block is why blood pressure drops after a spinal before any analgesia is felt.
  • Pelvic surgery and the pelvic splanchnics. Radical prostatectomy, rectal resection and hysterectomy can injure the S2–S4 parasympathetic fibers in the pelvic plexuses: bladder atony with retention, erectile dysfunction, constipation. Nerve-sparing technique exists because GVEp to these organs has no second route.
  • Lower motor neuron lesion at the ventral horn. Damage to GSE somata (poliomyelitis, a spinal cord infarct) or their axons gives flaccid paralysis, areflexia, atrophy and fasciculations in that myotome, with sensation spared — the ventral horn holds motor somata only.

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AI-authored study notes, not faculty-reviewed and not medical advice. Verify every structure against your atlas, your dissector, and your lab instructors; programs differ in what they tag.

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