Anatomy & physiology · Study sheet
Dermatome map and nerve study sheet
Every dermatome level with its landmark, the four plexuses, the twelve cranial nerves, and the sciatic, median and vagus nerves walked from origin to end. Then 18 practice questions, every answer explained. Free, and for study only.
- Dermatomes C2 to S5
- Plexuses and cranial nerves
- 18 practice questions
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Part 1 · The dermatome map
Dermatome map, one landmark per spinal level
A dermatome is the strip of skin served mainly by one spinal nerve. The table gives the standard key sensory point for every level from C2 to S5, with a memory hook where one is in common use. Filter by region, or cover the landmarks and name them yourself.
| Level | Key sensory point |
|---|---|
| C2, Cervical | Back of the head, about 1 cm beside the bump at the base of the skull (the occipital protuberance); also the skin behind the ear. |
| C3, Cervical | The hollow above the collarbone (supraclavicular fossa), in line with the middle of the clavicle. |
| C4, Cervical | Over the acromioclavicular joint, the top of the shoulder.C4: the shoulder cap. The phrenic nerve (C3–C5) is close by: “C3, 4, 5 keep the diaphragm alive.” |
| C5, Cervical | Thumb side of the elbow crease (lateral antecubital fossa), just above the elbow. |
| C6, Cervical | The thumb (back of its first bone).C6 thumb, C7 middle finger, C8 little finger: count out from the thumb. |
| C7, Cervical | The middle finger. |
| C8, Cervical | The little finger. |
| T1, Thoracic | Little-finger side of the elbow crease (medial antecubital fossa), beside the inner bump of the elbow (medial epicondyle).T1 and T2 run up the inner arm toward the armpit. |
| T2, Thoracic | The top of the armpit (apex of the axilla). |
| T3, Thoracic | Third rib space, in line with the middle of the clavicle. |
| T4, Thoracic | The nipple line (fourth rib space).T4: nipple line. |
| T5, Thoracic | Halfway between the nipple line and the bottom of the breastbone (xiphoid). |
| T6, Thoracic | The level of the xiphoid process, the tip of the breastbone.T6: xiphoid. |
| T7, Thoracic | A quarter of the way down from the xiphoid to the navel. |
| T8, Thoracic | Halfway between the xiphoid and the navel. |
| T9, Thoracic | Three quarters of the way down from the xiphoid to the navel. |
| T10, Thoracic | The navel (umbilicus).T10: belly but-TEN. |
| T11, Thoracic | Halfway between the navel and the groin crease (inguinal ligament). |
| T12, Thoracic | The midpoint of the groin crease (inguinal ligament).T12: the groin line. |
| L1, Lumbar | Halfway between the T12 point and the L2 point: the upper front of the thigh. |
| L2, Lumbar | The middle of the front of the thigh, halfway from the groin crease to the inner knee. |
| L3, Lumbar | The inner side of the knee, on the bony prominence just above the joint line (medial femoral condyle).L3: inner knee. L4: inner ankle. L5: top of the foot and big toe. S1: outer heel and little toe. |
| L4, Lumbar | The inner ankle bone (medial malleolus).L4: “down on all fours”: the knee and the inner side of the lower leg on simpler maps. |
| L5, Lumbar | The top of the foot at the base of the third toe; on simpler maps, the top of the foot and the big toe.L5: the big toe (five toes). |
| S1, Sacral | The outer side of the heel bone (lateral calcaneus); on simpler maps, the outer ankle and little toe.S1: the smallest toe. |
| S2, Sacral | The middle of the back of the knee (popliteal fossa). |
| S3, Sacral | The sit bone (ischial tuberosity) or the gluteal fold. |
| S4–S5, Sacral | The skin around the anus (perianal area).S4–S5: the saddle area. |
- There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. C1 has no dermatome, so the map starts at C2.
- A dermatome is the strip of skin served mainly by one spinal nerve. Neighboring dermatomes overlap, so one point of skin usually has more than one level feeding it.
- Published maps differ at the borders, and people differ from each other. The points above are the standard key sensory points; the simpler maps used in many textbooks put L5 on the big toe and S1 on the little toe.
Reflex levels that pair with the map
The five deep tendon reflexes, each tied to a few spinal levels. They are learned alongside the dermatomes because together they map the cord level by level.
| Reflex | Levels | Nerve and muscle |
|---|---|---|
| Biceps | C5–C6 | Musculocutaneous nerve · Biceps brachii |
| Brachioradialis | C5–C6 | Radial nerve · Brachioradialis |
| Triceps | C7–C8 | Radial nerve · Triceps brachii |
| Patellar (knee jerk) | L2–L4 | Femoral nerve · Quadriceps femoris |
| Achilles (ankle jerk) | S1–S2 | Tibial nerve · Gastrocnemius and soleus |
One, two: buckle my shoe (S1–S2, ankle). Three, four: kick the door (L3–L4, knee). Five, six: pick up sticks (C5–C6, biceps). Seven, eight: lay them straight (C7–C8, triceps).
Textbooks differ slightly on the exact spans: the knee jerk is often given as L3–L4 and the triceps as C7 (or C6–C8). The table follows StatPearls; your course materials decide.
Part 2 · Spinal nerves and the plexuses
The 31 spinal nerves and the four plexuses
Each spinal nerve forms from a dorsal (sensory) root and a ventral (motor) root, then splits into a dorsal ramus for the back and a ventral ramus for the front and the limbs. The ventral rami of the neck, arm, and leg levels braid together into plexuses before they reach their targets.
8
Cervical
C1–C8. One more nerve than there are cervical vertebrae (7): C1–C7 exit above their vertebra, C8 below C7.
12
Thoracic
T1–T12. T2–T11 form no plexus; they run as the intercostal nerves.
5
Lumbar
L1–L5.
5
Sacral
S1–S5.
1
Coccygeal
Co1.
Brachial plexus, layer by layer
- Roots: The anterior rami of C5, C6, C7, C8 and T1.
- Trunks: Upper (C5 + C6), middle (C7), lower (C8 + T1).
- Divisions: Each trunk splits into an anterior and a posterior division: six in all. Anterior divisions head for the front of the limb (flexors), posterior divisions for the back (extensors).
- Cords: Lateral cord (anterior divisions of the upper and middle trunks), medial cord (anterior division of the lower trunk), posterior cord (all three posterior divisions). The cords are named for where they sit around the axillary artery.
- Branches: Musculocutaneous (lateral cord), median (lateral + medial cords), ulnar (medial cord), axillary and radial (posterior cord).
Cervical plexus
Roots: Anterior rami of C1–C4
Deep to the sternocleidomastoid muscle in the neck. Its skin branches surface at the middle of the back edge of that muscle.
| Nerve | Roots | Supplies |
|---|---|---|
| Lesser occipital | C2 | Skin of the scalp behind and above the ear. |
| Great auricular | C2–C3 | Skin over the parotid area and the outer ear. |
| Transverse cervical | C2–C3 | Skin of the front of the neck. |
| Supraclavicular | C3–C4 | Skin above and below the collarbone and the top of the shoulder. |
| Ansa cervicalis | C1–C3 | The infrahyoid “strap” muscles (sternohyoid, sternothyroid, omohyoid). |
| Phrenic | C3–C5 (mainly C4) | The diaphragm, the main muscle of breathing. |
“C3, 4, 5 keep the diaphragm alive.”
Brachial plexus
Roots: Anterior rami of C5–T1
From the neck, behind the clavicle, into the axilla. Roots become trunks, trunks split into divisions, divisions regroup as cords around the axillary artery, and the cords give the terminal branches.
| Nerve | Roots | Supplies |
|---|---|---|
| Musculocutaneous | C5–C7 (lateral cord) | Elbow flexors (biceps brachii, brachialis, coracobrachialis); skin of the lateral forearm. |
| Axillary | C5–C6 (posterior cord) | Deltoid and teres minor; skin over the outer shoulder. |
| Radial | C5–T1 (posterior cord) | Triceps and every extensor of the forearm and wrist; skin of the back of the arm, forearm and hand. |
| Median | C6–T1, often C5 (lateral and medial cords) | Most forearm flexors and pronators, the thenar muscles and lateral two lumbricals; palm side of the thumb, index, middle and half the ring finger. |
| Ulnar | C8–T1, often C7 (medial cord) | Flexor carpi ulnaris, the medial half of flexor digitorum profundus and most small hand muscles; the little finger and half the ring finger. |
| Dorsal scapular | C5 (from the roots) | Rhomboids and levator scapulae. |
| Long thoracic | C5–C7 (from the roots) | Serratus anterior. |
| Suprascapular | C5–C6 (upper trunk) | Supraspinatus and infraspinatus. |
| Thoracodorsal | C6–C8 (posterior cord) | Latissimus dorsi. |
The five layers in order, Roots, Trunks, Divisions, Cords, Branches: “Real Texans Drink Cold Beer.”
Lumbar plexus
Roots: Anterior rami of L1–L4, with a branch from T12
Forms inside the psoas major muscle, beside the lumbar spine. L4 also sends fibers down to the sacral plexus as the lumbosacral trunk.
| Nerve | Roots | Supplies |
|---|---|---|
| Iliohypogastric | T12–L1 | Lower abdominal wall muscles; skin of the lateral hip and above the groin. |
| Ilioinguinal | L1 | Skin of the groin, the upper inner thigh and the front of the external genitals. |
| Genitofemoral | L1–L2 | Skin just below the groin crease; the cremaster muscle. |
| Lateral femoral cutaneous | L2–L3 | Skin of the outer and front-outer (anterolateral) thigh. Sensory only. |
| Obturator | L2–L4 | The hip adductors, gracilis and obturator externus; skin of the inner thigh. |
| Femoral | L2–L4 | Iliacus, pectineus, sartorius and the quadriceps; skin of the front of the thigh, and the inner leg and foot through its saphenous branch. The largest nerve of the plexus. |
The six branches, top to bottom: “I, I Get Leftovers On Fridays” (Iliohypogastric, Ilioinguinal, Genitofemoral, Lateral femoral cutaneous, Obturator, Femoral).
Sacral plexus
Roots: Lumbosacral trunk (L4–L5) with the anterior rami of S1–S4
On the front of the piriformis muscle at the back of the pelvis. Most of its branches leave the pelvis through the greater sciatic foramen.
| Nerve | Roots | Supplies |
|---|---|---|
| Superior gluteal | L4–S1 | Gluteus medius, gluteus minimus and tensor fasciae latae. |
| Inferior gluteal | L5–S2 | Gluteus maximus. |
| Sciatic | L4–S3 | The hamstrings and, through its tibial and common fibular parts, every muscle below the knee. The largest nerve in the body. |
| Posterior femoral cutaneous | S1–S3 | Skin of the back of the thigh. |
| Pudendal | S2–S4 | The pelvic floor and perineal muscles; skin of the perineum. |
| Nerve to quadratus femoris | L4–S1 | Quadratus femoris and the inferior gemellus. |
| Nerve to obturator internus | L5–S2 | Obturator internus and the superior gemellus. |
| Nerve to piriformis | S1–S2 | Piriformis. |
Part 3 · The cranial nerves
The twelve cranial nerves
Twelve pairs leave the brain directly. Three are purely sensory, five purely motor and four carry both, and four of them (III, VII, IX and X) also carry the parasympathetic outflow from the brain.
| Nerve | Type | What it does |
|---|---|---|
| I Olfactory | Sensory | Smell. |
| II Optic | Sensory | Vision. |
| III Oculomotor | Motor | Most eye movements, lifting the upper eyelid, and constricting the pupil (parasympathetic). |
| IV Trochlear | Motor | One eye muscle: the superior oblique. |
| V Trigeminal | Both | Sensation of the face in three divisions (ophthalmic, maxillary, mandibular); the chewing muscles. |
| VI Abducens | Motor | One eye muscle: the lateral rectus, which turns the eye outward. |
| VII Facial | Both | Muscles of facial expression; taste from the front two thirds of the tongue; tear and salivary glands (parasympathetic). |
| VIII Vestibulocochlear | Sensory | Hearing and balance. |
| IX Glossopharyngeal | Both | Taste and sensation from the back third of the tongue and the throat (the sensory side of the gag reflex); the parotid gland; one throat muscle. |
| X Vagus | Both | Muscles of the throat and larynx (the motor side of the gag reflex, swallowing, the voice); parasympathetic supply to the heart, lungs and most of the gut. |
| XI Accessory (spinal accessory) | Motor | Sternocleidomastoid and trapezius: turning the head and shrugging the shoulders. |
| XII Hypoglossal | Motor | The muscles of the tongue. |
- The names, I to XII: “Oh, Oh, Oh, To Touch And Feel Very Good Velvet, Ah, Heaven” (Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal).
- Sensory, motor or both: “Some Say Marry Money, But My Brother Says Big Brains Matter More” (S, S, M, M, B, M, B, S, B, B, M, M).
- Eye movements: LR6 SO4, the rest 3: the lateral rectus is cranial nerve VI, the superior oblique is IV, and every other eye muscle is III.
- The parasympathetic four: “1973”: cranial nerves III, VII, IX and X carry the parasympathetic outflow from the brain.
Part 4 · Three nerve pathways
Sciatic, median and vagus, from origin to end
The three nerves that come up most in anatomy courses, each as a route you can walk: where it starts, every landmark it passes, and what it supplies on the way.
Sciatic nerve
Roots: L4–S3, from the sacral plexus
The longest and widest nerve in the body. It is really two nerves travelling in one sheath, the tibial and the common fibular, which part company near the knee.
The route
- Forms on the front of the piriformis muscle in the pelvis.
- Leaves the pelvis through the greater sciatic foramen, passing under the lower edge of piriformis.
- Runs down the back of the thigh, between the sit bone and the greater trochanter at the top, then deep to the long head of biceps femoris and on top of adductor magnus.
- Splits into the tibial nerve and the common fibular (peroneal) nerve, usually at the top of the popliteal fossa behind the knee; in some people the split is higher.
- The tibial nerve runs straight down the back of the leg, behind the inner ankle bone, and ends as the medial and lateral plantar nerves of the sole.
- The common fibular nerve curves around the outer side of the knee, winds around the neck of the fibula where it is close to the skin, and divides into the superficial and deep fibular nerves.
- Motor
- Sciatic trunk: the hamstrings (semitendinosus, semimembranosus, biceps femoris) and the hamstring part of adductor magnus. Tibial nerve: the back of the leg (gastrocnemius, soleus, plantaris, popliteus, tibialis posterior, the long toe flexors) and the small muscles of the sole. Common fibular nerve: the superficial branch runs the outer leg (fibularis longus and brevis) and the deep branch the front of the leg (tibialis anterior, the long toe extensors, fibularis tertius).
- Sensory
- Through its branches, the skin of the whole leg below the knee except the inner side (that strip is the saphenous nerve, from the femoral), and the whole foot: the sole from the tibial nerve, most of the top of the foot from the superficial fibular nerve, and the web between the first and second toes from the deep fibular nerve.
- Tibial = toes down (plantarflexion and toe flexion); fibular = foot up (dorsiflexion and eversion).
- The common fibular nerve lies against the neck of the fibula, which is why it is the most exposed nerve of the leg.
- The root values L4–S3 are the sacral plexus’s middle: superior gluteal above it (L4–S1), pudendal below it (S2–S4).
Median nerve
Roots: C6–T1, often with C5, from the lateral and medial cords of the brachial plexus
The nerve of the front of the forearm and the thumb side of the hand. It supplies no muscle or skin in the arm itself; its work starts around the elbow.
The route
- Forms in the axilla where a lateral root (lateral cord) and a medial root (medial cord) join in a V in front of the axillary artery.
- Runs down the inner side of the arm alongside the brachial artery, crossing from its outer to its inner side on the way, and gives no skin branches and no muscle branches of note until it reaches the elbow.
- Crosses the elbow in the cubital fossa, lying on the inner (medial) side of the brachial artery, under the bicipital aponeurosis.
- Passes between the two heads of pronator teres into the forearm.
- Gives off the anterior interosseous nerve, which runs deep on the interosseous membrane: a motor branch to flexor pollicis longus, the lateral half of flexor digitorum profundus and pronator quadratus, with no skin supply.
- Continues down the forearm sandwiched between flexor digitorum superficialis above and flexor digitorum profundus below.
- Just above the wrist it gives the palmar cutaneous branch, which passes over the flexor retinaculum to the skin of the thumb side and middle of the palm.
- Enters the hand through the carpal tunnel, beneath the flexor retinaculum, alongside nine flexor tendons.
- Gives the recurrent (thenar) motor branch to the ball of the thumb, then digital branches to the first two lumbricals and the skin of the fingers.
- Motor
- Forearm: pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, and through the anterior interosseous nerve flexor pollicis longus, the lateral half of flexor digitorum profundus and pronator quadratus. Hand: abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, and the lateral two lumbricals. Not the median nerve: flexor carpi ulnaris and the medial half of flexor digitorum profundus (ulnar nerve).
- Sensory
- The palm side of the thumb, index and middle fingers and the lateral half of the ring finger, plus the backs of those fingertips; the thumb side and middle of the palm through the palmar cutaneous branch.
- LOAF, the hand muscles of the median nerve: lateral two Lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis.
- The palmar cutaneous branch leaves before the carpal tunnel and crosses over the retinaculum, so that patch of palm is served outside the tunnel.
- Three nerves, three sides of the hand: median for the thumb side of the palm, ulnar for the little-finger side, radial for the back of the hand on the thumb side.
Vagus nerve (cranial nerve X)
Roots: Cranial nerve X, from the medulla oblongata
The longest cranial nerve and the main parasympathetic nerve of the chest and abdomen. “Vagus” shares its root with “vagrant”: it wanders from the skull to the gut.
The route
- Leaves the medulla as a row of rootlets and exits the skull through the jugular foramen, together with cranial nerves IX and XI.
- Runs down the neck inside the carotid sheath, lying between the internal jugular vein and the carotid artery (internal carotid above, common carotid below).
- In the neck it gives the pharyngeal branches to the throat muscles and the superior laryngeal nerve, which splits into an internal branch (sensation inside the larynx above the vocal folds) and an external branch (the cricothyroid muscle).
- At the root of the neck the two sides differ: the right recurrent laryngeal nerve loops under the right subclavian artery; the left vagus crosses the aortic arch and its recurrent laryngeal nerve loops under the arch. Both recurrent nerves climb back up beside the trachea to the larynx.
- In the chest it gives cardiac branches to the heart and pulmonary branches to the lungs, passes behind the root of the lung, and breaks up into the esophageal plexus around the esophagus.
- The plexus re-forms as the anterior vagal trunk (mostly left vagus fibers) and the posterior vagal trunk (mostly right), which pass through the diaphragm with the esophagus.
- In the abdomen it supplies the stomach, liver, small intestine and the colon as far as about the last part of the transverse colon, near the left colic (splenic) flexure; textbooks place the handover point a little differently. Beyond it, the parasympathetic supply comes from the sacral cord (S2–S4).
- Motor
- The muscles of the pharynx and soft palate (through the pharyngeal plexus), the cricothyroid (external laryngeal branch) and every other intrinsic muscle of the larynx (recurrent laryngeal nerve). Parasympathetic fibers slow the heart, narrow the airways and increase gut movement and secretion.
- Sensory
- A small patch of skin in the ear canal and outer ear (auricular branch), the larynx, and sensation from the organs of the chest and abdomen; taste from the area around the epiglottis.
- Three nerves through the jugular foramen: IX, X, XI.
- Right recurrent laryngeal nerve: under the right subclavian artery. Left: under the aortic arch, so it has the longer loop.
- Cricothyroid is the one laryngeal muscle not served by the recurrent laryngeal nerve.
- Vagus to about the splenic flexure, then the sacral outflow (S2–S4) takes over.
Part 5 · The basics
The system, the neuron, the signal, the reflex, the cord, the autonomic split
The pieces every nervous-system unit starts with, as labeled lists you can read in place of a diagram.
The nervous system at a glance
- Central nervous system (CNS) · Integrates and decides.
- Brain · Cerebrum, diencephalon, brainstem, cerebellum.
- Spinal cord · Reflex center and the highway between brain and body.
- Peripheral nervous system (PNS) · 12 pairs of cranial nerves, 31 pairs of spinal nerves, and the ganglia.
- Sensory (afferent) division · Carries information in.
- Somatic sensory · Skin, muscles, joints, the special senses.
- Visceral sensory · The internal organs.
- Motor (efferent) division · Carries commands out.
- Somatic motor · Skeletal muscle, under voluntary control. One neuron from cord to muscle.
- Autonomic motor · Smooth muscle, cardiac muscle and glands. Two neurons: preganglionic and postganglionic.
- Sympathetic · Fight or flight; thoracolumbar.
- Parasympathetic · Rest and digest; craniosacral.
- Enteric · The gut’s own network, tuned by the other two.
- Sensory (afferent) division · Carries information in.
The neuron, labeled
Follow the signal: dendrites in, cell body, axon hillock, down the axon, out at the terminals.
- Cell body (soma)
- Holds the nucleus and most of the organelles.
- Dendrites
- Short branches that receive signals from other neurons at synapses and carry them toward the cell body.
- Axon hillock
- The tapered spot where the cell body narrows into the axon; the action potential starts here.
- Axon
- The single long fiber that carries the impulse away from the cell body. It may branch.
- Myelin sheath
- A fatty wrapping around the axon that insulates it and speeds the signal. Made by Schwann cells in the PNS and oligodendrocytes in the CNS.
- Nodes of Ranvier
- The gaps between stretches of myelin. The impulse jumps from node to node (saltatory conduction).
- Axon terminals and synaptic end bulbs
- The branched endings of the axon and the swellings at their tips, which release neurotransmitter onto the next cell at the synapse.
Three shapes of neuron
- Multipolar
- One axon and two or more dendrites. Nearly every neuron, including motor neurons and interneurons.
- Bipolar
- One axon and one dendrite at opposite ends of the cell body. Rare: the retina and the smell receptors.
- Unipolar (pseudounipolar)
- One process that acts as both dendrite and axon. Sensory neurons only; their cell bodies sit in ganglia such as the dorsal root ganglion.
The glial cells
- Astrocyte (CNS)
- Supports neurons, keeps the chemical environment steady, and helps form the blood-brain barrier.
- Oligodendrocyte (CNS)
- Myelinates axons in the brain and spinal cord; one cell wraps several axons.
- Microglia (CNS)
- The resident immune cells; they clear debris and pathogens.
- Ependymal cell (CNS)
- Lines the ventricles and central canal and makes cerebrospinal fluid.
- Schwann cell (PNS)
- Myelinates one segment of one peripheral axon.
- Satellite cell (PNS)
- Surrounds and supports the cell bodies of neurons in ganglia.
The sodium-potassium pump
- What it moves
- Three sodium ions (Na+) out of the cell and two potassium ions (K+) in, for every molecule of ATP it splits.
- Which way
- Both ions move against their concentration gradients, which is why it costs ATP (active transport).
- Why 3 and 2 matter
- Three positive charges leave for every two that enter, so the pump itself makes the inside a little more negative, and it keeps the gradients that the leak channels and the action potential run on.
- Resting potential
- About -70 mV inside relative to outside. Potassium leaking out through leak channels does most of the work; the pump keeps the gradients topped up.
The action potential, step by step
- Resting, about -70 mV. More Na+ outside, more K+ inside. Leak channels and the Na+/K+ pump hold the membrane here.
- Stimulus to threshold, about -55 mV. A stimulus lets some Na+ in. If the membrane reaches threshold, the voltage-gated Na+ channels open all at once; below threshold nothing happens (all or none).
- Depolarization, up to about +30 mV. Na+ rushes in and the inside turns positive. Every action potential peaks at about the same voltage.
- Repolarization. The Na+ channels’ inactivation gates close and the voltage-gated K+ channels open, so K+ flows out and the inside goes negative again.
- Hyperpolarization (the undershoot). The K+ channels close a little late, so the membrane dips below -70 mV before the leak channels and pump bring it back to rest. The whole event takes about 2 milliseconds.
- Refractory periods. Absolute: no new action potential is possible while the Na+ inactivation gates are shut. Relative: a stronger stimulus is needed while K+ is still leaving.
- Conduction. On a myelinated axon the Na+ channels sit only at the nodes of Ranvier, so the impulse jumps node to node (saltatory conduction). Unmyelinated axons conduct continuously, which is slower. Wider axons are faster too.
The reflex arc
Five parts, always in the same order. A reflex runs through the spinal cord and back out before the brain has weighed in.
- 1. Receptor. A sensory ending detects the stimulus: a stretched muscle spindle, or a pain receptor in the skin.
- 2. Sensory (afferent) neuron. Carries the signal in through the dorsal root. Its cell body sits in the dorsal root ganglion.
- 3. Integration center. The gray matter of the spinal cord. In the stretch reflex the sensory neuron synapses directly on the motor neuron (one synapse: monosynaptic). In the withdrawal reflex an interneuron sits in between (polysynaptic), and other interneurons inhibit the opposing muscle.
- 4. Motor (efferent) neuron. Its cell body sits in the ventral (anterior) horn; its axon leaves through the ventral root.
- 5. Effector. The muscle that contracts (or the gland that secretes). The brain learns about it afterward; the reflex does not wait for it.
Three examples
- Stretch reflex (knee jerk)
- A tap on the patellar tendon stretches the quadriceps; the spindle fires, the quadriceps contracts and the knee kicks. A side branch inhibits the hamstrings so they do not fight it. Levels L2–L4, femoral nerve.
- Withdrawal (flexor) reflex
- A pain receptor fires; interneurons excite the flexors that pull the limb away and inhibit the extensors. Polysynaptic, and usually paired with the crossed extensor reflex, which stiffens the opposite leg so you stay upright.
- Autonomic reflex
- The same loop with smooth muscle, cardiac muscle or a gland as the effector, for example the pupil narrowing to light (cranial nerves II in, III out).
The spinal cord in cross section
Picture a slice: a butterfly of gray matter in the middle, white matter around it, a dorsal root entering at the back and a ventral root leaving at the front.
- Gray matter
- The butterfly (or H) in the middle: cell bodies, dendrites and synapses.
- Posterior (dorsal) horn
- Sensory processing. Incoming sensory axons from the dorsal root synapse here.
- Anterior (ventral) horn
- The cell bodies of the somatic motor neurons that drive skeletal muscle. Their axons leave through the ventral root.
- Lateral horn
- Autonomic motor cell bodies. Present only in the thoracic and upper lumbar cord (sympathetic) and the sacral cord (parasympathetic).
- Central canal
- The small channel in the middle of the gray commissure, filled with cerebrospinal fluid.
- White matter
- The myelinated tracts around the gray matter, in three columns (funiculi) on each side.
- Posterior columns
- Ascending: fine touch, vibration and position sense. Fasciculus gracilis (legs, nearer the midline) and fasciculus cuneatus (arms, lateral). They cross in the medulla.
- Lateral columns
- Both directions: the spinothalamic tract climbs with pain and temperature (it crosses in the cord, near its entry level) and the lateral corticospinal tract descends with voluntary motor commands (crossed in the medulla).
- Anterior columns
- Both directions, including the anterior corticospinal tract.
- Dorsal root and ganglion
- Sensory axons entering the cord; the dorsal root ganglion holds their cell bodies.
- Ventral root
- Motor axons leaving the cord. Dorsal and ventral roots join to make the spinal nerve, which then splits into a dorsal ramus (back) and a ventral ramus (front and limbs; the plexuses are made of ventral rami).
- Enlargements and the tail
- The cord is thicker at the cervical and lumbar levels, where the limb nerves leave. It stops growing early in life while the spine keeps growing, so in an adult the cord ends near the upper lumbar vertebrae and the lower nerve roots run on down the canal as the cauda equina (“horse’s tail”).
Sympathetic vs parasympathetic
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Nickname | Fight or flight | Rest and digest |
| Where it leaves the CNS | Thoracolumbar: the lateral horn of the thoracic and upper lumbar cord (T1 to about L2) | Craniosacral: brainstem nuclei of cranial nerves III, VII, IX and X, and the sacral cord (S2–S4) |
| Ganglia | Close to the spine: the sympathetic chain (paravertebral) ganglia, and the collateral (prevertebral) ganglia in front of the aorta (celiac, superior and inferior mesenteric) | Terminal (intramural) ganglia, next to or inside the target organ |
| Fiber lengths | Short preganglionic, long postganglionic | Long preganglionic, short postganglionic |
| Preganglionic transmitter | Acetylcholine, on nicotinic receptors | Acetylcholine, on nicotinic receptors |
| Postganglionic transmitter | Norepinephrine, on alpha and beta adrenergic receptors (exception: most sweat glands get acetylcholine) | Acetylcholine, on muscarinic receptors |
| Adrenal medulla | Preganglionic fibers reach it directly; it releases epinephrine and norepinephrine into the blood | No supply |
| Pupil | Dilates | Constricts |
| Heart | Faster and stronger | Slower (vagus) |
| Airways | Bronchioles widen | Bronchioles narrow |
| Digestion | Slows; blood is sent to the muscles | Speeds up: more movement and secretion |
| Bladder | Wall relaxes, internal sphincter tightens (holds urine) | Wall contracts (empties) |
| Saliva | Thick, small amount | Watery, plentiful |
| Sweat glands | Sweating (the acetylcholine exception) | No supply |
- Sympathetic = “S” for spine: short first fiber, synapse in the chain by the spine. Parasympathetic = long first fiber all the way to the organ.
- “1973” names the parasympathetic cranial nerves: III, VII, IX, X.
- SLUDD lists what the parasympathetic system turns up: Salivation, Lacrimation, Urination, Digestion, Defecation.
Part 6 · Practice questions
18 practice questions, every answer explained
Dermatome points, reflex levels, the plexuses, the three pathways, the cranial nerves, the pump and the action potential. Your result is free and needs no sign-up.
Question 1 of 18
The dermatome at the nipple line is
The bonus
The printable study sheet and 20 more questions
Locked until you unlock the bonus above
- A one-page study sheet to print: every dermatome key point C2 to S5, the five reflex levels, the four plexuses with root values, and the twelve cranial nerves.
- 20 more practice questions on dermatomes, reflexes, the plexuses, the three pathways, the cranial nerves, the action potential and the cord, every answer explained.
Sources
Where these facts come from
Everything on this page is written in our own words from these public sources. Where a map or a textbook draws a border differently, your course materials decide.
- OpenStax, Anatomy and Physiology 2e, chapter 12 (nervous tissue and the action potential) · The parts of a neuron, the neuron types, the glial cells; and section 12.4 for the resting potential, threshold, peak and refractory periods.
- OpenStax, Anatomy and Physiology 2e, 3.1 The Cell Membrane · The sodium-potassium pump: three Na+ out and two K+ in per ATP.
- OpenStax, Anatomy and Physiology 2e, chapter 13 (the spinal cord and the peripheral nervous system) · The 31 spinal nerves, the plexuses, the cranial nerve table, the dorsal and ventral roots; and section 13.2 for the cord in cross section.
- OpenStax, Anatomy and Physiology 2e, chapter 14 (sensory pathways and reflexes) · The stretch and withdrawal reflexes; and section 14.2 for the dorsal column and spinothalamic pathways.
- OpenStax, Anatomy and Physiology 2e, chapter 15 (the autonomic nervous system) · Sympathetic vs parasympathetic: outflow, ganglia, fiber lengths, transmitters and effects.
- StatPearls (NIH Bookshelf), Anatomy, Skin, Dermatomes · Dermatome landmarks (thumb, middle and little fingers, nipple, xiphoid, umbilicus, knee, ankle, foot), the 31 pairs, and the overlap caveat.
- Wikipedia, Dermatome (anatomy) · The full list of key sensory points, C2 to S5, as the standard charts place them; C1 has no dermatome.
- StatPearls, Physiology, Deep Tendon Reflexes · The five deep tendon reflexes with their segmental levels and nerves; the Achilles reflex article on the NIH Bookshelf (NBK459229) for S1 as the key root.
- StatPearls (NIH Bookshelf), Anatomy, Shoulder and Upper Limb, Median Nerve · The median nerve’s course, branches, muscles and skin.
- StatPearls (NIH Bookshelf), Neuroanatomy, Cranial Nerve 10 (Vagus Nerve) · The vagus nerve’s origin, exit and reach.
- Wikipedia: Brachial plexus, Cervical plexus, Lumbar plexus, Sacral plexus, Sciatic nerve, Vagus nerve · Root values and the branches of each plexus, the sciatic nerve’s course and divisions, and the vagus nerve’s branches.
More free study tools: medical abbreviations and word parts, the phlebotomy order of draw, a printable nurse report sheet and the skills finder.
Questions
Common questions
Yes. The map, the plexus and cranial nerve tables, the three nerve pathways, the basics and the 18 practice questions are free, and your result needs no sign-up. An email unlocks the bonus: a printable one-page study sheet and 20 more questions.
The area of skin whose sensation is carried mainly by one spinal nerve. There are 31 pairs of spinal nerves, and every one from C2 down has a dermatome; C1 has none. The map on this page lists one key sensory point per level, the point standard charts use so that a level can be checked in the same place every time.
Neighboring dermatomes overlap, so the borders drawn on any map are approximate, and the published maps were made from different evidence. People differ too. The key sensory points are the steadiest part, which is why this page is built around them rather than around drawn borders. Where your course uses a particular map, follow that one.
A dermatome is the skin served by one spinal level; a myotome is the group of muscles served by one spinal level. The reflex table under the map is the bridge between them: a knee jerk checks the L2–L4 levels through the quadriceps, an ankle jerk the S1–S2 levels through the calf.
The sacral plexus, from the L4 to S3 levels. It leaves the pelvis through the greater sciatic foramen, runs down the back of the thigh and splits near the knee into the tibial nerve and the common fibular nerve. The lumbar plexus (L1–L4) gives the femoral and obturator nerves instead.
No. This is a study aid for people in anatomy and physiology, nursing, medical assisting, physical therapy assistant and similar programs. It describes anatomy only. It does not cover symptoms, diagnosis or treatment, it takes no patient details, and your course materials and instructor come first.
OpenStax Anatomy and Physiology 2e (the neuron, the action potential, the pump, the spinal cord, the plexuses, the cranial nerves, the reflexes and the autonomic system), StatPearls on the NIH Bookshelf (dermatomes, deep tendon reflexes, the median and vagus nerves) and the standard key sensory points used on dermatome charts. Every source is listed below.