Anatomy chapters
Somatic nervous system
How spinal nerves are formed and branch, dermatomes and myotomes, the reflex arc and the root values of the tendon reflexes, a summary of the cranial nerves, and the grades of peripheral nerve injury.
- The somatic system supplies skin, skeletal muscle and joints; it is under voluntary control.
- Each spinal nerve is formed by a dorsal (sensory) root and a ventral (motor) root, then splits into a dorsal ramus (back) and a ventral ramus (limbs and front).
- Learn the reflex roots: biceps C5–6, supinator C5–6, triceps C7–8, knee L3–4, ankle S1–2.
On this page
Overview
The somatic nervous system carries sensation from the skin, muscles and joints, and sends motor commands to skeletal muscle. It is under conscious, voluntary control.
It is made of the 12 pairs of cranial nerves and the 31 pairs of spinal nerves.
The autonomic system, which supplies organs and glands, is covered in autonomic nervous system.
Formation of a spinal nerve
- Dorsal (posterior) root: carries sensory fibres into the cord. The cell bodies of these neurons lie in the dorsal root ganglion.
- Ventral (anterior) root: carries motor fibres out of the cord, from anterior horn cells (and, at T1–L2 and S2–S4, autonomic fibres).
The two roots join in the intervertebral foramen to form the spinal nerve, which is mixed (motor and sensory).
Just outside the foramen, the spinal nerve divides:
- Dorsal ramus: supplies the deep muscles of the back and the skin over them.
- Ventral ramus: supplies the limbs and the front and sides of the trunk. Ventral rami form the cervical, brachial, lumbar and sacral plexuses.
Do not confuse a dorsal root (purely sensory) with a dorsal ramus (mixed, supplying the back).
There are 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Cervical nerves C1 to C7 exit above the vertebra with the same number; C8 exits below C7; from T1 down, each nerve exits below its vertebra.
Dermatomes and myotomes
A dermatome is the area of skin supplied by one spinal nerve. A myotome is the group of muscles supplied by one spinal nerve.
Useful dermatome landmarks:
| Landmark | Dermatome |
|---|---|
| Thumb | C6 |
| Middle finger | C7 |
| Little finger | C8 |
| Nipple | T4 |
| Umbilicus | T10 |
| Groin (inguinal region) | L1 |
| Front of the knee | L3–L4 |
| Big toe | L5 |
| Medial side of the leg and foot, medial malleolus | L4 |
| Little toe and sole | S1 |
| Perianal skin | S3–S5 |
See the upper and lower limb dermatome pages for full maps.
Neighbouring dermatomes overlap, so cutting a single spinal nerve may give little sensory loss. Shingles (herpes zoster) shows the dermatome clearly: a band of painful blisters along one nerve.
The reflex arc
A stretch (tendon) reflex has five parts:
- Receptor: the muscle spindle senses stretch when the tendon is tapped.
- Afferent neuron: carries the signal through the dorsal root.
- Synapse in the cord: a single synapse (monosynaptic) with the motor neuron.
- Efferent neuron: the anterior horn cell and its axon in the ventral root.
- Effector: the muscle contracts.
Root values of the tendon reflexes
| Reflex | Root |
|---|---|
| Biceps | C5, C6 |
| Supinator (brachioradialis) | C5, C6 |
| Triceps | C7, C8 |
| Knee (patellar) | L3, L4 |
| Ankle (Achilles) | S1, S2 |
Count up: "1, 2 buckle my shoe (ankle S1–2); 3, 4 kick the door (knee L3–4); 5, 6 pick up sticks (biceps C5–6); 7, 8 lay them straight (triceps C7–8)."
Superficial reflexes:
- Abdominal reflexes: T8–T12.
- Cremasteric reflex: L1–L2.
- Plantar reflex: S1; an upgoing toe (Babinski sign) means an upper motor neuron lesion.
Cranial nerves at a glance
| Nerve | Type | Main function |
|---|---|---|
| I Olfactory | Sensory | Smell |
| II Optic | Sensory | Vision |
| III Oculomotor | Motor (+ parasympathetic) | Most eye muscles, lid elevation, pupil constriction |
| IV Trochlear | Motor | Superior oblique |
| V Trigeminal | Both | Sensation of the face; muscles of mastication |
| VI Abducent | Motor | Lateral rectus |
| VII Facial | Both | Facial expression; taste front two thirds of tongue; glands |
| VIII Vestibulocochlear | Sensory | Hearing and balance |
| IX Glossopharyngeal | Both | Taste and sensation back third of tongue; stylopharyngeus; parotid |
| X Vagus | Both | Pharynx, larynx, heart, lungs, gut |
| XI Accessory | Motor | Sternocleidomastoid and trapezius |
| XII Hypoglossal | Motor | Muscles of the tongue |
Eye muscles: "LR6 SO4, the rest are 3": lateral rectus by VI, superior oblique by IV, all others by III.
Peripheral nerve injury
| Grade (Seddon) | What is damaged | Outlook |
|---|---|---|
| Neurapraxia | Myelin only; temporary conduction block; axon intact | Full recovery in days to weeks |
| Axonotmesis | Axon cut, but the connective tissue sheath is intact | Regrows along the sheath; good recovery, but slow |
| Neurotmesis | The whole nerve is cut through | No useful recovery without surgical repair |
After an axon is cut, the part beyond the cut breaks down (Wallerian degeneration). The axon then regrows at roughly 1 mm a day.
A Tinel sign (tingling when the nerve is tapped) moving down the limb over time shows that the nerve is regrowing.
Saturday night palsy (radial nerve compressed against the humerus) is usually neurapraxia, so it recovers fully.
Clinical relevance
Disc prolapse and root compression
How it happens: a disc bulges backwards and laterally and presses on a nerve root. In the lumbar spine, an L4/L5 disc usually compresses the L5 root, and an L5/S1 disc the S1 root.
What you see: pain along the dermatome (sciatica), weakness in the myotome and a reduced reflex (for example, a lost ankle jerk with S1).
Lower motor neuron lesion
How it happens: damage to the anterior horn cell, root or peripheral nerve.
What you see: weakness, wasting, low tone, reduced or absent reflexes and fasciculations in the muscles supplied.
Expect: the root values of the tendon reflexes, dermatome landmarks (nipple, umbilicus, groin), the parts of a reflex arc, the difference between roots and rami, the nerve supply of the eye muscles, and the three grades of nerve injury with their outcome.
Quick revision
- 31 spinal nerves: 8 C, 12 T, 5 L, 5 S, 1 Co.
- Dorsal root sensory; ventral root motor; dorsal ramus to the back; ventral rami form plexuses.
- Reflexes: ankle S1–2, knee L3–4, biceps C5–6, triceps C7–8.
- Dermatomes: nipple T4, umbilicus T10, groin L1.
- Eye muscles: LR6, SO4, rest III.
- Neurapraxia recovers fully; neurotmesis needs repair; regrowth about 1 mm a day.
Written by The House Job from standard neuroanatomy references (Snell's Clinical Neuroanatomy).
FAQ
What is a dermatome?
The area of skin supplied by the sensory fibres of a single spinal nerve.
What is the root value of the knee jerk?
L3 and L4.
What is neurapraxia?
A temporary block of nerve conduction without damage to the axon. It recovers fully.
Where are the cell bodies of sensory neurons?
In the dorsal root ganglion.