Week 1: Embryology + somatic innervation

Comprehensive practice questions covering embryonic origins of the nervous system, spinal nerve anatomy, dermatomes, myotomes, and peripheral nerve innervation.

Which three embryonic structures are involved in somatic innervation?

Neural tube, neural crest cells, and paraxial mesoderm.

What structures develop from the neural tube?

The brain and spinal cord.

What do neural crest cells develop into within the context of the peripheral nervous system?

Sensory neurons, including the dorsal root ganglia.

What are somites, and what is their embryonic origin?

Somites are segmented blocks of paraxial mesoderm that give rise to the axial skeleton, skeletal muscles, and dermis.

What are the two primary divisions of a somite?

Sclerotome and dermatomyotome.

What specific structures are formed by the sclerotome?

Vertebrae and ribs.

Where are the cell bodies of motor neurons located within the spinal cord?

Ventral (anterior) horn.

Through which root do sensory fibers enter the spinal cord, and where are their cell bodies located?

Sensory fibers enter through the dorsal root; their cell bodies are located in the dorsal root ganglion.

What type of nerve is formed when the dorsal and ventral roots join?

A spinal nerve, which is a mixed nerve containing both motor and sensory fibers.

What structures are supplied by the dorsal ramus?

Deep muscles of the back and skin over the back.

Why do ventral rami form plexuses while dorsal rami do not?

Ventral rami form plexuses to redistribute fibers for robust limb innervation; dorsal rami only supply the deep back muscles and skin.

Define a dermatome.

An area of skin supplied by the sensory fibers of one spinal nerve.

Define a myotome.

A group of muscles supplied predominantly by one spinal nerve.

Why is there no dermatome for the spinal level $C1$?

$C1$ does not normally supply an area of skin sensation.

What cranial nerve supplies most sensation to the face?

The trigeminal nerve ($CN V$).

What is the key test point for the $C2$ dermatome?

Approximately $3\,\text{cm}$ behind the ear.

Which dermatome mapping point is located at the umbilicus?

The $T10$ dermatome landmark.

What action and muscle are used to test the $C5$ myotome?

Arm or shoulder abduction, primarily testing the deltoid.

What action and muscles are used to test the $C6$ myotome?

Elbow flexion, using the biceps brachii and brachialis.

Which myotome is tested by first-toe extension?

$L5$ (Extensor hallucis longus).

What action is used to test the $L4$ myotome, and which muscle is primary?

Ankle dorsiflexion, primarily testing the tibialis anterior.

Which spinal levels are tested by the biceps reflex and the patellar reflex?

Biceps reflex tests $C5$-$C6$; patellar tendon reflex tests $L3$-$L4$.

What nerve innervates the trapezius muscle?

Accessory nerve ($CN XI$) with contributions from $C3$-$C4$ ventral rami.

What nerve supplies the serratus anterior?

Long thoracic nerve.

What muscles are supplied by the musculocutaneous nerve?

Coracobrachialis, biceps brachii, and brachialis (anterior compartment of the arm).

What nerve supplies most of the intrinsic muscles of the hand, and what are the median nerve exceptions?

Most are supplied by the ulnar nerve; exceptions are the thenar muscles and first two lumbricals (median nerve).

What phrase is used to remember the spinal levels for elbow flexors vs. elbow extensors?

$C5, C6$ pick up sticks (flexors); $C7, C8$ lay them straight (extensors).

What are the three innervation labels for the deltoid muscle?

$C5$-$C6$ spinal levels, $C5$ myotome, and axillary nerve.

How does the innervation of the flexor digitorum profundus ($FDP$) differ between its radial and ulnar halves?

The radial half (digits $2$ and $3$) is supplied by the median nerve; the ulnar half (digits $4$ and $5$) is supplied by the ulnar nerve.

What is the clinical difference between a spinal nerve lesion and a peripheral nerve lesion?

A spinal nerve lesion follows dermatome and myotome patterns; a peripheral nerve lesion follows that specific nerve's anatomical distribution.

Between which spinal levels are dermatomes present?

C2 to S4.

Why is there no C1 dermatome?

C1 does not normally supply an area of skin sensation.

Which nerve supplies most sensation to the face?

The trigeminal nerve, cranial nerve V.

Why do adjacent dermatomes overlap?

Skin regions commonly receive sensory fibres from neighbouring spinal levels.

Why do limb dermatomes appear to spiral?

Because the limbs rotate during embryonic development.

Why can dermatome maps differ slightly?

There is biological variation and overlap between adjacent dermatomes.

What is the C2 dermatome key test point?

Approximately 3 cm behind the ear.

What is the C3 dermatome key test point?

The supraclavicular fossa, posterior to the clavicle.

What is the C4 dermatome key test point?

Over the acromioclavicular joint.

What is the C5 dermatome key test point?

The lateral side of the cubital fossa, just proximal to the elbow crease.

What is the C6 dermatome key test point?

The dorsal surface of the thumb, over the proximal phalanx.

What is the C7 dermatome key test point?

The dorsal surface of the middle finger, over the proximal phalanx.

What is the C8 dermatome key test point?

The dorsal surface of the little finger, over the proximal phalanx.

What is the T1 dermatome key test point?

The medial arm, just proximal to the medial epicondyle.

What is the T2 dermatome key test point?

The apex of the axilla.

What is the T4 dermatome landmark?

The nipple line at the midclavicular line.

What is the T10 dermatome landmark?

The umbilicus.

What is the T12 dermatome key test point?

The midpoint of the inguinal ligament.

What is the L1 dermatome key test point?

Midway between the T12 and L2 test points.

What is the L2 dermatome key test point?

The anterior-medial thigh at mid-thigh.

What is the L3 dermatome key test point?

The medial femoral condyle above the knee.

What is the L4 dermatome key test point?

The medial malleolus.

What is the L5 dermatome key test point?

The dorsum of the foot at the third metatarsophalangeal joint.

What is the S1 dermatome key test point?

The lateral heel.

What is the S2 dermatome key test point?

The midpoint of the popliteal fossa.

What is the S3 dermatome key test point?

The ischial tuberosity or infragluteal fold.

What is the S4–S5 dermatome key test point?

The perianal area.

What action tests the C1–C2 myotomes?

Neck flexion and extension.

What action tests the C3 myotome?

Neck lateral flexion.

What action tests the C4 myotome?

Shoulder shrug.

What action tests the C5 myotome?

Arm or shoulder abduction.

What is the key muscle for the C5 myotome?

Deltoid.

What action tests the C6 myotome?

Elbow flexion.

What muscles are commonly used to test C6?

Biceps brachii and brachialis.

What action tests the C7 myotome?

Elbow extension.

What muscle is commonly used to test C7?

Triceps brachii.

What action tests the C8 myotome?

Third-finger flexion.

What muscle is commonly used to test C8?

Flexor digitorum profundus to the middle finger.

What action tests the T1 myotome?

Fifth-finger abduction.

What muscle is commonly used to test T1?

Abductor digiti minimi.

What action tests the L2 myotome?

Hip flexion.

What muscle is commonly used to test L2?

Iliopsoas.

What action tests the L3 myotome?

Knee extension.

What muscle group is commonly used to test L3?

Quadriceps.

What action tests the L4 myotome?

Ankle dorsiflexion.

What muscle is commonly used to test L4?

Tibialis anterior.

What action tests the L5 myotome?

First-toe extension.

What muscle is commonly used to test L5?

Extensor hallucis longus.

What action tests the S1 myotome?

Ankle plantarflexion.

What muscles are commonly used to test S1?

Gastrocnemius and soleus.

Why is shoulder abduction used to test C5 even though deltoid receives C5 and C6?

Because most deltoid fibres receive their dominant supply from C5.

Why is elbow flexion used to test C6?

Elbow flexors receive C5–C6 fibres, but C6 supplies the majority of fibres used in the test.

Why is elbow extension used to test C7?

Elbow extensors receive C6–C8 fibres, but C7 provides the dominant contribution.

Why is first-toe extension used to test L5 instead of ankle dorsiflexion?

First-toe extension is a more specific key action for L5, whereas dorsiflexion has a strong L4 contribution.

Does a myotome mean an entire muscle is supplied by only one spinal level?

No. It refers to the muscle fibres supplied by that level and the action used to assess its dominant contribution.

Why can weakness occur without complete paralysis after one spinal nerve is damaged?

Because most muscles receive fibres from multiple spinal levels. The deltoid example shows that it receives both C5 and C6 fibres, but C5 supplies most fibres, making it the C5 key test muscle.

Which spinal levels are tested by the biceps reflex?

C5–C6.

Which spinal level is mainly tested by the triceps reflex?

C7.

Which spinal levels are tested by the patellar tendon reflex?

L3–L4.

Which spinal levels are tested by the calcaneal or Achilles reflex?

S1–S2.

What innervates trapezius?

Accessory nerve, cranial nerve XI, with contributions from C3–C4 ventral rami.

Is trapezius innervated by the brachial plexus?

No.

What nerve supplies rhomboid major and rhomboid minor?

Dorsal scapular nerve.

What nerve mainly supplies levator scapulae?

Dorsal scapular nerve, with contributions from C3–C4 ventral rami.

What nerve supplies supraspinatus?

Suprascapular nerve.

What nerve supplies infraspinatus?

Suprascapular nerve.

What nerve supplies latissimus dorsi?

Thoracodorsal nerve.

What nerve supplies serratus anterior?

Long thoracic nerve.

What nerves supply pectoralis major?

Medial and lateral pectoral nerves.

What nerve mainly supplies pectoralis minor?

Medial pectoral nerve.