Session 5

Question 1

Why does transaction of a single spinal nerve not usually lead to anaesthesia of the entire dermatomal area? What is the exception?

Answer 1

There is considerable overlap between adjacent dermatomes. Exception concerns skin areas that touch an axial line - there is no overlap across an axial line.

Question 2

What is an axial line?

Answer 2

Junction between two dermatomes supplied from discontinuous spinal levels

Question 3

How is each dermatologist named?

Answer 3

According to the spinal nerve which provides MOST of its sensory innervation.

Question 4

What is a spinal nerve root and what are the two types?

Answer 4

They connect each spinal nerve to a segment of the spinal nerve. Dorsal roots contain afferent fibres only (there is a dorsal root ganglion here). Ventral roots contain efferent and autonomic nerve fibres only.

Question 5

What are spinal nerves and how many are there?

Answer 5

Mixed (motor and sensory), parallel bundles of axons encased in connective tissue. They exist briefly as they pass through the intervertebral foramen. 31 pairs.

Question 6

Where does the spinal cord start and end?

Answer 6

Starts at inferior margin of medulla oblongata. Ends at conus medullaris at L2.

Question 7

Where do long roots from inferior segments descend?

Answer 7

In the cauda equina (horses tail) to exit at their respective foramina.

Question 8

What innervates the upper limb?

Answer 8

Anterior primary rami of spinal nerves originating from NEURAL (not vertebral) levels C5 to T1. These rami form the brachial plexus.

Question 9

Describe where the cervical and thoracic nerves originate

Answer 9

C1 emerges between occipital bone and atlas. C1-C7 exit above corresponding vertebrae. C8 exits between vertebrae C7 and T1. T1-L5 exit below corresponding vertebrae.

Question 10

What innervates the lower limb?

Answer 10

Anterior primary rami of spinal nerves originating from the neural level L1-S4. These rami form the lumbosacral plexus.

Question 11

What is the fate of mixed spinal nerves?

Answer 11

They divide into anterior and posterior rami. The posterior rami divide again into medial and lateral branches and they supply the muscles and skin of the dorsal trunk.
The anterior rami supply the muscles and skin of the limbs and ventral and lateral trunk. They enter the plexuses.

Question 12

What is the spinal level of injury?

Answer 12

The lowest level of full sensation and function.

Question 13

What is the relationship between spinal and peripheral nerves?

Answer 13

Within each peripheral nerve there may be fibres from more than one spinal nerve.
Fibres from one spinal nerve may enter more than one peripheral nerve.

Question 14

What viral infection only affects the skin of a single dermatome and why?

Answer 14

Herpes Zoster (shingles). Virus remains dormant in a dorsal root ganglion after chickenpox.

Question 15

What are the extrinsic and intrinsic muscles of the hand responsible for?

Answer 15

Extrinsic - crude movements and produce a forceful grip.

Intrinsic - fine motor functions of the hand.

Question 16

What intrinsic muscles of the hand are located in each of the five compartments?

Answer 16

Thenar eminence - abductor pollicis brevis, flexor pollicis brevis and opponens pollicis.
Hypothenar eminence - abductor digiti minimi, flexor digiti minimi brevis and opponens digiti minimi.
Adductor compartment - adductor pollicis
Central compartment - lumbricals
Separate interosseous compartments - interossei

Question 17

Where are the muscles of the thenar eminence located in relation to each other?

Answer 17

Opponens pollicis is deep to the other two. Abductor pollicis brevis is proximal to flexor pollicis brevis.

Question 18

Where are the muscles of the hypothenar eminence located in relation to each other?

Answer 18

Opponens digiti minimi lies deep to the other two. Abductor digiti minimi is the most superficial. Flexor digiti minimi brevis lies lateral to the abductor digiti minimi

Question 19

How many lumbricals, palmar interossei and dorsal interossei muscles are there?

Answer 19

4 lumbricals, 4 dorsal interossei and 3 palmar interossei

Question 20

Describe how the thickness of the palmar fascia changes

Answer 20

Thin over the hypothenar eminences, thick centrally (palmar aponeurosis - fans out distally to become the fibrous digital sheath) and in the fingers (digital sheath).

Question 21

What is Dupuytren contracture?

Answer 21

A disease that presents with partial flexion of one or more fingers due to fibrosis of the palmar fascia. Treatment can involve surgery.

Question 22

What is the function of the extensor retinaculum?

Answer 22

Keeps the extensor tendons in position, preventing bowstring

Question 23

What structures help to limit hyperextension of the digits?

Answer 23

Palmar plates of IP and MCP joints

Question 24

Which parts of the hand do the radial and ulnar arteries mainly contribute to?

Answer 24

Radial - thumb and lateral side of index finger

Ulnar - rest of the digits and medial side of index finger

Question 25

Outline the anatomical course of the ulnar artery in the hand

Answer 25

Enters the hand anteriorly to the flexor retinaculum and laterally to the ulnar nerve. It divides into the superficial and deep palmar arch.
Common digital arteries arise from the superficial palmar arch (found just deep to the flexor retinaculum), supplying the digits. It then anastomoses with a branch of the radial artery.

Question 26

Outline the anatomical course of the radial artery in the hand

Answer 26

Enters the hand dorsally, crossing the floor of the anatomical snuffbox. It turns medially and moves between the heads of the adductor pollicis. It then anastomoses with the deep palmar arch, which gives rise to five arteries supplying the digits.

Question 27

What are the four classical signs of a flexor sheath infection of the digit?

Answer 27

Finger held in slight flexion
Fusiform swelling
Tenderness along flexor sheath
Pain with passive extension of digit