4) Dermatomes and myotomes

Question 1

Describe how we get dermatome and myotome from a somite

Answer 1

Somite > Sclerotome + dermamyotome

Sclerotome > Verterbrae and ribs (repeating structures)

Dermamyotome > Dermatome (dermis) + myotome (muscle)

Each develops at a specific neural levela and will take this inneravation with it wherever it goes in the adult body

(Neural tube = adult spinal cord)

Question 2

Describe the difference between a vertebral and neural level

Answer 2

• Each vertebrae makes up a vertebral segment, hence a vertebral level
• The spinal chord passes through the vertebral foreman giving off pairs (L&R) of segmental nerves through the intervertebral foreman created between stacked vertebra, hence a neural level.
• Vertebral and neural levels are not the same thing! (Spinal cord finishes at L1/2 hence before the end of the vertebral column)

Question 3

What is a dermatome?

Answer 3

strip or area of skin supplied by a single spinal nerve

Question 4

What is a myotome?

Answer 4

a mucles or group of muscles innervated by a single nerve root

Question 5

Describe the formation of a segmental nerve

Answer 5

1. Anterior (ventral) and posterior (dorsal) rootlets converge to form:
2. Anterior (motor efferent) and posterior (senory afferent) roots
3. These two roots unite to form a MIXED SPINAL NERVE / segmental nerve just proximal to the interverterbral foreman
4. The segmental nerve splits into a anterior and posterior rami (which are combinaions of moth motor and sensory fibers so are not purely motor or sensory)

e.g Upper limb is supplied by the anterior rami of C5-T1

*remember anterior = ventral and posterior = dorsal

Question 6

Give the neural roots for each branches of the brachial plexus

Answer 6

• Axillary C5,6
• MC C5,6,7
• Median C6,7,8,T1
• Radial C5,6,7,8,T1
• Ulnar C8,T1

Question 7

Describe the distribution of dermatomes for the upper limb

Answer 7

Question 8

Describe the myotomes for the upper and lower limb

Answer 8

S1,2 (Buckle my shoe) Ankle jerk

L3,4 (Kick the door) Knee jerk

C5,6 (Pick up the sticks) Flex at elbow

C7,8 (Shut the gate) Ext at elbow

Shoulder:

• 5, abduction
• 6,7,(8) adduction

Wrist:

• 6,7 extension + flexion

Hand:

• 1 adduction and abduction of fingers (interrosei)
• 6 Supination
• 7,8 Pronation
• 7,8 ext and flex of fingers

Question 9

Explain why a lesion at a spinal root may not always lead to loss of cutaneous sensation

Answer 9

• contiguous areas of skin are supplied by contiguous spinal nerves.
• There is considerable overlap between adjacent dermatomes and so lesion to a single dorsal spinal root does NOT USUALLY lead to anaesthesia of the entire dermatome area.
• EXCEPT skin areas around an axial line. Here contiguous areas of skin are NOT innervated by adjacent spinal segments and there is no overlap across the axial line.

Question 10

How the number of muscle fiberes effects the action of a motor unit?

Answer 10

Fewer fibres = more precise movement

e.g external ocular muscles (5-6) of eye vs buttocks (100)

Question 11

Distinguish difference between a dermamyotome and the peripheral cutaneous distibution of nerves

Answer 11

• A dermatome is the area of skin supplied by a single spinal nerve
• A myotome, in this context, is the group of muscles innervated by a single nerve root
• After nerve fibres leave the spinal cord, they are often redistributed via nerve plexi and become peripheral nerves.
• The cutaneous distribution of a peripheral nerve is an area of skin this peripheral nerve innervates. It often has nerve fibres from several spinal roots. It is not a dermatome!
• Lesion of a spinal root would cause loss of sensation of that dermatome and also loss of function in that myotome.
• A lesion in the periphery can damage a peripheral nerve; thus the patient would lose sensation and motor control beyond the point of the lesion to skin and muscle fibres contained in the peripheral nerve’s innervation.