S2) Dermatomes, Myotomes and Segmental Innervation of the Limbs Flashcards

1
Q

What is a dermatome?

A

A dermatome is the area of skin supplied by a single spinal nerve e.g. C5 supplies the skin on the lateral aspect of the arm

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2
Q

How are dermatomes named?

A

Each dermatome is named according to the spinal nerve which provides most of its sensory innervation

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3
Q

Why don’t spinal cord lesions/damage ever result in the loss of sensation in an entire dermatome?

A

There is considerable overlap between adjacent dermatomes so transection of a single dorsal spinal nerve root doesn’t usually lead to anaesthesia of the entire dermatome

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4
Q

Describe the embryonic development of dermatomes

A
  • Nerves grow into the developing limb buds
  • As the limb buds increase in size, the nerves are ‘dragged’ along with the structures they innervate thus eventually producing the adult pattern
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5
Q

Describe the anatomical distribution of the dermatomes in the upper limb

A

The upper limb is innervated by the anterior primary rami of spinal nerves originating from C5 to T1

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6
Q

Describe the anatomical distribution of the dermatomes in the lower limb

A

The lower limb is innervated by the anterior primary rami of spinal nerves originating from L2-5 and S1-3

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7
Q

What is a myotome?

A

A myotome is the group of muscles innervated by a single nerve root (motor equivalent of a dermatome) e.g. C5 myotome includes the flexors of the elbow and the abductors of the shoulder

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8
Q

In embryology, what does the term myotome refer to?

A

Myotome is also used in embryology to describe that part of the somite which develops into the muscles

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9
Q

State the segmental innervation of all movements in the upper limb

A
  • C5: shoulder abduction and lateral rotation
  • C6: elbow flexion, wrist extension, supination, medial rotation
  • C7: elbow extension, wrist flexion, pronation
  • C8: finger flexion, finger extension
  • T1: finger abduction and adduction
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10
Q

State the segmental innervation of all movements in the lower limb

A
  • L2: hip flexion
  • L3: knee extension, hip adduction
  • L4: ankle dorsiflexion
  • L5: great toe extension, ankle inversion, hip abduction
  • S1: ankle plantarflexion, ankle eversion, hip extension
  • S2: knee flexion, great toe flexion
  • S3-S4: anal wink
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11
Q

Describe the formation of a peripheral nerve

A

Nerve fibres leave the spinal cord to supply the limb and travel through the spinal nerve root and are redistributed via the nerve plexuses to become peripheral nerves

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12
Q

What is the cutaneous distribution of a peripheral nerve?

A

The cutaneous distribution of a peripheral nerve is the area of skin the peripheral nerve innervates and often contains nerve fibres from several spinal nerve roots

E.g. the radial nerve (C5-8,T1) innervates the skin of the anatomical snuff box

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13
Q

State the segmental sensory innervation of the skin of the upper limb

A
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14
Q

State the segmental sensory innervation of the skin of the lower limb

A
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15
Q

Describe the motor and sensory deficits caused by spinal nerve lesions

A

A lesion of a spinal nerve root (near the spinal cord) will cause loss of sensation in the relevant dermatome and also loss of function in the relevant myotome

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16
Q

Describe the motor and sensory deficits caused by peripheral nerve lesions

A

A lesion in a peripheral nerve will cause loss of sensation and muscle weakness in the distribution of the peripheral nerve

17
Q

Describe individual myotome testing in the upper limb

A
  • C5: elbow flexion
  • C6: wrist extension
  • C7: elbow extension
  • C8: grip
  • T1: finger abduction
18
Q

Describe individual myotome testing in the lower limb

A
  • L2: hip flexion
  • L3: knee extension
  • L4: ankle dorsiflexion
  • L5: big toe extension
  • S1: ankle plantarflexion
19
Q

Describe individual dermatome testing in the upper limb

A
  • C5: proximal third of lateral forearm
  • C6: tip of thumb
  • C7: tip of middle finger
  • C8: ulnar border of little finger
  • T1: proximal third of medial forearm
20
Q

Describe individual dermatome testing in the lower limb

A
  • L2: lateral anterior thigh
  • L3: medial anterior thigh
  • L4: medial anterior calf
  • L5: lateral anterior calf
  • S1: lateral foot, heel
  • S2: popliteal fossa
  • S3: central buttock