S2) Dermatomes, Myotomes and Segmental Innervation of the Limbs Flashcards
What is a dermatome?
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
How are dermatomes named?
Each dermatome is named according to the spinal nerve which provides most of its sensory innervation
Why don’t spinal cord lesions/damage ever result in the loss of sensation in an entire dermatome?
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
Describe the embryonic development of dermatomes
- 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
Describe the anatomical distribution of the dermatomes in the upper limb
The upper limb is innervated by the anterior primary rami of spinal nerves originating from C5 to T1
Describe the anatomical distribution of the dermatomes in the lower limb
The lower limb is innervated by the anterior primary rami of spinal nerves originating from L2-5 and S1-3
What is a myotome?
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
In embryology, what does the term myotome refer to?
Myotome is also used in embryology to describe that part of the somite which develops into the muscles
State the segmental innervation of all movements in the upper limb
- 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
State the segmental innervation of all movements in the lower limb
- 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
Describe the formation of a peripheral nerve
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
What is the cutaneous distribution of a peripheral nerve?
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
State the segmental sensory innervation of the skin of the upper limb
State the segmental sensory innervation of the skin of the lower limb
Describe the motor and sensory deficits caused by spinal nerve lesions
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
Describe the motor and sensory deficits caused by peripheral nerve lesions
A lesion in a peripheral nerve will cause loss of sensation and muscle weakness in the distribution of the peripheral nerve
Describe individual myotome testing in the upper limb
- C5: elbow flexion
- C6: wrist extension
- C7: elbow extension
- C8: grip
- T1: finger abduction
Describe individual myotome testing in the lower limb
- L2: hip flexion
- L3: knee extension
- L4: ankle dorsiflexion
- L5: big toe extension
- S1: ankle plantarflexion
Describe individual dermatome testing in the upper limb
- 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
Describe individual dermatome testing in the lower limb
- 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
