Dermatomes, Myotomes and Segmental Innervation of the Limbs Flashcards
How do dermatomes develop?
During embryonic development, nerves grow into the developing limb buds. As the limb bud increases in size, the nerves are ‘dragged’ along with the structures they innervate thus eventually producing the adult pattern.
How are dermatomes named?
Each dermatome is named according to the spinal nerve which provides most of its sensory innervation. In general, contiguous areas of skin are supplied by contiguous spinal nerves.
Why does division of a spinal nerve root not usually lead to anaesthesia of the entire dermatomal area?
Because there is considerable overlap between adjacent dermatomes. The exception to this rule concerns skin areas that abut onto an axial line.
Describe the position of C1-T1
There are seven cervical vertebrae and eight cervical nerves. The first cervical nerve emerges from the spinal cord and passes between the base of the skull and the 1st cervical vertebra; thus the 8th nerve emerges below C7. From T1 inferiorly, the spinal nerves exit below their corresponding vertebra.
Where does the lower limb receive its motor innervation from?
The lower limb receives its motor innervation from branches of either the lumbar or sacral plexus. These are formed from the anterior primary rami of L2, 3, 4 and 5 and S1, 2 and 3.
What is a myotome?
The muscles innervated by a single spinal nerve root are referred to as a myotome.
What is a motor unit?
The muscle fibres innervated by a single motor nerve fibre are called a motor unit.
How big are motor units?
The size of motor units varies considerably in different muscles. In the eye, the external ocular muscles may have motor units containing only 5-6 muscle fibres; in the muscles of the buttocks there may be more than 1000 muscle fibres. In general terms the smaller the motor unit, the more precise movements it may perform.
What will be the consequence of a lesion in a spinal nerve root vs a peripheral nerve?
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. A lesion in a peripheral nerve will cause loss of sensation and muscle weakness in the distribution of the peripheral nerve.
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, extending distally to just below the elbow.
What is the cutaneous distribution of peripheral nerves?
The cutaneous distribution of a peripheral nerve is the area of skin this peripheral nerve innervates. It often contains nerve fibres from several spinal nerve roots. It is not a dermatome. For example, the axillary nerve (C5,6) innervates the skin of the lateral shoulder and upper arm (regimental badge area) via its superior lateral cutaneous branch.
What is a peripheral nerve?
When nerve fibres leave the spinal cord to supply the limb, they travel through the spinal nerve root and are redistributed via the nerve plexuses (i.e. brachial or lumbosacral plexus) to become peripheral nerves.
