Dermatomes and peripheral nerves

[at599]

This might sound a stupid question but I was getting confused between dermatomes and peripheral nerves.

Are dermatomes regions supplied by the peripheral nerves, or directly from spinal nerves? For instance, the medial aspect of the hand and medial distal aspect of the forearm are C8 dermatome. However, the medial cutaneous nerve of the arm also has nerve fibres from c8 via the brachial plexus; yet the medial aspect of the arm is not part of the c8 dermatome. Can someone please explain this to me? I would be very grateful!

Thanks

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[richse]

Each dermatome indicates the spinal nerve that is responsible for the majority of sensation to the skin in that area. The afferent info is carried back by tons of small nerves so I wouldn't say the spinal nerves innervate the dermatome directly. In the example you give, the medial cutaneous nerve of the arm has innervation by C8 and T1 so it can supply multiple dermatomes. It might even carry some T2, because that supplies a small portion of the proximal part of the medial arm, hence the chest pain radiating to the upper arm. Basically, each dermatome doesn't need to be covered by only one nerve and one nerve can help innervate multiple dermatomes.