Answer:
Dominant innervation via the lateral branch of 4th intercostal nerve, with overlapping contribution from anterior and lateral cutaneous branches of 3rd-5th intercostal nerves. Decreased sensation post-op varies widely from negligible to almost half ; average is 15%. Prevention is through preservation of these branches at the lateral aspect of the breast by blunt dissection from medial to lateral to mobilize the branches out of the way. Easier to do this in the subpectoral plane (as the branches pass through the deep fascia into the gland they become relatively fixed and can be injured during dissection or electrocautery of lateral pocket).