Microanatomy and number of nerve fibres of the lower intercostal nerves with respect to a nerve anastomosis. Donor nerve analysis. I. (IV).

A human intercostal nerve has about 10,000 myelinated nerve fibres, of which about 1000 to 2000 are motor. At about the axillary line the nervus intercostalis splits into the ramus cutaneous lateralis and a deep branch to the musculus rectus abdominis and other muscles. The ramus cutaneous lateralis consists of 2 skin branches of about 3000 nerve fibres and a muscle branch to the musculus obliquus externus of about 400 to 1100 nerve fibres. The deep branch to the musculus obliquus internus, the musculus transversus and the musculus rectus abdominis and the anterior skin consists of about 2500 to 7000 fibres of which 400 to 1700 are skin fibres. There is indication that the muscle branch to the musculus obliquus externus has about 50% motor fibres. The intercostal nerve is, as it splits after about 20 cm into 4 branches, a suitable donor of at least 2 different muscle functions and a skin sensible function for a direct nerve anastomosis. A crude model for calculating matching probabilities of motor and sensory fibres between donor and acceptor nerves is introduced. Besides the importance of having enough donor nerve fibres, it is calculated that the small amount of motor nerve fibres can be best used if muscle branches of the donor nerves are connected to muscle branches of the acceptor nerves. Because of the separation between normal laboratory animals and humans on the phylogenetic scale, differences in regeneration and plasticity are discussed.