Fiber arrangements of nerves belonging to ventral and dorsal divisions in the proximal region of the brachial plexus: a study using fluorescence of DiI and DiO in adult rats

The motor and sensory fiber arrangements in the proximal region of the spinal nerves are important for understanding the relationship of the peripheral nerves to neuron distribution. On the other hand, the fiber arrangements are also important for the treatment of peripheral nerve grafting. We studied the fiber arrangements of two divisions (ventral and dorsal) in the proximal region of the brachial plexus and the fiber arrangements of the lateral cutaneous rami in Th7 and Th8 intercostal nerves in adult rats with a method using the fluorescent pigments DiI and DiO. Results showed that fiber arrangements belonging to the two divisions have a specific separate distribution in the distal region. However, this specific separate distribution form was absent in the proximal region of the spinal nerves in the plexus. Fiber arrangements of the lateral cutaneous ramus in the anterior branches of the thoracic nerves (intercostal nerves) were also observed in comparison with those in the brachial plexus by the same method. In the intercostal nerves, fibers of the lateral cutaneous ramus were distributed in the dorsal portion from distal to proximal. These results suggest that there are two types of fiber arrangement in the proximal regions of the spinal nerves: a ventrodorsal distributional type, comprising intercostal nerves and the dorsal branches of the spinal nerves; and a mixed type, comprising the anterior branches of the cervical and brachial (and perhaps lumbar) plexuses. On the other hand, fibers of the lateral cutaneous rami in the intercostal nerves were distributed on the dorsal part of the nerves. These results of fiber arrangement analysis for the intercostal nerves may offer an opportunity to improve the effect of treatments using peripheral nerve grafting and suturing in the brachial and lumbar plexus with intercostal nerves.