posterior branches of sacral nerves posterior branches of sacral nerves the posterior divisions of the sacral nerves are small and diminish in size as they move downward; they emerge, except the last, through the posterior sacral foramina. in some rare cases these nerves break and cause the person's legs to become weak and eventually wither away under the person's weight. posterior branches of sacral nervesthe posterior divisions of the sacral nerves.areas of distribution of the cutaneous branches of the posterior divisions of the spinal nerves. the areas of the medial branches are in black, those of the lateral in red.detailsfromsacral nervestomedial cluneal nervesidentifierslatinrami posteriores nervorum sacraliumanatomical terms of neuroanatomy the upper three are covered at their points of exit by the multifidus and divide into medial and lateral branches.