Final answer:
The pudendal nerve is responsible for the innervation of the inferior portion of the vagina, playing a significant role in voluntary control and contractions.
Step-by-step explanation:
The nerve that innervates the inferior portion of the vagina is the pudendal nerve. The pudendal nerve arises from the sacral micturition center and is associated with controlling voluntary functions, including contraction of the external urinary sphincter which is a skeletal muscle. It plays a crucial role in the mechanisms of both urination and vaginal contraction.