The point marked I is referred to as

The point marked "I" is referred to as the "node of Ranvier." The node of Ranvier is a small gap or space between the myelin sheaths that surround an axon. It is an important part of the nervous system because it allows for the efficient transmission of electrical impulses along the length of the axon. The myelin sheaths serve as insulation and help to speed up the transmission of the impulse, while the node of Ranvier allows the impulse to jump from one myelin sheath to the next, a process known as "saltatory conduction."