The point marked I is referred to as

Explanation

The diagram is that of neuron and the part labelled I is the node of ranvier.

A "node of Ranvier" in a neuron is a small gap in the myelin sheath that covers the axon, where the axon membrane is exposed to the extracellular fluid, allowing for the rapid transmission of electrical impulses along the axon by facilitating the influx of sodium ions at these specific points; essentially, it's a critical point where the signal can jump between sections of myelin to travel faster. 

(See diagram above)