Complex sentence: Electroreceptors detect electric signals by building up a potential difference between the outside environment and the fish body's internal environment; current flow due to such potential difference further results in a receptor potential that's presynaptic to the sensory fibers.
Split into simpler sentences: Electroreceptors detect electric signals by building up a potential difference between the outside environment and the fish body's internal environment. Current flow due to such potential difference further results in a receptor potential that is presynaptic to the sensory fibers.