I mean, impulse is electrical energy. So, its got to change its form.
The electric impulse is generated via chemical energy which is mediated via ion exchange mechanisms across the nerve membranes. Once, the person dies, the pumps are no longer maintained, regulated ion flux decreases and impulses can no longer be generated or conducted.
The net result is a conversion to a state of entropy, manifested by an increase in heat production and loss of membrane integrity.