The neuron solves the problem of conducting information over a distance by using electrical signals that sweep along the axon. Axons act like telephone wires, however, the type of signal used by the neuron is constrained by the special environment of the nervous system. Fortunately, the axonal membrane has properties that enable it to conduct a special type of signal, action potential, that overcomes these biological constraints. Action potentials do not diminish over distance; they are signals of fixed size and duration. Information is encoded in the frequency of action potentials of individual neurons, as well as in the distribution and number of neurons firing action potentials in a given nerve. Cells capable of generating and conducting action potentials are said to have excitable membrane. Thus the action in action potentials occurs at the cell membrane. When a cell is not generating impulse, it is said to be at rest.