Stimulation of sufficient intensity alters the permeability of the cell membrane. An action potential results from brief changes in the membrane's permeability to sodium and potassium ions. At the peak of the action potential, the inside of the axon becomes less negative (or even positive), and potassium ions are driven out of the cell while positively charged sodium ions rush in. The action potential originates near the soma and once it starts, it travels down the entire length of the axon until it reaches the terminal buttons. At the synapse, the action potential triggers the release of the neurotransmitter substance to affect postsynaptic cells. Neurons follow an all-or-none principle, meaning that either the neuron fires an action potential or it does not fire one. A weaker stimulus will not produce a weaker neural impulse. Neurons, however, can provide information about the strength of a stimulus by altering the rate at which they generate action potentials. A stronger stimulus will generally cause a cell to fire faster than a weaker stimulus will.