charge enters in one location and dissipates to others, losing intensity as it spreads, electrotonic spread is a graded response. It is important to contrast this with the all-or-none law propagation of the action potential down the axon of the neuron. EPSPs Electrotonic potential can either increase the membrane potential with positive charge or decrease it with negative charge. Electrotonic potentials that increase the membrane potential are called excitatory postsynaptic potentials (EPSPs). This is because they depolarize the membrane, increasing the likelihood of an action potential. As they sum together they can depolarize the membrane sufficiently to push it above the threshold potential, which will then cause an action potential to occur. EPSPs are often caused by either Na+ or Ca2+ coming into the cell. IPSPs Electrotonic potentials which decrease the membrane potential are called inhibitory postsynaptic potentials (IPSPs). They hyperpolarize the membrane and make it harder for a cell to have an action potential. IPSPs are associated with Cl− entering the cell or K+ leaving the cell. IPSPs can interact with EPSPs to "cancel out" their effect. Information Transfer Because of the continuously varying nature of the electrotonic potential versus the binary response of the action potential, this creates implications for how much information can be encoded by each respective potential. Electrotonic potentials are able to transfer more information within a given time period than action potentials. This difference in information rates can be up to almost an order of magnitude greater for electrotonic potentials. Cable theory Cable theory can be useful for understanding how currents flow through the axons of a neuron. In 1855, Lord Kelvin devised this theory as a way to describe electrical properties of transatlantic telegraph cables. Almost a century later in 1946, Hodgkin and Rushton discovered cable theory could be applied to neurons as well. This theory has the neuron approximated as a cable whose radius does not change, and allows it to be represented with the partial differential equation where V(x, t) is the voltage across the membrane at a time t and a position x along the length of the neuron, and where λ and τ are the characteristic length and time scales on which those voltages decay in response to a stimulus. Referring to the circuit diagram on the right, these scales can be determined from the resistances and capacitances per unit length. From these equations one can understand how properties