A shift in the electrical charge in a tiny area of a neuron is called a

• A. IPSP
• B. EPSP
• C. Action potential
• D. Graded potential

Answer: (D)

A shift in charge confined to a small patch of membrane is a graded potential. This local change arises when ion channels open in a limited region, so the size of the depolarization or hyperpolarization is proportional to the stimulus strength. Because it is local, the signal diminishes with distance from the site of origin. Graded potentials can be excitatory (depolarizing) or inhibitory (hyperpolarizing) and can sum with other signals to influence whether an action potential is triggered at the axon hillock. In contrast, an action potential is a brief, all-or-none event that travels along the axon without decreasing in strength. EPSP and IPSP are specific types of graded potentials at the postsynaptic membrane, illustrating the broader concept. Hence the shift in charge in a tiny area is best described as a graded potential.