Stimuli bring the membrane to the threshold potential, and voltage-gated sodium channels trigger the all-or-none action potential. How is the threshold potential attained, and how are action potentials actually generated in various types of neurons? In afferent neurons, the initial depolarization to threshold is achieved by a graded potential—here called a receptor potential, which is generated in the sensory receptors at the peripheral ends of the neurons. These are the ends farthest from the central nervous system where the nervous system functionally encounters the outside world. In all other neurons, the depolarization to threshold is due either to a graded potential generated by synaptic input to the neuron, known as a synaptic potential, or to a spontaneous change in the neuron’s membrane potential, known as a pacemaker potential. How synaptic potentials are produced is the subject of the next section.
Spontaneous generation of action potentials may occur in the absence of any identifiable external stimulus and is an inherent property of certain neurons (and other excitable cells, including certain smooth-muscle and cardiac-muscle cells). In these cells, the activity of different types of ion channels in the plasma membrane causes a graded depolarization of the membrane—the pacemaker potential. If threshold is reached, an action potential occurs; the membrane then repolarizes and again begins to depolarize. There is no stable, resting membrane potential in such cells because of the continuous change in membrane permeability. The rate at which the membrane depolarizes to threshold determines the action potential frequency. Pacemaker potentials are implicated in many rhythmical behaviors, such as breathing, the heartbeat, and movements within the walls of the stomach and intestines.