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Action potentials are electrical signals that propagate rapidly along the membrane of an excitable cell, such as a neuron. They are all-or-none events meaning that once initiated, they spread across the membrane with a constant amplitude and duration. Synaptic potentials are local potentials, changes in membrane electrical potential that are confined to one part of the membrane. They are graded, meaning their amplitude can vary depending on the strength of the stimulus and are generated at the synapse between two neurons.

Action potentials are initiated by a large enough stimulus causing the membrane potential to exceed a threshold, known as the threshold potential. After the threshold is reached, the action potential propagates along the membrane, maintaining its amplitude and duration until it reaches the terminals of the neuron. The propagation is unidirectional, preventing the action potential from traveling back to its point of origin. Synaptic potentials are generated at the synapse when a neurotransmitter released by a presynaptic neuron binds to a specific receptor in the postsynaptic neuron. The binding of the neurotransmitter can either cause depolarization or hyperpolarization in the postsynaptic membrane, depending on the characteristics of the neurotransmitter and receptor. These changes, called excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) respectively, are spatially and temporally summated to determine the overall potential change in the postsynaptic neuron.

Action potentials follow an all-or-none response, meaning that they either occur fully or not at all. Once the threshold is reached, the action potential will travel along the membrane with a constant amplitude and duration. Synaptic potentials, on the other hand, are graded, meaning their amplitude can vary based on the strength of the stimulus. The size of the EPSP or IPSP can change as the strength and rate of the input signal change.

Action potentials have a relatively short duration, usually lasting only a few milliseconds. Once the action potential is initiated, it travels rapidly along the membrane of the neuron. Synaptic potentials have a longer duration, usually lasting tens of milliseconds to several seconds. The duration of a synaptic potential depends on the amount of neurotransmitter released and the rate at which it is cleared from the synaptic cleft.

In summary, action potentials and synaptic potentials are both essential aspects of neuronal communication. Action potentials are all-or-none, rapidly propagated signals, while synaptic potentials are localized, graded responses generated at the synapse. The two processes differ in initiation, propagation, response type, and duration, but work together to enable complex signaling within the nervous system.