An action potential occurs when the summated EPSPs, minus the summated IPSPs, in an area of membrane reach the cell's threshold potential. Spatial summation occurs when postsynaptic potentials from adjacent synapses on the cell occur simultaneously and add together. Temporal summation occurs when graded potentials within the postsynaptic cell occur so rapidly that they build on each other before the previous ones fade. The typical neuron has a threshold potential ranging from –40 mV to –55 mV. The resting membrane potential is usually around –70 mV. As with EPSPs, the amplitude of the IPSP is directly proportional to the number of synaptic vesicles that were released. Hyperpolarization of membranes is caused by influx of Cl − or efflux of K +. Graded potentials that make the membrane potential more negative, and make the postsynaptic cell less likely to have an action potential, are called inhibitory post synaptic potentials (IPSPs). This shows the temporary and reversible nature of graded potentials. If the EPSP is not large enough to trigger an action potential, the membrane subsequently repolarizes to its resting membrane potential. The amplitude of the EPSP is directly proportional to the number of synaptic vesicles that were released. The membrane potential will begin at a negative resting membrane potential, will rapidly become positive, and then rapidly return to rest during an action potential. The transmitter diffuses across the synaptic cleft and activates ligand-gated ion channels that mediate the EPSP. The action potential is a brief but significant change in electrical potential across the membrane. When the presynaptic neuron has an action potential, Ca 2+ enters the axon terminal via voltage-dependent calcium channels and causes exocytosis of synaptic vesicles, causing neurotransmitter to be released. Depolarizing local potentials sum together, and if the voltage reaches the threshold potential, an action potential occurs in that cell.ĮPSPs are caused by the influx of Na + or Ca 2+ from the extracellular space into the neuron or muscle cell. Graded potentials that make the membrane potential less negative or more positive, thus making the postsynaptic cell more likely to have an action potential, are called excitatory postsynaptic potentials (EPSPs). The magnitude of a graded potential is determined by the strength of the stimulus. Glia are also essential to nervous system function, but they work mostly by supporting the neurons. They occur at the postsynaptic dendrite in response to presynaptic neuron firing and release of neurotransmitter, or may occur in skeletal, smooth, or cardiac muscle in response to nerve input. Neurons are the basic functional units of the nervous system, and they generate electrical signals called action potentials, which allow them to quickly transmit information over long distances. These impulses are incremental and may be excitatory or inhibitory. They do not typically involve voltage-gated sodium and potassium channels. They arise from the summation of the individual actions of ligand-gated ion channel proteins, and decrease over time and space. They include diverse potentials such as receptor potentials, electrotonic potentials, subthreshold membrane potential oscillations, slow-wave potential, pacemaker potentials, and synaptic potentials, which scale with the magnitude of the stimulus. Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none.
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