Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none. They arise from the summation of the individual actions of ligand-gated ion channel proteins, and decrease over time and space. They do not typically involve voltage-gated sodium and potassium channels. Also called local potentials, these impulses are decremental and may be excitatory or inhibitory. They occur at the postsynaptic dendrite as a result of presynaptic neuron firing. The magnitude of a graded potential is determined by the strength of a stimulus.
Graded potentials that make the membrane more negative (more polarized) and make the neuron less likely to have an action potential are called inhibitory post synaptic potentials (IPSPs). Hyperpolarization of membranes is caused by influx of Cl- from nearby synapses. These potentials are mediated by GABA receptors (chloride channels).
Graded potentials that make the membrane less negative (less polarized) and make the neuron more likely to have an action potential are called excitatory post synaptic potentials (EPSPs). Depolarizing local potentials sum together until the voltage reaches the threshold and an action potential fires in that area. EPSPs are caused by the influx of Na+ from nearby neurons. When the presynaptic cell has an action potential, Ca+2 enters the synaptic knob and causes exocytosis of synaptic vesicles, in the case of an excitatory synapse, acetylcholine (Ach) is released. Ach diffuses across the synaptic cleft and binds to ligand-gated ion channels that allow Na+ to flow in to the cell. (K+ is also flowing out of the cell through the same channels.) Na+ spreads along the membrane and causes a temporary local potential.
If the EPSP is not large enough to trigger an action potential, the membrane repolarizes to its resting membrane potential. This shows the temporary nature of local potentials- they can be reversed.
The resting membrane potential is usually around -70mV. The typical neuron has a threshold ranging from -40mV to -55mV. This balance is determined by each neuron having a slightly unique threshold, but each fires at the same threshold every time. An action potential is reached through summation. It typically takes at least 30 EPSPs for a neuron to reach threshold.
Temporal summation occurs when local potentials from the same presynaptic neuron fire so rapidly that they build on each other before the last one fades.
- Caused by intense stimulation by one neuron
Spatial summation occurs when local potentials from different synapses in the area fire and add together.
- These charges accumulate in the trigger zone
An action potential occurs when the graded potentials in an area sum to reach the neuron's threshold. Voltage-gated ion channels open and allow the influx of cations. The membrane depolarizes (gets more positive) until the peak is reached and the ion channels close. After an action potential, depolarization happens through active transport.
- Hille 2001, pp. 169–200. "Chapter 6. Ligand-gated channels of fast chemical synapses."
- Hille, Bertil (2001). Ion Channels of Excitable Membranes (3rd ed.). Sunderland, Massachusetts: Sinauer. ISBN 0-87893-321-2.
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