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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Graded potential</span></span>
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<p><b>Graded potentials</b> are changes in <a href="Membrane_potential" title="Membrane potential">membrane potential</a> that vary according to the size of the stimulus, as opposed to being <a href="All-or-none_law" title="All-or-none law">all-or-none</a>. They include diverse potentials such as <a href="Receptor_potential" title="Receptor potential">receptor potentials</a>, <a href="Electrotonic_potential" title="Electrotonic potential">electrotonic potentials</a>, <a href="Subthreshold_membrane_potential_oscillations" title="Subthreshold membrane potential oscillations">subthreshold membrane potential oscillations</a>, <a href="Slow-wave_potential" title="Slow-wave potential">slow-wave potential</a>, <a href="Pacemaker_potential" title="Pacemaker potential">pacemaker potentials</a>, and <a href="Synaptic_potential" title="Synaptic potential">synaptic potentials</a>. The magnitude of a graded potential is determined by the strength of the stimulus. They arise from the summation of the individual actions of <a href="Ligand-gated_ion_channel" title="Ligand-gated ion channel">ligand-gated ion channel</a> proteins, and decrease over time and space.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> They do not typically involve <a href="Voltage-gated_ion_channel" title="Voltage-gated ion channel">voltage-gated</a> <a href="Voltage-gated_sodium_channel" title="Voltage-gated sodium channel">sodium</a> and <a href="Voltage-gated_potassium_channel" title="Voltage-gated potassium channel">potassium channels</a>, but rather can be produced by neurotransmitters that are released at synapses which activate ligand-gated ion channels.<sup id="cite_ref-:0_2-0" class="reference"><a href="#cite_note-:0-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> They occur at the <a href="Chemical_synapse" title="Chemical synapse">postsynaptic</a> <a href="Dendrite" title="Dendrite">dendrite</a> in response to presynaptic <a href="Neuron" title="Neuron">neuron</a> <a href="Action_potential" title="Action potential">firing</a> and release of <a href="Neurotransmitter" title="Neurotransmitter">neurotransmitter</a>, or may occur in <a href="Skeletal_muscle" title="Skeletal muscle">skeletal</a>, <a href="Smooth_muscle" title="Smooth muscle">smooth</a>, or <a href="Cardiac_muscle" title="Cardiac muscle">cardiac muscle</a> in response to <a href="Peripheral_nervous_system" title="Peripheral nervous system">nerve</a> input. These impulses are incremental and may be excitatory or inhibitory.
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<div class="mw-heading mw-heading2"><h2 id="Ligand-gated_ion_channels">Ligand-gated ion channels</h2></div>
<p>Graded potentials are usually produced in the dendrites of a neuron where voltage-gated channels are not present. They are localized changes in the membrane potential in response to a stimuli, like neurotransmitters binding to receptor. This binding causes a change in conformation, which activates the receptor to interact with proteins. This reaction activates the opening of ion channels resulting in movement of Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, or Cl<sup>-</sup> ions across the membrane producing graded potentials. Unlike action potentials, graded potentials stay in the area where the stimulation occurred and each synapse will be either excitatory or inhibitory.<sup id="cite_ref-:02_3-0" class="reference"><a href="#cite_note-:02-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="Excitatory_postsynaptic_potentials_(EPSPs)">Excitatory postsynaptic potentials (EPSPs)</h2></div>
<p>Graded potentials that make the membrane potential less negative or more positive, thus making the postsynaptic cell more likely to have an <a href="Action_potential" title="Action potential">action potential</a>, are called <a href="Excitatory_postsynaptic_potential" title="Excitatory postsynaptic potential">excitatory postsynaptic potentials</a> (EPSPs).<sup id="cite_ref-Openstax_Anatomy_&_Physiology_attribution_4-0" class="reference"><a href="#cite_note-Openstax_Anatomy_&_Physiology_attribution-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> <a href="Depolarization" title="Depolarization">Depolarizing</a> local potentials sum together, and if the voltage reaches the <a href="Threshold_potential" title="Threshold potential">threshold potential</a>, an action potential occurs in that cell.
</p><p>EPSPs are caused by the influx of Na<sup>+</sup> or Ca<sup>2+</sup> from the extracellular space into the neuron or muscle cell. When the presynaptic neuron has an action potential, Ca<sup>2+</sup> enters the <a href="Axon_terminal" title="Axon terminal">axon terminal</a> via <a href="Voltage-dependent_calcium_channel" class="mw-redirect" title="Voltage-dependent calcium channel">voltage-dependent calcium channels</a> and causes <a href="Exocytosis" title="Exocytosis">exocytosis</a> of <a href="Synaptic_vesicle" title="Synaptic vesicle">synaptic vesicles</a>, causing neurotransmitter to be released. The transmitter diffuses across the synaptic cleft and activates ligand-gated ion channels that mediate the EPSP. The amplitude of the EPSP is directly proportional to the number of synaptic vesicles that were released.
</p><p>If the EPSP is not large enough to trigger an action potential, the membrane subsequently repolarizes to its <a href="Resting_membrane_potential" class="mw-redirect" title="Resting membrane potential">resting membrane potential</a>. This shows the temporary and reversible nature of graded potentials.
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<div class="mw-heading mw-heading2"><h2 id="Inhibitory_postsynaptic_potentials_(IPSPs)">Inhibitory postsynaptic potentials (IPSPs)</h2></div>
<p>Graded potentials that make the membrane potential more negative, and make the postsynaptic cell less likely to have an action potential, are called <a href="Inhibitory_post_synaptic_potential" class="mw-redirect" title="Inhibitory post synaptic potential">inhibitory post synaptic potentials</a> (IPSPs). <a href="Hyperpolarization_(biology)" title="Hyperpolarization (biology)">Hyperpolarization</a> of membranes is caused by influx of Cl<sup>−</sup> or efflux of K<sup>+</sup>. As with EPSPs, the amplitude of the IPSP is directly proportional to the number of synaptic vesicles that were released.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="Summation">Summation</h2></div>
<p>The resting membrane potential is usually around –70 mV. The typical neuron has a threshold potential ranging from –40 mV to –55 mV. Temporal summation occurs when graded potentials within the postsynaptic cell occur so rapidly that they build on each other before the previous ones fade. Spatial summation occurs when postsynaptic potentials from adjacent synapses on the cell occur simultaneously and add together. An action potential occurs when the summated EPSPs, minus the summated IPSPs, in an area of membrane reach the cell's threshold potential.
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<div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2></div>
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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
<ul><li><cite id="CITEREFHille2001" class="citation book cs1"><a href="Bertil_Hille" title="Bertil Hille">Hille, Bertil</a> (2001). <i>Ion Channels of Excitable Membranes</i> (3rd ed.). Sunderland, Massachusetts: Sinauer. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-87893-321-2</bdi>.</cite></li></ul></div><!--htdig_noindex--><div><div class="zim-footer">
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