 In this video, I will define and describe the electrochemical basis of the membrane potential, equilibrium potential, resting membrane potential, depolarization, repolarization, and hyperpolarization. Compare ligand-gated, voltage-gated, and ion-leak channels. Contrast graded potentials with the action potential. Define threshold potential, excitatory postsynaptic potentials, temporal summation, and spatial summation, and distinguish between absolute and relative refractory periods. Voltage, also known as electric potential difference, is the energy stored in the separation of positive and negative charges. This separation of charges creates a driving force known as the electromotive force that will drive the movement of charged particles. The membrane potential is a separation of charges across the plasma membrane. Typically, the membrane potential in a excitable cell at rest is approximately negative 70 millivolts. As you can see here, a voltmeter is a tool that can be used to measure the membrane potential of a cell by placing a recording mini-electrode in the cytosol and a reference electrode in the extracellular fluid. The voltmeter will compare the charge inside of the cell to the charge outside of the cell, and typically the resting membrane potential in a neuron is around negative 70 millivolts. The membrane potential results from the separation of ions by the plasma membrane. There's a high concentration of sodium and chloride ions in the extracellular fluid, and there's a relatively high concentration of potassium ions in the intracellular fluid. There's also a high concentration of organic anions that have a negative charge, which are mostly proteins found in the cytosol that are too large to cross the plasma membrane. The equilibrium potential for an ion is the value of the membrane potential when there is no net flow of that ion into or out of the cell because the electromotive force is equal and opposite to the force of diffusion. The Nernst equation can be used to calculate the equilibrium potential for an ion. The Nernst equation has three variables that will have to input to calculate the equilibrium potential for an ion. Z is the charge of that ion, so sodium and potassium have a plus one charge, whereas chloride has a negative one charge. Then the extracellular concentration of the ion and the intracellular concentration of the ion are the other variables that we will need to calculate the equilibrium potential for an ion. Using the Nernst equation, we can calculate the equilibrium potential for potassium because the potassium concentration inside of the cytosol is much higher than the potassium concentration in the extracellular fluid the force of diffusion resulting from the concentration gradient for potassium drives potassium across the plasma membrane from the cytosol out into the extracellular fluid. Therefore, it would require a negative membrane potential in order to counterbalance the force of diffusion. The electromotive force that is equal and opposite to the force of diffusion for potassium occurs when the membrane potential is equal to negative 97 millivolts, the equilibrium potential for potassium. Similarly, we can use the Nernst equation to calculate the equilibrium potential for sodium because there is a much higher extracellular concentration of sodium. The force of diffusion resulting from the concentration gradient for diffusion tends to move sodium into the cell across the plasma membrane from the extracellular fluid. Therefore, a positive value of the membrane potential would be required in order to create an electromotive force that is equal and opposite to the force of diffusion balancing the overall forces driving the movement of sodium so that there will be no net flow of sodium in or out of the cell. This occurs when the membrane potential equals positive 66 millivolts the electromotive force driving positively charged sodium ions out of the cell is equal and opposite to the force of diffusion driving sodium ions into the cell. We can also use the Nernst equation to calculate the equilibrium potential for chloride ions because there is a high extracellular concentration of chloride ions the force of diffusion will move chloride ions from the extracellular fluid into the cytosol across the plasma membrane. Now, chloride has a negative charge. Therefore, it will require a negative value of the membrane potential to create an electromotive force driving chloride out of the cell to oppose the force of diffusion. The equilibrium potential for chloride is approximately negative 90 millivolts which is a value of the membrane potential that creates an electromotive force equal and opposite to the force of diffusion therefore there will be no net flow of chloride into or out of the cell when the membrane potential is negative 90 millivolts. The resting membrane potential is the value of the membrane potential in a neuron at rest that is when the neuron is not producing a graded potential or action potential the value of the resting membrane potential is typically around negative 70 millivolts and this value results from the relative concentrations of sodium and potassium in the extracellular fluid and cytoplasm and also the relative permeability of the plasma membrane to those ions. In this illustration we can see three major types of integral membrane proteins that are involved in moving sodium and potassium across the plasma membrane the sodium-potassium transporter is the sodium-potassium pump that will use energy released from hydrolysis of ATP in order to force three sodium ions out of the cell and two potassium ions into the cell. This sodium-potassium pump is what creates the concentration gradients with a high sodium concentration in the extracellular fluid and a low potassium concentration in the extracellular fluid. Sodium and potassium can cross the plasma membrane through a variety of channels some of these ion channels are constantly open what we call ion leak channels that stay open all the time and are not gated other ion channels can be gated meaning that they can open or close and some of them are ligand gated ion channels that we'll see are chemically gated by a neurotransmitter and others are voltage gated that is they will be stimulated to open or close in response to changes in the membrane potential. The Goldman-Hodgkin-Katz equation enables us to calculate the value of the resting membrane potential the variables that we need to calculate the resting membrane potential include the charge of the ions and so for this equation we'll focus on sodium and potassium because these are the most important ions that influence the membrane potential and both sodium and potassium have a positive one charge we will also need the extracellular and intracellular concentrations for sodium and potassium ions and the relative permeability of the plasma membrane to these ions the plasma membrane is approximately 20 times more permeable to potassium than sodium and so the permeability factor for potassium will be set to one and the permeability factor for sodium is set to 0.05 or one twentieth of the permeability of potassium by entering these variables into the Goldman-Hodgkin-Katz equation you can see that the value of the resting membrane potential is approximately negative 70 millivolts ion leak channels determine the permeability of the membrane at rest and there are approximately 20 times more potassium ion leak channels than sodium ion leak channels in most neurons and this is why the resting membrane potential in a typical neuron which is approximately negative 70 millivolts is much closer to the equilibrium potential for potassium approximately negative 97 millivolts compared to the equilibrium potassium for sodium which is positive 66 millivolts however gated channels can open in order to influence the permeability of the membrane ligand gated ion channels also known as ion channel linked receptors or ionotropic neurotransmitter receptors are integral membrane proteins that detect a chemical message a paracrine signal known as a neurotransmitter the neurotransmitter is the ligand that binds to the ligand gated ion channel and this neurotransmitter binding to the receptor will cause the ion channel to open and this will change the permeability of the plasma membrane if the ligand gated ion channel is a sodium ion channel the relative permeability of the plasma membrane to sodium will increase and this will have an effect on the value of the membrane potential as the permeability of the membrane to sodium increases and sodium is rushing into the cell at a higher rate the membrane potential will become less negative which is what we call depolarization voltage gated ion channels can also influence the membrane potential the stimulus for opening of a voltage gated ion channel is a change in the value of the membrane potential so the previous example of ligand gated ion channels could create a small change in the membrane potential that would then trigger the opening of a voltage gated sodium ion channel as the voltage gated sodium ion channels open this would further increase the permeability of the plasma membrane to sodium which will further depolarize the membrane potential that is make the membrane potential less negative or more positive the voltage gated sodium channel is a channel that will open in response to a threshold value of the membrane potential that threshold value is approximately negative 55 millivolts so the resting membrane potential was negative 70 and if the membrane potential was depolarized by some ligand gated sodium ion channels being activated which depolarized the membrane enough to reach the threshold negative 55 millivolts that will stimulate voltage gated sodium channels to open and that will create even more depolarization as long as those channels remain open the voltage gated sodium channel will remain open for about one millisecond after it's activated then it will become inactivated during that period when the voltage gated sodium channel is inactivated it's impossible for it to become opened and activated again it will require repolarization of the plasma membrane to become more negative than the threshold in order to de-inactivate the voltage gated sodium channel which is de-inactivation is resetting the ion channel back to the starting point where it can now be activated by depolarization to threshold potential a graded potential is a localized change in the value of the membrane potential and a graded potential will become weaker as it spreads through the cell as it spreads out over time and it spreads out over space there are two major types of graded potentials the illustration here represents the excitatory postsynaptic potentials in green and the inhibitory postsynaptic potentials in red an excitatory postsynaptic potential involves depolarization this is a graded potential that causes the membrane potential to become less negative so an excitatory postsynaptic potential in EPSP is stimulated when a neurotransmitter activates a ligand gated sodium ion channel and this is called an excitatory postsynaptic potential because depolarization brings the membrane potential value closer to the threshold potential of negative 55 millivolts which is the critical level of the membrane potential that would stimulate opening of voltage gated ion channels so the other major type of graded potential is known as an inhibitory postsynaptic potential EPSP when a neurotransmitter stimulates opening of a ligand gated ion channel that is a chloride channel or potassium channel this would cause hyperpolarization hyperpolarization means the membrane potential becomes more negative the membrane potential moves away from the threshold potential and so an EPSP is known as an inhibitory postsynaptic potential because it's making it less likely that the membrane potential will reach the threshold for opening of voltage gated ion channels so a small stimulus could cause a relatively small size EPSP that depolarizes the plasma membrane but not enough to reach threshold for opening voltage gated ion channels and the duration of the graded potential is proportional to the amount of time that the ligand gated ion channels remain open we could have a longer duration EPSP or a shorter duration EPSP depending on how long the ion channel stays open for we could also have a larger magnitude of an EPSP or a smaller magnitude which would depend on the number of ligand gated ion channels that open if an EPSP is strong enough to stimulate depolarization of the plasma membrane to the threshold value of negative 55 millivolts voltage gated ion channels will open and this will cause an action potential so while EPSPs are depolarizing graded potentials that make it more likely for a neuron to reach threshold and fire an action potential IPSPs are hyperpolarizing graded potentials that make it less likely for a neuron to reach the threshold to fire an action potential at most synapses a single EPSP is not strong enough to cause the membrane potential to reach threshold and multiple EPSPs will have to overlap in the mechanism of summation of graded potentials in order for the membrane potential to reach threshold there are two major types of summation of graded potentials temporal summation occurs when a presynaptic neuron fires at a relatively high frequency stimulating multiple EPSPs in rapid succession that will overlap to produce enough depolarization that the membrane potential reaches threshold the other major type of summation is known as spatial summation spatial summation occurs when a postsynaptic neuron is stimulated by multiple presynaptic neurons when multiple synapses are stimulating the postsynaptic neuron at the same time this is what we call spatial summation so temporal summation can occur at one synapse when the presynaptic neuron stimulates the postsynaptic neuron at a high frequency in contrast to spatial summation that occurs when multiple neurons are simultaneously stimulating multiple synapses on one postsynaptic neuron summation can involve overlapping EPSPs that make it more likely for the postsynaptic neuron to fire an action potential or we can have summation of EPSPs with IPSPs which will cancel out the EPSPs making it less likely that the postsynaptic neuron will fire an action potential the action potential is a brief reversal of the membrane potential that spreads non decrementally down the axon from the axon hillock to the axon terminals the action potential is stimulated when the membrane potential is depolarized to the threshold value of negative 55 millivolts causing opening of voltage-gated sodium channels these channels open and sodium rushes into the cell causing depolarization during the rising phase of the action potential and then the voltage-gated sodium channels close after approximately one millisecond and become inactivated this inactivation of voltage-gated sodium channels causes a refractory period known as the absolute refractory period when the voltage-gated sodium channels are inactivated is impossible for another action potential to be generated this will help to ensure that the action potential spreads in one direction from the axon hillock to the axon terminals and this also will lead to a maximal frequency of action potentials it's impossible to fire action potentials any faster than about 1000 per second or 1000 hertz after the rising phase of the action potential when the voltage-gated sodium channels close voltage-gated potassium channels will open causing the falling phase of the action potential repolarization occurs during the falling phase of the action potential because the voltage-gated sodium channels have closed and the voltage-gated potassium channels have opened potassium will rush out of the cell, causing rapid repolarization, and there will be hyperpolarization that occurs. The voltage-gated potassium channels being open creates an even greater relative permeability of the plasma membrane to potassium, causing the membrane potential to become more negative than the resting membrane potential. This hyperpolarization that occurs at the end of the following phase of the action potential is the relative refractory period. That is, it's a brief period immediately following the action potential when it is more difficult to stimulate another action potential. So while it is possible to stimulate another action potential by having summation of EPSPs that occur during the hyperpolarization at the end of an action potential, these EPSPs will have to sum with that hyperpolarization and cancel it out in order to create enough depolarization to reach threshold. Therefore, at the relative refractory period, the open voltage-gated potassium channels are making it exceptionally difficult to trigger another action potential, but not impossible. So it's a relative refractory period in contrast to the absolute refractory period. The absolute refractory period will end during the following phase of the action potential when repolarization causes de-inactivation of the voltage-gated sodium channels. Once the voltage-gated sodium channels have been de-inactivated at the membrane potentials below negative 55, it is possible for them to become activated again during the relative refractory period, but it will just require a greater magnitude of depolarization to cancel out the hyperpolarization that results from the open voltage-gated potassium channels. You can see these voltage-gated potassium channels do close at the end of the action potential and cause the membrane potential to return to the value of the resting membrane potential of approximately negative 70 millivolts. The rising phase of the action potential occurs when the summation of excitatory post-enacted potentials depolarize the plasma membrane to threshold of negative 55 millivolts, causing opening of voltage-gated sodium channels. Some rushes into the cell, and the membrane potential depolarizes to a positive value, and then the voltage-gated sodium channel will close and become inactivated. The falling phase of the action potential occurs as the voltage-gated sodium channels close and become inactivated, as at the same time voltage-gated potassium channels open. The inactivated voltage-gated sodium channels will produce an absolute refractory period at the beginning of the falling phase of the action potential, until enough potassium rushing out of the cell through voltage-gated potassium channels causes repolarization of the membrane potential back below the threshold value of negative 55 millivolts. Then because the voltage-gated potassium channels are still open at the end of repolarization, there is hyperpolarization creating a relative refractory period. During that relative refractory period, the open voltage-gated potassium channels are creating a hyperpolarizing current, and it would make it more difficult for EPSPs to cause enough depolarization to reach threshold. However, if there is a great enough magnitude of depolarization from EPSPs to overcome the hyperpolarization from the voltage-gated potassium channels and push the membrane potential to the threshold value of negative 55 millivolts, voltage-gated sodium channels will open triggering another action potential. So the action potential in a neuron lasts around 2 milliseconds. The first millisecond is the rising phase where depolarization occurs as sodium is rushing in through voltage-gated sodium channels, then voltage-gated sodium channels close and become inactivated as voltage-gated potassium channels open causing repolarization during the falling phase of the action potential, and after the repolarization, hyperpolarization is produced as a result of open voltage-gated potassium channels. This hyperpolarization is responsible for the relative refractory period where it is more difficult for EPSPs to trigger another action potential. The action potential will be generated at the axon hillock as EPSPs and IPSPs are generated throughout the dendrites in SOMA. The summation of these graded potentials will occur at the axon hillock where voltage-gated sodium channels can respond. If the membrane potential at the axon hillock is depolarized to the threshold value of negative 55 millivolts, the voltage-gated sodium ion channels open and this produces a rapid depolarization that will spread from the axon hillock towards the axon terminals. Repolarization will spread as a wave and then repolarization will follow behind the wave of depolarization. The absolute refractory period for the voltage-gated sodium channels becoming inactivated prevents the action potential from traveling in the opposite direction, although voltage-gated sodium channels opening will cause depolarization that can spread in both directions through the axon. The region of the axon that has just fired an action potential will be in an absolute refractory period and cannot be triggered to fire again, therefore this depolarization can only trigger an action potential to spread in the direction from the axon hillock towards the axon terminals. When the action potential reaches the axon terminals, it will then stimulate the release of neurotransmitters from the axon terminals.