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Neuronal Physiology Chapter 4
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Nerve and muscle tissues are excitable. They can undergo rapid changes in their membrane potentials. They can change their resting potentials into electrical signals.
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NERVE CELLS USE THESE SIGNALS TO: INITIATE RECEIVE PROCESS TRANSMIT MESSAGES AROUND THE BODY
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DEPOLARIZATION & REPOLARIZATION The process of changing and then re- establishing resting membrane potential. There are four phases (parts) of this process.
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THE FOUR STATES OF MEMBRANE POTENTIAL ARE: Polarization Depolarization Repolarization Hyperpolarization
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Nerve and muscle cells can develop several electrical states at their membranes . Each state is characterized by a different electrical charge. The charge is a result of ion movement into or out of the cell. This change in charge can be used to send messages around the body.
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POLARIZATION Polarization - A -70mV charge has been established. (i.e. the membrane is polarized or has potential). Nothing is happening. This is the resting state, waiting for action.
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DEPOLARIZATION Depolarization - This change makes the membrane less negative (-70 towards 0mV). This is the change that sends the signal.
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RE-POLARIZATION Repolarization - A return to resting potential after depolarization. The electrical charge moves back towards negative -70mV. Re-establishing the original potential it had prior to depolarization.
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HYPER-POLARIZATION Hyperpolarization - This makes the membrane more polarized, more negative inside. For a brief period of time the electrical potential of the membrane goes beyond -70mV to -90mV. This makes it harder to depolarize the membrane again.
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Upward deflection = Decrease in potential Downward deflection = Increase in potential Repolarization Hyperpolarization Depolarization Resting potential
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Electrical changes occur by changes in ion permeability and ion movement across the plasma membrane of nerve and muscle cells.
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ION PERMEABILITY MEANS IONS (NA + & K +) CAN NOW MOVE ACROSS THE CELL MEMBANE A flow of positive ions into the ICF depolarizes the cell. (Na + influx) A flow of positive ions into the ECF can repolarize a cell after depolarization. (K + efflux) From the resting state, a flow of positive ions into the ECF can hyperpolarize the cell. (K + influx)
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Electrical changes occur by changes in ion permeability The process starts when ions (namely Na + ) move into the cell. This is what starts the depolarization process. Ion movement occurs across the plasma membrane of nerve and muscle cells.
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Ions can only cross the membrane through specific channels or gates. There are two types of gates or channels. Leak channels (open all the time--subject to concentration gradient). Voltage Gated channels (GATES) (selectively opened or closed depending on the membrane charge or potential).
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When ions move into the cell they create a triggering or depolarizing event. There are two types: Graded Potentials Action Potentials
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A graded potential is a local electrical change in the membrane.
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