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Module 4 Notes.docx

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Physiology 2130

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Physiology 2130 Module 4 Online Notes (Sec 4.1 to 4.31) Nerve Cells Introduction  Virtually all cells in the body have membrane potentials (inside being electrically negative relative to the extracellular fluid)  Recall the ion concentrations and electrical distribution of a typical cell  The sizes of the ions represent their relative extra- and intracellular concentrations and the arrows show the associated concentration gradients for each ion  Certain cells are considered “excitable” because they can use the resting membrane potential to generate an electrochemical impulse called an action potential. These excitable cells include nerve cells and muscle cells  The action potential is the way nerve cells communicate with one another  Action potentials are also necessary for muscle contractions Structure of a Nerve Cell Dendrites: Thin branching processes of the cell body, whose function is to receive incoming signals. Dendrites increase the overall surface area of the neurons so that it can communicate with many other neurons. The number of dendrites on a given nerve cell will vary depending on where in the nervous system the cell is located. Cell Body (soma): The cell body is the control center of the nerve cell, containing a nucleus and all necessary organelles for directing cellular activity in the nerve cell. Axon: Projection of the cell body that carries the outgoing signal to the target cell in the form of an action potential. It may or may not be myelinated. There is only one axon to a neuron and it is usually much longer and much less branched than dendrites. Myelin Sheath: A lipid sheath around a nerve fiber, formed from closely spaced spiral layers of the plasma membrane of a Schwann cell or oligodendrocyte that aids in the transmission of nerve impulses. Node of Ranvier: A gap in the myelin sheath of a nerve, between adjacent Schwann cells Collaterals: Branching of the axon near the terminal end, these collaterals serve to increase the number of possible target cells with which the nerve can interact Terminal Bouton or Axon Terminal: Swelling at the end of an axon collateral, this swelling contains mitochondria aswell as membrane bound vesicles containing various neruocrine molecules, the chemicals in the axon terminal facilitate the transimission of the signal across the synapse to the target cell. A Quick Look at the Action Potential  The action potential is a rapid reversal of the resting membrane  During an action potential, the membrane potential rapidly changes from resting (-70mV) to roughlt +35mV  This sudden change to a positive value is called depolarization  After this the membrane potential rapidly return to -70mV (repolarization)  The membrane potential then briefly becomes more negative, reaching roughly -90mV (hyperpolarization)  After this very negative phase it returns to resting levels (-70mV)  These changes are brought upon by the movement of ions across the membrane – principally sodium ions (Na ) and potassium ions (K ) Voltage-Gated Channels  There are two very special types of channels found in nerve and muscle cells: voltage-gated sodium and voltage-gated potassium channels  In a neuron these channels are generally found on the axon and are needed for the generation of an action potential  These channels are sensitive to changes in membrane potential and open when the inside of the cell becomes more positive (depolarization) Voltage-Gated Sodium Channels  This channel is specific for sodium and will allow no other molecules through  Gates only open when there is a depolarization  Summary of Events: o Depolarization of the membrane (inside becomes more positive) o Activation gate opens immediately o Na flow into the cell down the concentration gradient + o Inactivation gate closes and Na can no longer flow into the cell; the channel cannot open o Channel returns to resting configuration (inactivation gate open and activation gate closed) o Channel is now ready to open again  During the period when the inactivation gate is closed, the channel will not open, regardless of the strength of stimulation; the channel has become inactivated  Time period during inactivation is called the absolute refractory period Voltage-Gated Potassium Channel  Voltage-gated K channels contain only one gate, which opens when the + membrane depolarized, but they do not open immediately like the Na voltage channels  They begin opening when the Na voltage-gated start to become inactivated  Summary of Events: o Depolarization of the membrane occurs o After a brief pause, K voltage-gated channels open o K flow out of the cell, down their chemical gradients o Gate closes and channel returns to resting configuration o Channel is ready to open again  Unlike Na channels, these K channels do not have an inactivation period  K gates begin to open when the Na begin to enter their inactivation period  These channels are essential for the generation of an action potential The Action Potential  During an action potential, the membrane potential rapidly reverses from - 70mV to roughly +35mV  In the body, it generally begins at a region of the neuron called the axon hillock (region that contains largest number of voltage-gated channels)  Summary of Events: o Strong depolarization at the axon hillock triggers opening of most Na + voltage-gated channels o Na rushes into the neuron, down its electrochemical gradient o Membrane depolarizes rapidly to roughly +35mV o Na channels become inactivated while K channels begin opening o K rushes out of cell, down its electrochemical gradient o Membrane begins repolarizing back to normal (+35mV to -70mV) + o K continues to rush out of the cell and the membrane hyperpolarizes (reaches -90mV) o Membrane potential slowly returns to resting value (-70mV) Refractory Periods  Inactivation of the Na channels contributes to absolute refractory period o This is the period of time, when regardless of the strength of + depolarization, the Na gates will not open to fire another action potential  There is a second refractory period – relative refractory period  This is the period during the action potential when the membrane is hyperpolarized  This period is caused by the K voltage channels which are not only slow to open but are also slow to close +  This allows K to continue to leave the cell even after repolarization  During this period, it’s possible to fire another action potential, but it would require a stronger stimulus to reach threshold Threshold for Starting an Action Potential  Action potentials do not always occur, they require a strong depolarization at the axon hillock to open many voltage-gated Na channels  In a hypothetical situation where only a few voltage-gated channels would open up a small depolarization would occur, but the cell would attempt to maintain its resting membrane potential  The small buildup of positive charge would affect other ions inside and outside the cell, especially K and Cl -  Since K has a positive charge, it will leave the inside, at the same time, Cl - which is negative will be attracted into the cell  The movement of both of these ions will repolarize the membrane potential back to normal  In order to fire the action potential, the depolarizing force from the Na + moving in must exceed the natural repolarizing forces from K moving out and Cl coming in  The initial depolarization must be strong enough to open almost all of the sodium voltage-gated channels to allow a sufficient amount of Na into cell  This occurs when the membrane
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