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4 Nerve cells.docx

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Department
Physiology
Course
Physiology 2130
Professor
Anita Woods
Semester
Winter

Description
Module 4: Nerve Cells  Virtually all cells in the body have membrane potentials  Inside electrically negative  Outside electrically positive  Electrochemical gradients of the major intercellular and extracellular ions establish and maintain membrane potential  Excitable cells (nerve and muscle cells), use resting membrane potential to generate electrochemical impulse called an action potential - i.e. nerve cell (neuron) - action potential is the ―language‖ of nervous system because this is how cells communicate with one another - action potentials needed for muscles to contract Structure of a Nerve Cell  Picture of multipolar neuron  Several type of neurons Structure Function Dendrites - Thin branching processes of cell body that receive incoming signals - ↑ overall SA of neuron so it can communicate with other neurons - # dendrites vary depending on where nerve cell is located in the nervous system Cell Body (Soma) - Control centre of nerve cell - Contains nucleus and all other organelles needed to direct cellular activity Axon - Projection of the cell body that carries the outgoing signal to the target cell in the form of an action potential - May or may not be myelinated Myelin sheath - Layered phospholipid membrane sheath wrapped tightly around the axon - Insulated with a fatty acid material called myelin, produced by special cells: Schwann cells - Schwann cells: in PNS and oligodendrocytes in CNS - Effect of this myelin is to insulate the axon so few ions can leak out through the membrane - Insulator for the axon forcing the ionic changes that comprise the action potential to take place in only small exposed regions of the axon called the Nodes of Ranvier - This jumping of action potential from node to node results in significant increase in transmission down the length of the axon Node of Ranvier - Small exposed regions of the axon - Jumping of action potential from node to node increases the speed of transmission down the length of the axon Collaterals - Branching of the axon near its terminal end - Serve to ↑ # of target cells with which the neuron can interact Terminal Bouton - Swelling at the end of an axon collateral (axon terminal) - Swelling contains mitochondria & membrane bound vesicles contain various neurocrin molecules - Chemical in this terminal facilitate the transmission of the signal across the synapse to the target cell Action Potential  Action potential: it is a rapid reversal of the resting membrane 1. Membrane potential rapidly increases from resting (-70mV) to +35mV - called depolarization 2. Membrane potential rapidly returns to -70mv - called repolarization 3. membrane briefly becomes more negative, -90mV - called hyperpolarization 4. Membrane returns to resting membrane potential, -70mV Q: What causes these rapid changes in membrane potential? A: the movement of ions across the membrane—principally sodium ions + + (Na ) and potassium ions (K ). These ions are allowed to move across the membrane through special channels Voltage-Gated Channels  Types of channels 1. Voltage gated sodium channel In neurons, these channels are found on the axon 2. Voltage gated potassium channel  Essential for generation of action potential  Channels are sensitive to changes membrane potential - They open when inside of cell becomes more + - i.e. -70mV to -60mV, depolarization Voltage-Gated Sodium Channel  Only allows sodium through, when there is a depolarization of the membrane (inside +), both gates are on the intracellular site + Na Steps: 1. Depolarization of the membrane occurs (membrane potential becomes more positive/less negative) 2. Activation gate opens immediately + 3. Na flow into the cell, down the concentration gradient 4. Inactivation gate closes (10 of a millisecond later) and Na can no longer flow into the cell; the channel cannot open 5. Channel returns to resting configuration (inactivation gate open and activation gate closed) 6. Channel is now ready to open again. Inactivation of Na Voltage-Gated Channel – The Absolute Refractory Period  Absolute refractory period: time period when inactivation gate is closed, channel will not open (channel has become inactivated), regardless of the strength of stimulation Voltage-Gated Potassium Channel  Only 1 gate, which opens when the membrane depolarizes  Unlike Na+ channel, they begin to open when the sodium voltage-gated channels starts to become inactivated - Do not open immediately - This is essential to the generation of the action potential - Unlike the Na voltage-gated channel, these channels do not have an inactivation period K Steps: 1. Depolarization of membrane occurs (membrane potential becomes more +/less -) 2. After a brief pause, K+ voltage –gated channels open located at the intracellular side (unlike Na+ channels that open immediately) + 3. K flow out of the cell, down their electrical and chemical gradients 4. Gate closes and channel returns to resting configuration 5. Channel is now ready to open again  These channels are essential to create action potential  Na voltage-gated channels open first and then become inactivated, producing the absolute refractory period +  K voltage-gated channels then begin opening as the Na channels begin entering the inactivated period The Action Potential  Membrane reverses from -70mV to +35mV  Begins at axon hillock: a region of neuron, most electrically sensitive area of the nerve - This region contains largest # of voltage-gated channels Steps: 1. Strong depolarization at the axon hillock triggers opening of most Na voltage-gated channels 2. Na rushes into the neuron, down its electrochemical gradient 3. Membrane depolarizes rapidly to roughly +35 mV 4. Na channels become inactivated while K channels begin opening. + 5. K rushes out of the cell, down its electrochemical gradient slowly 6. Membrane begins repolarizing back to normal (+35 mV back to –70 mV) 7. K continues to rush out of the cell and the membrane hyperpolarizes (reaches –90 mV). + + 8. K channels begin to close and K no longer leaves the cell 9. Membrane potential slowly returns to resting value of –70 mV → → → → → → Refractory Periods  Inactivation of Na channels cause absolute refrectory period, peiod when Na+ gates won`t open to fire another action potential  Relative refractory period is the period during action potential when membrane is hyperpolarized (less than -70mV) - Caused by the K voltage-gated 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 it has repolarized to –70 mV - It is possible to fire another action potential, but it would require a stronger stimulus to reach threshold Threshold for Starting an Action Potential +  Action potentials require a strong depolarization at axon hillock → to open many Na voltage-gated channels + If small # of Na ions enter cell ↓ Small depolarization, small + charge build up ↓ Cell will attempt to maintain resting membrane potential (-70mV) ↓ + – + Charge build up affects other ions inside and outside the cell i.e. K and chloride (Cl ) ↓ + – K has a positive charge, it will leave the inside; at the same time, Cl (which is negative) will be attracted into the cell through leak channels which are ALWAYS open ↓ Movement of both of these ions (K out and Cl in) will repolarize the membrane potential back to normal +  In order to fire an action potention: depolarizing force from Na moving in must exceed the natural repolarizing forces from K moving out and Cl coming in +  If amount of positive charge (fr
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