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Module 4

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Western University
Physiology 2130
Anita Woods

Module 4 - Nerve and muscle cells are considered excitable because they can use resting membrane potential to generate an electrochemical impulse called an action potential - considered the language of the nervous system -cells communicate with one another using this - also used by muscle contractions - Nerve structure: - Cell body (soma) – control center – contains nucleus and all necessary organelles required for cellular activity - Dendrites- receive incoming signals – increases surface area to communicate with other neurons. # varies depending on where in the nervous system it is located. Thin, branch- like structures - Axon – projection of the cell body, carries signal to target cell in form of action potential– may be myelinated (layered phospholipid membrane sheath) o Myelin sheath – insulator for the axon – forces ionic changes that comprises the action potential to take place only at small exposed regions (nodes of ranvier) o Jumping of the action potential from node to node increases speed of transmission down the axon - Collaterals – branching of the axon near the terminal end - increases the number of target cell with which the neuron can interact - Terminal bouton (axon terminal) – swelling at the end of the axon collateral – contains mitochondria and membrane bound vesicles containing neurocrine molecules – chemicals here facilitates transmission of the signal across the synapse to the target cell - Action potential is a rapid reveral of the resting membrane – the membrane potential rapidly changes from -70 mV to +35 mV – called depolarization o After this the membrane quickly returns to -70 mV – repolarization o Membrane briefly becomes more negative (-90 mV) – hyperpolarization o Returns to resting levels of -70 mV o Movement of ions across the membrane (principally Na+ and K+) causes this change - Voltage-gated sodium and potassium channels are found on the axon and are essential for action potential generation o Sensitive to changes in the membrane potential and open when the inside of the cell becomes more positive (-70 mV to -60 mV) – depolarization o V-g sodium channels: depolarization opens activation gate immediately – sodium flows into the cell down the concentration (electrochemical) gradient  Inactivation gate closes and sodium can no longer flow into the cell, channel cannot open  Channel returns to resting configuration (inactivation gate open, activation gate closed) – channel is ready to open again  Inact+ act - Both gates on intracellular side of the channel  At rest – inactivation gate open, activation gate closed – depolarization causes change in protein to open activation gate quickly  Inactivation gate takes more time to react to the depolarization (1/10 of a millisecond) – returns to normal  When the inactivation gate is closed – channel will not open regardless of the strength of the stimulation – channel is inactivated  Time period during this inactivation called the absolute refractory period o V-g K+ channels contain only one gate – opens when the membrane depolarizes but not immediately like Na+ channels  Begin opening when Na+ v-g channels have been inactivated  After a pause, these channels are open – potassium flows out of the cell down their electrochemical gradients  Membrane begins repolarizing back to normal –K+ continues to rush out- reaches hyperpolarization  Gate closes, k+ can no longer leave cell and channel returns to resting configuration, channel ready to open again  no inactivation period,  Slowly returns to resting value (-70 mV) by passive movement of ions through leak channels - Action potential generally begins at region of neuron called the “axon hillock” – contains largest number of v-g channels – strong depolarization in this region - Second refractory period – the relative refractory period – period during the action potential when the membrane is hyperpolarized – caused by K+ channels which are not only slow to open but slow to close o Possible to fire another action potential but would require a stronger stimulus to reach threshold o K+ has a positive charge and will leave the inside, at the same time Cl- will be attracted into the cell – this repolarizes the membrane potential back to normal o In order to fire action potential – the depolarizing force of Na+ moving in must exceed the natural repolarizing forces from K+ moving out and Cl- moving in – if the movement of the incoming Na+ is large enough, the movement of other ions will not be sufficient enough to counteract the buildup of the positive charge  Initial depolarization must be strong enough to open almost all Na+ v-g channels  Most Na+ v-g channels open when membrane potential depolarizes to -55 mV – called the threshold for generating action potential  Na+ enters cell first – peaks rapidly – K+ leaves more slowly • The rounded peak of the action potential due to Na+ channels beginning to close while k+ channels beginning to ope
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