Lesson 13 12 4 Membrane Potential Ion Movements and Electrical Signals All plasma cell membranes produce electrical signals by ion movements Membrane potential is particularly important to neurons 2015 Pearson Education Inc 12 4 Membrane Potential Five Main Membrane Processes in Neural Activities 1 2 Resting potential The membrane potential of resting cell Graded potential Caused by stimulus Temporary localized change in resting potential 3 Action potential an electrical impulse Produced by graded potential Propagates along surface of axon to the synapse 2015 Pearson Education Inc 12 4 Membrane Potential Five Main Membrane Processes in Neural Activities 4 Synaptic activity Releases neurotransmitters at presynaptic membrane Produces graded potentials in postsynaptic membrane Information processing Response integration of stimuli of postsynaptic cell 5 2015 Pearson Education Inc 12 4 Membrane Potential The Membrane Potential Three important concepts 1 The extracellular fluid ECF and intracellular fluid cytosol differ greatly in ionic composition Concentration gradient of ions Na K Cells have selectively permeable membranes Remember water soluble substance including ions cannot pass through hydrophobic region of lipid bilayer need channel carrier protein 2 3 Membrane permeability varies by ion 2015 Pearson Education Inc 12 4 Membrane Potential Passive Forces Acting across the Plasma Membrane Chemical gradients Na K Electrical gradients Concentration gradients chemical gradient of ions Separate charges of positive and negative ions Result in potential difference 2015 Pearson Education Inc Figure 12 9 Resting Membrane Potential Active Na K Pumps Sodium potassium Na K exchange pumps maintain the concentration of sodium and potassium ions across the plasma membrane Passive Chemical Gradients The intracellular concentration of potassium ions K is relatively high so these ions tend to move out of the cell through potassium leak channels Similarly the extracellular concentration of sodium ions Na is relatively high so sodium ions move into the cell through sodium leak channels Both of these movements are driven by a concentration gradient or chemical gradient Cl K leak channel 2 K Protein 3 Na Na leak channel Sodium potassium exchange pump ATP ADP Protein KEY Sodium ion Na Potassium ion K Chloride ion Cl Resting Membrane Potential Whenever positive and negative ions are held apart a potential difference arises We measure the size of that potential difference in millivolts mV The resting membrane potential for most neurons is about 70 mV The minus sign shows that the inner surface of the plasma membrane is negatively charged with respect to the exterior Passive Electrical Gradients Potassium ions leave the cytosol more rapidly than sodium ions enter because the plasma membrane is much more permeable to potassium than to sodium As a result there are more positive charges outside the plasma membrane Negatively charged protein molecules within the cytosol cannot cross the plasma membrane so there are more negative charges on the cytosol side of the plasma membrane This results in an electrical gradient across the plasma membrane 70 30 mV 0 3 0 Plasma membrane Protein Protein KEY FLUID EXTRACELLULAR CYTOSOL Sodium ion Na Potassium ion K Chloride ion Cl 12 4 Membrane Potential Electrical Currents and Resistance Electrical current Movement of charges to eliminate potential difference Resistance The amount of current a membrane restricts 2015 Pearson Education Inc 12 4 Membrane Potential The Electrochemical Gradient For a particular ion Na K is the sum of chemical and electrical forces acting on the ion across a plasma membrane A form of potential energy 2015 Pearson Education Inc 12 4 Membrane Potential Active Forces across the Membrane Sodium potassium ATPase exchange pump Is powered by ATP Carries 3 Na out and 2 K in Balances passive forces of diffusion Maintains resting potential 70 mV 2015 Pearson Education Inc 12 4 Membrane Potential The Resting Potential Because the plasma membrane is highly permeable to potassium ions The resting potential of approximately 70 mV is fairly close to 90 mV the equilibrium potential for K The electrochemical gradient for sodium ions is very large but the membrane s permeability to these ions is very low Na has only a small effect on the normal resting potential making it just slightly less negative than the equilibrium potential for K 2015 Pearson Education Inc 12 4 Membrane Potential The Resting Potential The sodium potassium exchange pump ejects 3 Na ions for every 2 K ions that it brings into the cell It serves to stabilize the resting potential when the ratio of Na entry to K loss through passive channels is 3 2 At the normal resting potential these passive and active mechanisms are in balance The resting potential varies widely with the type of cell A typical neuron has a resting potential of approximately 70 mV 2015 Pearson Education Inc 12 4 Membrane Potential Changes in the Membrane Potential Membrane potential rises or falls In response to temporary changes in membrane permeability Resulting from opening or closing specific membrane channels 2015 Pearson Education Inc 12 4 Membrane Potential Sodium and Potassium Channels Membrane permeability to Na and K determines membrane potential They are either passive or active Passive Channels Leak Channels Are always open Permeability changes with conditions Active Channels Gated Channels Open and close in response to stimuli At resting potential most gated channels are closed 2015 Pearson Education Inc 12 4 Membrane Potential Three States of Gated Channels 1 Closed but capable of opening 2 Open activated 3 Closed not capable of opening inactivated 2015 Pearson Education Inc 12 4 Membrane Potential Three Classes of Gated Channels 1 2 3 Chemically gated channels Voltage gated channels Mechanically gated channels 2015 Pearson Education Inc 12 4 Membrane Potential 1 Chemically Gated Channels Open in presence of specific chemicals e g Ach acetylcholine at a binding site Found on neuron cell body and dendrites 2015 Pearson Education Inc 12 4 Membrane Potential 2 Voltage gated Channels Respond to changes in membrane potential Have activation gates open and inactivation gates close Characteristic of excitable membrane Found in neural axons skeletal muscle sarcolemma cardiac muscle 2015 Pearson Education Inc 12 4 Membrane Potential 3 Mechanically Gated Channels Respond to membrane distortion Found in sensory
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