Ch 12 Membrane Action Potential 10 25 2015 Describe the main membrane processes in neurons Membrane potential is particularly important to neurons The membrane potential is caused by a difference of charge between the inside and outside of the cell Five Main Membrane Processes in Neural Activities Resting potential Graded potential o The membrane potential of resting cell o Caused by stimulus o Temporary localized change in resting potential o Can make more positive or more negative Action potential an electrical impulse o Produced by graded potential if it is strong enough o Propagates along surface of axon to the synapse Synaptic activity o Releases neurotransmitters at presynaptic membrane o Neurotransmitter diffuses across synaptic cleft and binds to receptors on postsynaptic cell o Produces graded potentials in postsynaptic membrane Information processing o Response integration of stimuli of postsynaptic cell Explain the transmembrane potential Greater negative charge in inside of plasma membrane and positive outside membrane partitioning of charge creates a voltage difference across the membrane called the resting potential 1 Differences in ion concentrations across the membrane Sodium ions are found in a high concentration outside cells Potassium ions are found in a high concentration inside cells 2 The cell membrane is selectively permeable K channels are more permeable than Na channels Proteins cannot pass through channels too large 3 As a result of s 1 and 2 the cytoplasm tends to be negatively charged The extracellular area is positively charged The resistance of the lipid bilayer to a free flow of substances across the membrane helps to maintain this potential differenc across the membrane In a neuron this difference measures 70 millivolts Why is it important What are the main ions molecules that generate this potential o Potassium and sodium ion Explain the electrochemical gradient Why is it important 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 Explain the role of sodium and potassium ions on the membrane potential Do they enter leave through active or passive channels The extracellular fluid ECF and intracellular fluid cytosol differ greatly in ionic composition Concentration gradient of ions Na K More Na outside more K inside Cells have selectively permeable membranes Remember water soluble substance including ions cannot pass through hydrophobic region of lipid bilayer need channel carrier protein Membrane permeability varies by ion Due to the presence of specific passive leak channels Explain how gated channels function Active Channels Gated Channels Open and close in response to stimuli At resting potential most gated channels are closed What are the different types of gated channels 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 2 Voltage gated Channels Respond to changes in membrane potential A change in membrane potential causes the channels to open or close Have activation gates open and inactivation gates close Na has two gates while K only has one gate Characteristic of excitable membrane Found in neural axons skeletal muscle sarcolemma cardiac muscle 3 Mechanically Gated Channels Respond to membrane distortion Found in sensory receptors touch pressure vibration What are the different states to a gated channel o Closed but capable of opening o Open activated o Closed not capable of opening inactivated Describe the difference between action potentials and graded potentials Action potential an electrical impulse Produced by graded potential if it is strong enough Propagates along surface of axon to the synapse Propagated changes in membrane potential Affect an entire excitable membrane Link graded potentials at cell body with motor end plate actions Graded Potentials stimulation Also called local potentials Changes in membrane potential cannot spread far from site of o Like throwing a pebble into a pond the ripples that form travel a short distance before they disappear Includes any stimulus that opens a gated channel and produces a graded potential The resting state o Opening sodium channel produces graded potential Resting membrane exposed to chemical Sodium channel opens Sodium ions enter the cell Membrane potential rises Depolarization occurs why Remember more Na on outside so when opened Na flows into cell through the channel Are they always depolarizing or hyperpolarizing o Depolarizing o Depolarization A shift in membrane potential toward 0 mV Movement of Na through channel Produces local current Depolarizes nearby plasma membrane graded potential Change in potential is proportional to stimulus Drop a small pebble create small ripples in pond Drop a rock create bigger ripples Explain how graded potentials can generate action potentials AP s Action potentials originate in the axon hillock A threshold stimulus changes the shape of the voltage gated sodium channels causing their activation gates to open This event marks the beginning of phase one of the action potential known as depolarization An influx of sodium ions generates the action potential Which region of the neuron generates the AP o Axon hillock o Initiating Action Potential Initial stimulus A graded depolarization of axon hillock large enough 10 to 15 mV to change resting potential 70 mV to threshold level of voltage gated sodium channels 60 to 55 mV The graded depolarization can be from one source or multiple sources as long as it depolarizes the axon hillock to threshold What is the all or none principle o If a stimulus exceeds threshold amount Action potential is triggered The action potential is the same no matter how large the stimulus Describe the steps to the generation of an action potential Four Steps in the Generation of Action Potentials Step 1 Depolarization to threshold o Graded potential s reaches axon hillock to depolarize it It then passes to the initial segment of the axon If threshold is reached then step 2 occurs Step 2 Activation of Na channels o Rapid depolarization Voltage gated Na channels open allowing net flow of Na ions into cytoplasm Inner membrane of that area changes from negative to positive Step 3 Inactivation of Na channels and activation of K channels o At 30 mV Inactivation gates close Na channel inactivation K channels open net flow of K out of cell
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