LESSON 13 You should be able to Describe the main membrane processes in neurons 1 Resting potential 2 Graded potential The membrane potential of resting cell Temporary localized change in resting potential Caused by stimulus Can make inside more positive or more negative 3 Action potential an electrical impulse Produced by graded potential Propagates along surface of axon to the synapse 4 Synaptic activity Releases neurotransmitters at presynaptic membrane Produces graded potentials in postsynaptic membrane Neurotransmitter diffuses across synaptic cleft and binds to receptors 5 Information processing Response integration of stimuli of postsynaptic cell Explain the transmembrane potential Why is it important What are the main ions molecules that generate this potential Important to neurons Sodium and Potassium Three important concepts The extracellular fluid ECF and intracellular fluid cytosol differ greatly in ionic composition o Concentration gradient of ions Na K More Na outside more K inside Cells have selectively permeable membranes o Remember water soluble substances including ions cannot pass through hydrophobic region of lipid bilayer need channel carrier protein Membrane permeability varies by ion o Due to the presence specific passive leak channels 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 Combination of electrical and chemical gradients Explain the role of sodium and potassium ions on the membrane potential Do they enter leave through active or passive channels Sodium potassium ATPase exchange pump Is powered by ATP Carries 3 Na out and 2 K in Balances passive forces of diffusion occurs through passive channels Maintains resting potential 70 mV one of the numbers that we need to know 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 Net flow always greater to lesser concentration Active Channels Gated Channels Open and close in response to stimuli At resting potential most gated channels are closed Can go either way but net flow goes greater to lesser Describe the role of the sodium potassium pump in action potential generation Will cessation of this pump cause an immediate halt to action potential generation Powering the Sodium Potassium Exchange Pump To maintain concentration gradients of Na and K over time Requires energy 1 ATP for each 2 K 3 Na exchange Without ATP Na and K concentration gradient would disappear very slowly after tens of thousands of action potentials Neurons stop functioning Explain how gated channels function Open or close in response to specific stimuli an active channel What are the different types of gated channels Three Classes 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 channel to open or close Have activation gates open and inactivation gates close Some have 2 gates Such as Na others such as K only have 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 Hair follicles Physical Stimulus What are the different states to a gated channel Three States of Gated Channels 1 Closed but capable of opening 2 Open activated 3 Closed not capable of opening inactivated Describe the difference between action potentials and graded potentials Are they always depolarizing or hyperpolarizing Helpful Notes Hyperpolarization means the inside is becoming more negative Depolarization is more positive Graded Potentials Depolarizing or Hyperpolarizing Whether depolarizing or hyperpolarizing share four basic characteristics The membrane potential is most changed at the site of stimulation and the effect decreases with distance like ripples in water pool The effect spreads passively due to local currents The spread is in all directions The graded change in membrane potential may involve either depolarization or hyperpolarization The properties and distribution of the membrane channels involved determine the nature of the change For example in a resting membrane the opening of sodium channels causes depolarization whereas the opening of potassium channels causes hyperpolarization The change in membrane potential reflects whether positive charges enter or leave the cell The stronger the stimulus the greater the change in the membrane potential and the larger the area affected Action Potential is always depolarizing Explain how graded potentials can generate action potentials AP s Graded Potentials Also called local potentials Changes in membrane potential cannot spread far from site of stimulation Includes any stimulus that opens a gated channel and produces a graded potential The resting state 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 Effects of graded potentials At cell dendrites or cell bodies Trigger specific cell functions For example exocytosis of glandular secretions At motor end plate at the axon terminal Release ACh into synaptic cleft Which region of the neuron generates the AP Axon Hillock What is the all or none principle 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 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 form one source of multiple sources as long as it depolarizes the axon hillock to threshold Step 1 Depolarization to Threshold Graded potentials reach axon hillock to depolarize it to threshold 55mV Step 2 Activation of Na Channels 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
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