BIO 311D 2nd Edition Lecture 31 Outline of Last Lecture I Action of B and T cells in immune response II Introduction to neurons and membrane potential Outline of Current Lecture I Ion channels and Membrane Potential II Action Potential is all or none III Chemical Synapse Current Lecture I Ion channels and Membrane Potential A Ability of neurons to respond depends on differences of their membrane potential voltage changes B Ions can moved across membranes through passive transport most ion channels in neurons are passive or active transport e g sodium potassium pump a Pump uses ATP move against gradient C The charge across cell membranes a All cells have an electrical potential difference across their plasma membrane called a membrane potential b The cell s inside is negative relative to the outside c Sodium Na potassium K and large anions A are important in maintaining the membrane potential D Resting potential baseline membrane potential no signals and ions at overall equilibrium Resting potential about 70 mV inside cell more negative a Affected by many organic anions inside cell and by the equilibrium potential of various ions especially K E Recall The Na K pump sodium potassium pump ATPase produces large differences in Na and K ion concentrations inside and outside the cell F Passive ion channels are usually closed but have gates a Voltage gated 1 Open during certain voltage in cell b Ligand gated 1 Chemically gated respond to chemical signal 2 Acetylcholine binds to receptor II G In an experimental neuron the membrane resting potential was 60 mV inside negative and the concentrations of ions inside and outside the cell are shown above If passive sodium ion channels open the inside of the cell will become Outside Na 150 mM Inside Na 15 mM a More positive H Membrane potential becomes more negative hyperpolarization a Inhibitory moving farther away from threshold I Membrane potential becomes less negative depolarization a Excitatory moving closer to threshold Action Potential A The action potential is a brief event lasting only a few milliseconds B All or none because once threshold is reached the membrane potential change is always the same height duration cannot be stopped or increased C Stages of the Action Potential a Resting Potential b Some Na ions enter not yet to threshold c Depolarization Na channels open Na rush in voltage gated channels opening d Repolarization K channels open K rush out e Undershoot and back to f Resting Potential D When a neuron is stimulated the areas of the membrane at the point of stimulation become more permeable to Na From resting potential 70 mV a The membrane voltage will go above 70 mV because Na will move into the cell E Phase 5 of the action potential figure on handout 14 is called undershoot because the membrane voltage is briefly more negative than the resting potential Which of these causes it a Potassium ion channels are slow to close F What is the refractory period in terms of neuron response In terms of ion channel events a Positive Feedback action leading to faster version of the action b One way flow G The active toxin in the tissues of the puffer fish is a chemical called tetrodotoxin a Tetrodotoxin is a neurotoxin b Specifically tetrodotoxin blocks voltage gated sodium ion channels H III Chemical Synapse A Membrane at the end of an axon triggers the release of neurotransmitters into the synapse NT binds receptor causing response B How can the synapse be cleared of NT Why is that important a Enzyme splits the NT deactivating it b Reuptake into the pre synaptic cell clears the synapse and recycle the NT
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