BISC 421 1st Edition Lecture 29 Outline of Current LectureI. Ion channels and Membrane PotentialII. Action Potential is “all-or-none” III. Chemical SynapseCurrent LectureI. Ion channels and Membrane PotentialA. Ability of neurons to respond depends on differences of their membranepotential (voltage changes)B. Ions can moved across membranes through passive transport (most ion channelsin neurons are passive) or active transport (e.g. sodium-potassium pump)a. Pump: uses ATP, move against gradientC. The charge across cell membranesa. All cells have an electrical potential difference across their plasma membrane called a membrane potentialb. The cell’s inside is negative relative to the outsidec. Sodium (Na+) potassium (K+) and large anions (A-) are important in maintaining the membrane potentialD. Resting potential: baseline membrane potential, no signals, and ions at overall equilibrium. Resting potential about -70 mV inside cell more negativea. 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 largedifferences in Na+ and K+ ion concentrations inside and outside the cellF. Passive ion channels are usually closed but have gates a. Voltage-gated1. Open during certain voltage in cellb. Ligand-gated1. Chemically gated (respond to chemical signal)2. Acetylcholine binds to receptorG. In an experimental neuron, the membrane resting potential was -60 mV (insidenegative), and the concentrations of ions inside and outside the cell are shownabove. If passive sodium ion channels open, the inside of the cell will become:Outside Na+: 150 mM | Inside Na+: 15 mMa. More positiveH. Membrane potential becomes more negative, hyperpolarization a. Inhibitory: moving farther away from thresholdI. Membrane potential becomes less negative, depolarization a. Excitatory: moving closer to thresholdII. Action PotentialA. The action potential is a brief event, lasting only a few millisecondsB. “All-or-none”, because once threshold is reached, the membrane potentialchange is always the same (height, duration), cannot be stopped or increasedC. Stages of the Action Potential a. Resting Potentialb. Some Na+ ions enter, not yet to thresholdc. Depolarization - Na+ channels open, Na+ rush in (voltage-gated channels opening)d. Repolarization – K+ channels open, K+ rush oute. Undershoot (and back to…)f. Resting PotentialD. 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 thecellE. 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 closeF. 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 actionb. One-way flowG. 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 channelsH.III. Chemical SynapseA. Membrane (---) at the end of an axon triggers the release of neurotransmitters into the synapse; NT binds receptor, causing responseB. How can the synapse be cleared of NT? Why is that important?a. Enzyme splits the NT, deactivating itb. Reuptake into the pre-synaptic cell, clears the synapse and recycle the
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