PSB2000 Test #2:Resting Potential and Action Potential- 2.2:1) What ions are important in the action potential? When the cell is at rest, which ions are most highly concentrated inside of the cell, and which ones are most highly concentrated outside of the cell?a. The action potential is an electrical impulse down an axon.b. It depends on sodium (Na+) and potassium (K+) moving thru channels in the axon’s membrane. 2) Understand the forces working on the ions (electrical gradient and concentration gradient). a. Concentration gradient: ions flow from areas of high concentration to low concentration. i. Example: think about putting sugar in waterb. Electrical Gradient: ions flow to areas of opposite charge.i. Negative to positive, positive to negative. ii. Example: remember, opposites attract!3) What is a voltage-gated channel? Where are they?a. The voltage-gated channel is a sodium channel structure.i. Created from a single long polypeptideii. 4 domains, I-IV, that form the poreiii. 6 transmembrane alpha helices S1-S6iv. The sodium channel has a pore loop (b/t S5-S6) that functions as a selectivity filter making it 12X more selective for Na+ than K+. (Similar to the potassium channel)v. The channel is gated by a change in voltage across the membrane. vi. The voltage sensor resides in the S4 segment.vii. Positively charged amino acid residues are regularly spaced along the coils of the helix.viii. Depolarization pushes the S4 away from the inside of the membrane causing conformational change that opens the gate.ix. S4 is lined with amino acids that have positively charged side chains, so the channel is sensitive to voltage.x.4) Understand the steps of the action potential, and how one leads to the next. How is an action potential started and propagated? What ion enters first? Thru what type of channel does it enter? What forces drive it inside? Why does that channel close? What channel opens next? What ion moves thru that? What forces drive that ion? Etc etc.a. Propagation of the action potential depends on:i. Diameter of axonii. Insulation (myelin)b. Steps of propagation:i. Threshold is reached at the axon hillock.ii. Voltage gated Na+ channels open.iii. Na+ enters the cell.iv. Now, Voltage Gated Na+ Channels close and Voltage Gated Potassium channels open.v. K+ leaves the cell.vi. So much K+ exits the cell, that is becomes hyperpolarized!vii. When it reaches resting membrane potential again, it can conduct another action potential.viii. That period of hyperpolarization is called the refractory period.ix. Refractory period- the neuron cannot fire another action potential until it is back up to resting potential and repolarization. c. The action potential is the signal that conveys information over distances in the nervous system. The action potential is a rapid reversal of the situation at rest – for an instant, the inside of the membrane becomes positively charged relative to the outside. Action potentials are all or none. The frequency and pattern of action potentials is the code used by neurons to transfer information from one location to another. While rate can change, speed cannot. All of this occurs at the nodes of ranvier, between the segments of myelin.5) Terms to know and understand with regard to neurophysiology: polarized, depolarization, hyperpolarization, repolarization, resting potential, thresholda. With regard to those last 2 terms, What voltage is resting potential? What voltage is threshold? Where must threshold be reached for an action potential to occur?i. The membrane has a resting potential of -70 mV.ii. The membrane is polarized.iii. Polarized: at rest, an electrical gradient is maintained across the plasma membrane (negative charge is greater inside the cell)iv. Resting Potential: difference in voltage across the membrane of a cell ( ~ -.70 mV) at rest. At rest means not conducting an action potential.v. Hyperpolarized: as cell becomes more negative, it is “hyperpolarized” (more polarized). More negative than -70 is hyperpolarized. vi. Depolarized: as cell becomes less negative, it is “depolarized” (less polarized). Less negative than -70 is depolarized.b. Threshold the critical level of depolarization that must be achieved to trigger an action potential.6) What is the sodium/potassium pump? What does it do? What purpose does it serve?a. Acts to maintain proper concentrations of Na+ and K+. b. Needed for maintaining resting potential and for recovery from action potential.c. 3 Na+ out for every 2 K+ in, so more positive on outside.d. This pump uses 7-% of all ATP in the brain!e.7) What is the purpose of myelin? What happens at the nodes of Ranvier? What is saltatory conduction?a. Voltage-gated Na+ channels are concentrated at the Nodes of Ranvier.b. In myelinated axons, action potential can “jump” down axons much faster. It allows long distance rapid communication. 8) What does it mean that an action potential is “all or none”?a. Action potentials are generated in an “all-or-none” fashion. That is, in most (but not all) neurons, subthreshold depolarizations have no effect, but once the threshold is crossed, an action potential is generated.b. The rate (frequency) of action potentials depends on the magnitude and duration of the depolarizing stimulus.c. There is great variability in the firing frequency of neurons- some can only fire a single action potential even to a sustained depolarization, other can fire up to a limit of about 1000 Hz. Synaptic Potential- 3.1, 3.2:1) Know the different parts of a synapse (presynaptic cell, postsynaptic cell, synaptic cleft, neurotransmitters, receptors, vesicles)2) Know what happens at a synapse (chemical transmission).3) Understand different types of ion channels (ligand- and voltage-gated). Where are ligand-gated channels located? What is a “ligand”?a. Voltage-gated channel (VGC): i. On the axonii. Open when a cell reaches a certain voltageiii. Allows ions in or out1. Changes voltage of cellb. Ligand-gated channel (LGC): i. At synapsesii. Opens when a ligand (neurotransmitter) bindsiii. Allow ions thru,1. Changes voltage of cell2. May also start biochemical cascade within cell. 4) What is an excitatory synapse? What is an inhibitory synapse? For each one, what ion enters the cell? What does that do to the cell? Does it make an action potential more or less likely? a. Inhibitory Neurotransmitter: allow chloride (a negatively charged ion) into the
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