PSYC 220 WEEK 2 Notes
PSYC 220 WEEK 2 Notes PSYC 220
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This 3 page Class Notes was uploaded by Lynde Wangler on Friday January 8, 2016. The Class Notes belongs to PSYC 220 at University of North Carolina - Chapel Hill taught by Meghan Jones in Spring 2016. Since its upload, it has received 19 views. For similar materials see Biopsychology in Psychlogy at University of North Carolina - Chapel Hill.
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Date Created: 01/08/16
PSYC 220 WEEK 2 Notes: Chapter 2 (11 thEd.) Nerve Cells and Nerve Impulses Electrical versus Chemical Signaling: o Electrical: an action potential is propagated down an axon via salutatory conduction (of an electrical signal); electrical signal can travel anywhere from 1m/s to 100m/s o Chemical: communication between neurons occurs via chemical signaling (using ions and neurotransmitters) Membrane Potential Review – polarization (aka electrical gradient) is the difference in charge across the membrane (difference between inside and outside charges of a cell); membrane potential change is dependent upon ion movement o Diffusion: passive transport of ions from a high concentration to a low concentration o Electrostatic Pressure: refers to the quality of ions that causes ions of opposite charges to attract and those with like charges to repel one another For ex., because potassium is more concentrated within a cell at resting potential, the concentration gradient will want to push K+ out of the cell. However, electrostatic forces dictate that since the inside of the cell is more negative, K+ will come into the cell at resting membrane potential the K+ is approx. at equilibrium (that is, there is no net flow of this ion Recording Membrane Potential – place one electrode in a solution outside the cell and the other inside the cell; the number recorded in the difference in millivolts between the two o Resting Potential – (for humans) ~-70mV but different people and different neurons vary (ranges from about -60mV to -80mV) Selectively Permeable Cell Membrane: o Voltage-Gated Ion Channels: open and close in response to electrostatic changes across the membrane Potassium, Sodium, Chloride, Calcium o Resting Membrane Potential: approx. -70mV; refers to state of neuron membrane before an electrical signal (action potential) is sent Methods for Recording Activity of a Neuron: place electrodes in and around neuron to measure electrical differences; i.e., occurrence of action potentials can be observed Sodium-Potassium Pump: Exchanges internal Na+(3) for external K+(2) Maintains electrochemical gradient to maintain resting potential Does NOT cause large changes in membrane potential; works in the background Equilibrium Potential ions at resting potential: o Inside the Cell – many K+ ions and Proteins with negative(-) charges o Outside the Cell – many Na+, Ca2+, and Cl- ions Movement and Forces on Ions: o Sodium is pushed into the cell both by concentration gradient and electrical gradient o The concentration gradient would like to push K+ out of the cell so if the K+ channels were wide open (as they are during the falling phase of an action potential) K+ would flow out o Leaky potassium channels – not voltage-gated; allow a small number of K+ ions to exit the cell at resting potential while the Na+ channels are closed Action Potential Sequence of Events: o Action potentials originate at the axon hillock (the region on the soma right before the axon begins) strong changes in membrane potential are propagated down the axon regeneration at each node of Ranvier ensures that the signal does not decay the electrical signal communicates to the cell information about how to communicate with the next cell via processes in the axon terminal (coming soon) o Steps: Molecular Basis of the Action Potential 1) Resting Potential – K+ equilibrium, sodium channels are closed, leaky potassium channels are open 2) Depolarization – voltage-gated sodium channels open up allowing Na+ to flood into the cell depolarizing (making the inside of the cell less negative in comparison to the outside) the neuron 3) Reversal of Potential – occurs when the electrostatic potential exceeds 0; the inside of the cell is now positive relative to the outside; called the “overshoot” and this is when Na+ is almost at equilibrium 4) At (shortly after this point) sodium channels are INACTIVATED (absolute refractory period – they cannot open so an action potential CANNOT be generated) they then close; K+ flows in excess out of the cells with the concentration gradient and the charge then becomes more negative again 5) K+ ions continue to flow out causing hyperpolarization of the cell (relative refractory period); aka undershoot; occurs before returning to resting membrane potential when K+ channels close, which is maintained by the sodium-potassium pump Hyperpolarization – membrane potential moves away from zero; becomes more negative; -80mV; difference between inside and outside of the cell is greater Depolarization – less difference between inside and outside of cell; membrane potential is closer to zero more positive (but less than zero) Threshold of Excitation: subthreshold stimulation creates a response that then quickly decays; an action potential occurs only when the stimulation is strong enough to reach threshold of excitation Reversal Potential/ Reversed Polarity – the peak of the action potential reaches a positive number above zero; the inside of the cell is now positive relative to the outside of the cell Blocking Action Potential: o Scorpion venom keeps sodium channels open and blocks potassium channels; toxic levels of sodium build up in the neuron The All-Or-None Law: Amplitude and velocity of action potential are not dependent on the strength of the stimulus; If threshold of excitation is met, then an action potential will be generated stimulus intensity is coded by the frequency of firing Action Potentials Travelling in Reverse: they can back-propagate into soma and dendrites (structural changes in dendrites are associated with learning); normally action potentials travel in only one direction toward the axon terminal because of the absolute refractory period where the previously open sodium channels are inactive Propagation of an Action Potential: saltatory conduction the signal jumps and is regenerated down the axon at each node of Ranvier; speed is influenced by diameter of axon (larger = faster) and degree of myelination Demyelination Disorders: o Multiple sclerosis – muscle weakness; impaired motor function; impaired speech, vision, and cognition; genetic and environmental causes o Guillain-Barre syndrome – autoimmune disease; muscle weakness starting in legs and travelling upward in time; environmental causes Dysmyelination Disorders: o Tay-Sachs disease – progressive degeneration of motor and cognitive abilities; causes very young deaths (before age 4); genetic disease o Schizophrenia Local Neurons and Graded Potentials: o Do not have axons; exchange information with neurons that are very close; produce graded potentials (vary in magnitude and can be inhibitory or excitatory) depolarizes (excitatory)/hyperpolarizes (inhibitory) o Myth – only 10% of neurons are active at any given time……(UHM NOOO)
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