Test 101, Week 1 Notes
Test 101, Week 1 Notes Test 101
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ISE 3200 - 101
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This 5 page Class Notes was uploaded by Brianna Notetaker on Monday October 10, 2016. The Class Notes belongs to Test 101 at Arizona State University taught by Brianna Johnson in Fall 2016. Since its upload, it has received 2 views.
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Date Created: 10/10/16
IBB Chapter 4 Notes How Do Neurons Transmit Information? Early Clues that Linked Electricity and Neuronal Activity Electrical Stimulation Passing an electrical current from the uninsulated tip of an electrode onto a nerve produces a muscular contraction Electrical stimulation of the neocortex (a part of the cerebral cortex concerned w/sight and hearing) causes movement; Specific parts of neocortex cause arm/leg stimulation Brain of conscious person can be stimulated electrically to produce body movement Voltmeter Device that measures the flow and the strength of electrical voltage by recording the difference in electrical potential between two bodies Electroencephalogram (EEG) Graphs brain’s electrical activity; Monitors sleep stages, walking activity, and disruptions (e.g. epilepsy) Neurons send electrical messages that move as a wave and contains a chemical basis Consecutive waves constitute message conveyed by neuron Wave travels along axon, not the electrical charge (e.g. when dropping stone into pool, only pressure change moves, not water/ speaking carries sound waves/ flicking a towel moves wave to opposite end of towel) Tools for Measuring a Neuron’s Electrical Activity If single axon is stimulated, wave of excitation is produced, which can be recorded by electrode attached to voltmeter Most neurons are 120 um in diameter (giant squid studied w/ 1,000 um neurons) Oscilloscope Serves as a sensitive voltmeter by registering flow of electrons to measure voltage (electron beam leaves trace on screen and deflections are used) Microelectrode A microscopic insulated wire or a saltwaterfilled glass tube of which the uninsulated tip is used to stimulate or record from neurons Nerve impulses= changes in ion concentration across membrane Basis of electrical activity is movement of intracellular and extracellular ions carrying positive and negative charges How the Movement of Ions Creates Electrical Charge Intra/extracellular fluids of neurons filled with Na+ (sodium), K+ (potassium), and Cl (chloride) ions Cations Positively charged ions Anions Negatively charged ions Three factors that influence ion movement: diffusion, concentration gradient, charge Diffusion Spontaneous spreading out of ions from where they’re more concentrated to where they’re less concentrated (equilibrium results) Concentration Gradient Describes relative concentration of a substance in space or solution (e.g. pouring salt into water starts off in one area w/high concentration and diffuses to area with lower concentration/ spreads out) Ions carry an electrical charge (like charges repel and opposites attract) Voltage Gradient Difference in charge between two regions that allow a flow of current if the two regions are connected Concentration gradient and voltage gradient= e.g. of ion movement Ions always move from higher concentration/voltage to lower Concentration gradient e.g.= sodium and chloride Voltage gradient e.g.= positive and negative charges Efflux Outward flow; Influx Inward flow Concentration gradient= Voltage gradient Resting Potential Electrical charge across a resting cell membrane creates store of potential energy Electrical Potential Ability for a cell to use its stored power Resting Potential Electrical charge across the cell membrane in absence of stimulation; Store of energy produced by a greater negative charge on inner side relative to outside Extracellular side of membrane is given charge of 0mV and inside given charge of 70mV Four charged particles that take part in producing resting potential (all in unequal distributions): 1 ions of sodium (Na+) [more in extracellular fluid] 2 ions of potassium (K+) [more in intracellular fluid] 3 chloride ions (Cl) [more in extracellular fluid] 4 large protein molecules (A) [more in intracellular fluid] Cell membrane channels, gates, and pumps maintain resting potential Large protein molecules remain inside cell (due to selective permeability) K+ and Cl move more freely, but Na+ gates keep out sodium ions Na+ removed from intercellular fluid and replaced with K+ via the sodiumpotassium pump (exchanges 3 intracellular Na+ ions for 2 K+ ions when leaks occur) Resting Potential Inside the Cell Negative charge of A proteins alone is sufficient to produce transmembrane voltage or resting potential To balance this (^), K+ ions accumulate up to 20x as much in intracellular fluid vs. extracellular fluid Some K+ can’t enter because of high concentration already within cell Intracellular fluid remains negative despite influx of K+ ions because not enough K+ ions are able to balance charge of large proteins (A) because too much K+ influx is opposed by concentration gradient Resting Potential Outside the Cell Na+ doesn’t diffuse into cell because 1) Would eliminate charge produced by K+ ions and 2) Na+ ion channels usually closed Na+ could leak in, but cell has mechanism to prevent this neutralization (Na+/K+ pump) 10x as many Na+ ions on extracellular side vs. intracellular side (contributes to resting potential) Cl concentration gradient= Cl voltage gradient approximately same as resting potential; 12x as many Cl ions outside cell as inside Graded Potentials Graded Potentials Small voltage fluctuation in cell membrane restricted to vicinity on axon where ion concentrations change to cause brief increase (hyperpolarization) or decrease (depolarization) in electrical charge across membrane Graded potentials decay before traveling far as when a rock is dropped into smooth pond and causes ripples Axon must be stimulated for graded potential to occur If voltage applied to inside of membrane is negative, membrane potential increases in negative charge (70 mV to 73 mV) Hyperpolarization Increase in electrical charge across membrane, usually due to inward flow of Cl or outward flow of K+ (makes inside more negative) Charge=polarity Depolarization Decrease in electrical charge across membrane, usually due to inward flow of Na+ (makes inside more positive) Polarization typically occurs on soma membrane and dendrites of neuron The Action Potential Action potential Large, brief reversal in the polarity of an axon where voltage suddenly reverses, making intracellular side positive relative to extracellular, then abruptly reverses again after resting potential is restored Occurs when Na+, then K+ ions cross membrane rapidly Depolarizing phase due to Na+ influx Hyperpolarizing phase due to K+ efflux Action potential is triggered when cell depolarizes to 50 mV Na+ rushes in, then K+ rushes out Threshold Potential Voltage on a neural membrane at which an action potential is triggered by opening of Na+ and K+ voltagesensitive channels (about 50 mV) Relative voltage of membrane drops to 0 and continues to depolarize until charge or inside of membrane is as great as +30mVs (total voltage change of 100 mV) Reversal then occurs and cell becomes slightly hyperpolarized If TEA present, K+ channels become blocked and smaller action potential occurs from Na+ influx If Tetrodoxin present, Na+ channels become blocked and smaller action potential occurs from K+ efflux The Role of VoltageSensitive Ion Channels VoltageSensitive Channel Gated protein channel that opens or closes only at specific membrane voltages; Causes an action potential When membrane reaches threshold voltage, configuration of voltagesensitive channels alters, enabling them to open and let ions pass through (at 50 mV) Voltagesensitive Na+ channels more sensitive than K+ channels, so they open first Action Potentials and Refractory Periods Limit on how frequently action potentials occur Absolutely Refractory Refers to state of an axon in repolarizing period during which a new action potential cannot be elicited (w/some exceptions), because gate 2 of sodium channels, which is not voltage sensitive, is closed Sodium channels have two gates, potassium have one Relative Refractory Refers to state of an axon in later phase of an action potential, during which increased electrical current is required to produce another action potential; A phase during which K+ channels are still open; Stimulate during hyperpolarization Refractory Period 1) Gate 1 of Na+ channel is closed, but gate 2 of Na+ is open (resting potential) 2) Gate 1 of Na+ now opens, but gate 2 of Na+ closes membrane depolarizes when gate 1 opens, but ends when gate 2 closes (threshold level) 3) While gate 2 of Na+ channel is closed during repolarization, membrane is absolutely refractory 4) Opening of potassium channel repolarizes and eventually hyperpolarizes membrane 5) Since K+ channels open and close slowly vs. Na+ channels, hyperpolarization produced by efflux of K+ ions makes the membrane relatively refractory Because of refractory periods, there’s 5 millisecond limit on how frequently an action potential can occur (axon can produce action potentials at max. rate of 200/sec) Ex: Toilet flushing hard lever initiates flushing (action potential), during flush toilet is absolutely refractory (another flush can’t happen at time), refilling of bowl is relatively refractory (reflushing is possible, but harder), then rest The Nerve Impulse Nerve Impulse Propagation of an action potential on the membrane of an axon Propagate To give birth Each successive action potential gives birth to another down length of axon (domino effect) Two factors ensure single nerve impulse of constant size down axon: 1) Voltagesensitive channels produce refractory periods, which prevent it from reversing direction 2) All action potentials generated as nerve impulses travel of same magnitude Action potential is either generated or not at all “Domino Effect” causes opening of one channel to produce voltage change that triggers opening of neighbor’s channel Saltatory Conduction and Myelin Sheaths Glial cells play role in speeding nerve impulses in vertebrate nervous system Myelin Sheath Schwann cells in human peripheral nervous system and oligodendroglia in central nervous system wrap around each axon, insulating it besides small, exposed gap (node of Ranvier) Myelin prevents occurrence of action potentials because few channels through which ions can flow Nodes of Ranvier Part of an axon not covered by myelin, richly endowed w/voltagesensitive channels; Action potential at one node can trigger opening of voltagesensitive gates at adjacent node Saltatory Conduction Propagation of an action potential at successive nodes of Ranvier; speeds up rate at which action potential can travel along axon How Neurons Integrate Information Neuron contains dendritic tree covered w/spines, allowing it to establish more than 50,000 connections to other neurons Cell body between dendritic tree and axon, which too can receive connections from many other neurons Neurons that receive more than one kind of input sum up the information that they get Motor neurons receive input from skin, join, muscles, and brain Excitatory and Inhibitory Postsynaptic Potentials Excitatory Postsynaptic Potential (EPSP) Brief depolarization of a neuron membrane in response to stimulation, making the neuron more likely to produce an action potential; Reduce the charge on the membrane toward the threshold level; Associated with the opening of sodium channels, allowing influx of Na+ ions Inhibitory Postsynaptic Potential (IPSP) Brief hyperpolarization of a neuron membrane in response to stimulation, making the neuron less likely to produce an action potential; Increase the charge on the membrane away from the threshold level; Associated with the opening of potassium channels, allowing efflux of K+ ions (or influx of Cl ions) EPSPs and IPSPs last only a few milliseconds (decay and resting potential is restored) Action potential not produced on cellbody membrane; stimulation must reach axon hillock (area of cell where axon begins because full of voltagesensitive channels) Summation of Inputs Applies to both EPSPs and IPSPs Temporal Summation Graded potentials that occur at approximately the same time on a membrane are summated; Pulses separated in time produce two EPSPs/IPSPs similar in size, pulses close together in time partly sum, simultaneous pulses sum as one large EPSP/IPSP; Widely spaced Spatial Summation Graded potentials that occur at approximately the same location and time on a membrane are summated; Add to form a larger EPSP or IPSP; Pulses at same time, but different locations do not influence each other A neuron democratically sums all inputs that are close together in time and space Neuron analyzes these inputs before deciding what to do (decision made at axon hillock) VoltageSensitive Channels and the Action Potential Voltagechannel threshold level at axon hillock is 50 mV; Summed IPSPs and EPSPs on cellbody must depolarize membrane at hillock to 50 mV to produce action potential Dendritic branches don’t have many voltagesensitive channels, so don’t normally produce action potentials Back Propagation Reverse movement of an action potential into the dendritic field of a neuron; Serves as a signal to dendritic field that neuron is sending an action potential over its axon; Postulated to play a role in plastic changes that underlie learning Inputs close to axon hillock have much more say in influence than inputs occurring farther away on distant branches of dendrites; Some inputs have more say than others With sufficient excitation, the generation of an action potential occurs How Sensory Stimuli Produce Action Potentials We receive information about the world through tactile sensations (body senses such as touch and pain), auditory sensations (hearing), visual sensations (sight), and chemical sensations (taste and olfaction) StretchSensitive Channel Ion channel on a tactile sensory neuron that activates in response to stretching of the membrane, initiating a nerve impulse Hair receptors for hearing and balance also activate stretchsensitive channels In visual system, light particles strike chemicals in receptors within eye, causing chemical change that activates ion channels in membranes of relay neurons How Nerve Impulses Produce Movement Behavior is movement and in order for movement to occur, muscles must contract Axon of each motor neuron makes one or few contacts (synapses) with target muscle Axon terminal of motor neuron releases chemical transmitter acetylcholine onto end plate of musclecell membrane End Plate On a muscle, the receptorion complex that is activated by the release of the neurotransmitter acetylcholine from the terminal of a motor neuron TransmitterSensitive Channel Receptor complex that has both a receptor site for a chemical and a pore through which ions can flow Transmittersensitive channels depolarize to threshold for action potential, causing adjacent voltage sensitive channels to open, which produce action potential on muscle fiber, causing muscular contraction Transmittersensitive channels allow both Na+ influx and K+ efflux through same pore (making them larger than both two sodium and two potassium channels combined) To generate sufficient depolarization on end plate to activate neighboring voltagesensitive channels requires release of appropriate amount of acetylcholine
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