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Chapter 4

PSY2301 Chapter 4: How do Neurons Use Electrical Signals to Communicate and Adapt?

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Patrick Davidson

Chapter 4: ElectricalActivity Epilepsy: • an aura, or warning, of an impending seizure, which may take the form of sensation, such as an odor or sound, or may simply be a 'feeling' • abnormal movements such as repeated chewing or shaking; twitches that start in a limb and spread across the body; and in some cases, a total loss of muscle tone and postural support causes the person to collapse • loss of consciousness and later unawareness that the seizure happened • Stephen Gray in 1731 poses a foil and rubbing a rod experiment on a child to display that the nervous system runs on electrical activity • Luigi Galvani: looked at frog legs hanging off a wire in a storm, when the wire had electrical activity through it, the electricity conducted through the nerves of the leg and contracted the frog’s leg. o He had discovered the electrical stimulation  Definition: passage of an electrical current from the uninsulated tip of an electrode through tissue, resulting in changes in the electrical activity of the tissue. EEG • Richard caton o Was the first to measure the brain with a sensitive voltmeter, which measures the flow and the strength of electrical voltage by recording the difference in electrical potential between two bodies? o Now we call it the electroencephalogram  standard tool for recording brain measures for epilepsy and sleep/ wake stages  this proved that neurons use electrical activity to send messages to each other Hermann von Helmholtz • measured time that the electrical signal goes through a nerve to muscle contraction o result: this is much slower than the rate of electrical flow of electricity through a wire, thus the body must not be a flow of electricity Julius Bernstein • posed a theory that the chemistry of neurons produces electrical charges o these charges can changes, thus different signals o successive waves of electrical charge conveys the message between neurons o it is the not the electrical charge that passes through the axon, it is the waves Tools to measure Neuron's electrical activity • we can measure the neurons electrical 'wave' of excitation if the axon is big enough, the neuron is big enough , and if the voltmeter is sensitive enough to pick it up • Giant axon of the squid: o has very large axons (many axons fused together) • Oscilloscope: o Hodgkin and Huxley's experiments were made possible by the invention of the oscilloscope , a device that serves as a voltmeter sensitive enough to detect very small electrical signals from a nerve o the electron beam leaves a trace on a screen, deflections of the beam can be used to record voltage changes in an axon • Microelectrodes: this one is small enough to put on a tiny axon o can deliver an electrical current to one signal neuron or record from it o when put on an axon, it measures the extracellular measure of electrical current, when a second one is inserted inside the axon, we get the voltage of electrical current inside the cell o Hodgkin and Huxley discovered that the electrical current inside the cell was of the intracellular and extracellular flow of ions passing with negative and positive charges that stimulated the electrical activity Movement of Ions to create electrical charges • intracellular and extracellular fluids of the neuron are filled with various ions like K+ and NA+ (cations) and negatively charged (anions) like CL- • 3 factors influence the movement of anions and cations into and out of cells: o diffusion, concentration gradient, and charge o diffusion:  molecules move from places of high concentration to low  this requires no work  when diffusion is complete , everywhere in the cell is at equilibrium • Concentration Gradient: describes the relative concentration of a substance in space or in a solution. o Ions repel one another, so the negative charges repel and keep it flowing o a voltage gradient can be explained as the difference in charge between two regions Resting potential: electrical charge across the cell membrane in the absence of stimulation; a store of potential energy produced by a greater negative charge on the intracellular side relative to the extracellular side • in the absence of stimulation, the is difference is 70 millivolts o although the charge on the outside of the cell is positive, by convention it is given a charge of zero o therefore, the inside of the membrane at rest is -70 millivolt relative to the extracellular side o when there is no stimulation, the charge stays constant, but can change given certain changes in the membrane, but at rest, the difference in charge of the outside and the inside of the cell creates an electrical potential (ability to use its stored power) o the charge is thus a store of potential energy called the membrane's resting potential o a resting potential can range from -40 to -90 millivolts  the exact potential on an axon does not influence the neuron's ability to participate in generating brain activity  4 charged particles take part in producing the resting potential: ions of sodium (NA+), potassium (K+), CL- chlorine, and large protein molecules (A-) • these are distributed unequally in the axon's membrane with more protein anions and K +ions in the intracellular fluid and more CL- and NA+ ions in the extracellular fluid Maintaining the Resting Potential • the cell membrane's channels, gates and pumps maintain resting potential o because the membrane is relatively impermeable, large negatively charged protein molecules remain inside the cell o ungated potassium (and chloride) channels allow K+ and (CL-) ions to pass more freely, but gates on sodium channels keep out positively charged NA+ ions • NA+ - K+ pumps extrude NA+ from the intracellular fluid and inject K+ Graded potential: small voltage fluctuation in the cell membrane restricted to the vicinity on the axon where ion concentrations change to cause a brief increase (hyperpolarization) or decrease (depolarization) in electrical charge across the cell membrane Inside the cell • large protein anions are manufactured inside the cell • no membrane channels are large enough to allow these proteins to leave their cell, and their negative charge is sufficient enough to produce a transmembrane voltage or resting potential o because most cells produce these, they are negatively charged across their membrane • to balance the negative charge created by large protein anions in the intracellular fluid, cells accumulate positively charged K+ ions to the extent of 20x the amount of K+ is inside the cell versus outside of the cell • K+ ions cross the cell membrane by K+ channels o with such high K+ inside the cell, this limits it going into the cell o potassium is then drawn out of the cell because the K+ gradient is high inside the cell o however, not enough K+ ions can enter the cell to change the intracellular charge from negative to positively charged Outside the cell • the equilibrium of the potassium voltage gradient and concentration gradient results in some of the K+ to remain outside of the cell • only a few K+ ions on the outside are needed to keep the inside of the cell negatively charged and thus K+ ions contribute to the charge across the membrane • if NA+ could move freely into the cell's inner membrane it would but the channels to let NA+ are closed, because the NA+ would neutralize the cell's negative charge o though some NA+ do leak into the cell • when the NA+ leaks into the cell , it is shot out again by the sodium-potassium pumps inside the cell • constant exchange of 1 NA+ for 2 K+ • most of the NA+ reside on the outside of the cell's membrane contributing to the cell's resting potential • CL- moves in and out of the cell easily, the chloride gradient is equivalent to the chloride voltage gradient is the same as the cell's resting potential • CL- contributes the least of the cell's resting potential • the cell membrane's semi permeability and the actions of its channels, gates, and pumps thus create a voltage across the cell membrane: it's resting potentials Graded potentials • resting potentials provides an energy store that can be used for generating electricity and for releasing some energy by opened gates for irrigation • conditions under which ion concentrations across the cell membrane change produce graded potentials, small voltage fluctuations that are restricted to the vicinity on the axon where ion concentration changed • for a graded potential to arise, an axon must be stimulated o stimulating an axon electrically through a microelectrode mimics the way in which membrane voltage changes to produce a graded potential in the living cell o the voltage applied to the inside of the membrane is negative, the membrane potential increases in negative charge by a few millivolts – called hyperpolarization • hyperpolarization: the charge (polarity) of the membrane increases • If it decreases from a resting potential of 70 millivolts (inside the membrane the voltage is positive) to 65 millivolts this is called depolarization because the membranes charge decreases. • Hyperpolarization and depolarization usually occur on the soma (cell body) membrane and the dendrites of neurons o these areas contain channels that can open and close, causing the membrane potential to change • Three channels for potassium, chloride, and sodium ions underlie graded potentials 1. Potassium Channels: for the membrane to become hyperpolarized, its extracellular side must become more positive, which can be accomplished with an efflux or K+ ions. Even though potassium channels are open , some resistance remains to the outward flow of K+ ions, reducing this resistance allows for hyperpolarization 2. Chloride channels: the membrane can also become hyperpolarized if there is an influx of CL- ions. Though CL- ions can pass through the membrane, most CL- remains outside the membrane , so a decrease resistance to CL- flow can result in brief increases of CL- inside the cell 3. Sodium channels: depolarization can be produced by an influx of sodium ions and is produced by the opening of normally closed gated sodium channels • How we know that potassium pumps create hyperpolarization and depolarization? Use TEA chemical that blocks potassium channels inhibits both hyperpolarization and depolarization. Hyper polarization: is due to an efflux of K+, making the extracellular side of the membrane more positive or an influx of CL- • increases membrane voltage Depolarization: due to an influx of NA+ through NA+ channels • decrease membrane voltage Action Potential • an action potential is a brief but larger reversal in the polarization of an axon's membrane that lasts 1 millisecond • the voltage across the membrane suddenly reverses, making the intracellular side positive relative to the extracellular side, and then it abruptly reverses again to restore the resting potentials • many action potentials can occur at once • an action potential occurs when a large concentration of NA+ ions, and then K+ ions crosses the membrane rapidly • the depolarizing phase of the action potential due to NA1 influx, and hyperpolarizing phase to K+ efflux o NA+ rush in and then K+ ions rush out • an action potential is triggered when the cell membrane is depolarized to about -50 millivolts • Threshold potential: voltage on a neural membrane at which an action potential is triggered by the opening of NA+ and K+ voltage sensitive channels; about-50 millivolts relative to extracellular surround. • At this threshold potential, the membrane charge undergoes a further change without any stimulation o the relative voltage of the membrane drops to zero and then continues to depolarize until the charge on the inside of the membrane is as great as +30 millivolts- a total change of 100 millivolts o then the membranes potential reverses again, becoming slightly hyperpolarized , after this reversal it starts to slowly go back to resting potential charge -70 millivolts Role of Voltage- sensitive ion channels • cellular mechanism that underlie the movement of NA+ and K+ to produce action potential is the behaviour of the class of gated NA+ and K+ ion channels that are sensitive to the membranes voltage • these voltage sensitive channels are closed when an axon's membrane is at its resting potential, so ions cannot pass through them • when the membrane reaches threshold voltage, the shape of the voltage sensitive channels change; they open briefly, enabling ions to pass through , then close again to restrict their flow 1. Both NA+ and K+ voltage sensitive channels are attuned to the threshold voltage -50 millivolts. If the cell membrane changes to -50 millivolts then both ions can pass the voltage sensitive sodium channels are more sensitive than the potassium, so they open first. 2. As a re
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