Chapter 2

StudyBlue printing of Chapter 2 html, body, div, span, applet, object, iframe, h1, h2, h3, h4, h5, h6, p, blockquote, pre, a, abbr, acronym, address, big, cite, code, del, dfn, em, font, img, ins, kbd, q, s, samp, small, strike, strong, sub, sup, tt, var, b, u, i, center, fieldset, form, label, legend, table, caption, tbody, tfoot, thead, tr, th, td { margin: 0; padding: 0; border: 0; outline: 0; font-size: 100%; background: transparent; } body { line-height: 1; } blockquote, q { quotes: none; } blockquote:before, blockquote:after, q:before, q:after { content: ''; content: none; } /* remember to define focus styles! */ :focus { outline: 0; } /* remember to highlight inserts somehow! */ ins { text-decoration: none; } del { text-decoration: line-through; } /* tables still need 'cellspacing="0"' in the markup */ table { border-collapse: collapse; border-spacing: 0; } /* end RESET */ .header { min-width:800px; } .logo { padding:6px 20px 2px 20px; margin:0; font-size:25px; font-weight:bold; color:#808285; position:relative; border-bottom: 1px solid #c5c5c5; } .logo-blue { color:#70adc4; } .logo-desc { font-weight:normal; font-size:19px; color:#cccccc; margin-top:50px; position:absolute; display: none; } .back-button { position:absolute; top:20px; right:20px; font-size:13px; line-height:25px; color:rgb(0,175,225); font-weight:normal; } .back-button a { color:rgb(0,175,225); } .instructions { padding:0; margin:0; width:100%; position:relative; color:rgb(100,100,100); } .step-holder { border-left:1px solid #ededed; margin-left:20px; } .steps { padding:15px 0; float:left; width:24%; border-right:1px solid #ededed; text-align:center; } .steps-01 { } .steps-02 { } .steps-03 { } .steps-04 { } .label { padding:5px 10px; } .print-button { } .print-button a { background-color:rgb(0,175,225); color:white; line-height: 19px; padding:9px 8px 5px 30px; font-size:14px; text-decoration:none; background-image: url(images/printer.png); background-repeat: no-repeat; background-position: 7px 50%; -moz-border-radius: 5px; -webkit-border-radius: 5px; } .print-button a:hover { background-color:black; } .theNote .content { width: 8.0in !important; margin: 5px auto; padding:20px; background-color:white; } .theNote .header { border-bottom: 1px dashed #C8C8C8; font-size: 17px; padding: 0 0 10px; line-height: 19px; color: #00ADE1; min-width:500px; } .theNote .body { font-size: 14px; line-height: 19px; padding: 10px 0; } .theNote{ padding:6px 0; clear:both; background-color: rgb(200,200,200); } .theNote h3{ color: rgb(100,100,100); } .theNote h1, .theNote h3{ background-color:white; padding:2px 20px; width:8.0in !important; margin: 0 auto; font-size: 15px; } .theNote h1{ padding-top: 10px; font-size: 15px; } .theNote h1:first-child{ font-size: 20px; } .theNote h3 { font-size: 14px; font-weight: normal; } #options { border: 3px double #ccc; padding: 5px 12px; margin: 10px 50px 10px 20px; float: left; } #info { border-top: 1px solid #ccc; padding-top: 5px; font-style: italic; } li { margin: 5px 10px 5px 25px; } ul li { list-style: disc; } ol li { list-style: decimal; } img { border: 0; } table { clear: both; width: 100%; border: 1px solid #c5c5c5; border-width: 1px 0; margin: 0; page-break-after: always; } table#page { page-break-after: auto; } td { text-align: center; font-size: 12px; border-bottom: 1px dashed #c5c5c5; height: 1.75in; width: 50%; padding-left: 15px; } .leftside { border-right: 1px solid #cccccc; padding: 0 15px 0 0; } .bottom td { border-bottom: none; } .clearfix { clear:both; line-height:1px; height:1px; } img { max-width:80%; max-height:150px; margin:20px; } @media print {.header { display: none; } .content .header{ display:inherit; } table { border: 1px dashed #bbb; border-width: 1px 0; } .theNote{ background-color:white; } } 1.) Electrical Potentials across Nerve Cell Membranes This elecrical phenomenon can be observed as soon as a microelectrode is inserted through the membrane of the neuron. Upon entering the cell, the microelectrode reports a negative potential, indicating that neurons have a means of generating a constant voltage across their membranes when at rest, this voltage, called the resting potential depends on the type of neuron being examined but it is always a fraction of a volt (typically -40 to -90mV). Receptor potentials are due to the activation of sensory neurons by external stimuli such as light, sound, or heat. Similar receptor potentials are observed in all other sensory neurons during the transduction of sensory signals. Synaptic potentials allow transmission of info from one neuron to another. Activation of a synaptic terminal innervating a hippocampal pyramidal neuron causes a very brief change in the resulting membrane potential in the pyramidal neuron. Serve as the means of exchanging info in complex neural circuits in both CNS and PNS. Action potentials are the booster system for electrical conductivity in neurons. To elicit an aciton potential, pass an electrical current across the membrane of the neuron. Normally, this current would be generated by receptor or synaptic potentials. In the lab, electrical current can be initiated by inserting microelectrode. Hyperpolarization makes the membrane potential more negative, nothing dramatic happens to the neuron. Such hperpolarizing do not require any unique property of neurons and are called passive electrical responses. Depolarization occurs if the current of the opposite polarity is delivered so that the membrane potential of the nerve cell becomes more positive than the resting potential. In this case, at a certain level of membrane potential, called the threshold potential, an action potential occurs. The action potential is a quick change from negative to positive in the transmembrane potential. The aplitude of the action potential is independent of the magnitude of the current used to evoke it. Larger currents do not elicit larger action potentials. The action potentials of a given neuron are said to be all or none becausre either they occur fully or they do not occur at all. Aplitude or duration of stimulus current is increased, multiple action potentials occur. In receptor potentials, amplitudes are graded in proportion to the magnitude of the sensory stimulus or from synaptic potentials whose amplitudes vary according to the number of synapses activated and the previous amount of synaptic activity. 2.) How Ionic Movements Produce Electrical Signals Why are electrical potentials generated across the membranes of neurons? Because there are differences in the concentrations of specific ions across nerve cell membranes and because the membranes are selectively pereable to some of these ions. Active transporters are proteins which establish the ion concentration gradients. The selective permeability of membranes is due largely to ion channels, proteins that allow only certain ions to cross the membrane in the direction of their concentration gradients. Channels and transporters work against each other, thus generating electrical signals produced by neurons: resting membrane potential, action potentials, synaptic potentials and receptor potentials. If ion concentration is not the same on either side of the membrane, then electrical potential will be generated. Potassium ions flow down the concentration gradient. At electrochemical equilibrium, there is an exact balance between two opposing forces: the concentration gradient thta causes K+ to move from compartment 1 to copartment 2, taking along positive charge, and 2 an opposing electrical gradient that increasingly tends to stop K+ from moving across the membrane. The number of ions that needs to flow to generate this electrical potential is very small. This means that the concentrations of permeant ions on each side of the membrane remain essentially constant, even after the flow of ions has generated the potential. Also, the tiny fluxes of ions required to establish the membrane potential do not disrupt chemical electronuetrality bc each ion has an oppositely charged counter-ion to maintain the neutrality of the sulutions on each side of the membrane. 3.) The Forces that Create Membrane Potentials Equilibrium potential can be predicted by the Nernst equation E x =(RT/zF)1n[X] 2 /[X] 1 Ex is the equilibrium potential for any ion X, R is the gas constant, T is the absolute temperature on the Kelvin scale. and z is the valence electrical charge of the permeant ion. an dF is the Faraday constant( the amount of electrical charge contained in one mole of a univalent ion. The brackets indicate the concentrations of ion X on each side of the membrane and 1n indicates the natural logarithm of the concentration gradient. Because there is no permeability term in the Nernst equation, which only considers the simple case of a single permeant ion species, a more elaborate equation is needed that takes inot account both the concentration gradients of the pereant ions and the relative permeability of the membrane to each permeant species. Goldman equation is an extended version of the Nernst equation that takes into account the relative permeabilities of each of the ions involved. 4.) Electrochemical Equilibrium in an Environment with More than One Permeant Ion Measurements of ion concentrations indicate that there is more K+ i8nside the neuron than outside and that there is much more Na+ outside than inside. The ionic strength of mammalian blood is lower than that of sea-dwelling animals such as squid, ion concentration is significantly lower. When you find the ion concentration gradients across various neuronal membranes, the Nernst equation can be used to calc the equilibrium potential for K+ and other major ions. Hodgkin and Katz (1949) asked what happens to the resting membrane potential if the concentration of K+outside the neuron is altered. If the resting membrane were permeable only to K+, then the Goldman equation predicts that the membrane potential will vary in proportion to the logarithm of the K+ concentration gradient across the membrane. They showed that the inside-negative resting potential arises because the membrane of the resting neuron is more permeable to K+ than to any of the other ions present and that there is more K+ inside the neuron than outside 5.) The Ionic Basis of the Resting Membrane Potential While the resting neuronal membrane is only slightly pereable to Na+, the membrane becomes extraordinarily permeable to Na+ during the rising phase and overshoot phase of the action potential. This temporary increase in Na+ permeability results from the opening of Na+ selective channels that are essentially closed in the resting state. Membrane pumps maintain a large electrochemical gradient for Na+ which is in much higher concentration outside the neuron than inside. When Na channels open, Na flow into the neuron causing the membrane potential to depolarize and approach E Na. . 6.) The Ionic Basis of Action Potentials The membrane potential rapidly repolarizes to resting levels and is actually followed by a transient undershoot. During an undershoot, the membrane potential is transiently hyperpolarized because K+ permeability becomes even greater than it is at rest. The action potential ends when this phase of enhanced K+ permeability subsides and hte membrane potential thus returns to its normal resting level. The ion substitution experiments by Hodgkin and Katz provided convincing evidence that the resting membrane potential results from high resting membrane permeability to K+ and that depolarization during and action potential results from a transient rise in membrane Na+ permeability