NROB60 Chapter 4.doc

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Human Biology
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Janelle Le Boutillier

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NROB60 – Chapter 4 - Action potential is often called a spike, a nerve impulse or a discharge o They are generated by a cell and are similar in size and duration o They don’t diminish as they are conducted down the axon - The frequency and pattern of action potentials constitute the code used by neurons to transfer information from one location to another Properties of the Action Potential The Ups and Downs of an Action Potential - A voltmeter is used to measure the electrical potential difference between the tip of the intracellular microelectrode and another placed outside the cell o This is to determine membrane potential, Vm, by inserting a microelectrode in the cell - When the neuronal membrane is at rest, the voltmeter reads a steady potential difference of about -65 mV. o During the action potential, however, the membrane potential briefly becomes positive o Because this occurs so rapidly, a special type of voltmeter called an oscilloscope is used to study action potentials  The oscilloscope records the voltage as it changes over time - The action potential has certain identifiable parts: o Rising phase (first part) – characterized by a rapid depolarization of the membrane  This change in membrane potential continues until Vm reaches a peak value of about 40 mV  The part of the action potential where the inside of the neuron is positively charged with respect to the outside is called the overshoot o Falling phase – rapid repolarization until the membrane is actually more negative than the resting potential  Last part of this phase is called the undershoot or after- hyperpolarization o From beginning to end, the action potential lasts about 2 milliseconds (msec) The Generation of an Action Potential - The perception of sharp pain when a thumbtack enters your foot is caused by the generation of action potentials in certain nerve fibers in the skin o The membrane of these fibers is believed to possess a type of gated sodium channel that opens when the nerve ending is stretched o The initial chain of events is therefore:  1) the thumbtack enters the skin  2) the membrane of the nerve fibers in the skin is stretched  3) Na+ permeable channels open • Due to the large concentration gradient and negative charge of the cytosol, Na+ ions enter the fiber through these channels - Entry of Na+ depolarizes the membrane; the cytoplasmic (inside) surface of the membrane becomes less negative o If this depolarization, called a generator potential, achieves a critical level, the membrane will generate an action potential o The critical level of depolarization that must be crossed in order to trigger an action potential is called threshold  Action potentials are caused by depolarization of the membrane beyond threshold - The depolarization that causes action potentials arises in different ways in different neurons o In interneurons, depolarization is usually caused by Na+ entry through channels that are sensitive to neurotransmitters released by other neurons o Neurons can also be depolarized by injecting electrical current through a microelectrode, a method commonly used by neuroscientists to study action potentials in different cells - Applying increasing depolarization to a neuron has no effect until it crosses threshold, and then one action potential o Action potentials are said to be all-or-none The Generation of Multiple Action Potentials - If we pass continuous depolarizing current into a neuron through a microelectrode, we will generate many action potentials in succession o The rate of action potential generation depends on the magnitude of the continuous depolarizing current - The firing frequency of action potentials reflects the magnitude of the depolarizing current o Although firing frequency increases with the amount of depolarizing current, there is a limit to the rate at which a neuron can generate action potentials.  The maximum firing frequency is about 1000 Hz; once an action potential is initiated, it is impossible to initiate another for about 1 msec.  This period of time is called the absolute refractory period - It can be relatively difficult to initiate another action potential for several milliseconds after the end of the absolute refractory period o During this relative refractory period, the amount of current required to depolarize the neuron to action potential threshold is elevated above normal The Action Potential, In Theory - The action potential is a dramatic redistribution of electrical charge across the membrane o Depolarization of the cell during the action potential is caused by the influx of sodium ions across the membrane o Repolarization is caused by the efflux of potassium ions Membrane Currents and Conductances (read page 80) - The membrane of the cell has 3 types of protein molecules: o Sodium-potassium pumps o Potassium channels o Sodium channels  The pumps work continuously to establish and maintain concentration gradients - We consider 3 points when focusing on the movement of K+ that took the membrane potential from 0 mV to -80 mV (according to Fig. 4.4) o 1) The net movement of K+ ions across the membrane is an electrical current  Representing this current using the symbol (Ik) o 2) The number of open potassium channels is proportional to an electrical conductance  Representing this conductance by the symbol (gK) o 3) Membrane potassium current, Ik, will flow only as long as Vm does not equal Ek  The driving force on K+ is defined as the difference between the real membrane potential and the equilibrium potential • Can be written as Vm – Ek - There is a simple relationship between the ionic driving force, ionic conductance and the amount of ionic current that will flow o For K+ ions, this may be written as Ik = gK (Vm – Ek) o More generally written as I-ion = g-ion (Vm – Eion)  Basically an expression of Ohm’s law, I=gV - Since the membrane is impermeable to K+, the potassium conductance, gK = 0 and Ik = 0 o Potassium current only flows when we stipulate that the membrane has open potassium channels and therefore gK > 0 - When Vm = Ek, the membrane is at equilibrium and no net current will flow The Ins and Outs of an Action Potential (read page 82) - The action potential could be accounted by the movement of ions through channels that are gated by changes in the membrane potential The Action Potential, in Reality - When the membrane is depolarized to threshold, there is a transient increase in gNa o The increase allows the entry of Na+ ions, which depolarizes the neuron o The increase must be brief in duration to account for the short duration of the action potential - Restoring the negative membrane potential would be further aided by a transient increase in gK during the falling phase, allowing K+ ions to leave the depolarized neuron faster - Voltage clamp – clamps the membrane potential of an axon at any value they choose o Could then deduce the changes in membrane conductance that occur at different membrane potentials by measuring the currents that flowed across the membrane - New molecular biological techniques have enabled neuroscientists to determine the detailed structure of the proteins - New neurophysiological techniques have enabled neuroscientists to measure the ionic currents that pass through single channels The Voltage-Gated Sodium Channel - The protein forms a pore in the membrane that is highly selective to Na+ ions and the pore is opened and closed by changes in the electrical potential of the membrane Sodium Channel Structure: - The voltage gated sodium channel is created from a single long polypeptide o the molecule has 4 distinct domains, numbered I-IV  each domain consists of 6 transmembrane alpha helices, numbered S1- S6 - The 4 domains are believed to clump together to form a pore between them o The pore is closed at the negative resting membrane potential o When the membrane is depolarized to threshold, the molecule twists into a configuration that allows the passage of Na+ through the pore - The sodium channel has pore loops that are assembled into a selectivity filter o This filter makes the sodium channel 12 times more permeable to Na+ than it is to K+ o The Na+ ions are stripped of most of their associated water molecules as they pass into the channel  The retained water serves as a molecular chaperone for the ion and is necessary for the ion to pass the selectivity filter  The ion-water complex can be used to select Na+ and exclude K+ - The sodium channel is gated by a change in voltage across the membrane o The voltage sensor resides in segment S4 of the molecule  In this segment, positively charged amino acid residues are regularly spaced along the coils of the helix  Depolarization twists S4, and
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