Week 5 - Psy1bna Lecture Notes PDF

latrobe.edu.au PSY1BNA Lecture 5: The action potential Week 5 La Trobe University CRICOS Provider Code Number 00115M latrobe.edu.au The action potential Neural membrane Ion channels The action potential The role of ion channels in generating the action potential Action potential conduction and neura...

latrobe.edu.au PSY1BNA Lecture 5: The action potential Week 5 La Trobe University CRICOS Provider Code Number 00115M latrobe.edu.au The action potential Neural membrane Ion channels The action potential The role of ion channels in generating the action potential Action potential conduction and neural integration Neurotransmitter synthesis Synaptic transmission latrobe.edu.au Summary of week 4 The resting membrane potential: https://learninglink.oup.com/access/content/neuroscience-sixth-editionstudent-resources/animation-2-1?previousFilter=tag_animations Electrochemical Equilibrium: https://learninglink.oup.com/access/content/neuroscience-sixth-editionstudent-resources/animation-2-2?previousFilter=tag_animations The sodium-potassium pump: https://learninglink.oup.com/access/content/neuroscience-sixth-editionstudent-resources/animation-4-2?previousFilter=tag_animations latrobe.edu.au Summary of week 4 latrobe.edu.au Summary of week 4 latrobe.edu.au Summary of week 4 latrobe.edu.au Readings Recommended reading: Breedlove, S.M., & Watson, N.W. (2023). Behavioral Neuroscience (10th ed.). Sunderland, MA: Sinauer Associates, Inc. (Chapter 3; pp. 72-81). Breedlove, S.M., & Watson, N.W. (2020). Behavioral Neuroscience (9th ed.). Sunderland, MA: Sinauer Associates, Inc. (Chapter 3; pp. 68-77). Breedlove, S.M., & Watson, N.W. (2017). Behavioral Neuroscience (8th ed.). Sunderland, MA: Sinauer Associates, Inc. (Chapter 3; pp. 66-74). latrobe.edu.au Key Knowledge and Understanding How an action potential is triggered Voltage-gated ion channels, ionic movements, and membrane potential shifts at each stage of the action potential (rest, rising phase, peak, falling phase) How action potentials propagate along the axon Part 1 The Action Potential latrobe.edu.au The Action Potential The fundamental mechanism by which information is conveyed within the neuron Action potentials are brief but large changes in membrane potential. They originate in the axon hillock and are propagated along the axon. Patterns of action potentials carry information to postsynaptic targets. latrobe.edu.au The Action Potential The inside of the membrane rapidly becomes positively charged in relation to the outside When membrane potential is monitored during an action potential, a rapid “spike” or “discharge” is observed All action potentials are similar in size and duration “All or None” Do not diminish as they travel along the axon latrobe.edu.au The Properties of the Action Potential Resting Rising phase Rapid depolarisation of the membrane Rises until membrane potential (Vm) reaches about 40mV Overshoot Inside of the membrane is now positively charged in relation to the outside Falling phase Rapid repolarisation Undershoot/ hyperpolarisation When repolarisation becomes more negative than the resting membrane potential latrobe.edu.au latrobe.edu.au Investigating the Properties of the Action Potential Extracellular recording Recording electrode is placed near the membrane, not into the neuron As an action potential occurs at the recording electrode, a sharp drop in potential is recorded as positive charge moves away from the electrode into the cell Useful technique for measuring AP propagation Movement of AP signal along an axon latrobe.edu.au latrobe.edu.au Investigating the Properties of the Action Potential Intracellular Recording The potential difference between an electrode inside the neuron and another electrode outside the neuron in a bathing solution Requires careful placement of a microelectrode into the cell Studies using squid; large neurons The microeletrode is often a micropipette filled with a highly conductive salt solution (eg. AgCl, NaCl, KCl) The electrodes are connected to an amplifier and readings are recorded with an oscilloscope latrobe.edu.au The effects of hyperpolarizing and depolarizing stimuli on a neuron Hyperpolarization—the interior of the membrane becomes even more negative, relative to the outside. Depolarization—the interior of the cell becomes less negative. latrobe.edu.au Membrane Responses Membrane potential changes more slowly than current injection because of passive electrical properties of the neuron Local potential: An electrical potential that spreads passively across the membrane, diminishing as it moves away from the point of stimulation. latrobe.edu.au Membrane Responses Small, hyperpolarising & depolarising current injections elicit “passive” membrane potential changes Large depolarising current injection pushes membrane potential over “threshold” and an “active”, constant size action potential is generated If the membrane potential reaches the threshold (about –40 mV), an action potential is triggered. The membrane potential reverses and the inside of the cell becomes positive. latrobe.edu.au latrobe.edu.au Action Potential Generation All-or-none property of action potential Neuron fires at full amplitude or not at all. Does not reflect increased stimulus strength. Information is coded in the frequency of action potentials. Increased frequency = increased stimulus strength Afterpotentials are changes in membrane potential after action potentials. latrobe.edu.au The Generation of Multiple Action Potentials latrobe.edu.au Ionic Driving Force (DFion) The difference between the overall membrane potential (Vm) and the equilibrium potential (Eion) of a particular ion; DFion = (VM- Eion) Driving force with membrane at rest: Sodium: DFNa = (-65-62) = - 127mV - a large driving force into the cell Potassium: DFK = (-65 - -80) = +15mV - moderate force out of the cell latrobe.edu.au Membrane Currents and Conductances Consider: The net movement of K+ ions across the membrane is an electrical current. We represent this current as IK The number of open potassium channels is proportional to an electrical conductance. We can represent this conductance with the symbol gK Membrane potassium current, IK will flow only as long as VM does not equal EK as DF = Vm – EK Iion = gion (Vm – Eion) latrobe.edu.au Movement of Ions During an AP 1. Voltage gated Na+ channels open → Na+ rushes into the cell (depolarisation) 2. K+ channels very slowly begin to open 3. Na+ channels inactivate → no more Na+ enters the cell (the peak of the AP) 4. K+ channels are fully open → K+ ions are leaving the cell (repolarisation) 5. K+ channels begin to close → K+ is still slowly leaving the cell (hyperpolarisation) 6. K+ channels close → excess ions diffuse away; membrane recovers to resting Vm (recovery) latrobe.edu.au latrobe.edu.au Things to Remember AP generation Methods of cellular recording intracellular extracellular The AP waveform Resting, rising, overshoot, repolarisation, undershoot, recovery Neural coding AP – all or none; frequency coding Ion movement & channels during the AP Na+/K+ pump Part 2 The Action Potential In Reality latrobe.edu.au The Action Potential in Reality When the membrane is depolarized to threshold, there is a transient increase in gNa → allows the entry of Na+ ions, which depolarizes the neuron. Hodgkin and Huxley (1950’s) proposed: To account for the transient changes in gNa there are sodium gates in the axonal membrane. They are “activated” (opened) by depolarisation above threshold and “inactivated” (closed and locked) when the membrane potential acquires a positive potential These gates are “de-inactivated” (unlocked) when the membrane potential returns to a negative value. latrobe.edu.au  The action potential: https://learninglink.oup.com/access/content/neuroscience-sixth-edition-studentresources/animation-2-3?previousFilter=tag_animations latrobe.edu.au The Voltage Gated Sodium Channel The protein making up the sodium channel 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. The voltage-gated sodium channel is created from a single long polypeptide. The molecule has four distinct domains (I-IV) with each domain consisting of six transmembrane alpha helices (S1-S6). latrobe.edu.au At S4 there is a voltage sensor The pore loop also contributes to the selectivity filter The four domains are believed to clump together to form a pore between them The pore is closed at the negative resting potential When the membrane is depolarised to threshold, the molecule twists into a new configuration allowing Na+ through the pore latrobe.edu.au The Voltage Gated Sodium Channel latrobe.edu.au latrobe.edu.au Selective Filter Like the K+ channel, the sodium channel has pore loops that are assembled into a selective filter → makes Na+ channel 12x more permeable to Na+ than to K+ Na+ ions are striped of their associated water molecules as they pass the channel The retained water serves as a sort of molecular chaperone for the ion → necessary for the ion to pass the selective filter The ion-water complex can then be used to select Na+ and exclude K+ latrobe.edu.au Na+ is gated by a change in voltage across the membrane 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 Depolarisation pushes S4 away from the inside of the membrane, and this conformational change in the molecule causes the gate to open latrobe.edu.au Functional Properties of the Sodium Channel Patch-Clamp Method of patch-clamp is used to study the ionic currents passing through individual ion channels https://learninglink.oup.com/access/content/neuroscience-sixth-editionstudent-resources/animation-4-1?previousFilter=tag_animations This method: Sealing the tip of an electrode to a very small patch of neuronal membrane This patch is then thorn away from the neuron and the ionic currents across it can be measured as the membrane potential is clamped at any value the experimenter selects latrobe.edu.au latrobe.edu.au Functional Properties of the Sodium Channel Changing the membrane potential of a patch of axonal membrane from -80mV to -65 mV has little effect on the voltage gated Na+ channels → they remain closed because depolarisation of the membrane has not reached threshold Changing the membrane potential from -65mV to -40mV causes these channels to pop open latrobe.edu.au Pattern of Behaviour for Sodium Channels  Na+ channels: 1. Open with little delay 2. Stay open for about 1ms and then close (inactivate) 3. Cannot be opened again by depolarisation until the membrane potential returns to a negative value near threshold latrobe.edu.au latrobe.edu.au Na+ channels conformational change yields functional properties: 1. The closed channel 2. Opens upon membrane depolarisation 3. Inactivation involves globular portion of the protein swings up and occludes the pore 4. De-inactivation occurs when the pore closes by movement of the transmembrane domains latrobe.edu.au Sodium Channels and the Action Potential A single channel does not make an action potential The membrane of an axon may contain 1000’s of Na+ channels per square micrometer Concentrated action of all these channels is required to generate what we measure as an action potential The short time the channels stay open before inactivating (about 1ms) partly explains why the action potential is so brief → absolute refractory period latrobe.edu.au Refractory Period Relative Refractory Period During the undershoot of a previous AP or when membrane potential is below resting, a larger stimulus than normal is needed to exceed threshold and generate an AP. Na+ channels are closed The membrane potential stays hyperpolarised until the voltage-gated potassium channels close. Therefore, more depolarisation is required to bring the membrane to threshold. Absolute Refractory Period During the “spike” of an AP, another AP cannot be generated, no matter how large the stimulus is. Na+ channels are inactivated latrobe.edu.au Effects of Toxins on Channels Currents through the Na+ channel can be blocked with tetrodotoxin (TTX) TTX originally isolated from the ovaries of Japanese puffer fish – delicacy in Japan TTX clogs the Na+-permeable pore by binding tightly to a specific site on the outside of the channels latrobe.edu.au Voltage Gated Potassium Channels The falling phase of the action potential was explained only partly by the inactivation of gNa A transient increase in gK also occurs during this falling phase which functions to speed the restoration of a negative membrane potential after a spike → voltage gated potassium channels They do not open immediately upon depolarisation → 1ms to open → the K+ conductance serves to rectify, or reset, the membrane potential → conductance known as delayed rectifier Function to diminish any further depolarisation by giving K+ ions a path to leave the cell across the membrane latrobe.edu.au Voltage Gated Potassium Channels The channel protein consists of four separate polypeptide subunits that come together to form a pore between them When the membrane is depolarised, the subunits twist into shape that allow the K+ ions to pass through the pore latrobe.edu.au Effects of Toxins on Channels Currents through the K+ channel can be blocked with tetraethylammonium (TEA) TEA blocks autonomic ganglia TEA is a competitive inhibitor at nicotinic ACh receptors TEA may inhibit aquaporin channels latrobe.edu.au Review of AP Generation Threshold Rising phase Overshoot Falling phase Undershoot Absolute refractory period Relative refractory period Na+/K+ pump Part 3 Action Potential Conduction latrobe.edu.au Action Potential Conduction To transfer information from one point of the nervous system to another it is necessary that the action potential, once generated, be conducted down the axon When the axon is depolarised sufficiently to reach threshold, voltage gated Na+ channels open and AP is initiated Influx of charge depolarises the segment of membrane just ahead of it → when threshold is reached it generates its own AP → AP works its way down the axon until it reaches the axon terminal → synaptic transmission latrobe.edu.au latrobe.edu.au latrobe.edu.au Action Potential Conduction AP propagates in one direction membrane just behind it is refractory because of inactivation of Na+ channels Just like the fuse – AP can be generated at either end of the axon Orthodromic activation – down axon to the axon terminal Antidromic (experimental) activation – backward propagation Impulse propagates with no decrement AP conduction velocities vary → 10 m/sec is a typical rate (the length of the AP is 2ms) latrobe.edu.au latrobe.edu.au Factors Influencing Conduction Velocity For positive current spreading down the axon there are two paths that positive current can take: Down the inside of the axon Across the axonal membrane If the axon is narrow and there are many open pores, most of the current will flow out across the membrane If the axon is wide and there are few open pores, most of the current will flow down the inside of the axon latrobe.edu.au Factors Influencing Conduction Velocity AP conduction velocity increases with increasing axonal diameter Neural pathways that are especially important for survival have evolved unusually large axons – giant squid axon ( mm in diameter!!) important in mediating escape reflex in response to strong sensory stimulation Axonal size and number of voltage gated channels in the membrane also affect axonal excitability Smaller axons require greater depolarisation to reach AP threshold and are more sensitive to blockage by local anaesthetics – smaller fibers that convey information about painful stimuli latrobe.edu.au Myelin and Saltatory Conduction The good thing about fat axons is that they conduct APs faster → BUT they take up a lot of space → head would be as big as a barn door if all axons would be as thick as the giant squid axon Solution to increase action potential velocity – ensheathing the axon with insulation called myelin Myelin consists of many wraps of membrane provided by glial support cells Schwann cells in the peripheral NS Oligodendrocytes in the CNS Facilitates current flow down the inside of the axon → increasing AP conduction velocity latrobe.edu.au Myelin and Saltatory Conduction The myelin sheath does not extend continuously along the entire length of the axon There are breaks in the insulation where ions cross to generate APs i.e nodes of Ranvier → voltage gated Na+ channels are concentrated in the membrane of the nodes (0.2-2mm) latrobe.edu.au Myelin and Saltatory Conduction In myelinated axons, action potential propagation is called saltatory conduction (Latin to leap) latrobe.edu.au  Impulse conduction in axons https://learninglink.oup.com/access/content/neuroscien ce-sixth-edition-student-resources/animation-32?previousFilter=tag_animations latrobe.edu.au Multiple Sclerosis A demyelinating condition Importance of myelin for normal transfer of information is revealed by the neurological disorder known as multiple sclerosis (MS) Common complaints: Weakness Lack of coordination Impaired vision and speech MS attacks the myelin sheaths of bundles of axons in the brain, spinal cord and optic nerves Simple test involves stimulating the eye with a checkerboard pattern, then measuring the elapsed time until an electrical response occurs from the scalp over the part of the brain that is the target of the optic nerve latrobe.edu.au Multiple Sclerosis latrobe.edu.au Guillain-Barre Syndrome A demyelinating disease Attacks the myelin of the peripheral nerves that innervate muscle and skin Disease may follow minor infections, illnesses and inoculations, and appears to results from an anomalous immunological response against one’s own myelin Symptoms stem directly from the slowing and/or failure of action potential conduction in the axons that innervate the muscles Demonstrated clinically by stimulating the peripheral nerves electrically through the skin, then measuring the time it takes to evoke a response (eg. Muscle twitch) latrobe.edu.au Action Potentials, Axons, and Dendrites Action potentials are a feature mainly of axons The membranes of dendrites and neuronal cell bodies do not generate sodium dependent action potentials → very few voltage gated sodium channels The part of the neuron where an axon originates from the soma, the axon hillock is often also called the spikeinitiation zone The depolarisation of the dendrites and soma caused by synaptic input from other neurons leads to generation of action potentials if the membrane of the axon hillock is depolarised beyond threshold In most sensory neurons, spike initiation zone occurs near the sensory nerve endings latrobe.edu.au latrobe.edu.au Concluding remarks  Depolarisation of the membrane to threshold opens voltage gated sodium channels.  Sodium ions rush into the cell further depolarising the membrane.  The inside of the membrane will change polarity for a short period of time.  This is followed by the movement of potassium ions out of the cell, making the inside of membrane negative compared to the outside.  Small changes in ionic concentrations can result in large membrane voltage changes.  The sodium-potassium pump maintains resting membrane potential.  Action potential spreads down the axon.  Myelin increases the speed of the action potential.