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Lecture 4

PSYC 271 Lecture 4: PSYC271 Chapter 4 Textbook Notes

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PSYC 271
Amanda Maracle

Week 3&4 – Chapter 4: Neural Conduction and Synaptic Transmission 4.1 Resting Membrane Potential - Resting membrane potential is the difference in electrical charge between the inside and the outside of the cell Recording the Membrane Potential - Put tip of one electrode inside the neuron and the tip of the other outside in extracellular fluid - Intracellular electrodes are called microelectrodes - When each tip is placed in their location, a steady potential of -70 mV is recorded, so inside is 70mV less than outside o This is the resting potential Ionic Basis of the Resting Potential - Salts in neuron separate into positive and negative ions - We focus on sodium (Na ) and potassium (K ) ions, each carrying a single positive charge - Unequal distributions of ions is maintained through ion channels, each specialized for a particular ion - Two sources of pressure on Na+ to enter resting neurons o Electrostatic Pressure: comes from the resting membrane potential – opposite charges attract (-70mV charge attracts positive Na+ ions into resting neuron o Random Motion: pressure for Na+ ions to move down their concentration gradient; more likely to move from areas of high concentration to low - Remember that in a resting neuron: o The sodium ion channels are closed o The potassium channels are open, but only a few exit as they are held inside the negative resting membrane potential - How does a resting membrane potential always stay the same if some Na+ enter and some K+ leave? Answer: sodium-potassium pump o Same rate that Na+ ions are leaked in, Na+ is actively transported out o Same rate that K+ ions are leaked out, K+ is actively transported in - Other transporters also exist to help the potential Sodium-Potassium Pump “Leaky” Ion Channels Active Transport Passive Transport Week 3&4 – Chapter 4: Neural Conduction and Synaptic Transmission 4.2 Generation and Conduction of Postsynaptic Potentials - Neurons fire and release neurotransmitters that diffuse across synaptic clefts and can interact with next neurons by receptors - Depending on the neurotransmitter, receptor and postsynaptic neuron, they have one of two effects: o Excitatory Postsynaptic Potentials (EPSPs): depolarize the resting membrane potential (ex. -70 to -67 mV) and increase the likelihood that a neuron will fire o Inhibitory Postsynaptic Potentials (IPSPs): hyperpolarize resting membrane (ex. -70 to -72) and decrease the likelihood that a neuron will fire o Both of these are graded responses meaning that intensity of EPSPs and IPSPs depend on the intensity of the signals that cause them ▪ Weak signals: elicit small postsynaptic potentials ▪ Strong signals: elicit large “ “ - EPSPs and IPSPs travel from site of generation (usually dendrites or cell body) always at a rapid pace, despite the signal strength, and transmission is decremental o Decrease in amplitude as they travel through a neuron and usually do not travel more than a couple millimetres before they fade out o As ions come in and out of neuron, they spread out very quickly, hence fading - PSPs use ionic basis of transmission where when the cell receives the info, receptors open causing a concentrated area inside the neuron  charge propagates outward 4.3 Integration of Postsynaptic Potentials and Generation of Action Potentials - Whether or not a neuron fires is determined by the net activity of PSP activity - More specifically, on balance between excitatory and inhibitory signals reaching axon - Action potentials generated by axon initial segment, an adjacent section to hillock - If the graded PSPs are great enough to depolarize membrane, threshold of excitation is reached (~-65mV) and action potential is generated o Action potential: massive, momentary reversal of membrane potential from about -70mV to +50mV ▪ Unlike PSPs, are not graded; instead all or nothing response meaning, they either fire or do not (no strong/weak signals) - Combining individual signals is called integration; neurons integrate signals by: o Spatial Summation: EPSPs/IPSPs on different parts of membrane sum to form greater EPSP/IPSP and how simultaneous EPSP and IPSP sum to cancel out o Temporal Summation: PSPs created in rapid succession form greater signal; activated again before potential has completely faded - Each neuron continuously integrates signals over time and space; most neurons receive thousands of such contacts - Location of synapse is important: because EPSPs and IPSPs are transmitted decrementally, synapses near axon trigger zone have the most influence on firing - Like firing a gun: as a neuron is stimulated, it becomes less polarized until the threshold of excitation is reached (all or none response for neural firing) Week 3&4 – Chapter 4: Neural Conduction and Synaptic Transmission 4.4 Conduction of Action Potentials - Voltage gated ion channels open or close in response to changes in the level of the membrane potential Ionic Basis of Action Potentials - Recall: membrane potential of neuron at rest is constant - When membrane potential is depolarized to threshold by EPSP, voltage-gated Na+ channels open and sodium floods in, driving a change of -70mV to ~+50mV - Triggers opening of K+ gated channels where ions near the membrane are driven out, first by high internal concentration (gradient), then by positive internal change at peak of action potential o This is the end of the rising phase where sodium channels will close and the beginning of repolarization by influx of potassium ions o Once repolarization is achieved, potassium channels will gradually close, causing hyperpolarization due to too many K+ ions leaving o Will quickly return to resting phase - Only ions very close in proximity to membrane will interact with channels thus, single action potential has little effect on relative concentrations of various ions inside and outside neuron o Resting state also re-established by random movement of ions in and out Week 3&4 – Chapter 4: Neural Conduction and Synaptic Transmission Refractory Periods - ~1-2 milliseconds after initiation of an action potential, there is a time where no other action potential can be elicited, called the absolute refractory period - Followed by relative refractory period where it is possible to fire neuron again, but only by applying a higher than normal level of stimulation o After the end of this second phase, levels return to normal - Important because signals only travel down axon in one direction because as recently- travelled areas are left refractory, AP cannot reverse direction - Also, allows high level of stimulation cause another firing just after absolute period instead of both; if level needed to fire was just large enough to move it out of its resting phase, then we would have to wait for both refractory periods to finish Axonal Conduction of Action Potentials - Nondecremental: do not grow weaker as they travel along the axonal membrane - Speed: conducted more slowly than PSPs - Reasons for differences is that action potentials are active and PSPs are passive - Once an AP has been generated, it is passively sent down axon, signals opening of channels where Na+ will rush in, initiating the action potential o Continuous process: signal conducted passively to next channels where another AP is triggered in all terminal buttons o Think of it as wave of excitation at constant speed instead of discrete events o Wave travels back through cell body an dendrites; unknown why - Think about mouse traps in a row analogy o Cannot be sent again until re-set o Still as strong as all the other ones before (nondecremental) - Antidromic Conduction: action potential generated and travels back to cell body if enough electrical stimulation is applied at terminal end of axon - Orthodromic Conduction: conduction in the natural direction from cell body to terminal Conduction in Myelinated Axons - Ions pass through axonal membrane only at nodes of Ranvier - Sodium channels are concentrated at nodes - Signal conducted passively and decrementally along 1 segment of myelin to next node - Somewhat diminished once it gets to the node, still strong enough to open channels o Generates a new full blown action potential – continues across every node - Because conduction is passive in myelinated axons, it happens instantly and signal “jumps” across nodes o Delay at every node, but still much faster than normal conduction due to jumping effect of nodes - Called saltatory conduction - Diseases attacking myelin have devastating effects on neural activity and behaviour (ex. Multiple sclerosis) Week 3&4 – Chapter 4: Neural Conduction and Synaptic Transmission The Velocity of Axonal Conduction - Conduction is faster in large-diameter axons as well as, myelinated axons - Mammalian motor neurons (synapse at skeletal muscles) are large and myelinated so, can conduct at speeds of 100m per second - Small, unmyelinated conduct at 1m per second - In humans specifically, max speed is 60m per second; cats can have APs at 100m/s Conduction in Neurons Without Axons - Action potentials is where axons conduct a signal (all or none) nondecrementally over long distances - But, many neurons in mammalian brain are interneurons with short or no axons at all - Conduction in these are passive and decremental The Hodgkin-Huxley Model in Perspective - Based on study of squid motor neurons because simple, large and very accessible in PNS o Properties make it difficult to apply to mammalian brain as many of these have actions not found in motor neurons o Squid motor neurons are specifically large Week 3&4 – Chapter 4: Neural Conduction and Synaptic Transmission - Properties not shared: o Many cerebral neurons fire continually even without any input o Axons of some cerebral neurons can actively conduct graded & action potentials o APs of different
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