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Chapter 4

Chapter 4- How Neurons send and receive signals.docx

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Psychology 2221B
Derek Quinlan

Chapter 4: Neural Conduction and Synaptic Transmission - Talking about how neurons chat with each other Resting Membrane Potential - Neuron has an electrical potential. And a charge within a neuron is -70Mv with respect to other fluids just outside the neuron -  Recording the membrane potential: difference in electrical charge between inside and outside of cell  Inside of the neuron is negative with respect to the outside  Resting membrane potential is about –70mV  Membrane is polarized (carries a charge) - With this difference in charge- refer to the membrane being polarized- if depolarized, bring those charges together and hyper is the opposite - Ionic Basis of the Resting Potential  Factors contributing to even distribution of ions (charged particles) Random motion – particles tend to move down their concentration gradient Electrostatic pressure – like repels like, opposites attract  Factors contributing to uneven distribution of ions Selective permeability to certain ions Sodium-potassium pumps - When we were trying to figure this out, there must be forces pushing these particles. So the two main things that can push things is 1. Concentration Gradient- random motion of molecules and neurons that will cause a concentration of those particles in the environment. - Perfume example of dispersing in Equilibrium - 2. Electrostatic Force- ex magnets, opposites attract, positive and negative charges attract and same charges repel each other - If we were to filled corner with negative charges they will push themselves as far as they can away from each other until they reach equilibrium. - These two forces we need to reconcile - Some things tend to leak through membrane and channels - We have active processes that maintain the charges Ions Contributing to Resting Potential  Sodium (Na ) +  Chloride (Cl ) -  Potassium (K ) + -  Negatively charged proteins (A ) Synthesized within the neuron Found primarily within the neuron - Charged particles moving through channels or being actively moved* Neuron at Rest  Ions+move in -nd out through ion-specific channels  K and Cl pass readily +  Little movement of Na  And the neg. protein ions don‟t move at all, trapped inside Equilibrium Potential (Hodgkin-Huxley model) - HH went about trying to calculate the pressures that are needed to keep things where they are. What force do we have to exert to keep that -70 in the neuron - They ended up finding that Chloride is neg. charged - some inside and some not. o Theres a lot more chloride outside then inside,  but the charge difference is pushing back with that force and theres no movement. o Potassium- 90 Mv charge that we have to deal with - lots more inside than out. So we 90 mv pressure outward. The positive wants to come in to make the charge equal o Sodium **- a lot of sodium outside huge concentration gradient pushing sodium in to the cell body. But its positively charged***  The potential at which there is no net movement of an ion – the potential it will move to achieve when allowed to move freely +  Na =+120mV  K = 90mV  Cl = -70mV (same as resting potential)  Na is driven in by both electrostatic forces and its concentration gradient +  K is-driven in by electrostatic forces and out by its concentration gradient  Cl is at equilibrium  Sodium-potassium pump – active (uses ATP) force that exchanges 3 Na inside + for 2K outside - HH figure d out these pressures, and there has to be a process to maintain this - Theres a sodium-potassium pumps, ATP as energy source, they will grab 3 sodium from cell body and force them out into the extra fluid and grab potassium and pull it inside so we end up maintaining the -70 Mv - Necessary for us to signal down the neurons Generation and Conduction of Postsynaptic Potentials (PSPs) - Theres no direct contact between most cells. But there has to be means for how one neuron chats to another-- NT - chemical signal to be sent across to a neighbouring neuron. - These messengers, are doing 1-2 things - either going to depolarize- -70 and get it close to 0 and if hyperpolarization get it even more negative - You’re controlling if its going to create an Action Potential  Neurotransmitters bind at postsynaptic receptors  These chemical messengers bind and cause electrical changes Depolarizations (making the membrane potential less negative) Hyperpolarizations (making the membrane potential more negative) - Some NT act as excitatory (Depolarization) or inhibitory, they‟re going to drive it to less likely having AP. ESPs (excitatory) and IPSPs (inhibitory)  Travel passively from their site of origination  Decremental (graded) – they get smaller as they travel - The post synaptic neuron- it may have 1000 of synapses coming in from multiple neurons some are ESP some are IPS- going to cause a local charge change - The charge difference spreads instantly - but dies as it go and weakens as it goes further - It’s a passive process. - At the axon hillock- is where the AP occurs. So the charge diff. needs to make it all the way there so we need a pretty strong charge difference. Integration of PSPs and Generation of Action Potentials (Aps) - we can summate temporarily or spatially.  One EPSP typically will not suffice to cause a neuron to “fire” and release neurotransmitter – summation is needed  In order to generate an AP (or “fire”), the threshold of activation must be reached near the axon hillock  Integration of IPSPs and EPSPs must result in a potential of about -65mV in order to generate an AP Integration  Adding or combining a number of individual signals into one overall signal  Spatial summation – integration of events happening at different places  Temporal summation – integration of events happening at different times - I have 5 charges at the same time, they add together so that charge has more power and can get through the neuron. But they have to be around the same time - If they’re close together they will add and combine Conduction of APs - Once u hit -65 mv you have a series of events that cant stop - You hit -65, theres a bunch of ion channels on the axon, first are the sodium channels open- sodium rushes into the axon, it depolarizes it and makes it more positive. After a short period of time this charge diff. is going to cause potassium channels to open and allowing them to go in and then positive K rushing out, then when you reach 50- the sodium channels closes and now it becomes more negative cause the potassium is rushing out and making it more negative and when they close off, it goes below resting potential and hyperpolarize temporarily. - If we have a ton of signal coming in (excitatory) and comes in during time of repolarization, nothing is going to happen- absolute refractory period, you cant make it fire again - But once you get back to -70 mv- now you’re in a relatively refractory period.  Now you can cause another AP. But now it has to be even stronger because it’s more hyperpolarized. - What’s interesting- that pattern of ions moving, starts at the axon hillock  All-or-none – when threshold is reached the neuron “fires” and the action potential either occurs or it does not  When threshold is reached, voltage-activated ion channels are opened - Because this takes ‘time’, its not instantaneous, theres a limit to how many APs can go. - In myelin neuron its about 1 millisecond Refractory Periods  Absolute – impossible to initiate another action potential  Relative – harder to initiate another action potential  Prevent the backwards movement of APs and limit the rate of firing PSPs vs. Action Potential (APs) EPSPs/IPSPs Action Potentials  Decremental - Nondecremental  Fast - Conducted more slowly  Passive (energy is not used) - Passive and Active (use ATP) Axonal Conduction of APs  Passive conduction (instant and decremental) along each myelin segment to next node of Ranvier  New action potential generated at each node  In myelinated axons: instant conduction along myelin segments results in faster conduction than in unmyelinated axons  Bunch of ion channels in between the „beads‟, the gaps in between we have the active charge  Underneath he myelin it‟s a passive process- because we electrically isolated it. And once charge gets there- its so fast and on to the next  Signal with leap frog  Soltatory conduction ***  No AP in the Myelin - it jumps on the next that‟s why it goes to fast. Velocity of Axonal Conduction - Depending on what mammal you look at- the AP varies.  Maximum velocity of conduction in human motor neurons is about 60 meters per second Conduction in Neurons without Axons  Conduction in interneurons is typically passive and decremental  If we look at interneuron- that doesn’t have axons- there is No APs that happens. You have just passive instantaneous signal happening and degrades as it t
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