Psych 115 Week 2 Day 1: Neurophysiology

What's the action potential propagation & how they travel down the neuron?

-All-or-none property --Action potentials will fire at its full amplitude (e.g., strength) and speed or it won't fire at all. ---There's not half/minor action potential -Action potential propagation ·--One action potential triggers another and another and another and another... --Does not decay in amplitude or speed as it propagates down a neuron! -Kinda like the WAVE! ---(image of an electrode on an axon) Here you are able to see with the 3 electrodes the amount of action potentials that occur with one signal going down the axon. The time is different, but the speed of it and the strength of it will always be the same.

Why do action potentials (aka neurons firing) happen?

-Because the membrane maintained the concentration gradient and electrostatic pressure to favor Na+ entering the cell, the neuron can respond strongly and quickly. Because you have these 2 forces already pulling sodium in, but can't the sodium continues to build and because of that it's ready to enter the cell strongly & quickly. & What happens is that an action potential facilitates sodium flooding in.

What are post-synaptic potentials?

-Here we see a post-synaptic neuron and we have 2 pre-synaptic neurons sending a signal to the post-synaptic neuron o excitatory pre-synaptic neuron (EPSP) --Fires action potential (all or nothing), causes Excitatory Post-Synaptic Potential in post-synaptic neuron, which are graded and can take a little bit of a bump. ---It's excitatory because it moves the post-synaptic neurons closer to 0, closer to being more positively charged. --Small depolarization (+) because local potentials are graded, not "all-or- nothing" like action potentials. o Inhibitory pre-synaptic neuron (IPSP) -Fires action potential, but it has an inhibitory effect on the post-synaptic neuron and causes a dip which is an Inhibitory Post-Synaptic Potential in post- synaptic neuron. --Inhibitory because it makes it even more negative. --· Small hyperpolarization (-) because local potentials are graded, it's not all or none it takes minor bumps in potentials.

What's temporal (timing) summation?

-Rapid repeated input from the same pre-synaptic neuron at can have a cumulative effect and can produce an action potential and the difference here is the repetition of the signal from the same neuron. -Here again, if the overall sum of EPSPs and IPSPs can depolarize the cell at the axon hillock, an action potential will occur. The basic idea is that either of these summation methods it's a cumulative effect and it can add up so the post-synaptic neuron can reach threshold > action potential, it's just the way it which it can happen which differs.

Whats the relative refractory phase?

-The phase where the membrane starts dipping into the negative -A brief period immediately after an action potential where only strong stimulation (of positive charge) can produce another action potential.

What's the sodium potassium pump?

-This pump (ion channel) ONLY allows sodium (Na+) and potassium (K+) to pass through. -Simultaneously exchanges sodium (Na+) ions for potassium (K+) ions every time. --Transports 3 Na+ ions OUT --Transports 2 K+ ions IN -Why is the sodium-potassium pump so important? --Maintains higher sodium concentration OUTSIDE the cell --Maintains higher potassium concentration INSIDE the cell. --Balancing the Na+/K+ concentrations stabilizes a cell's resting potential. ---Because if it were to only let out sodium ions then it'll decrease the positive charge of the inside of the cell, but because of the system it currently has it maintains a balance.

What's the conduction (speed) of action potentials in MYELINATED axons?

-Travels down faster -Myelin sheath -- does not contain any ion channels, it's an insulation --But increases the speed of action potentials travelling down the axon...HOW? . . with saltatory conduction. -Saltatory Conduction --There's an action potential, Na+ rushes in (increase in positive charge in the nodes of ranvier) through the nodes to depolarize the membrane, the Na+ ions (since there are no channels) in the myelin boosts the depolarization (increase of positive charge) to the next Node of Ranvier. ---& because of that the positive charge will open up the flood gates of the voltage gated sodium channels which then causes more sodium to rush in to the next node and then propagate down ----Making action potentials look like they're "jumping" from node to node. When in fact it's the myelin facilitating this. --Significantly increases conduction speed! 150 meters/sec or 492ft/sec!! 15 times as fast compared to an unmyelinated axon

What are local potentials?

-Unlike Action Potentials (all or none), local potentials are NOT "all-or-none". -Can diminish as they move away from point of stimulation, they are graded. -With local potentials you can have a small bump in positive charge or less bump, but it's not all or none. It can take a bunch of positive charges & kind of add them up whereas action potentials is just one swift increase in positive charge. o Note: Pre-synaptic(sending) neurons can work to turn on (excite) or shut down (inhibit) post-synaptic (receiving) neurons. (Is this specific to the local potentials or this in general pre and post neurons can do this?)

What's the axon hillock?

-a cone-shaped area of the cell body that gives rise to the axon and where the axon starts. -Plays an important role on how action potentials start because it happens here first and then it travels down. --The first Action Potential happens at the Axon Hillock, then propagates the signal down the entire axon! --How does the Axon Hillock get its first action potential?!

What's spatial (space) summation ?

-synaptic input from several (3) pre-synaptic neurons at the same time can have a cumulative effect and trigger an action potential. -If the overall sum of EPSPs and IPSPs can depolarize the cell at the axon hillock, an action potential will occur.

What makes ions move during the resting potential?

1. Diffusion causes ions to flow from areas of high to low concentration (i.e., concentration gradient) --· An example of diffusion, is having a cup of water and you add drops of food coloring and you would see the food coloring start to diffuse and you can think of the food coloring as ions. So it diffuses from areas of high concentration to the areas of low concentration. It spreads it out so it's even across the entire cup of water (aka concentration gradient). The ions would flow from areas of high con > low con and typically diffusion is separated by a permeable membrane and it's what we saw with the lipid bi-layer and these channels. The channels can filter specific ions to let them in while refusing others. ---Semipermeable membranes can select out which neurons can be diffused 2. Electrostatic pressure causes ions to flow towards oppositely charged areas (opposite charges attract!). Having two ions of opposite charges they will attract or pull each other towards each other. --This is opposite to the concentration gradient in a way, in that this is dependent entirely on the charges of an ion. o A balance of these two forces determines the an ion's equilibrium potential, and in turn the neuron's resting potential.

How do action potentials happen?

1. Increase in + charge moves resting potential to threshold (-55mV) (some Na+ channels open) --It has to pass the -55mV for the action potential to be granted. --The membranes resting potential is -60mV and with the increase of positive charge it's going to move the charge to be more positive (towards the -55mV) & once it's reached, it's where the action potential occurs and depolarizes (coming more positively charged) the cell. 2. Depolarizes, threshold met, voltage gated Na+ channels open (goes from -60mV to 0mV inside) 3. Repolarizes, voltage gated Na+ channels close & voltage gated K+ channels open (goes from 0mV to -60mV) 4. Hyperpolarization, some voltage gated K+ channels still open. (goes from -60mV to -80mV) 5. Back to resting potential

What determines a cell's resting potential?

1. Membrane permeability (how easy it is for ions to pass through(or move back & fourth) a membrane). These ions carry charges and they can be positive/negative which contribute to the charge. 2. Equilibrium of Potassium (K+) movement · The rate in which potassium (K+) moves in/out of the membrane --This is the biggest contributor to the resting potential. 3. Sodium-Potassium (Na+, K+) pump --Exchanging 3 sodium (Na+) for 2 potassium (K+), which helps maintain the charge of the cell.

What's involved in an action potential?

1. Voltage-gated sodium (Na+) channels · As stated in the previous slide sodium always want to get in, but can't. They can't because they're voltage gated. What that means is that. These channels need a certain electrical charge in order to open & let sodium pass through. -Ion channel that ONLY lets sodium pass -This channel opens/closes ONLY when the membrane resting potential reaches a certain voltage threshold. --When it does meet that threshold the channel doors open and it allows the sodium to go in. 2. Voltage Gated Potassium (K+) channels -Ion channel that ONLY lets K+ pass -This channel opens/closes ONLY when the membrane resting potential reaches a certain voltage threshold. --When it does meet that threshold the channel doors open and it allows the K+ to go in. · NOTE: NOT the same as the sodium- potassium pump! (lets potassium and sodium flow through the same channel and these don't & it's not voltage gated) These 2 channels and this specific process underlies action potentials.

What's hyperpolarization?

Inside becoming more negatively charged than before -from -60mV down to -80mV. -& once it hyperpolarizes it'll rebound back to the -60mV (resting potential)

What's the membrane permeability?

Membranes are more permeable to potassium (K+) ions --Special K+ ion channels that only allow K+ to pass in/out K+ channels are open all the time, K+ leaves and enters the cell at about an equivalent rate --This is why K+ has such a big influence on the resting potentials (next slide) Sodium (Na+) ions want to get in, but can't! --The reason why sodium always wants to get into the cell is because of the concentration gradient, less sodium inside the cell than outside which is why the sodium wants to push to even it out (concentration gradient) --Inside is more negatively charged than outside (electrostatic pressure). ---Sodium ions are more positively charged and the outside of the cell is more positively charged than the inside and the EP, the negative charge inside the cell wants to bring in sodium ions inside the cell. --But Na+ channels are always closed! Unlike the K+ that are always open. & because they're always closed they can't get in whenever they want.

What's the resting potential of a neuron?

The resting potential of a typical neuron is between. -50to-80mV -In the example you are able to see -60mV, The actual resting potential of a neuron is determined by the size or type of a neuron. you are able to see the reaction on the neuron with the electrode, where the inside is more negative, you can see the activity dip down to more negative because of the charge inside that the. Electrode emits

· What is "Neurophysiology"?

is the study (or branch of neuroscience) of electrical and chemical processes in a neuron and there are 2 main methods in which neurons use to communicate -includes neuroconduction, neurotransmission

What's the absolute refractory phase?

o A brief period immediately after an action potential where absolutely no stimulation can cause another action potential, because it's already at its peak.

What happens when a neuron is not at rest?

o Action Potentials happen! --When a positive charge causes the membrane's resting potential to quickly and briefly move towards 0mV (depolarize) or become positively charged. --Must past a certain threshold for this to happen -usually greater than -55mV. In order for an action potential to happen.

What's the conduction (speed) of action potentials in UNMYELINATED axons?

o Action potentials are regenerated along the axon --each adjacent section is depolarized (inside becoming more positively charged) and a new action potential occurs. -You are able to see that there are voltage gated sodium channels and the signal propagates in one direction followed by a refractory period (rest time) before another one can occur. Which is why. Action potentials can only go in one direction from the input zone > output zone. Because of the refractory period it can't travel backwards, so it forces to go down one way. -o Action potentials travel in one direction because of the refractory period of the membrane after a depolarization. o Conduction velocity of unmyelinated axon --Is much slower because there's no myelin, no insolation which forces these action potential to occur at every single point down the axon. --Action potential travels along the unmyelinated axon at 10 meters/sec (e.g., 32.8 ft/sec!) ---With the myelin sheath and the nodes of Ranvier work together to help speed up the conduction of the neuron.

What's EPSP & IPSP?

o If more Excitatory Post-Synaptic Potential (EPSP) is received: --When you have these excitatory post-synaptic inputs coming in, it increases the positive charge of the post-synaptic cell. ---Inside membrane will be more positively charged (red meter). --It will add up to reach threshold and trigger action potential at Axon Hillock o The Inhibitory Post-Synaptic Potential (IPSP) is received: --· Inhibitory meaning it works to make the post-synaptic neuron more negative > decreasing its ability to fire an action potential (which requires the neuron to be more positively charged). --Inside membrane will be more negatively charged (blue meter) --Pushing it away from threshold and prevent action potential at Axon Hillock ---Serves as an active "brake" that suppresses excitation and tell neurons that shouldn't be firing the whole time because if they do it can be bad (i.e., scorpion venom example) Summing up EPSP & IPSP: What determines the input zone from summing up all of these inputs together. Two ways in which inputs can be integrated to cause an action potential in the post-synaptic neuron.

What are the electrically charged molecules that are involved in neuroconduction?

o Ions are electrically charged molecules. --Anions(-) are negatively charged (Chloride ions, Cl-) --Cations(+) are positively charged (Sodium ions Na+, Potassium ions, K+). o Ions are dissolved in intracellular fluid (inside the cell), separated from the extracellular fluid (outside the cell) by the plasma membrane. o A neuron at rest it reflects that it's at balance of electrochemical forces.

What's the plasma membrane?

o Lipid Bi-layer --The structure of the plasma membrane (i.e., walls of a neuron) --Lipids = fatty acids, so two layers of fatty acids. o Embedded in the lipid bilayer membrane are protein channels --Allows charged ions across. --Small uncharged ions pass through easily (water, carbon dioxide, oxygen). -Inside & Outside of the cell is hydrophilic (likes water) -Middle of the bilayer it's hydrophobic (dislikes water, so no water passes through). -Extracellular space, fluid outside of the cell -Intracellular fluid, fluid inside of the cell.

What are the ions involved in resting potential?

o Positively charged --Sodium (Na+) - more concentration of these ions outside of the cell than inside --Potassium (K+) - more concentration inside of the cell. --Calcium (Ca2+) - more concentration of these ions outside of the cell than inside o Negatively charged --Chloride (Cl-) - more concentration outside of the cell. o The movement of each of these ions in to outside of the cell is what contributes to resting potential and the movement to each ion has its own equilibrium potential --The electrical difference (in millivolts) across the membrane contributed just by the single ion.

What are a sub-category of local potentials?

o Post synaptic potentials a sub-category of local potential that's generated at the (input zone) synapse/dendrites of the receiving neuron. --Excitatory post-synaptic potential (EPSP) ---Increases the probably that the post-synaptic (receiving) neuron will fire an action potential --Inhibitory post-synaptic potential(IPSP) ---Decreases the probability that the post-synaptic (receiving) neuron will fire an action potential

What's resting potential?

o The difference of voltage outside vs. inside the cell · The voltage inside the neuron when it is at "rest" --Outside of neuron is more positively charged --Inside of neuron is more negatively charged.

What's the battle of post-synaptic potentials?

o The excitatory/inhibitory inputs from multiple pre- synaptic neurons "compete" to excite or inhibit the post-synaptic neuron and to see if it will make it fire or not. o A postsynaptic neuron will fire an action potential if a depolarization that exceeds threshold, as seen in the image, reaches its axon hillock.

What's the summary of Summation?

o The likelihood of an action potential depends upon the ratio of IPSPs to EPSPs at a given moment. o Inhibitory & Excitatory can influence how a neuron fires if & when it fires an action potential -Gives different scenarios om how the summation methods can happen and influence and action potential --3. Has different bumps, but has an additive effect which keeps building up to reach the threshold. --4. The Spatial Summation

Whats a neurotransmission?

o chemical, between neurons --A neurotransmitter (i.e, dopamine, serotonin, norepinephrine)is a chemical messenger between neurons. (which facilitates the messages between neurons).

What's neuroconduction?

o electrical, within neurons to pass the signal down --An action potential is a rapid electrical signal that travels along the axon of a neuron.

What's the K+ Equilibrium?

the rate that potassium moves in and out of the cell -Higher concentration of K+ inside of cell than outside. -K+ moves to outside of cell to "even out" the concentration between outside vs. inside (Concentration gradient). --Leaves behind a lot of negatively charged ions, now inside becomes VERY negative & because of the EP it's going to drag more positively charged potassium. -· The increased negative charge on the inside will start PULLING IN K+ because opposite charges attract (K+ vs Cl-, a.k.a. electrostatic pressure!) -Balance of two forces! --Concentration gradient pushes K+ outside, because there's more K+ inside than outside and wants to diffuse across. --Electrostatic pressure pull K+ back in ---The balance of the EP and the concentration gradient is what underlies the equilibrium of potassium ---But once with the concentration gradient the EP will pull the K+ back in, because inside of the cell is more negative now and the inside of the cell attracts the positive charge of the K+ ions. --Equilibrium of K+ movement = equilibrium potential (a difference in the voltage across the membrane)! -K+'s equilibrium potential is mainly responsible for the neuron's resting potential at -60mV.

What's depolarization?

§ Inside becoming more positively charged · E.g., goes from -60mV to +0mV inside

What's repolarization?

§ Inside becoming negatively charged again (from +0mV back down to -60mV) · This happens because is that those voltage gated sodium channels will close, so the flood of sodium ions will stop and then the voltage-gated potassium channels will begin to open. Because there's an abrupt stop to all of the sodium channels flooding in that's when the cell starts to become more negative. · When the voltage gated potassium channels open at once & because of that you see a dip that goes below -60mV, which is called hyperpolarization.

What does the axon hillock integrate?

· Integrates Post-Synaptic Potentials -(image of a neuron receiving synaptic inputs) This is a post-synaptic neuron receiving the input and the axon hillock . . . o The Axon Hillock integrates all inputs and determine if post- synaptic neuron will fire an action potential or not. (you can look at the gauge at the image) --The input from multiple pre-synaptic neurons "compete" to excite or inhibit the post-synaptic neuron.

What's the refractory period?

· There's an important periods for these action potentials that prevent from firing again & doing another action potential. · When the membrane is briefly inactivated after an action potential (rest period) --Once an action potential happens there needs to be some brief period for the membrane to recharge before it can actually fire another action potential.