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PNB - Exam 2
Terms in this set (77)
Electricity that causes muscle contraction comes from ions. The charges of ions influence their behavior: Like charges repel, and opposite charges attract (come together). Separating charges requires energy
Membranes in which very few things (oxygen) can freely cross. Intact membranes do not allow ions to pass freely. Ion channel and carriers permit the movement of ion across the membrane, which changes the concentration gradient
Created by the uneven distribution of electrical charge across the membrane. The movement of ions causes change to the membrane potential
The Cell Membrane as a Battery
The plasma membrane separates charged ions on both sides of the membrane. The Potential (electrical) energy stored in the membrane is used to send/receive signals inside the cell
Recording Membrane Potential (Vm)
Vm is the potential inside the cell relative to the potential outside of the cell. Expressed as VOLTAGE. The inside of the call is ALWAYS negative than the outside. These are relative terms, "the inside is more something to the outside".
Key Contributions to Vm (1)
Ions are not distributed equally across the cell membrane. Ion distribution depends on ATP-driven pumps (Sodium/Potassium/ATPase)
Key Contributions to Vm (2)
Membranes are unequally permeable to various ions (More permeable to potassium than sodium at rest). Results in different concentration gradients present on both sides of the membrane
The Two Opposing Forces across Cell Membranes
Concentration Gradients and Electrical Potentials. If original electrical potential is zero, and a concentration gradient is present, molecules move in the direction of the concentration gradient (1). If the original concentration gradient is zero, BUT THERE IS AN ELECTRICAL GRADIENT PRESENT, then molecules move down the electrical gradient (2).
Equilibrium Potential (Ex)
Also known as the Nernst Potential. The membrane potential at which the concentration gradient and electrical potential forces are equal and opposite. This is the balancing point for ONLY ONE ION.
Common Misconception about Vm
Despite movement of ions, relative concentrations of ions do not change. It only takes a very small movement of ions to develop an electrical potential, NOT massive quantities.
The Nernst Equation
Used to calculate membrane potential is the cell is permeable to a SINGLE ion. Every ion has a different membrane potential.
Predicting Changes in Ek
Is there a concentration gradient? Does it become larger or smaller? Which direction will ions move? What is the charge of the ion? Would the movement make the cell more positive, or negative?
Problem with the Nernst Equation
Cells are permeable to multiple ions, so the nernst equation can rarely be used to calculate Vm.
The cell membrane potential at which the net flux of ALL ions together equals zero. Vm is a steady state, but NOT an equilibrium
Goldman Hodgkin Katz (GHK) Equation
If a cell is permeable to multiple ions, Vm is a weighted average of the Ex for each permeable ion. Can also be referred to as "Parallel Conductance". Which ion is "Pulling" on the electrical charge the hardest
GHK Equation properties
In this equation, Vm must fall between the Largest and Smallest Ex. Ions with greater permeability have greater influence on Vm. The larger the influence of an ion, X, the closer Vm will be to Ex. If the membrane is permeable to only 1 ion, the equation reduces down to the Nernst Equation.
GHK Equation and Vm
The GHK equation shows us that you can change the membrane potential of a neuron (Vm) by changing the permeability of the different ions
Leaky Potassium Channels
One family of Potassium Channels that are open at resting membrane potential. Causes potassium to be relatively higher inside the cell. The potassium efflux makes the resting Vm negative.
Uses ATP to pump BOTH Sodium (3) and Potassium (2) against the concentration gradient. Maintains the concentration gradient. Sodium moves out of the cell, and Potassium moves in. Results in a net loss of 1, causing the cell to lose a charge (-1) and become slightly more negative.
Importance of the Na/K - Pump
Important for Osmolarity, Maintains the Concentration Gradient, and is Electrogenic (Helps to make the Vm negative). Results in a net loss of 1 solute, which balances hypertonicity
Passive Electrical Signals
A transient change in membrane potential that dissipates as it propagates in space and time (a graded/synaptic potential)
Active Electrical Signals
A change in membrane potential that is maintained over a long distance (an action potential)
Action Potentials - "Firing"
Action potentials are a burst of electrical activity that rapidly propagates through the cell
Structure of a Neuron
Dendrites (input), Some (integration), Axon Hillock (Where the Action potential is generated), Axon (Where the Action potential propagates) and Synapse (Signal output - Chemical)
Initiation of an Action Potential
The density of sodium channels is highest at the axon hillock. Therefore, action potentials originate in this region of the neuron
Voltage-Gated Sodium Channels
Propagates the action potential in neutrons with Sodium Ions. Can be closed, activated and open, OR inactivated and open. A sensor on the channel moves as the cell becomes more positive, and the more positive the cell gets, the more the channel opens.
Two Gates in Sodium Channels
Activation and Inactivation gates.
Voltage Sensor Domain
Senses the membrane potential.
Steps of an Action Potential
At the resting membrane potential, the activation gate closes the channel. A depolarizing stimulus arrives at the channel, and the activation gate opens. With the activation gate open, sodium ions enter the cell. The inactivation gate then closes and sodium entry stops. During repolarization, which is caused by potassium ions leaving the cell, the two gates rest to their original positions
Protein channels within a cell's semipermeable membrane that are selective towards K, Na and Cl. Moves these ions into, or out of, the cell to change the concentration gradient. Always move ions from high to low concentration.
Potassium Distribution Across a Membrane
Intracellular: 100 (mM)
Extracellular: 5 (mM)
Sodium Distribution Across a Membrane
Intracellular: 10 (mM)
Extracellular: 100 (mM)
Chlorine Distribution Across a Membrane
Intracellular: 10 (mM)
Extracellular: 105 (mM)
Calcium Distribution Across a Membrane
Intracellular: 0.0001 (mM)
Extracellular: 1.2 (mM)
Different Ion Channel Gating Mechanisms
Voltage-Gated (sodium), Ligand gated (both extracellular and intracellular), and Stress-Activated gated channels
Result of No Ion Channel Being Open
No movement. Membrane is not permeable to ions even if there is a concentration gradient present
Result of Ion Channels that are Non-Selective (Example 2 in Slides)
Positive and Negative Ions move down the concentration gradient until they reach equilibrium. Concentration gradients are equal AND electrical potential will be equal on both sides of the membrane
Results of Ion Channels that are Selective to a Particular Ion (Example 3 in Slides)
Positively charged ions move down the concentration gradient. Some positively charged ions THEN move back into the cell to equalize the charge on both sides of the membrane to eliminate the electrical potential. This is due to the fact that the attractive forces between opposite charges pull the ion back into the cell.
Determining Factors of the Nernst Equation
The charge of the ion is important, and the concentration gradient of the ion across the membrane is important. If no gradient is present, than Ex = 0.
Equilibrium Potential for Potassium
Equilibrium Potential for Sodium
Equilibrium Potential for Chlorine
Result of the Concentration of Potassium Outside the Cell Increasing (-75 mV Ex)
Concentration gradient decreases, so less Potassium leaves the cell. Fewer positive charges are leave the cell, so the membrane potential goes up (gets more positive)
Result of Concentration of Potassium Outside the Cell Being Equal to Inside
No concentration gradient is present, so there is no movement. Ex becomes 0
GHK Equation Explanation
If the ion is + charges, and the membrane potential is positive, than there is a higher concentration of the ion outside the cell. If the ion is + charged, and the membrane potential is NEGATIVE, than there is a higher concentration of the ion inside the cell.
Solving GHK Equation Problems
If the permeability of 2 of the ions present decreases, or the permeability of 1 increases, the membrane potential moves towards the membrane potential of the ion "pulling hardest" on the charges
Importance of the Membrane Potential of Potassium
With so much K inside the cell from leaky K channels, the inside of the cell can always become more negative by moving K out (why Vm for K is -75)
Situation where the membrane potential becomes more positive. Inside of the cell becomes more negative, because as the inside of the cell becomes more negative, there is a greater chance that positive ions will move into the cell.
Period of an Action Potential where the membrane potential becomes more negative, back down to resting potential
Period of an Action Potential where the membrane potential becomes MORE negative than the resting potential.
Action Potentials Traveling Forward
After one segment of axon membrane is very depolarized nearby membrane patches are also depolarized. This neighboring patch then goes through the action potential steps. Sections that just finished the action potential are refractory.
Refractory of Action Potentials
After just finishing the action potential, sections are resistant to depolarization because the sodium channels need time to recover. Relates to inactivation gate; if inactivated, it cannot open again. Make the action potential move in only one direction
Allows further, faster depolarization/AP propagation. Increases the speed of conduction by (up to) a factor of 100. Insulation that wraps around axons. Makes it so we don't need as many voltage gated sodium channels
Action potentials appear to jump from node to node along the Nodes of Ranvier
Myelin and the Nodes of Ranvier Na and K Channels
Alter the distribution of both channels. The node has roughly 10,000 Na channels per squared micrometer
Depolarization at the axon terminal opens calcium channels. Because the intracellular concentration of calcium is low at rest, opening these channels allows the ions to enter presynaptic terminal. Calcium enters from the extracellular space, into the cell
Synaptic transmission using neurotransmitters
Synaptic transmission that passes electrical signals
Steps Producing the Release of Neurotransmitters
An action potential depolarizes the axon terminal. Opens volted-gated calcium channels, and calcium enters the cell. Calcium entry triggers exocytosis of synaptic vesicle contents. The neutrotransmitters then diffuses across the synaptic cleft and binds with receptors on the postsynaptic cell. This binding initiates a response in the postsynaptic cell
Graded Synaptic Potentials
Synaptic potentials are produced primarily by ligand gated ion channels. Binding of a neurotransmitter causes receptors to open which increases the permeability of the ion that the receptor prefers. If this increases permeability to Sodium, Vm will increases towards the equilibrium potential of sodium (GHK equation). Decrease in strength as they spread out from the point of origin
Excitatory Post-Synaptic Potential
Depolarization if the membrane potential is more positive than Vm
Inhibitory Post-Synaptic Potential
Hyperpolarization if the membrane potential is more negative than Vm.
Propagation of Synaptic Potentials
Travel from the synapse in all directions. They get smaller and slower over distance. Synaptic potentials are due to passive membrane properties
Integration of Synaptic Potential
The soma receives many EPSPs and IPSPs. If the threshold in the axon hillock is reached, the AP fires
Integration of Potentials
Post synaptic potentials exibit Spatial (same spot) and Temporal (same time) summation.
Mechanism of Voltage Gated Sodium Potentials
Depolarization caused positive charges to move into cell. Sensor on channel picks up on negative charges outside the cell, and door starts to open slightly. Past -55 mV, the channel opens completely, resulting in larger depolarization. At +30 mV, the inactivation gate swings open and sodium entry stops (inactive, NOT CLOSED)
Action Potential as a Positive Feedback Loop
Sodium entry and depolarization leads to more depolarization. This keeps moving forward until activation gate swings closed and stops it
No channel response can occur. No action potentials can fire
Relative Refractory Period
Some channels can respond to action potential but it is much smaller
Reason why Action Potential Does Not occur Along the Entire Length of Axon SImultaneously
Diffusion is limited by distance. Each axon is divided into segments, with each segment responding to depolarization of the previous segment.
How a Signal Propagates Down the Axon
As the 1st segment undergoes repolarization, the influx of sodium into the next segment causes it to go into its depolarization phase. The first segment's inactivation gates closes, preventing them from firing. ONLY the forward segments have voltage gated sodium channels can be open/activated because they are at rest
Myelinated region of the axon. Action potential moves very fast through this region
Nodes of Ranvier
Part of the axon with no myelin. Action potential moves very slow through this region
Involved in chemical signaling. Present are membrane vesicles with neurotransmitters inside neurons. The synaptic knob contains voltage gated calcium channels
Point of communication between 2 cells. Neurons/Neurons OR Neurons/Muscles. Space between presynaptic and postsynaptic cells
Sends the chemical signal into the synapse
Receives chemical signal from presynaptic cell. Channels located along the synapse are only stimulated (concerned) by neurotransmitters. These are ligand gated channels
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