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85 Cards in this Set
- Front
- Back
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Describe lipid bilayer of cell membrane
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Phospholipids with polar head and two hydrophobic fluid-like tails. Lipid rafts (cholesterol) found in between.
Spontaneously organizes into bilayer with polar (hydrophilic) groups facing out to H20. |
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Why does the phospholipid bilayer not form planar shapes?
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It is energetically unfavorable to have any hydrophobic groups (tails) exposed to water. Phospholipid bilayer edges curl and come together to form sphere so that only hydrophilic groups (heads) touch water.
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Functions of cholesterol in cell membrane?
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*Can increase the membrane permeability to small water-soluble molecules.
*Stiffens membranes and forms thick lipid rafts that better accommodate proteins. |
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What is sphingomyelin?
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Acts with cholesterol to stiffen cell membrane and form lipid rafts.
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Typically, cell membrane has (low/high?) permeability to H2O and (low/high?) permeability to O2 and CO2
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LOW permeability to H2O
HIGH permeability to O2, CO2 |
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What happens to ions such as H+, Na+, HCO3-, K+, Ca2+, Cl-, Mg2+ when they try to get through the cell membrane?
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Cannot get through. Require "tunnels".
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What happens to hydrophobic molecules such as O2, CO2, N2, and benzene when they try to get through the cell membrane?
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They go right through
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What happens to small uncharged polar molecules like H2O, urea, and glycerol when they try to go through the cell membrane?
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They permeate through somewhat
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What happens to large uncharged polar molecules like glucose and sucrose when they try to go through the cell membrane?
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They hardly permeate at all.
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Membrane proteins comprise ____% of the area of the cell membrane?
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50%
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Describe the most common type of alpha helix transmembrane protein?
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Single α-helix. Single helical portion of protein within bilayer. Hydrophilic ends stick out. Most common. Forms receptor for signals.
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What interconnects membrane proteins (and lipids) with extracellular matrix?
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Carbohydrates
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Carbohydrate layer binds ____________ which mediate cell-cell adhesion (e.g. blood clotting and inflammation).
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lectins
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What protects cell from inappropriate protein-protein interactions?
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Extracellular matrix.
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Describe the type of transmembrane protein that forms aqueous pores in the cell membrane?
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Multi α-helix. Single protein with multiple helical chains that cross membrane several times to form "tunnel" in bilayer. Hydrophilic ends stick out.
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What is membrane potential (Vm)?
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It is the electrical potential (voltage difference) across a cell membrane. It is generated by the net flux of all ions. The net flux is the product of the net driving force and permeability.
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Do all cells have a membrane potential?
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Yes, and they are all produced by the same mechanism.
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Describe ion flux?
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The passage of ions through ion-specific, cylindrical, membrane protein-based ion channels.
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Ion specificity of a channel depends on what two factors?
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Distribution of charges. (Channels lined with neg. charges permit passage of pos., but not neg., ions).
Diameter and shape of channel. (Small channels permit only small ions through. Size of ion determined more by hydration shell than by atom.) |
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Current
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Movement of ions
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Sodium-Potassium Pump
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Na-K and ATP produce concentration gradients.
3 Na+ ions out and 2 K+ ions in, against their concentration gradients. |
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How is ATP used to create the Sodium-Potassium Pump?
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ATP donates 3rd phosphate group, which alters protein conformation.
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Describe movement of K+ due to its concentration gradient
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Concentration of K+ is higher inside the cell than it is outside. Net outward flux of K+ down its concentration gradient produces a neg. charge on cell membrane interior.
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Describe movement of K+ due to its electrical gradient.
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Accumulation of neg charge in interior (because of movement of K+ by concentration gradient) electrostatically pulls K+ inward via an electrical gradient.
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Equilibrium Potential
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For a specific ion, the electrical potential difference that exactly counterbalances diffusion due to the concentration difference is called the equilibrium potential for that specific ion.
http://courses.washington.edu/conj/membpot/equilpot.htm |
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When achieving equilibrium potential, is there a net change in the concentration of K+ inside or outside the cell?
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No. It is a steady state equilibrium. There is no net loss of K+ because what leaves the cell is transported back in by the Na-K pump.
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Describe what the Nernst equation calculates
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Expresses the equilibrium potential (E) quantitatively by calculating the theoretical potential for only one ion. (In reality, several ions contribute to membrane potential through channels of variable permeability.)
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Net flux of an ion across a membrane is determined by the product of what two factors?
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1. Net driving force -- sum of concentration and electrical gradients.
2. Permeability -- ease of flux through a channel. |
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Why is K+ net flux high?
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Concentration and electrical gradients are opposite, reducing net driving force, but K+ channel permeability is high.
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Why is Na+ net flux low at resting conditions?
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Na+ concentration and electrical gradients are in the same direction, increasing net driving force, but Na+ channel permeability is very low.
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What is the difference between equilibrium potential and membrane potential?
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Equilibrium potential is for one ion only. Membrane potential measures what you actually have in the cell (more than one ion).
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Goldman Equation
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Quantifies Vm by weighting each ionic flux with its permeability. It is a balance between concentration gradient and permeability.
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What is the range of typical membrane potentials for cells?
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-90 to +55 millivolts mV
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Depolarization
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Stimuli reduce the polarization of a membrane to "depolarize" it and make it closer to zero mV.
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What determines ion permeability?
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The gating properties of channels.
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What type of channel is stimulated by depolarizing or hyperpolarizing current?
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Voltage-gated channels
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Where are voltage-gated channels found?
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Myelinated axons => Axon hillocks (base of axon between axon and cell body), Nodes of Ranvier
Unmyelinated axons => Distributed evenly along the length of the axon. |
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Ligand-gated channels are opened by what kind of stimulus?
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Ligands
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How are transmitter-gated channels opened?
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Neurotransmitters bind to part of channel and cause it to change shape resulting in opening of channel.
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Ligand-gated and transmitter-gated channels are found in what types of cells?
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Neural or other cell surfaces
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Mechanically-gated channels are affected by what type of stimuli?
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Pressure, touch, light
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Mechanically-gated channels are found in what types of cells?
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Skin, retina
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How do voltage-gated channels open?
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Build up of depolarizing or hyperpolarizing charge alters conformation of channel proteins (for example membrane depolarization causes charged helix to rotate) to open pores and change permeability.
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What are the types of non-gated channels?
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Na+ , K+
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What is the most ubiquitous type of channel?
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K+ channels
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How do K+ channels open?
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They are "leak" channels. They are non-gated but flicker between open and closed. Very permeable to K+.
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What type of channel is the primary determinant of a cell's resting membrane potential?
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K+ channels
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Describe the action of non-gated channels
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They do not change their permeability in response to stimuli. Up or down regulation can alter number or permeability of channels.
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This type of channel is generally open most of the time (non-gated or gated)?
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Non-gated
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What is the difference between the dendrites of a sensory neuron vs the dendrites of a motor neuron?
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Sensory neurons have fewer dendrites. Instead have divided axon w/ sections on either side of cell body. In motor neurons, dendrites conduct synaptic signals from other cells toward the neuron. In sensory neurons, part of the axon does this job.
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What is the difference between the axon of a sensory neuron and the axon of a motor neuron?
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SENSORY = Axon is divided into peripheral and central branches that carry electrical impulses from periphery to spinal cord. MOTOR = Dendrites carry signals from other cells to the axon. Axon carries signals from cell body to muscle.
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What type of cell connects sensory neurons to motor neurons?
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Interneuron
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In what type of cells are Receptor Potentials generated? What is the purpose of the potential?
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Neurons: purpose is communication
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In what type of cells are Action Potentials generated? What is the purpose of the potential?
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Muscle cells: purpose is contraction
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What do Receptor and Action Potentials have in common?
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(1) changes in membrane potential in response to some signal and (2) occur in excitable tissues: neurons and muscle tissue
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Describe the reflex arc (in general) in terms of the types of cells that are involved from sensation to action.
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Sense organ (ie skin) --> Afferent neuron --> Synapse --> Efferent neuron --> Neuromuscular junction (motor end plate) --> Muscle
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Describe the reflex arc (in general) in terms of the types of potentials that are generated.
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Sense organ/Receptor potential --> Afferent neuron/Action potential --> Synapse/Postsynaptic potentials (PSP) --> Efferent neuron/Action Potentials --> Neuromuscular junction/End-plate potentials --> Muscle/Action Potentials
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Describe the first phase of the Action Potential
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Resting State:
All gated Na+ and K+ channels closed. Incoming positive ionic current depolarizes membrane to threshold level of voltage. Na channel opens. |
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Describe the second phase of the Action Potential
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Depolarizing phase:
Abrupt activation of most Na channels that bring mV closer to Na equilibrium potential, E (Na). |
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Describe the third phase of the Action Potential
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Repolarization phase.
Na channel spontaneously inactivates. K channels open slowly. mV approaches initial resting mV of cell. |
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Describe the fourth phase of the Action Potential
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Hyperpolarization.
K channel still open; K flux now through both leak and voltage-gated channels. Potential almost reaches E (K). Na channels resetting. |
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Do action potentials involve gated or non-gated Na and K channels?
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Action potentials involve voltage-gated channels. Non-gated Na+ and K+ channels do not participate in action potentials
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T or F? Shifts in polarization during the action potential are due to sequential changes in voltage gated Na and K channel permeability.
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True
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What causes inactivation of Na+ channels?
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Prolonged depolarization will close the interior side of the channel.
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Why is the initial depolarization so strong?
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Na has a high driving force - both concentration and electrical gradients are in the same direction and their values are additive. This is why membrane almost reaches the E (Na) at the peak of the action potential.
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What does a cell's excitability refer to?
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The sensitivity of the cell's ability to produce action potentials. It depends on the proximity of the membrane potential to threshold.
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Excitability can be altered by changing what two variables?
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(1) Concentration gradient, especially of K+ or
(2) channel permeability and/or threshold potential. |
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What effect does hyperkalemia (increasing extracellular K+ level) have on a cell's excitability INITIALLY?
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Hyperkalemia depolarizes the membrane nearer to threshold. Initially this increases probability that stimulus will trigger action potential.
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What effect does hyperkalemia (increasing extracellular K+ level) have on a cell's excitability OVER TIME?
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With prolonged depolarization, Na+ channels are inactivated, the threshold is raised and the cell membrane becomes less excitable. This leads to impairment of neuromuscular, cardiac, and GI systems.
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How can channel permeability be altered?
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Protein reconfiguration or channel blockage. Ex. Lidocaine alters configuration of voltage sensors and decreases sensitivity of voltage gated Na+ channels to depolarizing current. This raises threshold potential, less likely to be able to initiate.
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What are unmyelinated axons surrounded by?
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Unmyelinated axons are surrounded by a single layer of glial (Schwann) cell membrane.
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What are myelinated axons surrounded by?
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Myelinated axons are surrounded by several layers of glial (Schwann cell) membrane which forms myelin.
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How is action potential conduction different in UNmyelinated axons compared to myelinated axons?
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IN UNMYELINATED: 1) voltage-gated Na+ and K+ channels are located along the whole axon, 2) continuous production of action potentials along the whole axon, 3) slower, 4) maintain fidelity (ensure amplitude of AP along axon).
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What is the role of non-gated channels in AP conduction in UNmyelinated axons?
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They don't do anything but passively leak out K+
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Describe the phases in action potential conduction in unmyelinated axons?
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1) Na+ channels open (stimulus), 2) depolarizing current passively flows down axon, 3) local depolarization downstream causes Na+ channels to open and generate AP, 4) Upstream Na+ channels inactivate, K+ channels open, 5) Process is repeated.
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Refractory period
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In unmyelinated axons, it is the period of time after an action potential during which the membrane channels are less responsive to stimuli.
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Absolute Refractory Period
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Channels cannot be activated.
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Relative Refractory Period
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Requires greater levels of stimulation to trigger action potentials.
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Where are action potentials produced in myelinated axons? Why?
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Only in Nodes of Ravier and the axon hillock because this is where voltage-gated channels are.
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Saltatory conduction
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The skipping of AP generation from one node to the next (from the Latin "to leap")
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What is the effect of myelination on action potential conduction velocity? Why?
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Impulses travel rapidly through myelinated areas (in between nodes) electronically without producing action potentials. Passive conduction.
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What is the effect of myelination on action potential amplitude? Why?
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Amplitude of impulses decreases because current leaks out through non-gated K+ channels. Less leakage, however, because of myelination (layers of membrane).
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How is the action potential maintained down the length of a myelinated axon?
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Impulses retain sufficient depolarizing strength through myelinated areas to initiate a new action potential at the next Node of Ravier.
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Demyelination
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Common cause of neuropathy, esp. in autoimmune conditions like MS and Guillain Barre syndrome. Glial cells unwind and decrease thickness of myelin, facilitating current leakage through the membrane.
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How does demyelination disrupt AP conduction?
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In demyelinated areas, AP conduction is slowed or halted because excess current leaks out through non-gated K+ channels. This reduces the available current for the next node of Ravier.
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