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20 Cards in this Set
- Front
- Back
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Definition of the Trigger Zone
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The spike initiation zone in a neuron
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Continuous conduction
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Conduction in an unmyelinated axon
The leading edge of a propagating action potential moves to the next patch of membrane w/ sufficient v-gated channels |
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At the peak of the AP, what is the relationship btwn the influx of Na+ and efflux of K+?
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Na+ influx = K+ efflux
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The structural similarities of v-gated K+, Na+ and Ca2+ channels (4)
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• Each channel has 4 identical subunits clustered around a central pore
• Each of the 4 subunits has 6 transmembrane-spanning segments • Segment 4 of each subunit has a charged component that acts as a voltage sensing element for the channel • Each subunit has a pore loop that acts as a selectivity filter |
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Absolute vs Relative Refractory periods:
• responsive other impulses/stimuli? • State of v-gated Na+ channels • State of v-gated K+ channels |
Absolute period:
• Never other impulses/stimuli • All are open • gradually opening Relative Refractory periods: • Only responsive to strong impulses/stimuli • Na+ channels are inactivated • a large amount are open and are slow to inactivate |
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How Na+ and K+ channels are distributed in unmyelinated axons and skeletal muscle fibers
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They are distributed uniformly
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In myelinated axons, where v-gated Na+ and K+ channels are found
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v-gated Na+ channels are found at:
• the initiation zone (the initial segment of the axon) • the pre-seynaptic terminals • nodes of Ranvier v-gated K+ channels are found: • at the paranodal & juxtaparanodal portions of peripheral and centrally myelinated axons • in presynaptic terminals • the initiation zone |
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Impulses in myelinated axons:
• only occur at this region • are repolarized at these regions |
Impulses in myelinated axons:
• nodes of Ranvier • paranodal and juxtaparanodal position |
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An important point about how impulses are propagated in myelinated and unmyelinated axons
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They are propagated without decrement
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Factors limiting the conduction velocity of unymyelinated axons (2)
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1. They have small diameters, decreasing the effectiveness of current spread along the axon's interior
2. The axonal membrane must be driven to threshold sequentially |
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Rule of thumb to find the conduction velocities of unmyelinated and myelinated axons
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Unmyelinated: the value (m/s) is ~2x the fiber diameter (μm)
Myelinated: ~6x |
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The advantage of myelination of axons
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• With impulses only being generated at nodes of Ranvier, there is an increase in the conduction velocity
• Because impulses are confined to nodes, energy is saved • Large velocity can be achieved in fibers w/ relatively small diameters |
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Two toxins of v-gated Na+ channels and their mode of action
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Tetrodotoxin (TTX) and Saxitoxin (STX)
They bind to the outside (extracellular site) of the channel |
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Local Anesthetics:
• how they get into the cell • what they do once they get inside the cell • The parts of the AP they block • The hierarchy of fibers they block (3) |
Local Anesthetics:
• They cross the cell membrane in an uncharged form • Enter the Na+ channel and block the entry site • Initiation and conduction • 1st - small myelinated axons (A δ) 2nd - unmyelinated axons (C-fibers) 3rd - large myelinated axons |
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Define the safety factor and what it means for myelinated axons
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It reflects that the current flowing from one active node to the next is
exceeds the requirement to excite the next node. Myelinated axons have a large safety factor, ~5x, and the excitation of its node occurs in <20μs. |
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Three types of impairment caused by demyelination and a brief description of each
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• Frequency-related block:
Once the current reaches the demyelinated region, it dissipates such that future nodes cannot reach threshold • Total conduction block: The severity of the demyelination is such that an active node cannot depolarize the demyelinated region sufficiently • "Cross talk": In the demyelinated regions, a dissipated current may depolarize an adjacent hyperexcitable axon and generate impulses in both directions on that axon |
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The most common type of impairment caused by demyelination in diseases
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Total conduction block
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Multiple Sclerosis:
• is relatively rare outside the US except for those with these types of genes • Lesions in the spinal cord result in this group of sx • its effect on cranial nerves |
"Scandinavian genes"
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Guillain-Barr syndrome:
• mode of action • etiology • when it becomes life threatening |
Guillain-Barr syndrome:
• autoimmune d/o against Schwann cells • usually, onset is several wks following a pre-disposing respiratory or intestinal infection • is life threatening when the diaphragm and other respiratory muscles become paralyzed |
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Congential neuropathies:
• age of onset • state of neurons |
Congenital neuropathies:
• infancy - 2yrs • loss of of myelination or dysmyelination of neurons |
