Physiology of the Neuron
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| What permits outward diffusion of K+ from the neuron? | Continuously open "leaky" K+ channels located in the lipid bilayer of the surface membrane of the neuron
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| How does K+ diffuse out of the neuronal cell? | down a concentration gradient (K is highly concentrated inside the cell)
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| Describe the role of K+ diffusion on membrane charge. | Negatively charged proteins are left behind when K+ diffuses out of the cell. The outside of the cell, thus, becomes lined with + charged potassium ions and the inside of the cell membrane becomes lined with - charged proteins
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| Why are all other ions impermeable at this time? | Because Na+ and Ca++ channels are closed in the resting neuron
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| The resting membrane potential in excitable tissues (neurons, skeletal muscle cells, smooth muscle cells, and cardiac muscle cells) is determined mostly by ___. | K+
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| Describe the effect of hyperkalemia on resting membrane potential. | With acute hyperkalemia, the diffusion gradient for K is reduced. Because the rate of diffusion out of the cell is decreased, the RMP is diminishes; the cell depolarizes.
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| Describe the effect of hypokalemia on resting membrane potential. | With hypokalemia, the diffusion gradient for K+ is enhanced. Because the rate of diffusion out of the cell is increased, the RMP is increased; the cell hyperpolarizes.
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| Cells depolarize with acute _____ and hyperpolarize with acute _____. | depolarize- hyperkalemia; hyperpolarize- hypokalemia
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| What ion is responsible for depolarization of the neuron in an action potential? | Na+
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| Where and how does Na+ move in the neuron? | Na+ moves into the cell via fast voltage gated sodium channels
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| Where are voltage gated sodium channels found? | principally in the axon and are concentrated in the nodes of Ranvier
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| How is Na+ that entered the cell during depolarization removed from the cell when the neuron is at rest? | Na/K pump
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| When fast voltage gated Na channels are in the inactivated state, what cannot happen? | another action potential cannot be fired, now matter how intese the stimulus
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| What is the absolute refractory period? | the period corresponding to the time when the gated sodium channel is in the inactivated state.
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| What are three important clinical examples related to the inactivated state concept? 1 | 1. the high K concentration in cardioplegia solutions causes membrane depolarization, which locks the sodium channels in the inactive state so the heart electrically arrests.
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| What are three important clinical examples related to the inactivated state concept? 2 | Depolarization of the skeletal muscle motor end-plate by succinylcholine causes the gated sodium channels to become inactivated, thereby electrically arresting skeletal muscle.
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| What are three important clinical examples related to the inactivated state concept? 3 | Local Anesthetics interrupt nerve conduction by locking the sodium channel in the inactivated state
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| Describe the 4 steps of a Nerve action potential. | 1. The cell body of the axon depolarizes 2. The depolarization spreads to the axon hillock 3. At the threshold of the axon hillock, voltage gated Na channels open and the AP is initiated 4. The AP travels uninterrupted along the axon to the nerve terminal
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| What causes depolarization of the axon? | a chemical, the neurotransmitter on the surface of the cell body
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| The axon hillock is the ___ segment of the axon. | initial
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| Describe threshold. | the critical level of depolarization
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| The neuronal action potential is a ___-__-__ event. | all-or-none
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| Describe ho the neuronal action potential is self propagating. | The action potential causes the segment of nerve just ahead of itself to depolarize to threshold; it does not stop until it reaches the nerve terminal
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| The action potential is like a spark traveling along a fuse; the spark triggers the release of what? | neurotransmitters
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| How long does the nerve action potential last? | 1 ms (one thousandth of a second)
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| What is most responsible for resting membrane potential? | potassium efflux through leak channels
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| What is responsible for depolarization of the axon? | diffusion of sodium ions into the cell
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| What is responsible for repolarization of the axon? | diffusion of potassium ions out of the cell
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| What happens when the neuron is in the absolute refractory period? | the sodium channel is the inactivated state and another action potential cannot be fired
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