Term
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Definition
| Ion channels which are always open; responsible for the resting membrane potential |
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Term
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Definition
| Channels which open/close in response to a stimulus. Three types -> voltage-gated, ligand-gated and mechanically gated. |
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Term
| Define voltage-gated channel |
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Definition
| Channels that open/close in response to a change in membrane potential |
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Term
| Define ligand-gated channels |
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Definition
| Ion channels that open/close in response to a chemical messenger binding to the channel. |
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Term
| Define mechanically gated channels |
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Definition
| Open/close in response to a mechanical force on the membrane. |
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Term
| Define resting membrane potential |
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Definition
| The voltage that exists across a membrane when the cell is not transmitting electrical signals; polarity is such that the inside of the cell is negative in respect to the outside. |
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Term
| How is resting membrane potential established by passive processes? |
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Definition
| Na+ ions are more highly concentrated outside of the cell than inside the cell, causing a chemical driving direction of outside the cell for Na+. K+ are more concentrated inside the cell than outside so the chemical driving direction is outside the cell. So Na+ in, K+ out. |
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Term
| How is resting membrane potential maintained by active processes? |
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Definition
| The cell membrane has both K+ and Na+ channels, although more K+ channels than Na+ channels. This makes the membrane more permeable to K+ than Na+. Both ions move in the direction of their chemical driving forces due to these channels: K+ out and Na+ in, however more K+ out than Na+ in making a negative charge in relative to the outside charge. As membrane potential gets more negative, outward K+ slows down while inward Na+ speeds up. Eventually, flows of Na+ and K+ are equal and opposite so that the membrane potential stays steady at -70mv. |
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Term
| Explain what it means that at resting membrane potential, the cell is polarized. |
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Definition
| It means that the -70mv charge in the cell is relative to the outside of the cell. The inside is negative and the outside is positive = polar (1 +, 1 -). |
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Term
| Define graded depolarization |
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Definition
| Stimulation of a neuron causes sodium gates to open and the membrane becomes partially depolarized as sodium ions enter the neuron making it less -, or +. This type of depolarization is called "graded" because the amount of depolarization depends on the strength of the stimulus. |
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Term
| Where does graded depolarization occur? |
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Definition
| Along the cell membrane. When a change in potential occurs across a cell membrane, it moves from the site of stimulation across the membrane, causing voltage changes in these areas. The goal is to create an action potential. |
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Term
| Define summation of graded potentials |
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Definition
| Due to the fact that a single graded potential is not of sufficient strength to elicit an action potential, the graded potentials can be added/summed up together to create a huge graded potential, strong enough to elicit an action potential. |
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Term
| Define temporal summation. |
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Definition
| The addition of graded potentials generated at a particular site that occurs when stimulated at a high frequency. |
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Term
| Define spatial summation. |
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Definition
| The addition of graded potentials generated at different locations that occurs when they are stimulated more or less simultaneously. |
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Term
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Definition
| The critical value of the membrane potential at which the cell must be depolarized in order to trigger an action potential. |
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Term
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Definition
| A stimulus too weak to generate an action potential. |
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Term
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Definition
| A stimulus that exceeds the threshold to generate an action potential |
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Term
| Describe the sequence of events that occur during an action potential. |
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Definition
1) Sudden increase in permeability to Na+ causes the Na+ to come into the cell at a very fast and sudden rate. This influx of positive ions changes the membrane potential from -70mv to a positive or less negative potential (-70 -> +30). This rapid change in potential is depolarization.
2) An action potential occurs because the threshold has been crossed at a range of (-70 -> +30mv). Na+ permeability decreases rapidly and K+ increases rapidly, causing K+ to move out faster than Na+ is coming in. The potential is now decreasing and becoming negative again.
3) K+ permeability remains increased for a brief time after resting membrane potential has been achieved (-70mv), causing the membrane potential to be hyperpolarized (very negative). This is the refraction period. This goes on until resting membrane potential is achieved again at -70mv. |
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Term
| Absolute refractory period |
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Definition
| Period during and immediately following an action potential during which a 2nd action potential cannot be generated in response to a 2nd stimulus, regardless of the strength of that stimulus. |
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Term
| Define relative refractory period. |
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Definition
| Period immediately following the absolute refractory period during which it is possible to generate a 2nd action potential, but only w/ a stimulus stronger than that needed to reach threshold under resting conditions. |
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Term
| Explain the differences between graded and action potentials in origin of stimulation, types of channels, conduction, strength of response and duration. |
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Definition
Origin of stimulation:
Graded- Mainly in dendrites and cell bodies
Action- Axon
Types of channels involved in producing change in potential:
Graded- Ligand-gated and mechanically gated
Action- Voltage-gated
Conduction:
Graded- Localized, can't travel long distances and signal strength fades w/ distance
Action- Can travel long distances, the signal doesn't diminish in strength w/ distance.
Strength of response:
Graded- Relatively weak, proportional to strength of stimulus
Action- All or none, typically 100mv.
Duration:
Graded- Typically longer
Action- Typically shorter |
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Term
| What are the factors that influence conduction velocity (speed of action potential change). |
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Definition
-Larger diameter of axon = action potentials are propagated faster from axon hillock to axon terminal.
-Myelinated axons have faster conduction rates than unmyelinted axon b/c action potentials traveling on myelinated axons have fewer potentials to change and takes less time to reach destination. Unmyelinated axons have more action potentials to change and reach the destination slower. |
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