Term
| List the different components of a nerve cell along with their functions |
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Definition
Cell body - contains the nucleus of the nerve cell
axon hilock - contains a large amount of voltage-gated Na+ channels so this is where the action potential can start
node of ranvier - a break in myelination where membrane resistance is low and action potentials can easily occur, important in propagation
myelin sheath - enables propagation to the next node of ranvier by preventing dissipation of the AP by stopping Na+ efflux
dendrite - receives the signal from a neighboring nerve cell |
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Term
| What are the factors that determine the rate of propagation of the action potential? |
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Definition
the space (length) constant
the time constant
myelinated vs unmyelinated
note that the space and time constants are not dependent on voltage but instead on physical properties of the membrane |
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Term
| What are the ionic principles behind action potential propagation? |
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Definition
the initial segment of the axon fires an action potential
at the active site positive charges flow from the initial segment to the inactive adjacent area and activate that area
the original active region repolarizes back to RMP
this repeats over and over down the nerve cell |
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Term
| What is time constant and how is action potential propagation velocity related? |
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Definition
time constant (tau) = RmCm
Rm = resistive properties of the membrane and is the inverse of the permeability
Cm = membrane capacitance that describes the ability of the membrane to store charge
The smaller the time constant, the higher the propagation velocity.
This is because the membrane has a greater ability to respond rapidly to stimulus currents.
Note the time constant refers to the time it takes for the potential change to reach 63% of its final value |
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Term
| What is the space (length) constant and how does it relate to the action potential propagation velocity? |
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Definition
The length constant (gamma) is defined as the square root of ((Rm*d)/Ri*4)).
Rm = membrane resistance
d = diameter of the axon
Ri = longitudinal resistance
The greater the length constant, the greater the propagation velocity
Note the length constant is the distance it takes for the depolarizing displacement to decay by 63% of its initial value. |
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Term
| How is conduction velocity related to axonal properties on unmyelinated nerve fibers? |
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Definition
Conduction velocity is proportional to the diameter of the axon.
The larger the diameter of the axon, the greater the speed of propagation.
Remember length constant is proportional to the diameter and a higher length constant means a higher conduction velocity. |
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Term
| How is conduction velocity related to axonal properties on myelinated nerve fibers? |
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Definition
The velocity of propagation is determined by the distance between the nodes of Ranvier.
The greater the distance between the nodes of Ranvier, the greater the velocity of action potential propagation. |
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Term
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Definition
Myelin is a lipid insulator of nerve fibers that greatly increases membrane resistance.
Myelinating a nerve fiber causes the depolarizing current to flow down the interior of the nerve where resistance is low. |
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Term
| What is the importance of the nodes of ranvier? |
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Definition
At intervals of 1 to 2 mm there are breaks in the myelin sheath called nodes of ranvier.
at the node of ranvier the change from myelinated to unmyelinated allows the depolarizing current to flow across from a high resistance area to a low resistance area to propagate the AP.
without the breaks in myelination the action potential would simply dissipate (remember the length constant) because there wouldn't be any changes in resistance. |
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Term
| What is saltatory conduction? |
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Definition
This is where action potentials "jump" long distances between one node of ranvier to the next.
This is possible because at the nodes, membrane resistance is low so current can flow across the membrane and cause an action potential. |
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Term
| What axonal characteristics will give cause a faster conduction velocity? |
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Definition
Increased Rm - this encourages to current to flow internally
Increased diameter - this decreases the Ri
Decreased Ri - less internal resistance means more internal travel
Decreased Cm - the shorter the membrane stores the charge, the faster the current can travel
myelination - saltatory conduction is faster than normal conduction
This is because of the principle of traveling along the path of least resistance.
If the Rm is high and the Ri is low then the current will want to flow in the cytoplasm |
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