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| What are the factors that determine the rate of propagation of the action potential? |
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Definition
Passive properties of axonal membranes
- Space (length) constant
- Time constant
The space and time constants are not directly dependent on metabolism or any voltage-dependent permeability changes. They are reflections of the physical properties of the neuronal membrane. |
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Term
| Mechanism of conductance down an axon is called? |
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Definition
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Term
Tm= 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 greater the ability of a membrane to respond rapidly to stimulus currents and the greater the propagation velocity. |
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Definition
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Term
| True or False: The greater the length constant , the greater will be the propagation velocity. |
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Definition
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Term
Conduction velocity is proportional to the diameter of the axon.
The larger the diameter of the axon, the greater the speed of propagation. |
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Definition
| Unmyelinated Nerve Fibers |
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Term
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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Definition
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Term
| The ??? is the distance it takes for the depolarizing displacement to decay by 63% of its initial value. |
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Definition
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Term
| The ??? refers to the time it takes for the potential change to reach 63% of its final value. |
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Definition
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Term
Compare the conduction velocity of an action potential propagated down a 1 μm-diameter unmyelinated axon to the propagation velocity of a 10 μm-diameter myelinated axon. Include in your discussion the effects that length constant, fiber diameter, and myelination have on conduction velocity.
Myelination will dramatically increase conduction velocity (increases Rm; decrease in membrane capacitance, capacitance is charge separation)
Length constant: Increase in diameter will increase LC and also increases conduction velocity. Myelin will increase the resistance of the axonal membrane, Rm. This will increase LC and this will increase conduction velocity.
Therefore, a smaller, unmyelinated axon will be slower than a larger, myelinated axon
In the body, smallest myelinated axon has a larger diameter than the biggest unmyelinated axon, and therefore will be much faster by combined effects. |
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Definition
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Discuss the ways in which the passive properties of axonal membranes of unmyelinated nerve fibers affect propagation velocity. Include in your discussion how the space or length constant, the time constant, and fiber diameter influence conduction velocity
The space constant increases with an increase in axon diameter and time constant decreases. If you increase the diameter, then there is a decrease in internal resistance and therefore propagation velocity is increasesd.
The longer the length constant, the stronger the potential along the length of the axon (the further potential can travel). Ideal situation is a high Rm and low RI to make propagation faster.
Length constant is inversely related to internal resistance. If you increase the LC by 2, then you will increase the diameter by 4.
An increase in the time constant (Resistance x Capacitance) decreases propagation velocity. Time constant is the time it takes to store charge. Ideal situation is a short time constant, but long length constant.
Propagation velocity is directly related to Length constant, but inversely related to time constant. |
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Definition
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Term
The major player in conducting nerve impulses is the plasma membrane, which depolarizes under sufficient stimulus.
The dendrites receive signals; axons propagate them to other cells or tissues. Myelin insulates axons to achieve higher efficiency and velocity.
The bouton releases NT into the synapse.
Propagation through unmyelinated nerves is called ELECTROTONIC CONDUCTION |
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Definition
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Term
| Saltatory conduction is conduction of an action potential through depolarization at nodes of Ranvier, which is between sheets of myelin on an axon. Myelination helps saltatory conduction by increasing the resistance of the axon membrane. This prevents sodium from leaving the cell and thus allows the action potential to propagate to the next node. Mammals benefit from this type of conduction because action potentials propagate faster. Squid axons have a diameter of 1000, therefore we cannot accommodate such size in our body. Instead, we have myelination so that we do not have to accommodate such large fibers to get a faster conduction. |
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Definition
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The speed at which APs are carried along a nerve or muscle fiber is called the ???
Note: This property is of great physiologic importance because it determines the speed at which information can be transmitted in the nervous system. |
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Definition
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Term
The amount of time it takes folliwing the injection of current for the potential to change to 63% of its final value. In other words, indicates how quickly a cell membrane depolarizes in response to an inward current or how quickly it hyperpolarizes in response to an outward current.
Note: The smaller this property, the greater will
be the propagation velocity. |
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Definition
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Term
Inversely related to the permeability. When this property is high, it means low permeability decreasing the ability of the membrane to allow Na
influx to occur as rapidly, effectively increasing the time constant. |
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Definition
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Term
| Describes the ability of the membrane to store charge to make the membrane potential change necessary. The larger this property, the greater the amount of charge that must flow to depolarize the membrane, the longer the time constant will be. |
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Definition
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Term
| True or False: The time constant is greater (i.e., takes longest) when Rm and Cm are high. |
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Definition
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Term
| The distance it takes for the depolarization to decay by 63% or decays to 37% its initial value. The greater the space constant, the greater will be the propagation velocity. |
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Definition
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Term
Length constant = Square root(Rm/Ri)
Indicates how far a depolarizing current will spread along a nerve. In other words, the longer the length constant, the farther the current spreads down the nerve fiber. |
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Definition
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| Rm represents the membrane resistance. Internal resistance, Ri, is inversely related to the ease of current flow in the cytoplasm of the nerve fiber. Therefore, the length constant will be greatest (i.e., current will travel the farthest) when the diameter of the nerve is large, when membrane resistance is high, and when internal resistance is low. Current flows along the path of least resistance. |
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Definition
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Term
| True or False: The propagation velocity is directly proportional to the space constant and inversely proportional to the time constant. |
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Definition
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Term
| There are two mechanisms that increase conduction velocity along a nerve: increasing the size of the nerve fiber and myelinating the nerve fiber. |
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Definition
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Term
| Lipid insulator of nerve fibers that greatly increase membrane resistance. |
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Definition
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Term
| At the nodes of ranvier, current can flow across the membrane, and action potentials can occur. Thus, conduction of action potentials is faster in myelinated nerves than in unmyelinated nerves because action potentials "jump" long distances from one node to the next, a process called ??? |
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Definition
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Term
1. Depolarize each adjacent region in axon for action potential to be propagated.
2. Allows propagation of action potential without decrement the entire length of the fiber.
Conduction in unmyelinated nerve fibers or neurons is called ??? |
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Definition
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Term
| Conduction in myelinated nerve fibers or neurons |
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Definition
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Term
Axon is wrapped in sheaths of ??? to form the myelin, which insulate the lipid membrane and prevent the passage of ions through the axonal membrane, effectively increasing Rm, membrane
resistance. |
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Definition
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Term
| This is decreased by the coating the membrane with the thick insulating sheath, myelin. |
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Definition
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Term
| Extremely high concentration at the nodes and few in the region between the nodes. The myelin effectively reduces the Cm and increases Rm to decrease time constant and increase length constant if one was to compare an unmyelinated axon and myelinated axon of the same diameter. |
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Definition
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Term
| True or False: A myelinated axon has a greater conduction velocity than an unmyelinated axon fiber that is 100 times larger in diameter. |
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Definition
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