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Excitable Cells - Lecture 7 (20/02/2018)
An action potential at one part of a neuron can spread along the neuron as a wave of excitation.
It cannot 'double back' on itself due to the refractory period of the previously excited part of the neuron.
Transmission can be broken down into two main components: electrical and chemical.
Electrical transmission is largely kept within neurons, but communication between neurons is usually chemical.
Attenuation = The reduction of the electrical energy.
The dendrites lose current as they are not as well insulated as neurones, so some charge is lost.
In Dendrites the passage of the current becomes attenuated.
In Neurones the passage of the current doesn't become attenuated.
Similarly attenuation is very important in electrical wiring so signals don't get diminished.
V =V 0 exp
V = Voltage.
V0 = Initial Voltage.
X = Distance
Λ = Length Constant.
The Length constant (λ)) is the distance over which the voltage drops to 37% of its initial value.
The Solution to Decreasing Attenuation
In order to get efficient transmission along a cable, it is desirable to get cables with big length constants.
The Length constant is dependent upon the Leakiness (Rm), Conductivity (Ri) and Diameter of the cable.
The options to get better transmissions are:
Increase the Rm through better insulation.
Decrease the Ri by using better conducting cores.
Increase d by using thicker cables.
Attenuation in Dendrites vs Axons
Attenuation is not a big problem in Dendrites as short distances are involved and there are many inputs, with a large starting signal.
Dendrites can generate action potentials; however it is usually passive and does not involve a wave of action potentials.
If axons behaved like Dendrites, this would present a great problem as signals would not be able to pass through the body effectively.
In addition, axons capable of 1 m passive transmission would need to have diameter of 1 cm, and this would cause a massive problem with space, especially within the brain.
The vertebrate nervous system has to transmit signals very quickly, over quite large distances.
Properties of Axons
Axons have a far higher density of sodium channels than
This is the key to non-attenuated transmission: an AP
This picture shows the distribution of sodium channels, which plays a key role in generating the action potential.
The axon input into the Dendrite has a particularly high concentration of channels.
To get fast transmission along the axon a big length constant (Λ) is required. Λ) is required. ) is required.
So in order to achieve a larger length constant axons increase their Rm by improving their insulation.
They don't decrease their Ri, and increasing the diameter is not preferable.
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