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13.1 The Nervous System -- Part 3
The ion densities in the axon are not changed because the number of ion flow in and out of the axon is small.
The ion concentrations are kept at the appropriate levels by the metabolic pumps.
The Eq is being used.
We can estimate the number of sodium ion that enter the axon during the rising phase of the action potential.
The amount of electrical charge inside the axon is affected by the initial inrush of sodium ion.
The axon voltage in the resting state is -70 mV.
A net voltage change across the 100 mV is caused by the change in the voltage during the pulse.
It is 100 mV.
There are 87 x 1011 sodium ion entering per meter of axon length.
The number of potassium ion leaves is the same.
The resting state of the axon has 7 x 1014 and 7 x 1015, which is 7 x 1014 and 7 x 1015, respectively.
The inflow and outflow of ion are small compared to the equilibrium density.
Another simple calculation.
B.6 can be used to estimate the minimum energy required for the impulse to travel along the axon.
5 x 10 W/m to replenish it's capacitance.
The mechanism can be incorporated into the circuit by connecting small signal generators.
The analysis of a complex circuit is outside the scope of this text.
We are going to simplify this circuit by ignoring the axon membrane.
The representation is valid if the capacitors are fully charged.
When a steady voltage is applied to one end, we will be able to calculate the voltage attenuation along the cable.
Predicting the time-dependence of the axon is not possible with the simplified model.
The resistivities inside and outside the axon are the same.
Now going back to Eq.
We can apply it.
At a distance of 0.8mm from the point of application, the voltage decreases to 37% of its value.
Myelinated axons have a smaller conductance because of their outer sheath.
The result helps to explain how the myelinated axons work.
The sheath is in 2-mm segments.
The action potential is only generated between segments.
The pulse travels through the myelinated segments.
The propagation of an electrical impulse down the axon has been considered so far.
The pulse is transmitted from the axon to other cells.
The axon branches into nerve endings at the far end.
The axon sends signals through the nerve endings.
The action potential can be transmitted from the nerve endings to the cells.
The signal is usually transmitted by a chemical substance.
The nerve endings are not in contact with the cells.
There is a small gap between the nerve ending and the cell body.
A chemical substance is released at the nerve end which diffuses across the gap and stimulates the adjacent cell.
The chemical is released into the air.
A neuron is in contact with many sources.
The action potential in the target cell can often be started by a number of synapses being activated at the same time.
The neuron can either produce an action potential of the standard size or not.
The chemicals that are released at the synechia are not stimulating the cell but are preventing it from responding to impulses coming from a different channel.
These types of interactions allow decisions to be made on a cellular level.
The details of these processes are not fully understood.
The same way as neurons, muscle fibers produce electrical impulses.
The impulses coming from motor neurons initiate the action potential in the muscle fiber.
This stimulation causes a reduction of the potential across the fiber.
The shape of the action potential is the same as in the neuron, but its duration is usually longer.
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