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13.1 The Nervous System -- Part 2
When the axon isn't conducting an electrical pulse, the axon is only slightly permeable to sodium ion.
The large organic ion is not able to pass through the impermeable membrane.
The axon can leak out of the sodium ion.
The large negative ion can't follow the leak out of the axon because they can't see it.
A negative potential is produced inside the axon with respect to the outside.
The negative potential of 70 mV holds back the outflow of potassium so that the concentration of ion is the same.
The pumping process transports sodium ion out of the cell and brings in an equal number of potassium ion.
The description of the axon is applicable to other types of cells as well.
Most cells have a negative potential with respect to their surroundings.
The neuron has a special ability to conduct electrical impulses.
In order to study the properties of nerve impulses, a probe is inserted into the axon and measured with respect to the surrounding fluid.
The nerve impulse is caused by something on the axon.
An injected chemical, mechanical pressure, or an applied voltage are some of the stimuli.
A nerve impulse can only be produced if the stimuli exceed a certain thresh old value.
An impulse is generated at the point of stimulation and travels down the axon.
Most neurons have scales of time and voltage.
The axon has an electrical response.
The potential decreases to about -90 mV and returns slowly back to the initial resting state.
In a few milliseconds, the pulse passes a point.
The type of axon affects the propagation speed.
The pulse can be sent at speeds up to 100 m/s.
The action potential is discussed in the following section.
The impulses produced by a given neuron are always the same size and travel down the axon.
The rate at which the nerve impulses are produced is determined by the intensity of the stimulation.
Some of the techniques of electrical engineering will be used in the analysis of the electrical properties of the axon.
The methods used in the other sections of the text are more complex.
Quantitative understanding of the nervous system requires added complexity.
There are differences between the axon and electrical cable.
It is possible to get some insight into the functioning of the axon by analyzing it as an insulated electric cable submerged in a conducting fluid.
The resistance of the fluids inside and outside the axon must be taken into account in the analysis.
The membrane is characterized by both resistance and capacitance.
Four electrical parameters are needed to specify the cable properties.
The resistance of the axon is distributed along the length of the cable.
It is not possible to represent the whole axon with only four components.
The axon is a series of small electrical-circuit sections.
The whole axon is made up of many different parts.
The sample values of the circuit parameters for both myelinated and nonmyelinated are listed in Table 13.1.
Table 13.1 quotes the values for a 1-m length of the axon.
A pulse along an axon travels at a speed that is less than 100 m/s, while an electrical signal travels at a speed that is more than three times the speed of light.
The axon was an electrical cable.
The propagation of an impulse along the axon is well understood after many years of research.
The potential inside the axon is driven to a positive value by the rush of sodium ion into the axon.
The initial rise of the action potential pulse is produced by this process.
The spike in one portion of the axon increases the permeability to the sodium immediately ahead of it, which in turn causes a spike in that region.
This is similar to how a flame is propagating down a fuse.
The axon renews itself.
At the peak of the action, the axon membrane closes its gates to sodium and opens them to potassium.
The axon potential drops to a negative value due to the rush out of the potassium ion.
A portion of the axon is ready to receive another pulse after a few milliseconds when the axon potential returns to its resting state.
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