13.1 The Nervous System

The major tools for the study of life processes have been provided by this technology.

Many life processes involve electrical phenomena.
The nervous system of animals and the control of muscle movement are governed by electrical interactions.
In Chapter 14 we will discuss the applications of electrical technology in biology and medicine, and in this chapter we will describe some of the electrical phenomena in living organisms.
There is a brief review of electricity in Appendix B.

Animals have the most remarkable use of electrical phenomena in their nervous system.

The brain is located in the center of the network and has the ability to store and analyze information.

The nervous system is in charge of various parts of the body.

The human nervous system is very complex.

It is not known how information is stored and processed by the nervous system.

Some aspects of the nervous system are well known.

The method of signal propagation through the nervous system has been firmly established over the past 40 years.
The messages are sent by the brain.
When a neuron is stimulated, it produces electrical impulses that travel along its cablelike structure.

The pulse is constant in magnitude and duration.

The number of pulse produced is a good indicator of the strength of theStimulus.
When the pulse reaches the end of the cable, other cells are activated.

The internal and external environment of the body are monitored by the sensory organs.

Depending on the function of the sensory neurons, they convey messages about factors such as heat, light, pressure, muscle tension, and odor to higher centers in the nervous system.
The motor neurons communicate with the muscle cells.

The central nervous system located in the brain provides information to these messages.

The interneurons communicate.

Nerve endings at the far end of the axon transmit signals to other cells.

Nerve impulses from a muscle travel to the spine.
The signal is sent to a motor neuron, which sends impulses to control the muscle.
Simple circuits are associated with actions.

The axon conducts electrical impulses away from the cell body.

The axons connecting the spine with the fingers and toes are more than a meter in length.
The myelin sheath increases the speed of pulse propagation.

Many axons share a common path within the body.

These axons are usually grouped together.

The special elec trical characteristics of the axon allow the neuron to transmit messages.

The data about the electrical and chemical properties of the axon can be obtained by injecting small needlelike probes into the axon.
It is possible to measure currents flowing in the axon with probes.
The diameter of most axons is very small.
Experiments with the squid axon yielded a lot of information about signal transmission in the nervous system.

Salt and other substances are positive and negative in the body.

Body fluids are good conductors of electricity.
The fluids' resistivity is 100 million times greater than that of copper.

The inside of the axon has an ionic fluid that is separated from the surrounding body fluid.

The axon membrane is not a perfect electrical insulator.

The electrical resistivities of the internal and external fluids are the same.

Its ionic solutes are mostly positive.

The axon has positive and negative ion concentrations.

There is a large concentration of sodium ion outside the axon and a large concentration of potassium ion inside the axon.

The answer is in the properties of the axon.

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.

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.

Some aspects of muscle contraction were discussed in Chapter 5.

The process is not fully understood.

This information is relayed to the brain.

The movement of muscles is under control.

An electric current can be applied to muscle fibers.

The frog's leg twitched when an electric current passed through it, which was first observed in 1780.

External muscle stimulation can be used to maintain muscle tone in cases of temporary muscle paralysis.

The voltages and currents associated with the electrical activities in the cells extend to other parts of the body.

The action potential along the axon can be considered as an example.
There is a voltage difference between this point and the adjacent regions when the axon drops suddenly.
The outer surface of the axon develops a voltage drop.

Experiments can be performed on a whole nerve.

Currents can be produced inside and outside the axon.

The surface potentials produced by the individual axons are the sum of the measured voltage.

The electric fields in the cells extend all the way to the animal's body.

Along the surface of the skin, we can measure electric potentials representing the collective cell activities associated with certain processes in the body.
Chapter 14 will discuss the measurement of these surface signals.

There are many activities that are associated with surface signals.