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Q & A: Electrical signals in nerves

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Most recent answer: 10/22/2007
Q:
Does the electrical signals our brain send to our nerves have the same properties of electricity? Can the brain waves our brain send out be conducted using an electrical conductor like metals?
- Benjie
A:

Hi Benjie,

Well, yes and no. Nerves carry signals from one end to another using a complicated process involving the exchange of charged ions inside and outside of the cell membrane. The main ions involved are sodium, potassium, chloride, and calcium. The membranes are specially set up so that as a pulse arrives at a particular spot along an axon, the proper ions are invited into the axon from outside, through special ion channels, which can differentiate one kind of ion from another. The ion channels open and close, and some even have more complicated gates, forming "ion pumps".

A sample web site from an expert: .

On the ends of nerve cells, pulses are transmitted from one neuron to another by using special chemicals, called "neurotransmitters". When a pulse reaches the end of a neuron, the end releases these chemicals, which are then absorbed by receptor sites on neighboring neuron ends. A single neuron may propagate its impulse to many, many other neurons in this way. The neurotransmitters are quickly reabsorbed so that the pulse does not persist and so the junction can get ready for the next pulse.

Electrical conduction along a wire involves electrons flowing through a material which offers little resistance to them, like copper or aluminum. The speed of a pulse down a wire depends on the resistance, capacitance, and inductance of the wire per unit length. To carry the pulse from one conductor to another means electrically connecting them, but care has to be taken. If the resistance, capacitance, and inductances of the conductors don’t match properly, the pulse may be reflected backwards from the junction! And if the pulse is shared out to too many places, it must be re-amplified with an amplifier. The ion channels in nerves get their energy from cellular metabolism. The signals in wires typically travel at a large fraction of the speed of light, while the slow ion transport in nerves means the signals in nerves travel much much slower.

Signals on neurons may be measured with electronic equipment. One merely has to arrange conductors in contact with the end of a neuron, measuring the voltage difference inside and outside the neuron’s membrane. It may then need to be amplified before being sent elsewhere, depending on what needs the information. Neurons may also be stimulated with electrical signals.

Very sensitive measuring devices which can pick up tiny magnetic fields can detect neural activity without needing to poke electrodes directly into neurons, although one is never quite sure what one is measuring with this, as billions of neurons contribute to any signal measured in this way. To try to get a picture of which part of the brain is active, arrays of several hundred magnet sensors are used.

Tom


(published on 10/22/2007)

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