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Learning About the Impulse "Messages"

Neurons respond to stimulation in various ways that communicate to other neurons what the stimulus was. By putting metal wires next to a neuron, and viewing the electrical pulses on an oscilloscope or computer screen, we can see that nerve cell impulses are generated whenever the cell is stimulated.   Examples:

If you apply a stimulus, such as touching the skin, at the same time you can record electrical activity from the nerve that supplies that part of the skin. You would see that the stimulus appears a few thousandths of a sec (millisecond) before impulses appear.

When the touch is applied and held on the skin, the neuron fires a few impulses and then quits, even though the stimulus is still there. 

Other sensory neurons respond in other ways, depending on the kind of stimulus they can detect.

A similar approach can be used for any stimulus, such as flashing visual patterns on the eye and recording from electrodes placed at various points along the visual pathway that leads from the eye to the visual centers in the cerebral cortex.

How do you measure impulses? From a given nerve cell, they are all the same size, but big differences can be seen in:

  • how many occur in a given time
  • the intervals between spikes
  • the sequential ordering of intervals.

Learning About The Secretion of Chemicals

We can collect and analyze the chemicals that nerve cells secrete into the blood. Such chemicals are called hormones. But neurons also secrete chemicals directly on each other. The chemicals acts as messengers (neurotransmitters) to convey information from one neuron to others. The action of transmitters varies with their chemical nature, but in general they either:
  • excite
  • inhibit
  • modulate (create a bias for being excited or inhibited)

Neurotransmitters can be collected from the brain itself in experimental animals.  If different parts of the brain of a euthanized animal are examined, we see that the chemicals released vary with different parts of the brain. Over 100 chemical secretions have been found in nerve tissue. 

What these chemicals do can be tested by injecting them in the form of drugs or, in experimental animals, applying them directly on nerve cells by way of an implanted tube. One popular practice is to use several small glass microtubes bundled together. One is used as an electrode to record responses of nerve cells to chemicals that are injected by way of the other tubes.

These chemicals can be seen to do one of three things when applied to neurons:

  • Excite neurons (make them fire impulses)
  • Inhibit neurons (reduce impulse firing)
  • Change the sensitivity of neurons (make them more or less responsive to other chemicals or electrical input)

How Do We Know How the Brain is Wired?



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