Bodily Defenses Image Map

Generating Electricity

Neurons are like batteries. The battery is created because electrically charged atoms, especially sodium and potassium, have different concentrations inside and outside of the neuron. When the neuron battery is discharged or "shorted," the charged atoms move across the membrane, creating an electrical current. The current of each impulse spreads down the extensions of a neuron, like a burning fuse.  The neuronal battery can be discharged by electrical stimulation or chemical stimulation. 

LEFT: Animation showing how a pulse of electricity ("nerve impulse") spreads throughout a neuron and its processes. When a target is reached (either another neuron, a muscle, or a gland), the impulse may trigger the release of chemical messengers (purple) onto the target cell. A very clever animation of the details of how these impulses are generated can be found at another Web site; click here.

RIGHT: Ions, not electrons, are the carriers of current in brain. When a neuron is stimulated, molecular receptors in its membrane open. Sodium ions are the first to move. They move  in because their is an electrical pulling force (inside is negative and sodium ions are positive) and an osmotic force (sodium is more concentrated on the outside). But as sodium rushes in, the  inside becomes positive, and that forces out potassium. In the resting state, potassium is concentrated on the inside of cells. For video of the experiments that discovered these currents, click here.

Secreting Chemicals

When the electrical impulses reach the end of an axon, they trigger the release of neurotransmitter chemicals. These chemicals then move in the body fluids and come in contact with receptor molecules on the membranes of target cells (see "Proteins " and "Membranes" in the "Cells are Us" Module).

Target cells of neurons are glands, muscles, or other neurons. The message differs with the chemical and the target, but basically it is to promote activity in the target cell or suppress it. This is done in steps: 
  1. Neurotransmitter binds with a protein on the target cell membrane
  2. Binding activates a protein on the inside face of the membrane that serves to trigger actions on other cell chemicals.
  3. One of the activated chemicals may even migrate into the nucleus to change the expression of genes.

What The Spinal Cord Does

If you cut the spinal cord of any higher animal in cross section, it looks like this. There is a central "H" zone containing millions of neurons surrounded by white substance, which is actually many nerve fibers cut in cross section. Some fiber bundles go to the brain, while others are bringing information to the spinal cord from the brain. 



Pain information is carried in fiber bundles in  the lateral part of the cord. Common pain killer drugs act at different levels of the pathway.

Two-way Cabling

Like electrical wiring in your car or your house, neurons in the spinal cord communicate with the brain by insulated cables. The axons that descend from the brain or ascend to the brain are covered with an insulating coat of membrane. The membrane coating comes from nearby cells ("glial cells") that wrap their membranes around and around the axons. 

Knowing this, can you see why damage to the spinal cord causes paralysis? Can you see why that paralysis might be permanent?

Reflex Action




Diagram of a reflex circuit in the spinal cord (shown as a cross section, with the top of the drawing nearest the back). 

Pain information comes in by way of spinal nerves and excites neurons (open-circled cell bodies) to activate muscles that flex the hip and knee of the leg on the same side as the stimulus. At the same time, the painful stimulus excites an inhibitory neuron (closed-circle cell body) that prevents contraction of the muscles that extend the leg. Why is this necessary? What would happen if both flexor and extensor muscles of the same leg contracted at the same time?

At the same time, opposite effects occur on the other leg.  Can you explain why? Do you see how the wiring diagram assures that the opposite leg extends?

Note that all of this occurs locally. There is no need for the brain. Of course, the painful information is sent to the brain by way of some of the neuron cables that run from the spinal cord to the brain.


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