Content Background

"You’re wired!" That's what the text tells your students, and they immediately imagine a "body electric." That's far from true, but it represents a persistent misconception and a good place to start to build more accurate ideas about the nervous system.

It's easy to stress the relationship between structure and function in a neuron. Most nerve cells have long processes, either axons or dendrites, to conduct messages. Potential energy is stored in the neuron as a difference in charges inside and outside its membrane (inside about -70 millivolts more negative than outside.) That charge difference (resting potential) is maintained by channels and enzyme "pumps" in the membrane itself. The sodium-potassium pump system moves more Na⁺ out of the cell than K⁺ in.

Students understand that turning the switch on a flashlight can start the use of the energy in a battery. Similarly, stimulating the plasma membrane of a neuron starts a process through which that cell's potential energy is used. Special voltage-gated channels are normally closed within the cell membrane. When the cell is stimulated, the Na⁺ channels open, allowing that ion to flood into the interior of the neuron. When enough positive ions pass through the channels, an action potential is reached (about +35 mV). Once the action potential is reached, the Na⁺ close and K⁺ channels open, letting potassium ions rush out. (These channels close slowly, so a graph of the potential across the cell membrane normally shows a small dip in potential below the -70 mV average.) The cell returns to a resting state.

The animation shows how this scenario plays out. You may want to check this one out as well.

The action potential is a localized event - it happens at a single point on the neuron. But its effect is much like tipping one domino in a row. As the Na⁺ ions enter the cell, they affect the membrane before and after them. This triggers the opening of Na⁺ channels and the action potential is propagated. Since the changes can't occur in sections of the cell where K⁺ ions are rushing out, the action potential moves in one direction.

An action potential (a nerve signal) is an "all-or-nothing" reaction. You can't have a partially lit fuse, or a partial action potential. The intensity of a nerve impulse depends upon the frequency of impulses, or the number of nerve cells that are stimulated.

Between two adjacent nerves the reaction is propagated by chemicals called neurotransmitters. These are important molecules which play a vital role in maintaining the body's balance or homeostasis. Most neurotransmitters are small molecules, like acetylcholine. Others in the same group (biogenic amines) include epinephrine (adrenaline), norepinephrine, serotonin, and dopamine.

Neurotransmitters have subtle yet vital functions in cells. Imbalances have been related to mental illnesses such as bipolar disease, Parkinson's disease, and chronic depression. The actions of many psychoactive drugs (like mescaline and LSD) appear to disturb the balance of neurotransmitters in the brain, mimicking the biology of chronic mental illness. Cocaine and amphetamines increase the effects of the neurotransmitter norepinephrine. Caffeine (in coffee and chocolate) appears to stimulate the body by decreasing the effectiveness of the chemicals that inhibit neurotransmitters.

As students study the structure of nerves, it's important to remind them of the body’s parallel message delivery scheme, the endocrine system. Hormones and nervous messages work in tandem to maintain homeostasis, and interact with dramatic effects - especially in the bodies of growing students.