"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.
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,
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