permeability of its membrane and the presence of electrically charged particles called ions
near the inside and outside surfaces of the membrane in different concentrations. When
sufficiently stimulated (to threshold), a net flow of sodium ions into the cell causes a rapid
change in potential across the membrane, known as the action potential(see Figure 7.4).
If stimulation is not strong enough, your neuron doesn’t fire. The strength of the action
potential is constant whenever it occurs. This is the “all-or-none principle.”
The wave of depolarization and repolarization is passed along the axon to the terminal
buttons, which release neurotransmitters. Spaces between segments of myelin are called
nodes of Ranvier. When the axon is myelinated, conduction speed is increased since depo-
larizations jump from node to node. This is called saltatory conduction. Chemical neuro-
transmitters are released into the synapsewhere they attach to specific receptor sites on
membranes of dendrites of your postsynaptic neurons, like a key fitting into the tumbler of
a lock (the lock and key concept). Some of your synapses are excitatory, the neurotrans-
mitters cause the neuron on the other side of the synapse to generate an action potential (to
fire); other synapses are inhibitory, reducing or preventing neural impulses. The sum of all
excitatory and inhibitory inputs determines whether your next neuron will fire and at what
rate. The constant flow of these neurochemical impulses gives your behavior its amazing
complexity. It regulates your metabolism, temperature, and respiration. It also enables you
to learn, remember and decide.Reflex Action
The simplest form of your behavior, called a reflex, involves impulse conduction over a few
(perhaps three) neurons. The path is called a reflex arc. Sensory or afferent neuronstrans-
mit impulses from your sensory receptorsto the spinal cord or brain. Interneurons, located
entirely within your brain and spinal cord, intervene between sensory and motor neurons.
Motor or efferent neuronstransmit impulses from your sensory or interneurons to muscle
cells that contract or gland cells that secrete. Muscle and gland cells are called effectors.72 ❯ STEP 4. Review the Knowledge You Need to Score High
Figure 7.4 Action potential.