Synapses and Communication Between Neurons

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Cells of the Nervous System
FIGURE 2 .3
Communication Between Neurons
When stimulation of a neuron reaches a
certain level, the neuron fires, sending an
action potential shooting to the end of
its axon and triggering the release of a
neurotransmitter into the synapse. This
process stimulates neighboring neurons
and may cause them to fire their own
action potentials.
Cell
body
1. An action potential
shoots down the axon,
away from the cell body.
Axon
1
Neurotransmitters
2. A neurotransmitter is
released into the synapse,
where the dendrites of
neighboring neurons
detect it. See enlarged
area.
Synapse
2
3. If there is a receptor for
this neurotransmitter
on the dendrites, the
neurotransmitter and
receptor bind, creating
an electrochemical signal.
3
Dendrite
Receptors for
neurotransmitters
4
4. If that signal is strong
enough, it spreads down
the dendrites and across
the cell body of the next
neuron, and begins
another action potential.
Cell body
affair: The cell either fires its action potential at full strength or it does not fire at all.
Once a cell has fired, a very short recovery time called the refractory period follows,
during which the cell cannot fire again. Even so, neurons are able to fire as often as
1,000 times per second. The speed of an action potential ranges from about 5 to about
260 miles per hour and depends on the thickness or diameter of the axon—larger ones
are faster—and on the presence of myelin (pronounced “MY-a-lin”). Myelin is a fatty
substance that wraps around some axons like a stocking and speeds up action potentials. When a neuron fires, dendrites in the next cell detect the message and send the
signal to their cell body.
Synapses and Communication Between Neurons
refractory period A short recovery
time after cell firing, during which the
cell cannot fire again.
neurotransmitter A chemical that
transfers messages across synapses.
synapse The tiny gap between the
axon of one neuron and the dendrites
of another.
How do the dendrites detect a signal from another neuron? As shown in Figure 2.3, it
works a little like the game of tag you played as a child. In this neural communication
tag game, however, one neuron “sends” a tag without actually touching the next neuron.
When an action potential reaches the ends of an axon’s branches, it stimulates the release
of a chemical that is stored there in little “bags,” called vesicles (pronounced “VESS-ickels”). This chemical is called a neurotransmitter because it acts as a kind of messenger between neurons. Neurotransmitters flow across a tiny gap, less than a millionth of
an inch wide, which separates the axon of one neuron and the dendrites of another. This
is the synaptic gap, often referred to simply as the synapse (see Figure 2.4) When they
reach the dendrite of the next cell, neurotransmitters chemically fit, or bind, to proteins
52
Chapter 2 Biology and Behavior
FIGURE
A Synapse
2.4
This photograph taken with an electron
microscope shows part of a synapse
between neurons, magnified 50,000 times.
The end of one neuron’s axon is shaded
green; the green ovals are mitochondria.
The red spots are neurotransmittercontaining vesicles. The synapse itself
appears as the narrow gap between the
first cell’s axon and the dendrite of the
second cell, which is shaded blue.
Removed due to copyright
permissions restrictions.
in review
called receptors. Like a key fitting into the right lock, a neurotransmitter snugly binds
to its own receptors, but not to receptors for other neurotransmitters. The receptors
“recognize” only one type of neurotransmitter. In the dendrite, this binding creates an
electrochemical signal that is called a postsynaptic potential because it occurs after the
neurotransmitter has crossed the synapse. The postsynaptic potential, in turn, passes the
message to the cell body for the signaling process to continue.
Generally, more than one message must go to a cell to make it fire. Signals from
groups of cells often arrive at the same postsynaptic cell at about the same time.
The messages from these many cells may conflict with one another. Some messages
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NEURONS, NEUROTR ANSMIT TERS, AND RECEPTORS
Part
Function
Type of Signal Carried
Axon
Carries signals away from the
cell body
The action potential, an all-ornothing electrochemical signal
that shoots down the axon to
vesicles at the tip of the axon,
releasing neurotransmitters
Dendrite
Detects and carries signals to
the cell body
The postsynaptic potential, an
electrochemical signal
moving toward the cell body
Synapse
Provides an area for the
transfer of signals between
neurons, usually between
axon and dendrite
Chemicals that cross the
synapse and reach receptors
on another cell
Neurotransmitter
A chemical released by one
cell that binds to the
receptors on another cell
A chemical message telling
the next cell to fire or not to
fire its own action potential
Receptor
Proteins on the cell
membrane that receive
chemical signals
Recognizes certain
neurotransmitters, thus
allowing it to begin a
postsynaptic potential in the
dendrite
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Tutorial: Action Potential
?
1. For one neuron to communicate with another, a
between them.
,
2. The nervous system’s main functions are to
information.
3. The two main types of cells in the nervous system are
has to cross the
, and
and
.