THE PRINCIPLES OF NERVE CELL
COMMUNICATION
The nerve cell, or neuron, is the key player in the
activity of the nervous system. It conveys information
both electrically and chemically. Within the
neuron itself, information is passed along through the
movement of an electrical charge (i.e., impulse). The neuron
has three main components: (1) the dendrites, thin
fibers that extend from the cell in branched tendrils to
receive information from other neurons; (2) the cell body,
which carries out most of the neuron’s basic cellular functioning;
and (3) the axon, a long, thin fiber that carries
nerve impulses to other neurons.
Nerve signals often travel over long distances in the
body. For example, if you step barefooted on a sharp object,
the sensory information is relayed from your foot all the way
to the brain; from there, nerve signals travel back to the leg
muscles and cause them to contract, drawing back the foot.
Dozens of neurons can be involved in such a circuit, necessitating
a sophisticated communication system to rapidly
convey signals between cells. Also, because individual neurons
can be up to 3 feet long, a rapid-relay mechanism within
the neurons themselves is required to transmit each signal
from the site where it is received to the site where it is
passed on to a neighboring cell. Two mechanisms have
evolved to transmit nerve signals. First, within cells, electrical
signals are conveyed along the cell membrane. Second,
for communication between cells, the electrical signals generally
are converted into chemical signals conveyed by small
messenger molecules called neurotransmitters.
SIGNAL TRANSMISSION WITHIN NERVE CELLS
The mechanism underlying signal transmission within
neurons is based on voltage differences (i.e., potentials)
that exist between the inside and the outside of the cell.
This membrane potential is created by the uneven distribution
of electrically charged particles, or ions, the most
important of which are sodium (Na+), potassium (K+),
chloride (Cl-), and calcium (Ca2+). Ions enter and exit the
cell through specific protein channels in the cell’s membrane.
The channels “open” or “close” in response to
neurotransmitters or to changes in the cell’s membrane
potential. The resulting redistribution of electric charge
may alter the voltage difference across the membrane. A
decrease in the voltage difference is called depolarization.
If depolarization exceeds a certain threshold, an impulse
(i.e., action potential) will travel along the neuron. Various
mechanisms ensure that the action potential propagates in
only one direction, toward the axon tip. The generation of
an action potential is sometimes referred to as “firing.”
Signal Transmission Between Cells
Communication among neurons typically occurs across
microscopic gaps called synaptic clefts. Each neuron may
communicate with hundreds of thousands of other neurons.
A neuron sending a signal (i.e., a presynaptic neuron) releases
a chemical called a neurotransmitter, which binds to
a receptor on the surface of the receiving (i.e., postsynaptic)
neuron. Neurotransmitters are released from presynaptic
terminals, which may branch to communicate with several
postsynaptic neurons. Dendrites are specialized to receive
neuronal signals, although receptors may be located elsewhere
on the cell. Approximately 100 different neurotransmitters
exist. Each neuron produces and releases only one
or a few types of neurotransmitters, but can carry receptors
on its surface for several types of neurotransmitters.
To cross the synaptic cleft, the cell’s electrical message
must be converted into a chemical one. This conversion
takes place when an action potential arrives at the axon tip,
resulting in depolarization. The depolarization causes Ca2+
to enter the cell. The increase in intracellular Ca2+ concen-
VOL. 21, NO. 2, 1997 107
NEUROTRANSMITTER REVIEW
Signal transmission across the synaptic cleft. The binding
of neurotransmitters (shown as triangles) to receptors that
act as ligand-gated ion channels causes these channels to
open, leading in some cases to a depolarization of the part
of the membrane closest to the channel. Depolarization
results in the opening of other ion channels, which in turn
may generate an action potential. Neurotransmitters
(shown as circles) that bind to second messenger-linked
receptors initiate a complex cascade of chemical events
that can produce changes in cell function. In this schematic,
the first component of such a signaling cascade is a G protein.
Postsynaptic
nerve cell
Presynaptic
nerve cell
G
protein
Receptor
Ligand-gated
ion channel
Receptor
binding
Cell
membrane
Neurotransmitter
release
Cell
membrane
Storage vesicle
108 ALCOHOL HEALTH & RESEARCH WORLD
tration triggers the release of neurotransmitter molecules
into the synaptic cleft.
Two large groups of receptors exist that elicit specific
responses in the receptor cell: Receptors that act as ligandgated
ion channels result in rapid but short-lived responses,
whereas receptors coupled to second-messenger systems
induce slower but more prolonged responses.
Ligand-Gated Channel Receptors. When a neurotransmitter
molecule binds to a receptor that acts as a ligand-gated
ion channel, a channel opens, allowing ions to flow across
the membrane (see figure). The flow of positively charged
ions into the cell depolarizes the portion of the membrane
nearest the channel. Because this situation is favorable to
the subsequent generation of an action potential, ligandgated
channel receptors that are permeable to positive ions
are called excitatory.
Other ligand-gated channels are permeable to negatively
charged ions. An increase of negative charge within the cell
makes it more difficult to excite the cell and induce an action
potential. Such channels accordingly are called inhibitory.
Second Messenger-Linked Receptors. Second messengers
(e.g., G proteins) are molecules that help relay signals from
the cell’s surface to its interior. Neurotransmitters that bind
to second messenger-linked receptors, such as dopamine,
initiate a complex cascade of chemical events that can
either excite or inhibit further electrical signals (see figure).
The neurotransmitters also may attach to receptors on
the transmitting cell’s own presynaptic sites, beginning a
feedback process that can affect future communication
through that synaptic cleft.
With so many different receptors on its cell surface, some
of the signals the neuron receives will have excitatory effects,
whereas others will be inhibitory. In addition, some of
the signals (e.g., those transmitted through ligand-gated
channels) will induce fast responses, whereas others (e.g.,
those transmitted through second messenger-linked proteins)
will trigger slow responses. The integration by the neuron of
these often conflicting signals determines whether the neuron
will generate an action potential, release neurotransmitters,
and thereby exert an influence on other neurons.
NEUROTRANSMITTERS AND ALCOHOL
Among the neurotransmitters of most interest to alcohol
researchers are dopamine, serotonin, glutamate, gammaaminobutyric
acid (GABA), opioid peptides, and adenosine,
all of which are featured in this special section. These
molecules generally fall into three categories: (1) excitatory
neurotransmitters (e.g., glutamate), which activate the
postsynaptic cell; (2) inhibitory neurotransmitters (e.g.,
GABA), which depress the activity of the postsynaptic cell;
and (3) neuromodulators (e.g., adenosine), which modify
the postsynaptic cell’s response to other neurotransmitters.
Neurons that release these substances form the basis of
neural circuits that link different areas of the brain in a
complex network of pathways and feedback loops. The
integrated activity of these circuits regulates mood, activity,
and the behaviors that may underlie disorders such as
alcoholism.
( source internate )
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