Summary CNS: Nervous System Control and Communication
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Information is transmitted in the central nervous system mainly in the form of nerve action potentials,
called simply "nerve impulses," through a succession of neurons, one after another. Each impulse (1) may
be blocked in its transmission from one neuron to the next, (2) may be changed from a single impulse into
repetitive impulses, or (3) may be integrated with impulses from other neurons to cause highly intricate
patterns of impulses in successive neurons. All these functions can be classified as synaptic functions of
neurons.
Types of Synapses-Chemical and Electrical
There are two major types of synapses: (1) the chemical synapse and (2) the electrical synapse.
Almost all the synapses used for signal transmission in the central nervous system of the human being are
chemical synapses. In these, the first neuron secretes at its nerve ending synapse a chemical substance
called a neurotransmitter (or often called simply transmitter substance), and this transmitter in turn acts
on receptor proteins in the membrane of the next neuron to excite the neuron, inhibit it, or modify its
sensitivity in some other way. More than 40 important transmitter substances have been discovered thus
far. Some of the best known are acetylcholine, norepinephrine, epinephrine, histamine, gamma-
aminobutyric acid (GABA), glycine, serotonin, and glutamate.
Electrical synapses, in contrast, are characterized by direct open fluid channels that conduct electricity
from one cell to the next. Most of these consist of small protein tubular structures called gap junctions
that allow free movement of ions from the interior of one cell to the interior of the next. Only a few
examples of gap junctions have been found in the central nervous system. However, it is by way of gap
junctions and other similar junctions that action potentials are transmitted from one smooth muscle fiber
to the next in visceral smooth muscle and from one cardiac muscle cell to the next in cardiac muscle.
"One-Way" Conduction at Chemical Synapses
Chemical synapses have one exceedingly important characteristic that makes them highly
desirable for transmitting most nervous system signals. They always transmit the signals in one direction:
that is, from the neuron that secretes the transmitter substance, called the presynaptic neuron, to the
neuron on which the transmitter acts, called the postsynaptic neuron. This is the principle of one-way
conduction at chemical synapses, and it is quite different from conduction through electrical synapses,
which often transmit signals in either direction or simply, all directions.
As many as 10,000 to 200,000 minute synaptic knobs called presynaptic terminals lie on the
surfaces of the dendrites and soma of the motor neuron, about 80 to 95 percent of them on the dendrites
and only 5 to 20 percent on the soma. These presynaptic terminals are the ends of nerve fibrils that
originate from many other neurons. Many of these presynaptic terminals are excitatory-that is, they
secrete a transmitter substance that excites the postsynaptic neuron. But other presynaptic terminals are
inhibitory-they secrete a transmitter substance that inhibits the postsynaptic neuron.
, Presynaptic Terminals
Electron microscopic studies of the presynaptic terminals show that they have varied anatomical
forms, but most resemble small round or oval knobs and, therefore, are sometimes called terminal knobs,
boutons, end-feet, or synaptic knobs.
The basic structure of a synapse shows a single presynaptic terminal on the membrane surface of
a postsynaptic neuron. The presynaptic terminal is separated from the postsynaptic neuronal soma by a
synaptic cleft. The terminal has two internal structures important to the excitatory or inhibitory function
of the synapse: the transmitter vesicles and the mitochondria. The transmitter vesicles contain the
transmitter substance that, when released into the synaptic cleft, either excites or inhibits the postsynaptic
neuron-excites if the neuronal membrane contains excitatory receptors, inhibits if the membrane contains
inhibitory receptors. The mitochondria provide adenosine triphosphate (ATP), which in turn supplies the
energy for synthesizing new transmitter substance.
Mechanism by Which an Action Potential Causes Transmitter Release from the Presynaptic Terminals-
Role of Calcium Ions
The membrane of the presynaptic terminal is called the presynaptic membrane. It contains large
numbers of voltage-gated calcium channels. When an action potential depolarizes the presynaptic
membrane, these calcium channels open and allow large numbers of calcium ions to flow into the
terminal. The quantity of transmitter substance that is then released from the terminal into the synaptic
cleft is directly related to the number of calcium ions that enter.
Action of the Transmitter Substance on the Postsynaptic Neuron-Function of "Receptor Proteins"
The membrane of the postsynaptic neuron contains large numbers of receptor proteins. The
molecules of these receptors have two important components: (1) a binding component that protrudes
outward from the membrane into the synaptic cleft-here it binds the neurotransmitter coming from the
presynaptic terminal-and (2) an ionophore component that passes all the way through the postsynaptic
membrane to the interior of the postsynaptic neuron. The ionophore in turn is one of two types: (1) an
ion channel that allows passage of specified types of ions through the membrane or (2) a "second
messenger" activator that is not an ion channel but instead is a molecule that protrudes into the cell
cytoplasm and activates one or more substances inside the postsynaptic neuron. These substances in turn
serve as "second messengers" to increase or decrease specific cellular functions.
Ion Channels
The ion channels in the postsynaptic neuronal membrane are usually of two types: (1) cation
channels that most often allow sodium ions to pass when opened, but sometimes allow potassium and/or
calcium ions as well, and (2) anion channels that allow mainly chloride ions to pass but also minute
quantities of other anions. The cation channels that conduct sodium ions are lined with negative charges.
These charges attract the positively charged sodium ions into the channel when the channel diameter
increases to a size larger than that of the hydrated sodium ion. But those same negative charges repel
chloride ions and other anions and prevent their passage. For the anion channels, when the channel
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