Human Physiology, 14th edition (2016)

The Nervous System 181

of these composes half of the gap junction, called a hemichan-

nel. When the hemichannels of two plasma membranes dock

together, they form a complete gap junction ( fig.  7.21 ) that

spans both membranes and allows ions and molecules to pass

from one cell to the other.

Gap junctions are present in cardiac muscle, where they

allow action potentials to spread from cell to cell so that the

myocardium can contract as a unit. Similarly, gap junctions

in most smooth muscles allow many cells to be stimulated

and contract together, producing a stronger contraction (as in

the uterus during labor). The functions of gap junctions in the

nervous system are less well understood, but they are known

to be present in many regions of the brain. Although once

thought to be relatively simple and static, newer information

demonstrates that gap junctions can be modified by the addi-

tion or removal of channels to regulate their conductance, and

that they can interact functionally with chemical synapses.

Gap junctions are also found between neuroglial cells, where

they are believed to allow the passage of Ca^2 1 and perhaps

other ions and molecules between the connected cells.

Chemical Synapses

Transmission across the majority of synapses in the nervous

system is one-way and occurs through the release of chemical

neurotransmitters from presynaptic axon endings. These pre-

synaptic endings, called terminal boutons (from the Middle

French bouton 5  button) because of their swollen appearance,

are separated from the postsynaptic cell by a synaptic cleft so

narrow (about 10 nm) that it can be seen clearly only with an

electron microscope ( fig. 7.22 ).

Chemical transmission requires that the synaptic cleft stay

very narrow and that neurotransmitter molecules are released

axon of the first (or presynaptic ) neuron to the second (or
postsynaptic ) neuron. Most commonly, the synapse occurs
between the axon of the presynaptic neuron and the dendrites
or cell body of the postsynaptic neuron.
In the early part of the twentieth century, most physiolo-
gists believed that synaptic transmission was electrical —that
is, that action potentials were conducted directly from one cell
to the next. This was a logical assumption, given that nerve
endings appeared to touch the postsynaptic cells and that
the delay in synaptic conduction was extremely short (about
0.5 msec). Improved histological techniques, however, revealed
tiny gaps in the synapses, and experiments demonstrated that
the actions of autonomic nerves could be duplicated by certain
chemicals. This led to the hypothesis that synaptic transmis-
sion might be chemical —that the presynaptic nerve endings
might release chemicals called neurotransmitters. The neu-
rotransmitters would then change the membrane potential of
the postsynaptic cell, thereby producing action potentials if a
threshold depolarization were achieved.
In 1921 a physiologist named Otto Loewi published the
results of experiments suggesting that synaptic transmission
was indeed chemical, at least at the junction between a branch
of the vagus nerve (chapter 9; see fig. 9.6) and the heart. He had
isolated the heart of a frog and, while stimulating the branch
of the vagus that innervates the heart, perfused the heart with
an isotonic salt solution. Stimulation of the vagus nerve was
known to slow the heart rate. After stimulating the vagus nerve
to this frog heart, Loewi collected the isotonic salt solution and
then gave it to a second heart. The vagus nerve to this second
heart was not stimulated, but the isotonic solution from the first
heart caused the second heart to also slow its beat.
Loewi concluded that the nerve endings of the vagus
must have released a chemical—which he called Vagusstoff —
that inhibited the heart rate. This chemical was subsequently
identified as acetylcholine, or ACh. In the decades following
Loewi’s discovery, many other examples of chemical synapses
were discovered, and the theory of electrical synaptic transmis-
sion fell into disrepute. More recent evidence, ironically, has
shown that electrical synapses do exist in the nervous system
(though they are the exception), within smooth muscles, and
between cardiac cells in the heart.

Electrical Synapses: Gap Junctions

In order for two cells to be electrically coupled, they must
be approximately equal in size and they must be joined by
areas of contact with low electrical resistance. In this way,
impulses can be regenerated from one cell to the next without
interruption. Adjacent cells that are electrically coupled are
joined together by gap junctions. In gap junctions, the mem-
branes of the two cells are separated by only 2 nanometers
(1 nanometer  5  10 2 9 meter). In the plasma membrane of each
apposed cell, six proteins called connexins come together to
form a transmembrane structure with an aqueous core. Each

Figure 7.21 The structure of gap junctions. Gap

junctions are water-filled channels through which ions can pass

from one cell to another. This permits impulses to be conducted

directly from one cell to another. Each gap junction is composed

of connexin proteins. Six connexin proteins in one plasma

membrane line up with six connexin proteins in the other plasma

membrane to form each gap junction.

Cytoplasm

Connexin

proteins

forming gap

junctions

Cytoplasm

Two cells,

interconnected

by gap

junctions

Plasma

membrane

of one cell

Plasma

membrane

of adjacent

cell