Page 357 - Color Atlas of Biochemistry

348 Tissues and organs
Signal transmission in the CNS
A. Structure of nerve cells
Nerve cells (neurons) are easily excitable cells trigger exocytosis of the neurotransmitter
that produce electrical signals and can react to stored in the presynaptic cell (for details, see
such signals as well. Their structure is mark-
edly different from that of other types of cell.
Numerous branching processes project from
their cell body (soma). Neurons are able to
receive signals via dendrites and to pass
them on via axons.The axons, which can be
up to 1 m long, are usually surrounded by
Schwann cells, which cover them with a
lipid-rich myelin sheath to improve their
electrical insulation.
The transfer of stimuli occurs at the
synapses, which link the individual neurons
to each other as well as linking neurons func-
tionally to muscle fibers. Neurotransmitters
(see p. 352) are stored in the axonal nerve
endings. These signaling substances are re-
leased in response to electrical signals in or-
der to excite neighboring neurons (or muscle
cells). It is estimated that eachneuroninthe
brain is in contact via synapses with approx-
imately 10 000 other neurons.
There is a noticeably high proportion of
lipids in the composition of nerve cells, rep-
resenting about 50% of their dry weight. In
particular, there is a very wide variety of
phospholipids, glycolipids, and sphingolipids
(see p. 216).
B. Neurotransmitters and neurohormones
Neurosecretions are classed into two groups:
neurotransmitters are released into the syn-
aptic cleft in order to influence neighboring
cells (C). They have a short range and a short
lifespan. By contrast, neurohormones are re-
leased into the blood, allowing them to cover
larger distances. However, the distinction be-
tween the two groups is a fluid one; some
neurotransmitters simultaneously function
as neurohormones.
C. Synaptic signal transmission
All chemical synapses function according to a
similar principle. In the area of the synapse,
the surface of the signaling cell (presynaptic
membrane) is separated from the surface of
the receiving cell (postsynaptic membrane)
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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only by a narrow synaptic cleft. When an ac-
tion potential (see p. 350) reaches the presyn-
aptic membrane, voltage-gated Ca 2+ channels
integrated into the membrane open and
p. 228).
Each neuron usually releases only one type
of neurotransmitter. Neurons that release
dopamine are referred to as “dopaminergic,”
for example, while those that release acetyl-
choline are “cholinergic,” etc. The transmit-
ters that are released diffuse through the
synapticcleft andbindon the othersideto
receptors on the postsynaptic membrane.
These receptors are integral membrane pro-
teins that have binding sites for neurotrans-
mitters on their exterior (see p. 224).
The receptors for neurotransmitters are
divided into two large groups according to
the effect produced by binding of the trans-
mitter (for details, see p. 354).
Ionotropic receptors (bottom left) are
ligand-gated ion channels. When they open
as a result of the transmitter’s influence,
ions flow in due to the membrane potential
(see p. 126). If the inflowing ions are cations
+
+
2+
(Na ,K ,Ca ), depolarization of the mem-
brane occurs and an action potential is trig-
gered on the surface of the postsynaptic cell.
This is the way in which stimulatory trans-
mitters work (e. g., acetylcholine and gluta-
mate). By contrast, if anions flow in (mainly
–
Cl ), the result is hyperpolarization of the
postsynaptic membrane, which makes the
production of a postsynaptic action potential
more dif cult. The action of inhibitory trans-
mitters such as glycine and GABA is based on
this effect.
A completely different type of effect is ob-
served in metabotropic receptors (bottom
right). After binding of the transmitter, these
interact on the inside of the postsynaptic
membrane with Gproteins (see p. 384), which
in turn activate or inhibit the synthesis of
second messengers. Finally, second messen-
gers activate or inhibit protein kinases,which
phosphorylate cellular proteins and thereby
alter the behavior of the postsynaptic cells
(signal transduction; see p. 386).