356
UNIT THREE
T
he signs of a migraine are unmistakable—
a pounding head, waves of nausea,
sometimes shimmering images in the
peripheral visual field, and extreme sensitiv-
ity to light or sound. Inherited susceptibilities
and environmental factors probably cause
migraines. Environmental triggers include sud-
den exposure to bright light, eating a particular
food (chocolate, red wine, nuts, and processed
meats top the list), lack of sleep, stress, high alti-
tude, stormy weather, and excessive caF
eine or
alcohol intake. Hormonal in±
uences may also be
involved, because two-thirds of the 300 million
people who suffer from migraines worldwide
are women between the ages of 15 and 55.
A migraine attack may last only a few hours,
or days. It is due to a phenomenon called “cortical
spreading depression,” in which an intense wave of
excitation followed by a brief period of unrespon-
siveness in certain neurons stimulates the trigemi-
nal nucleus at the base of the brain to produce
pain sensations. The excitation and dampening
of the activity level of these neurons also triggers
changes in blood ±
ow in the brain that were once
thought to be the direct cause of migraine.
Drugs called triptans can very effectively
halt a migraine attack, but must be taken as soon
as symptoms begin. Triptans block the release
of neurotransmitter from the trigeminal nerves.
Because triptans constrict blood vessels through-
out the body, making them dangerous for some
people, newer migraine drugs have been devel-
oped that block the specific neurotransmitter
that the trigeminal nerves release (calcitonin
gene-related peptide), better targeting the thera-
peutic eF
ect.
Several drugs developed to treat other con-
ditions are used on a long-term, daily basis to
lessen the frequency of migraines. These drugs
include certain antidepressants, anticonvulsants,
and drugs used to treat high blood pressure (cal-
cium channel blockers and beta blockers). A phy-
sician must consider an individual’s family and
health history before prescribing these drugs to
prevent migraine.
10.1
CLINICAL APPLICATION
Migraine
like spines (dendritic spines) on their surfaces, which are
contact points for other neurons.
A neuron may have many dendrites, but only one axon.
The axon, which often arises from a slight elevation of the cell
body (axonal hillock), is a slender, cylindrical process with a
nearly smooth surface and uniform diameter. It is specialized
to conduct nerve impulses away from the cell body. The cyto-
plasm of the axon includes many mitochondria, microtubules,
and neuroF brils (ribosomes are found only in the cell body).
The axon may give off branches, called
collaterals.
Near its
end, an axon may have many F ne extensions, each with a
specialized ending called an
axon terminal.
This ends as a
synaptic knob
close to the receptive surface of another cell,
separated only by a space called the
synaptic cleft.
In addition to conducting nerve impulses, an axon conveys
biochemicals produced in the neuron cell body, which can be
quite a task in these long cells. In this activity, called
axonal
transport,
vesicles, mitochondria, ions, nutrients, and neuro-
transmitters move from the cell body to the ends of the axon.
In the PNS, neuroglia called
Schwann cells
encase the
large axons of peripheral neurons in lipid-rich sheaths. These
tight coverings form as layers of cell membrane and wind
around the axons somewhat like a bandage wrapped around a
F nger. The layers are composed of
myelin
(mi
e
˘-lin), which has
a higher proportion of lipid than other cell membranes. This
coating is called a
myelin sheath.
The parts of the Schwann
cells that contain most of the cytoplasm and the nuclei remain
outside the myelin sheath and comprise a
neurilemma
(nur
ıˉlem
ah)
,
or
neurilemmal sheath,
which surrounds the
myelin sheath. Narrow gaps in the myelin sheath between
Schwann cells are called
nodes of Ranvier
(f g. 10.4)
.
can be subdivided into the somatic and the autonomic ner-
vous systems. Generally the
somatic nervous system
over-
sees conscious (voluntary) activities, such as skeletal muscle
contraction. The
autonomic nervous system
controls vis-
cera, such as the heart and various glands, and thus controls
subconscious (involuntary) actions.
10.3
DESCRIPTION OF CELLS OF
THE NERVOUS SYSTEM
Neurons vary in size and shape. They may differ in the
lengths and sizes of their axons and dendrites and in the
number of processes. Despite this variability, neurons share
certain features. Every neuron has a
cell body,
dendrites,
and an axon.
Figure 10.3
shows some of the other structures
common to neurons.
A neuron’s cell body (soma or perikaryon) contains
granular cytoplasm, mitochondria, lysosomes, a Golgi appa-
ratus, and many microtubules. A network of F
ne threads
called
neuroF
brils
extends into the axon and supports it.
Scattered throughout the cytoplasm are many membranous
packets of
chromatophilic substance
(Nissl bodies), which
consist mainly of rough endoplasmic reticulum. Cytoplasmic
inclusions in neurons contain glycogen, lipids, or pigments
such as melanin. Near the center of the neuron cell body is a
large, spherical nucleus with a conspicuous nucleolus.
Dendrites are typically highly branched, providing recep-
tive surfaces with which processes from other neurons com-
municate. (In some types of neurons, the cell body provides
such a receptive surface.) Some dendrites have tiny, thorn-
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