Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
391
11
axon collaterals
, extend from the axon at more or less right
angles. Whether an axon is undivided or has collaterals, it usu-
ally branches profusely at its end (terminus): 10,000 or more
terminal branches
(also called
terminal arborizations
) per neu-
ron is not unusual. Te knoblike distal endings of the terminal
branches are called
axon terminals
or
terminal boutons
(boo-
tonz
9
; “buttons”). ±ake your pick!
The Axon: Functional Characteristics
Functionally, the axon
is the
conducting region
of the neuron (Figure 11.4). It
gener-
ates nerve impulses
and
transmits them
, typically away from the
cell body, along the plasma membrane, or
axolemma
(ak
0
so-
lem
9
ah). In motor neurons, the nerve impulse is generated at
the junction of the axon hillock and axon (which for this reason
is called the
trigger zone
) and conducted along the axon to the
axon terminals, which are the
secretory region
of the neuron.
When the impulse reaches the axon terminals, it causes
neurotransmitters
—signaling chemicals usually stored in vesicles—
to be released into the extracellular space. Te neurotransmitters
either excite or inhibit neurons (or effector cells) with which the
axon is in close contact. Because each neuron both receives signals
from and sends signals to scores of other neurons, it carries on “con-
versations” with many different neurons at the same time.
An axon contains the same organelles found in the dendrites
and cell body with two important exceptions—it lacks rough
Dendrites, the main
receptive
or
input regions
, provide an
enormous surface area for receiving signals from other neurons.
In many brain areas, the finer dendrites are highly specialized
for collecting information. Tey bristle with
dendritic spines
thorny appendages having bulbous or spiky ends—which rep-
resent points of close contact (synapses) with other neurons
(Figure 11.4b).
Dendrites convey incoming messages
toward
the cell body.
Tese electrical signals are usually
not
action potentials (nerve
impulses) but are short-distance signals called
graded potentials
,
as we will describe shortly.
The Axon: Structure
Each neuron has a single
axon
(
axo
5
axis, axle). Te initial region of the axon arises from a cone-
shaped area of the cell body called the
axon hillock
(“little hill”)
and then narrows to form a slender process that is uniform in
diameter for the rest of its length (Figure 11.4a). In some neu-
rons, the axon is very short or absent, but in others it accounts
for nearly the entire length of the neuron. For example, axons of
the motor neurons controlling the skeletal muscles of your great
toe extend a meter or more (3–4 feet) from the lumbar region of
your spine to your foot, making them among the longest cells in
the body. Any long axon is called a
nerve fiber
.
Each neuron has only one axon, but axons may have oc-
casional branches along their length. Tese branches, called
Dendrites
(receptive
regions)
Cell body
(biosynthetic center
and receptive region)
Nucleolus
Nucleus
Chromatophilic
substance (rough
endoplasmic
reticulum)
Axon
(impulse-generating
and -conducting
region)
Axon hillock
Terminal branches
Myelin sheath gap
(node of Ranvier)
Impulse
direction
Schwann cell
Axon terminals
(secretory
region)
Dendritic
spine
Neuron cell body
(b)
(a)
Figure 11.4
Structure of a motor neuron.
(a)
Diagrammatic view.
(b)
Scanning electron
micrograph showing the cell body and dendrites with obvious dendritic spines (2000
3
).
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