Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
423
11
surrounding environment. Receptors for these signals generate
various second messengers that cause the filopodia to move by
rearranging their actin protein cores.
Once the axon reaches its target area, it must select the right site
on the target cell to form a synapse. Special cell adhesion molecules
couple the presynaptic and postsynaptic membranes together and
generate intracellular signals that recruit vesicles containing pre-
formed synaptic components. Tis results in the rapid formation
of a synapse. In the brain and spinal cord, astrocytes provide both
physical support and the cholesterol essential for constructing
synapses. Both dendrites and astrocytes are active partners in the
process of synapse formation. In the presence of thrombospondin
released by astrocytes, dendrites actually reach out and grasp mi-
grating axons, and synapses begin sprouting.
If neurons fail to make appropriate or functional synaptic
contacts, they die. Besides cell death resulting from unsuccess-
ful synapse formation,
apoptosis
(programmed cell death) is
also a normal part of the developmental process. Of the neu-
rons formed during the embryonic period, perhaps two-thirds
die before we are born or shortly thereaFer. Tose that remain
constitute most of our neural endowment for life.
Te generally amitotic nature of neurons is important because
their activity depends on the synapses they’ve formed, and if neu-
rons were to divide, their connections might be hopelessly dis-
rupted. Tis aside, there
do
appear to be some special neuronal
populations where stem cells are found and new neurons can be
formed—notably olfactory neurons and some cells of the hippo-
campus, a brain region involved in learning and memory.
processing. A single neuron sends information along several
pathways instead of just one, so you process a large amount of
information much more quickly.
Check Your Understanding
21.
Which types of neural circuits would give a prolonged output
after a single input?
22.
What pattern of neural processing occurs when your finger
accidentally touches a hot grill? What is this response called?
23.
What pattern of neural processing occurs when we smell
freshly baked apple pie and remember Thanksgiving at our
grandparents’ house, the odor of freshly cooked turkey,
sitting by the fire, and other such memories?
For answers, see Appendix H.
Developmental Aspects
of Neurons
Describe how neurons develop and form synapses.
We cover the nervous system in several chapters, so we limit our
attention here to the development of neurons. Let’s begin with two
questions, How do neurons originate? and How do they mature?
Te nervous system originates from a dorsal
neural tube
and the
neural crest
, formed from surface ectoderm (see ±ig-
ure 12.33
2
,
3
, p. 475). Te neural tube, whose walls begin as
a layer of
neuroepithelial
cells, becomes the CNS. Te neuroepi-
thelial cells then begin a three-phase process of differentiation,
which occurs largely in the second month of development:
1.
Tey
proliferate
to produce the appropriate number of cells
needed for nervous system development.
2.
Te potential neurons,
neuroblasts
, become amitotic and
migrate
externally into their characteristic positions.
3.
Te neuroblasts sprout axons to
connect with
their func-
tional targets and in so doing become neurons.
How does a neuroblast’s growing axon “know” where to go—
and once it gets there, where to make the proper connection?
Te growth of an axon toward an appropriate target requires
multiple steps and is guided by multiple signals.
Te growing tip of an axon, called a
growth cone
, is a prickly,
fanlike structure that gives the axon the ability to interact with
its environment
(Figure 11.25)
. Extracellular and cell surface
adhesion proteins such as laminin, integrin, and
nerve cell ad-
hesion molecule
(
N-CAM
) provide anchor points for the growth
cone, saying, “It’s okay to grow here.”
Neurotropins
are chemi-
cals that signal to the growth cone “come this way” (netrin) or
“go away” (ephrin, slit) or “stop here” (semaphorin). Troughout
this growth and development, neurotrophic factors such as
nerve
growth factor
(
NGF
) must be present to keep the neuroblast alive.
±ailure of any of these guiding signals results in catastrophic
developmental problems. ±or example, lack of N-CAM action
causes developing neural tissue to fall into a tangled, spaghetti-
like mass and hopelessly impairs neural function.
Te growth cone gropes along like an amoeba, with oozing
processes called
filopodia
which detect the guiding signals in the
Figure 11.25
A neuronal growth cone.
Fluorescent stains show
the locations of cannabinoid receptors (green), tubulin (blue), and
actin (pink) in this photomicrograph (1400
3
).
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