Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
421
11
are transmembrane protein complexes. Tey include muscarinic
ACh receptors and those that bind the biogenic amines and neu-
ropeptides. Because their effects tend to bring about widespread
metabolic changes, G protein–linked receptors are commonly
called
metabotropic receptors
.
When a neurotransmitter binds to a G protein–linked recep-
tor, the G protein is activated
(Figure 11.21)
. (±o orient yourself,
refer back to the simpler G protein explanation in Figure 3.16 on
p. 82.) Activated G proteins typically work by controlling the pro-
duction of second messengers such as
cyclic AMP
,
cyclic GMP
,
diacylglycerol
, or
Ca
2
1
.
Tese second messengers, in turn, act as go-betweens to regu-
late (open or close) ion channels or activate kinase enzymes that
initiate a cascade of enzymatic reactions in the target cells. Some
second messengers modify (activate or inactivate) other pro-
teins, including channel proteins, by attaching phosphate groups
to them. Others interact with nuclear proteins that activate genes
and induce synthesis of new proteins in the target cell.
Check Your Understanding
19.
ACh excites skeletal muscle and yet it inhibits heart muscle.
How can this be?
20.
Why is cyclic AMP called a second messenger?
For answers, see Appendix H.
Basic Concepts
of Neural Integration
Until now, we have concentrated on the activities of individual
neurons. However, neurons function in groups, and each group
contributes to still broader neural functions. In this way, the
organization of the nervous system is hierarchical.
Any time you have a large number of
anything
—people
included—there must be
integration
. In other words, the parts
must be fused into a smoothly operating whole.
In this section, we move to the first level of
neural integration
:
neuronal pools
and their patterns of communicating with other
parts of the nervous system. In Chapter 12 we discuss the highest
levels of neural integration—how we think and remember. With
this understanding of the basics and of the larger picture, in Chap-
ter 13 we examine how sensory inputs interface with motor activity.
Organization of Neurons: Neuronal Pools
Describe common patterns of neuronal organization and
processing.
Te billions of neurons in the CNS are organized into
neuronal
pools
. Tese functional groups of neurons integrate incoming in-
formation received from receptors or different neuronal pools and
then forward the processed information to other destinations.
In a simple type of neuronal pool
(Figure 11.22)
, one in-
coming presynaptic fiber branches profusely as it enters the
pool and then synapses with several different neurons in the
pool. When the incoming fiber is excited, it will excite some
postsynaptic neurons and facilitate others. Neurons most likely
to generate impulses are those closely associated with the in-
coming fiber, because they receive the bulk of the synaptic con-
tacts. Tose neurons are in the
discharge zone
of the pool.
Neurons farther from the center are not usually excited to thresh-
old, but they are facilitated and can easily be brought to threshold by
stimuli from another source. For this reason, the periphery of the
pool is the
facilitated zone
. Keep in mind, however, that our figure is
a gross oversimplification. Most neuronal pools consist of thousands
of neurons and include inhibitory as well as excitatory neurons.
Types of Circuits
Individual neurons in a neuronal pool both send and receive infor-
mation, and synaptic contacts may cause either excitation or inhibi-
tion. Te patterns of synaptic connections in neuronal pools, called
circuits
, determine the pool’s functional capabilities.
Figure 11.23
describes four basic circuit patterns and their properties: diverging,
converging, reverberating, and parallel a²er-discharge circuits.
Patterns of Neural Processing
Distinguish between serial and parallel processing.
Input processing is both
serial
and
parallel
. In serial processing,
the input travels along one pathway to a specific destination.
In parallel processing, the input travels along several different
pathways to be integrated in different CNS regions. Each mode
has unique advantages, but as an information processor, the
brain derives its power from its ability to process in parallel.
Serial Processing
In
serial processing
, the whole system works in a predict-
able all-or-nothing manner. One neuron stimulates the next,
which stimulates the next, and so on, eventually causing a spe-
cific, anticipated response. Te most clear-cut examples of se-
rial processing are spinal reflexes, but straight-through sensory
Presynaptic
(input) fiber
Facilitated zone
Discharge zone
Facilitated zone
Figure 11.22
Simple neuronal pool.
Postsynaptic neurons in the
discharge zone receive more synapses and are more likely to discharge
(generate APs). Postsynaptic neurons in the facilitated zone receive
fewer synapses and are facilitated (brought closer to threshold).
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