Fundamentals of the Nervous System and Nervous Tissue
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
When a neurotransmitter binds to a G protein–linked recep-
tor, the G protein is activated
. (±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
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
ACh excites skeletal muscle and yet it inhibits heart muscle.
How can this be?
Why is cyclic AMP called a second messenger?
For answers, see Appendix H.
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
included—there must be
. In other words, the parts
must be fused into a smoothly operating whole.
In this section, we move to the ﬁrst level of
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
Te billions of neurons in the CNS are organized into
. 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
, 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
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
. 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
, determine the pool’s functional capabilities.
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
. 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.
, 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
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).