376
UNIT THREE
Divergence
A neuron has a single axon, but axons may branch at several
points. Thus, impulses leaving a neuron of a neuronal pool
may exhibit
divergence
(di-ver
jens) by reaching several
other neurons. For example, one neuron may stimulate two
others; each of these, in turn, may stimulate several others,
and so forth. Such a pattern of diverging axons can amplify
an impulse—that is, spread it to increasing numbers of neu-
rons within the pool (±
g. 10.21
b
).
As a result of divergence, an impulse originating from a
single neuron in the CNS may be ampli±
ed so that suf± cient
impulses reach the motor units in a skeletal muscle to cause
forceful contraction. Similarly, an impulse originating from
a sensory receptor may diverge and reach several different
3. Neuro±
brils support axons.
4. Chromatophilic substance is mostly rough ER and is
scattered throughout the cytoplasm of neurons.
5. Dendrites and the cell body provide receptive
surfaces.
6. A single axon arises from the cell body and may be
enclosed in a myelin sheath and a neurilemma.
7. White matter consists of myelinated axons, and
gray matter consists of unmyelinated axons and cell
bodies.
10.4
CLASSIFICATION OF CELLS OF THE
NERVOUS SYSTEM (PAGE 359)
1. Classi±
cation of neurons
a. Neurons are structurally classi±
ed as multipolar,
bipolar, or unipolar.
b. Neurons are functionally classi±
ed as sensory
neurons, interneurons, or motor neurons.
2. Classi±
cation of neuroglia
a. Neuroglia are abundant and have several
functions.
b. They ± ll spaces, support neurons, hold nervous
tissue together, help metabolize glucose, help
regulate potassium ion concentration, produce
myelin, carry on phagocytosis, rid synapses
of excess ions and neurotransmitters, nourish
neurons, and stimulate synapse formation.
c. They include astrocytes, oligodendrocytes,
microglia, and ependymal cells in the CNS and
Schwann cells and satellite cells in the PNS.
d. Malfunctioning neuroglia can cause disease.
e. Neuroglia are involved in axonal regeneration.
(1) If a neuron cell body is injured, the neuron is
likely to die; neural stem cells may proliferate
and produce replacements.
CHAPTER SUMMARY
10.1
INTRODUCTION (PAGE 354)
1. The nervous system is a network of cells that
sense and respond to stimuli in ways that maintain
homeostasis.
2. The nervous system is composed of neural tissue,
including neurons and neuroglia, blood vessels and
connective tissue.
3. Neurons have processes that receive (dendrites) and
send (axons) bioelectric signals (neurotransmitters)
that cross spaces (synapses) between them.
4. Organs of the nervous system are divided into the
central and peripheral nervous systems.
10.2
GENERAL FUNCTIONS OF THE NERVOUS
SYSTEM (PAGE 355)
1. Sensory receptors detect changes in internal and
external body conditions.
2. Integrative functions gather sensory information and
make decisions that affect motor functions.
3. Motor impulses stimulate effectors to respond.
a. The motor portion of the PNS that carries out
voluntary activities is the somatic nervous
system.
b. The motor portion of the PNS that carries out
involuntary activities is the autonomic nervous
system.
10.3
DESCRIPTION OF CELLS OF THE
NERVOUS SYSTEM (PAGE 356)
1. Neurons vary in size, shape, sizes and lengths of
axons and dendrites, and number of dendrites.
2. A neuron includes a cell body, cell processes, and the
organelles usually found in cells.
regions of the CNS, where the resulting impulses can be pro-
cessed and acted upon.
The nervous system enables us to experience the world
and to think and feel emotion. This organ system is also
sensitive to outside infl
uences. Clinical Application 10.5 dis-
cusses one way that an outside infl uence can affect the ner-
vous system—drug addiction.
PRACTICE
18
Def
ne
neuropeptide
.
19
What is a neuronal pool?
20
Def
ne
facilitation
.
21
What is convergence?
22
What is the relationship between divergence and amplif
cation?
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