CHAPTER 14
Answers to “What Did You Learn?”
1.
(1) Collect information, (2) Process and evaluate information, and (3) Respond to
information.
2.
Afferent implies “input” and means transmission of information toward the CNS. The
sensory (afferent) division of the nervous system is responsible for transmitting
sensory information from the body to the CNS. Efferent implies “output” and means
transmission of information from the CNS. The motor (efferent) division is
responsible for transmitting motor information from the CNS to muscles and glands.
3.
Dendrites are usually small and branching; they may be quite numerous; and they
conduct nerve impulses toward the neuron cell body. Most neurons have only a
single axon; it may be very long; and it conducts nerve impulses away from the
cell body.
4.
The perivascular feet of astrocytes wrap completely around and ensheathe the
outer surface of capillary walls to strictly control the entry substances entering the
nervous tissue in the brain from the bloodstream.
5.
As a result of an inflammation or nervous system infection, microglial cells
usually respond
6.
Satellite cells are flattened cells arranged around neuronal cell bodies in PNS
ganglia. They support the neurons, physically separate them from the
surrounding interstitial fluid, and regulate the exchange of nutrients and waste
between the neurons and their environment.
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7.
Neurolemmocytes myelinate PNS axons and oligodendrocytes myelinate CNS
axons. A neurolemmocyte can myelinate only a small part of a single axon (about
1 millimeter along the length of the axon), whereas a single oligodendrocyte can
myelinate small parts of several axons.
8.
Neurofibril nodes, also called the nodes of Ranvier, are the gaps or small spaces
interrupting the myelin sheath between adjacent neurolemmocytes or
oligodendrocytes along myelinated axons.
9.
PNS axon regeneration is dependent on (1) the amount of damage, (2) the
secretion of nerve growth factors, and (3) the distance between the damaged axon
and the effector organ.
10.
The perineurium is a cellular, dense irregular connective tissue layer that wraps
around nerve fascicles.
11.
Regeneration of severed axons has a lesser chance to succeed in the CNS because
(1) oligodendrocytes do not secrete a nerve growth factor to stimulate axonal
outgrowth, and (2) oligodendrocytes do produce secretions that inhibit axonal
outgrowth.
12.
The two types of synaptic communication are electrical and chemical.
13.
The rate of nerve impulse conduction is influenced by the diameter of the axon
and the presence (or absence) of a myelin sheath.
14.
A diverging circuit spreads information from one presynaptic neuron to several
postsynaptic neurons, or from one pool to multiple pools. Reverberating circuits
utilize feedback to produce a repeated, cyclical stimulation of a neuronal pathway
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or circuit, which continues until inhibitory stimuli or synaptic fatigue break the
cycle of stimulation.
Answers to “Content Review”
1.
The three structural types of neurons are classified based on the number of
processes emanating directly for the cell body: unipolar (one process), bipolar
(two processes), and multipolar (three or more processes). The three functional
types of neurons are classified according to the direction the nerve impulse travels
relative to the CNS: sensory (afferent, from sensory receptors to the CNS),
interneurons (entirely within the CNS), and motor neurons (efferent, from the
CNS to muscles or glands).
2.
Sensory neurons are called afferent neurons: they are unipolar neurons
specialized to detect changes in their environment. These changes, called
stimuli [touch, pressure, temperature, light, chemicals], result in the
transmission of information about the stimuli to the CNS. They form the
afferent division of the PNS. They begin in the body periphery [external
environment] or a visceral organ [internal environment] and end in the CNS.
3.
(1) Astrocytes are the most numerous and largest glial cells in the CNS. They
help form the blood-brain barrier, regulate tissue fluid composition, strengthen
and reinforce the nervous tissue in the CNS, replace damaged neurons, and assist
with neuronal development. (2) Ependymal cells and nearby blood capillaries
form the choroid plexus, which produces CSF. The ependymal cells have patches
of cilia on their apical surfaces that help to circulate the CSF. (3) Microglial cells
are small phagocytic cells that wander through the CNS and phagocytize cellular
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debris from dead nervous tissue, microorganisms, waste products, and other foreign
matter. (4) Oligodendrocytes myelinate the axons in the CNS. (5) Satellite cells,
located in the PNS, function to separate peripheral nervous system neuron cell bodies
from their surrounding interstitial fluid and control / regulate the continuous exchange
of nutrients and waste products between peripheral neurons and their environment.
(6) Neurolemmocytes myelinate the axons in the PNS.
4.
Oligodendrocytes form the myelin sheath in the CNS; neurolemmocytes form it in
the PNS. Each oligodendrocyte can myelinate several small portions of different
axons in the CNS, however each neurolemmocyte can only myelinate a small
portion of one axon in the PNS.
5.
A nerve cell may repair itself through a process called Wallerian degeneration.
After an axon in the PNS is severed, the proximal portion of the severed end seals
and begins to swell. The distal severed region degenerates and is phagocytized.
The neurolemmocytes in the distal region survive and together with the remaining
endoneurium form a regeneration tube. The axon regenerates and remyelination
occurs. The regeneration tube guides the axon sprout as it grows under the
influence of nerve growth factor released by the neurolemmocytes. Innervation is
restored as the growing axon contacts the original effector.
6.
Individual axons in the PNS are surrounded by neurolemmocytes and then wrapped in
a delicate layer of loose connective tissue called the endoneurium. Groups of axons
are wrapped into bundles, called nerve fascicles, by a cellular dense irregular
connective tissue layer called the perineurium. All of the fascicles are bundled
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together by a superficial dense irregular connective tissue covering termed the
epineurium.
7.
Neurons the basic structural unit of the nervous system. They conduct nerve
impulses between body parts and are called nerve cells. An axon is the long
process emanating from the cell body of a neuron that transmits nerve impulses
toward other cells. Often, it is referred to as the nerve fiber. A nerve is a bundle
of many parallel axons, their myelin sheaths, and some successive wrappings of
connective tissue.
8.
Electrical synapses exhibit direct physical contact between pre-synaptic and
post-synaptic cells, which are tightly bond together by their plasma
membranes. They are connected by gap junctions, which facilitate ion flow
between the cells, and results in a voltage change between the cells and the
initiation of a nerve impulse. In a chemical synapse, the most common type in
humans, a signalling molecule called a neurotransmitter passes between the
presynaptic and postsynaptic cell. Binding of the neurotransmitter to a
receptor in the post-synaptic cell membrane cause a brief voltage change
across its membrane.
9.
In a converging circuit, a single post-synaptic neuron receives input from several
presynaptic neurons. In a parallel after-discharge circuit, several neurons or
neuronal pools process the same information at one time. A single presynaptic
neuron stimulates different groups of neurons, each of which passes the nerve
impulse along a pathway that ultimately synapses with a common postsynaptic
cell.
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10.
In the early embryo, a thickened region of ectoderm, called the neural plate forms.
These cells undergo the process of neurulation to form the nervous system. The
neural plate grows, extends neural folds and forms a neural tube. The neural tube
develops into the central nervous system.
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