CH. 14 Nervous Tissue Flashcards
What are the 3 general functions of the CNS and PNS components of the sensory and motor nervous system?
■ Collect information. Specialized PNS structures called
receptors (dendrite endings of sensory neurons or cells) detect
changes in the internal or external environment (called stimuli)
and pass them on to the CNS as sensory input (discussed in
section 19.1).
■ Process and evaluate information. After processing sensory
input, the CNS determines what, if any, response is required.
■ Initiate response to information. After selecting an
appropriate response, the CNS initiates specific nerve impulses
(rapid movements of an electrical charge along the neuron’s
plasma membrane), called motor output. Motor output travels
through structures of the PNS to effectors (the cells that
receive impulses from motor neurons: muscles or glands).
What is the sensory nervous system responsible for?
The sensory (or afferent; af′ĕr-ĕnt) nervous system is responsible
for receiving sensory information from receptors and transmitting
this information to the CNS. (The term afferent means
“inflowing,” which indicates that nerve impulses are transmitted
to the CNS.) Thus, the sensory nervous system is responsible for
input.
What are the two components of the sensory nervous system?
The somatic sensory components are
the general somatic senses—touch, pain, pressure, vibration, temperature,
and proprioception (sensing the position or movement of joints
and limbs)—and the special senses (taste, vision, hearing, balance,
and smell). These functions are considered voluntary because we
have some control over them and we tend to be conscious of them.
The visceral sensory components transmit nerve impulses from
blood vessels and viscera to the CNS. The visceral receptors detect
chemical composition of blood or stretch of an organ wall. These
functions are said to be involuntary because most of the time you do
not have voluntary control over them and are not conscious of them
What is the motor nervous system responsible for?
The motor (or efferent; ef′ĕr-ent) nervous system is responsible for
transmitting motor impulses from the CNS to effectors (muscles
or glands). (The term efferent means “conducting outward,” which
indicates that nerve impulses are transmitted from the CNS.) Thus,
the motor nervous system is responsible for output
What are the two components of the motor nervous system?
The somatic motor component
(somatic nervous system; SNS) conducts nerve impulses from the
CNS to the skeletal muscles, causing them to contract. The somatic
motor component is often called the voluntary nervous system
because the contractions of the skeletal muscles are under conscious
control;
The autonomic motor component is often called the autonomic
nervous system (ANS). Because it innervates internal organs and
regulates smooth muscle, cardiac muscle, and glands without our control,
it is also known as the visceral motor system or the involuntary
nervous system. For example, we cannot voluntarily make our hearts
stop beating, nor can we prevent our stomachs from growling
What two distinct cell types form the nervous tissue?
neurons, which are
excitable cells that are able to generate, transmit, and receive nerve
impulses, and glial cells, which are nonexcitable cells that support
and protect the neurons
What are the special characteristics of neurons?
■ Neurons have a high metabolic rate. Their survival depends
upon continuous and abundant supplies of glucose and oxygen.
■ Neurons have extreme longevity. Most neurons formed
during fetal development are still functional in very elderly
individuals.
■ Neurons typically are nonmitotic (unable to divide and produce
new neurons). During the fetal development of neurons, mitotic
activity is lost, except possibly in certain areas of the brain and
the sense organ for smell.
■ Neurons are excitable because they respond to a stimulus
(exposure to a chemical, stretch, or pressure change).
■ Neurons exhibit conductivity when an electrical charge is
quickly propagated along their plasma membrane.
Describe a cell body as well as its surrounding organelles.
The cell body, also called a soma, serves as the neuron’s control
center and is responsible for receiving, integrating, and sending
nerve impulses. The cell body is enclosed by a plasma membrane
and contains cytoplasm surrounding a nucleus. The nucleus contains
a prominent nucleolus, reflecting the high metabolic activity
of neurons, which require the production of many proteins. Numerous
mitochondria are present within this cytoplasm to produce
the large amounts of ATP needed by the neuron. Large numbers
of free ribosomes and rough ER produce proteins for the active
neuron. Together, both free and bound ribosomes go by two names:
chromatophilic (krō-mă-tō-fil′ik; chromo = color; phileo = to
love) substance, because they stain darkly with basic dyes, or
Nissl bodies, because they were first described by the German
microscopist Franz Nissl. Cytologists believe that the chromatophilic
substance together with dendrites and cell bodies account for
the gray color of the gray matter, as seen in brain and spinal cord
areas containing collections of neuron cell bodies
Describe the structure of a dendrite.
Dendrites (den′drīt; dendrites = relating to a tree) tend to be
shorter, smaller processes that branch off the cell body. Some neurons
have only one dendrite, whereas others have many. Dendrites conduct
nerve impulses toward the cell body; in essence, they receive input
and then transfer it to the cell body for processing. The more dendrites
a neuron has, the more nerve impulses that neuron can receive
from other cells. On many, but not all, neurons, dendrite surface area
is increased by small knoblike protuberances called dendritic spines.
These small projections permit a greater number of nerve impulses
to be received by the neuron.
Describe an axon and its surrounding structures.
be received by the neuron.
The axon (ak′son; axon = axis), or nerve fiber, is typically a
longer nerve cell process emanating from the cell body to make contact
with other neurons, muscle cells, or gland cells.
Neurons have either one axon or no axon at all (neurons with
only dendrites and no axons are called anaxonic [an-ak-son′ik; an =
without]. They are small neurons that provide no clues to distinguish
axon from dendrite; they are found only in CNS; they are uncommon
and their function is unknown.) Most neurons, however, have
a single axon. The axon transmits a nerve impulse away from the
cell body toward another cell; in essence, the axon transmits output
information to other cells. The axon extends from a triangular region of the cell body called the axon hillock (hil′lok). Unlike
the rest of the cell body, the axon hillock is devoid of chromatophilic
substance, and so it lacks those dark-staining regions when
viewed under the microscope. Although an axon remains relatively
unbranched for most of its length, it may give rise to a few side
branches called axon collaterals. Most axons and their collaterals
branch extensively at their distal end into an array of fine terminal
extensions called terminal arborizations. The extreme tips of
these fine extensions are slightly expanded regions called synaptic
(si-nap′tik) knobs (also called synaptic bulbs, axon terminals, end
bulbs, or terminal boutons)
Describe the difference between unipolar, bipolar, and multipolar neurons.
Unipolar neurons have a single, short process that emerges
from the cell body and branches like a T. These neurons are also
called pseudounipolar (sū′dō-yū′ni-pō′lăr; pseudo = false, uni =
one) because they start out as bipolar neurons during development,
but their two processes fuse into a single process. The naming of the
branched processes in unipolar neurons has been a source of confusion
as it relates to the common definitions of dendrites and axons. It
seems most appropriate to call the short, multiple-branched receptive
endings dendrites. The combined peripheral process (from dendrites
to the cell body) and central process (from the cell body into the
CNS) together denote the axon, because these processes generate and
conduct impulses and are often myelinated. Most sensory neurons of
the PNS are unipolar neurons.
Bipolar neurons have two processes that extend from the
cell body—one axon and one dendrite. These neurons are relatively
uncommon in humans and primarily limited to some of the special
senses. For example, bipolar neurons are located in the olfactory
epithelium of the nose and in the retina of the eye.
Multipolar neurons are the most common type of neuron.
Multiple processes—many dendrites and a single axon—extend from
the cell body. Examples of multipolar neurons include motor neurons
that innervate muscle and glands.
Describe the differences between sensory, motor, and interneurons, and how they all relate with one another.
Sensory neurons, or afferent neurons, transmit nerve impulses
from sensory receptors to the CNS. These neurons are specialized to
detect changes in their environment called stimuli (sing., stimulus).
Stimuli can be in the form of touch, pressure, heat, light, or chemicals.
Most sensory neurons are unipolar, although a few are bipolar
(e.g., those in the olfactory epithelium of the nose and the retina of
the eye, as previously mentioned). The cell bodies of unipolar sensory
neurons are located outside the CNS and housed within structures
called posterior (dorsal) root ganglia.
Motor neurons, or efferent neurons, transmit nerve impulses
from the CNS to muscles or glands. They are called motor neurons because
most of them extend to muscle cells, and the nerve impulses they
transmit cause these cells to contract. The muscle and gland cells that
receive nerve impulses from motor neurons are called effectors, because
their stimulation produces a response or effect. The cell bodies of most
motor neurons lie in the spinal cord, whereas the axons primarily travel
in cranial or spinal nerves to muscles and glands. All motor neurons are
multipolar.
Interneurons, or association neurons, lie entirely within the
CNS and are multipolar structures. They receive nerve impulses from
many other neurons and carry out the integrative function of the nervous
system—that is, they retrieve, process, and store information
and “decide” how the body responds to stimuli. Thus, interneurons
facilitate communication between sensory and motor neurons.
Figure 14.5 shows a sensory neuron transmitting stimuli (sensory information)
to an interneuron, which then processes that information and
signals the appropriate motor neuron(s) to transmit a nerve impulse to
the muscle. Interneurons outnumber all other neurons in both their total
number and different types; it is estimated that 99% of our neurons are
interneurons. The number of interneurons activated during processing
or storing increases dramatically with the complexity of the response.
How do glial cells assist neurons?
Collectively, the glial cells physically protect and help
nourish neurons, and provide an organized, supporting framework for all the nervous tissue. During development, glial cells form the
framework that guides young migrating neurons to their destinations.
Recent research on glial cells suggests they also play a role in learning
and memory, by adjusting the speed of action potentials between
neurons. More research is forthcoming on this exciting new finding.
List and describe the 4 different types of glial cells in the central nervous system.
Astrocytes Astrocytes (as′trō-sīt; astron = star) exhibit a starlike
shape due to many projections from their surface (figure 14.7a).
These numerous cell processes touch both capillary walls and
different parts of neurons. Astrocytes are the most abundant glial cell in the CNS, and they constitute over 90% of the nervous tissue in some areas of the brain
Ependymal Cells Ependymal (ĕ-pen′di-măl) cells are cuboidal epithelial
cells that line the internal cavities (ventricles) of the brain and the
central canal of the spinal cord (figure 14.7b). These cells have slender
processes that branch extensively to make contact with other glial cells
in the surrounding nervous tissue. Ependymal cells and nearby blood
capillaries together form a network called the choroid (kor′oyd) plexus
(see figure 15.7). The choroid plexus produces cerebrospinal fluid (CSF),
a clear liquid that bathes the CNS and fills its internal cavities. The
ependymal cells have cilia on their apical surfaces that help circulate
the CSF. (Section 15.2c describes ependymal cells, the choroid plexus,
and CSF in more detail.)
Microglial Cells Microglial (mī-krog′le-ăl; micros = small) cells
represent the smallest percentage of CNS glial cells; some estimates of
their prevalence are as low as 5%. Microglial cells are typically small
cells that have slender branches extending from the main cell body
(figure 14.7c). They wander through the CNS and replicate in response to
an infection. They perform phagocytic activity and remove debris from
dead or damaged nervous tissue. Thus, the activities of microglial cells
resemble those of the macrophages of the immune system.
Oligodendrocytes Oligodendrocytes (ol′i-gō-den′drō-sīt; oligos
= few) are large cells with a bulbous body and slender cytoplasmic
extensions or processes (figure 14.7d). The processes of
oligodendrocytes ensheathe portions of many different axons, each
repeatedly wrapping around part of an axon like electrical tape
wrapped around a wire. This protective covering around the axon is
called a myelin sheath, which we discuss in section 14.3a.
What are the functions of an astrocyte.
■ Help form the blood-brain barrier. Ends of astrocyte
processes called perivascular feet wrap completely around and
cover the outer surface of capillaries in the brain. Together,
the perivascular feet and the brain capillaries, which are less
“leaky” than other capillaries in the body, contribute to a
blood-brain barrier (BBB) that strictly controls substances
entering the nervous tissue in the brain from the blood. This
blood-brain barrier protects the delicate brain from toxins (such
as certain waste products and drugs in the blood), but allows
needed nutrients to pass through. Sometimes this barrier is
detrimental; for example, some medications are not allowed to
exit the capillaries and enter the nervous tissue in the brain.
■ Regulate tissue fluid composition. Astrocytes help regulate
the chemical composition of the interstitial fluid within the
brain by controlling movement of molecules from the blood
to the interstitial fluid.
■ Form a structural network. The cytoskeleton in astrocytes
strengthens and organizes nervous tissue in the CNS.
■ Replace damaged neurons. When neurons are damaged and
die, the space they formerly occupied is often filled by cells
produced by astrocyte division, a process termed astrocytosis.
■ Assist neuronal development. Astrocytes help direct the
development of neurons in the fetal brain by secreting
chemicals that regulate the connections between neurons.
■ Help regulate synaptic transmission. A two-way
communication pathway is established between astrocytes
and neurons at the synapse.
