C h a p t e r 1
Cell Structure and Function
27
areas where neurogenesis occurs from neural stem cells/
progenitor cells throughout life. Embryonic development
of the nervous system and the structure and functions of
the nervous system are discussed more fully in Chapter 34.
Structurally, nervous tissue consists of two cell types:
nerve cells or neurons and supporting cells or neuroglia.
Most nerve cells consist of three parts: the soma or cell
body, dendrites, and the axon. The cytoplasm-filled den-
drites, which are multiple elongated processes, receive
and carry stimuli from the environment, sensory epithe-
lial cells, and other neurons to the cell. The axon, which
is a single cytoplasm-filled process, is specialized for gen-
erating and conducting nerve impulses away from the cell
body to other nerve cells, muscle cells, and glandular cells.
Neurons can be classified as afferent and efferent
neurons according to their function. Afferent or sensory
neurons carry information toward the central nervous
system; they are involved in the reception of sensory
information from the external environment and from
within the body. Efferent or motor neurons carry infor-
mation away from the central nervous system (CNS);
they are needed for control of muscle fibers and endo-
crine and exocrine glands.
Communication between neurons and effector organs,
such as muscle cells, occurs at specialized structures called
synapses
. At the synapse, chemical messengers (i.e., neu-
rotransmitters) alter the membrane potential to conduct
impulses from one nerve to another or from a neuron to
an effector cell. In addition, electrical synapses exist in
which nerve cells are linked through gap junctions that
permit the passage of ions from one cell to another.
Neuroglia (
glia
means “glue”) are the cells that sup-
port neurons, form myelin, and have trophic and phago-
cytic functions. Four types of neuroglia are found in the
CNS: astrocytes, oligodendrocytes, microglia, and epen-
dymal cells. Astrocytes are the most abundant of the
neuroglia. They have many long processes that surround
blood vessels in the CNS. They provide structural sup-
port for the neurons, and their extensions form a sealed
barrier that protects the CNS. In last decade, investiga-
tions have established important roles for astrocytes in
the response of the brain to injury, calcium ion-based
excitation, CNS metabolism, neural stem cell source,
blood–brain barrier maintenance, and numerous dis-
eases and pathological conditions. The oligodendrocytes
provide myelination of neuronal processes in the CNS.
The microglia are phagocytic cells that represent the
mononuclear phagocytic system in the nervous system.
Ependymal cells line the cavities of the brain and spinal
cord and are in contact with the cerebrospinal fluid. In
the peripheral nervous system, supporting cells consist
of the Schwann and satellite cells. The Schwann cells
provide myelination of the axons and dendrites, and the
satellite cells enclose and protect the dorsal root ganglia
and autonomic ganglion cells.
ExtracellularTissue Components
The discussion thus far has focused on the cellular com-
ponents of the different tissue types. Within tissues, cells
are held together by cell junctions, and adhesion mol-
ecules form intercellular contacts.
Cell Junctions
The junctions between tissue cells are important in govern-
ing the shape of the body, transmitting mechanical stresses
fromone cell to another, and creating pathways for commu-
nication. Cell junctions occur at many points in cell-to-cell
contact, but they are particularly plentiful and important
in epithelial tissue. These specialized junctions enable cells
to form barriers to the movement of water, solutes, and
cells from one body compartment to the next. Three basic
types of intercellular junctions are observed: tight junctions,
adhering junctions, and gap junctions (Fig. 1-20).
Tight
or
occluding junctions
(i.e., zonula occludens),
which are found in epithelial tissue, seal the surface
membranes of adjacent cells together. This type of inter-
cellular junction prevents fluids and materials such as
macromolecules present in the intestinal contents from
entering the intercellular space.
Adhering junctions
represent sites of strong adhesion
between cells. The primary role of adhering junctions
may be that of preventing cell separation. Adhering junc-
tions are not restricted to epithelial tissue; they provide
adherence between adjacent cardiac muscle cells as well.
Adhering junctions are found as continuous, beltlike
adhesive junctions (i.e., zonula adherens) or scattered,
spotlike adhesive junctions, called
desmosomes
(i.e.,
macula adherens). A special feature of the adhesion belt
junction is that it provides an anchoring site to the cell
membrane for actin filaments. In epithelial desmosomes,
bundles of keratin intermediate filaments (i.e., tonofila-
ments) are anchored to the junction on the cytoplasmic
area of the cell membrane. A primary disease of desmo-
somes is pemphigus, which is caused by antibody binding
to the desmosome proteins and the resulting separation of
neighboring cells. Affected persons have skin and mucous
membrane blistering.
Hemidesmosomes
, which resemble
a half desmosome, are another type of junction. They are
found at the base of epithelial cells and help attach the
epithelial cell to the underlying connective tissue.
Gap
or
nexus junctions
involve the close adherence of
adjoining cell membranes with the formation of channels
that connect the cytoplasm of the two cells. Because they
are low-resistance channels, gap junctions are impor-
tant in cell-to-cell conduction of electrical signals (e.g.,
between cells in sheets of smooth muscle or between
adjacent cardiac muscle cells, where they function as
electrical synapses). Gap junctions also enable ions and
small molecules to pass directly from one cell to another.
Extracellular Matrix
Tissues are not made up solely of cells. A large part of
their volume is made up of an extracellular matrix. This
matrix is composed of a variety of proteins and polysac-
charides (i.e., polymers made up of many sugar mono-
mers). These proteins and polysaccharides are secreted
locally and are organized into a supporting meshwork
in close association with the cells that produced them.
The amount and composition of the matrix vary with
the different tissues and their function. In bone, for
example, the matrix is more plentiful than the cells that
surround it; in the brain, the cells are much more abun-
dant and the matrix is only a minor constituent.
