Regulation and Integration of the Body
In this chapter, we have examined how the amazingly com-
plex neurons, via electrical and chemical signals, serve the body
in a variety of ways. Some serve as “lookouts,” others process
information for immediate use or for future reference, and still
others stimulate the body’s muscles and glands into activity.
With this background, we are ready to study the most sophisti-
cated mass of neural tissue in the entire body—the brain (and its
continuation, the spinal cord), the focus of Chapter 12.
Check Your Understanding
What is the name of the growing tip of an axon that “sniffs
out” where to go during development? What is the general
name for the chemicals that tell it where to go?
For answers, see Appendix H.
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Functions and Divisions of the Nervous System
Te nervous system bears a major responsibility for maintaining
body homeostasis. It monitors, integrates, and responds to
information in the environment.
Te nervous system is divided anatomically into the central
nervous system (brain and spinal cord) and the peripheral
nervous system (mainly cranial and spinal nerves).
Te major functional divisions of the PNS are the sensory
(aﬀerent) division, which conveys impulses to the CNS, and the
motor (eﬀerent) division, which conveys impulses from the CNS.
Te eﬀerent division includes the somatic (voluntary) system,
which serves skeletal muscles, and the autonomic (involuntary)
system, which innervates smooth and cardiac muscle and glands.
Histology of Nervous Tissue
Neuroglia (supporting cells) segregate and insulate neurons and
assist neurons in various other ways.
CNS neuroglia include astrocytes, microglial cells, ependymal
cells, and oligodendrocytes. PNS neuroglia include Schwann cells
and satellite cells.
Neurons have a cell body and cytoplasmic processes called axons
A bundle of nerve ﬁbers is called a tract in the CNS and a nerve
in the PNS. A collection of cell bodies is called a nucleus in the
CNS and a ganglion in the PNS.
Te cell body is the biosynthetic (and receptive) center of the neuron.
Except for those found in ganglia, cell bodies are found in the CNS.
Most neurons have many dendrites, receptive processes that
conduct signals from other neurons toward the nerve cell body.
With few exceptions, all neurons have one axon, which generates
and conducts nerve impulses away from the nerve cell body.
Axon terminals release neurotransmitter.
Bidirectional transport along axons uses A±P-dependent motor
proteins “walking” along microtubule tracks. It moves vesicles,
mitochondria, and cytosolic proteins toward the axon terminals
and conducts substances destined for degradation back to the
Large nerve ﬁbers (axons) are myelinated. Te myelin sheath
is formed in the PNS by Schwann cells and in the CNS by
oligodendrocytes. Te myelin sheath gaps are also called nodes
of Ranvier. Nonmyelinated ﬁbers are surrounded by supporting
cells, but the membrane-wrapping process does not occur.
Anatomically, neurons are classiﬁed according to the number of
processes issuing from the cell body as multipolar, bipolar, or unipolar.
Functionally, neurons are classiﬁed according to the direction of
nerve impulse conduction. Sensory neurons conduct impulses
toward the CNS, motor neurons conduct away from the CNS,
and interneurons (association neurons) lie between sensory and
motor neurons in the neural pathways.
Nervous System I; Topic: Anatomy Review, pp. 1–12.
Basic Principles of Electricity
Te measure of the potential energy of separated electrical charges
is called voltage (
) or potential. Current (
) is the ﬂow of electrical
charge from one point to another. Resistance (
) is hindrance to
current ﬂow. Ohm’s law gives the relationship among these:
In the body, ions provide the electrical charges; cellular plasma
membranes provide resistance to ion ﬂow. Te membranes contain
leakage channels (nongated, always open) and gated channels.
Nervous System I; Topic: Ion Channels, pp. 1–10.
The Resting Membrane Potential
A resting neuron exhibits a resting membrane potential, which is
70 mV (inside negative). It is due both to diﬀerences in sodium
and potassium ion concentrations inside and outside the cell and
to diﬀerences in permeability of the membrane to these ions.
Te ionic concentration diﬀerences result from the operation of
the sodium-potassium pump, which ejects 3 Na
from the cell for
each 2 K
Nervous System I; Topic: The Membrane Potential, pp. 1–16.
Membrane Potentials That Act as Signals
Depolarization is a reduction in membrane potential (inside
becomes less negative); hyperpolarization is an increase in
membrane potential (inside becomes more negative).
Graded potentials are small, brief, local changes in membrane
potential that act as short-distance signals. Te current produced
dissipates with distance.