C h a p t e r 3 4
Organization and Control of Neural Function
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of cell, the
ependymal cell,
forms the lining of the neural
tube cavity, the ventricular system. In some areas, these
cells combine with a rich vascular network to form the
choroid plexus,
where production of the cerebrospinal
fluid (CSF) takes place.
Metabolic Requirements of Nervous
Tissue
Nervous tissue has a high rate of metabolism. Although
the brain makes up only 2% of the body’s weight, it
receives approximately 15% of the resting cardiac out-
put and consumes 20% of its oxygen. Despite its sub-
stantial metabolic requirements, the brain can neither
store oxygen nor engage in anaerobic metabolism. An
interruption in the blood or oxygen supply to the brain
rapidly leads to clinically observable signs and symp-
toms. Without oxygen, brain cells continue to function
for approximately 10 seconds. Unconsciousness occurs
almost simultaneously with cardiac arrest, and the death
of brain cells begins within 4 to 6 minutes. Interruption
of blood flow also leads to the accumulation of meta-
bolic by-products that are toxic to neural tissue.
Glucose is the major fuel source for the nervous
system. Unlike muscle cells, neurons have no glycogen
stores and must rely on glucose from the blood or the
glycogen stores of supporting glial cells to meet their
energy needs. Persons receiving insulin for diabetes may
experience signs of neural dysfunction and unconscious-
ness (i.e., insulin reaction or shock) when blood glucose
drops because of insulin excess (see Chapter 33).
Nerve Cell Communication
Neurons are characterized by their ability to commu-
nicate with other neurons and body cells through elec-
trical impulses or action potentials, which are abrupt,
pulsatile changes in the membrane potential that last a
few ten thousandths to a few thousandths of a second.
The frequency and pattern of action potentials consti-
tute the code used by neurons to transfer information
from one location to another.
Action Potentials
The cell membranes of excitable tissue, including
those of nerve and muscle cells, contain ion channels
that are responsible for generating action potentials
(see Chapter 1, Understanding Membrane Potentials).
These ion channels are guarded by voltage-dependent
gates that open and close with changes in the membrane
potential. Separate voltage-gated channels exist for the
sodium, potassium, and calcium ions. Each type of ion
channel has a characteristic membrane potential that
opens and closes its channels. Also present are ligand-
gated channels that respond to chemical messengers
such as neurotransmitters, mechanically gated channels
that respond to physical changes in the cell membrane,
and light-gated channels that respond to fluctuations in
light levels.
Action potentials can be divided into three phases:
the resting or polarized state, depolarization, and repo-
larization (Fig. 34-4). The
resting membrane potential
(approximately −90 mV for large nerve fibers) is the
undisturbed period of the action potential during which
the nerve is not transmitting impulses. During this
period, the membrane is said to be “
polarized”
because
of the −90 mV negative membrane potential (i.e., posi-
tive on the outside and negative on the inside) that is
present. The resting phase of the membrane potential
continues until some event causes the membrane to
increase its permeability to sodium. A
threshold potential
SUMMARY CONCEPTS
■■
Anatomically, the nervous system can be divided
into two basic components: the central nervous
system (CNS), consisting of the brain and spinal
cord; and the peripheral nervous system (PNS),
which relays afferent or sensory input to the
CNS for processing and transmitting efferent or
motor output from the CNS to effector organs
throughout the body.
■■
The nervous system contains two major types
of cells: neurons, which are functioning cells of
the nervous system, and neuroglial cells, which
protect the nervous system and supply metabolic
support.
■■
Neurons (nerve cells), which are the functional
components of the nervous system, are composed
of three parts: a cell body, which controls cell
activity; dendrites, which conduct information
toward the cell body; and an axon, which carries
impulses from the cell body.
■■
The neuroglial cells that provide support
and protection for the neurons consist of the
Schwann and satellite cells of the PNS and the
oligodendrocytes, astrocytes, microglial cells,
and ependymal cells of the CNS.The Schwann
cells of the PNS and the oligodendrocytes of the
CNS form the myelin sheath that allows for rapid
conduction of impulses.
■■
The nervous system has a high level of metabolic
activity, requiring a continuous supply of
oxygen and glucose. Although the brain makes
up only 2% of the body’s weight, it receives
approximately 15% of the resting cardiac output
and consumes 20% of its oxygen.
