Nerve Cells "Talk" to Each Other 192
• The nervous system is made of neurons and neuroglia.
Neurons have cell bodies, dendrites, axons, and axon ter-
minals and can transmit information in the form of electro-
chemical signals. A layer of myelin, which helps to speed
up nerve impulse transmissions, covers some nerve fibers.
Neuroglial cells (astrocytes, microglia, Schwann cells, and
ependymal cells) support and maintain neurons.
• Like other excitable cells, neurons have a resting membrane
potential (approximately -70 mV), which is caused by an
imbalance of ions across the cell membrane. Various stimuli
cause sodium or potassium channels to open, which can
change the membrane potential slightly. If enough sodium
channels open, the cell will reach the threshold potential.
Then the membrane will depolarize rapidly. This triggers
openings of potassium channels, which eventually repo-
larize the membrane. An action potential is transmitted
rapidly down the length of the neuron, causing other nerve
segments to go through the same process.
• At the synapse, the action potential triggers the release of
a chemical called a neurotransmitter into the gap or cleft
between neurons, as shown. The neurotransmitter diffuses
across the cleft, binds to receptors on the postsynaptic cell,
and triggers an action potential.
Synaptic transmission • Figure 7.6
o c J
Axon terminal of presynaptic neuron
The Central Nervous System Coordinates
All Nervous Activity 200
• The central nervous system (CNS) consists of the brain and
spinal cord. The brain lies in the cranial cavity, is encased
in the protective meninges, and is bathed in cerebrospinal
fluid. The spinal cord is connected to the brain and lies in
the vertebral column, where it is also encased in meninges
and bathed in cerebrospinal fluid. The CNS tissues are iso-
lated from harmful chemicals by the blood-brain barrier.
• As shown, the brain is composed of four parts: the brain-
stem, diencephalon, cerebellum, and cerebrum. The brain
receives sensory information, processes it, and sends out
motor commands using various pathways. It also takes care
of involuntary functions (breathing, adjustments to heart
rate, and so on) and voluntary muscle actions, processes
information from special senses, controls emotions, and
allows thought processes to occur. Each different part of the
brain performs designated tasks.
The brain • Figure 7.10
• The spinal cord is divided into segments. Sensory and mo-
tor neurons enter and leave the various segments through
spinal nerves. Nerves are conduits that consist of wrapped
bundles of nerve fibers (axons, dendrites, or a combination
of both). Spinal nerves make up pathways that relay senso-
ry information to the brain through ascending spinal tracts
and receive motor information from the brain via descend-
ing spinal tracts. Interneurons within the gray matter of the
CNS help process information and relay it to the appropriate
portion of the brain.
220 CHAPTER 7
The Nervous System