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MODU LE 2.1
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CONCEPT CHART 2.1
Parts of the Neuron
Concept 2.2
The nervous system has three types
of neurons: sensory neurons, motor
neurons, and interneurons.
neurons
Nerve cells.
brain The mass of nerve tissue encased
in the skull that controls virtually everything we are and everything we do.
soma The cell body of a neuron that
contains the nucleus of the cell and carries out the cell’s metabolic functions.
axon The tubelike part of a neuron
which carries messages away from the
cell body toward other neurons.
terminal buttons Swellings at the tips
of axons from which neurotransmitters
are dispatched into the synapse.
neurotransmitters Chemical messengers that transport nerve impulses from
one nerve cell to another.
synapse The small fluid-filled gap between neurons through which neurotransmitters carry neural impulses.
dendrites Rootlike structures at the
end of axons that receive neural impulses from neighboring neurons.
sensory neurons Neurons that transmit information from sensory organs,
muscles, and inner organs to the spinal
cord and brain.
motor neurons Neurons that convey
nerve impulses from the central nervous
system to muscles and glands.
glands Body organs or structures that
produce secretions.
hormones Secretions from endocrine
glands that help regulate bodily
processes.
interneurons Nerve cells within the
central nervous system that process information.
nerve A bundle of axons from different
neurons that transmit nerve impulses.
Part
Description
Functions
Soma
Cell body containing the
nucleus
Performs metabolic, or life-sustaining,
functions of the cell
Axon
Long cable projecting
from the soma
Carries neural impulses to the terminal
buttons
Terminal
buttons
Swellings at ends of axons
Release chemicals, called neurotransmitters, that carry neural messages to
adjacent neurons
Dendrites
Fibers that project from
the soma
Receive messages from neighboring
neurons
The axons of the neurons in your brain may be only a few thousandths of
an inch long. Other axons, such as those that run from your spinal cord to your
toes, are several feet long. Axons may branch off like the stems of plants, fanning out in different directions. At the ends of these branches are knoblike
swellings called terminal buttons. It is here that chemicals called neurotransmitters are stored and released. These chemicals are synthesized in the soma and
ferry outgoing messages to neighboring neurons across the synapse, a tiny gap
that separates one neuron from another.
Dendrites are treelike structures that project from the soma. Dendrites have
receptor sites, or docking stations, that enable them to receive neurotransmitters
released by neighboring neurons (Häusser, Spruston, & Stuart, 2000). Through its
dendrites, each neuron may receive messages from thousands of other neurons
(Kennedy, 2000).
The nervous system has three types of neurons: sensory neurons, motor neurons, and interneurons. These different types play specialized roles in the nervous system.
Sensory neurons (also called afferent neurons) transmit information about the
outside world to the spinal cord and brain. This information first registers on
your sensory organs. So when someone touches your hand, sensory receptors
within the skin transmit the message through sensory neurons to the spinal cord
and brain, where the information is processed, resulting in the feeling of touch.
Sensory neurons also carry information from your muscles and inner organs to
your spinal cord and brain.
Motor neurons (also called efferent neurons) convey messages from the brain
and spinal cord to the muscles that control the movements of your body. They
also convey messages to your glands, causing them to release hormones, chemical substances that help regulate bodily processes.
Interneurons (also called associative neurons) are the most common type of
neuron in the nervous system. They connect neurons to neurons. In the spinal
cord, they connect sensory neurons to motor neurons. In the brain, they form
complex assemblages of interconnected nerve cells that process information from
sensory organs and control higher mental functions, such as planning and
thinking.
A neuron is not the same thing as a nerve. A nerve is a bundle of axons from
different neurons. An individual nerve — for example, the optic nerve, which
transmits messages from the eyes to the brain — contains more than a million axons. Although individual axons are microscopic, a nerve may be visible to the
naked eye. The cell bodies of the neurons that contain the axons are not part of
the nerve itself.