Chapter 6 The Nervous System 197
Two Major Divisions
The central nervous system (CNS) includes
the brain and spinal cord. The CNS directs the
activity of the entire nervous system. Injuries to
either the brain or the spinal cord have serious
consequences and can be life threatening.
Fortunately, these delicate structures are well
protected inside the skull and vertebral column.
The parts of the nervous system other than
the brain and spinal cord make up what is called
the peripheral nervous system (PNS). For
example, the PNS includes spinal nerves that
transmit information to and from the spinal cord
and cranial nerves that transmit information
to and from the brain. The PNS also includes
specialized nerve endings called sensory receptors,
which respond to stimuli such as pressure, pain,
or temperature.
Nerves that transmit impulses from the
sensory receptors in the skin, muscles, and joints to
the CNS are known as afferent (sensory) nerves.
Those that carry impulses from the CNS out to the
muscles and glands are efferent (motor) nerves.
The Efferent Nerves
There are two functional subdivisions of
the efferent, or motor, nerves. The somatic
(voluntary) nervous system stimulates our
skeletal muscles, causing them to develop
tension. The autonomic (involuntary) nervous
system controls the cardiac muscle of the heart
and the smooth muscles of the internal organs.
The autonomic nervous system prompts the
heart to beat faster when we exercise and causes
the smooth muscle activities that move food
through the digestive system.
Thanks to our autonomic nervous system,
we do not have to think about everyday body
functions that sustain life. And under certain
Memory Tip
Afferent nerves tell the body how it is
being affected by stimuli such as light,
heat, and pressure.
Efferent nerves stimulate muscles to produce
effort.
Figure 6.1 This diagram represents the organization of the nervous system and summarizes the relationships
among the subdivisions. If you were asked to put the word voluntary into one of the six boxes above and
involuntary into another one of the boxes, where would you put each word?
Central Nervous System (CNS)
(brain and spinal cord)
Peripheral Nervous System (PNS)
(parts of the nervous system other
than brain and spinal cord)
Autonomic Nervous System
(to cardiac and smooth muscles
and glands)
Somatic Nervous System
(to skeletal muscles)
Parasympathetic Nervous System
(routine involuntary functions)
Sympathetic Nervous System
(high alert)
Sensory
Receptors
Sensory (afferent)
Motor (efferent)