Biology 12

Chapter 5 The Nervous System • MHR 139

The autonomic nervous system is not consciously
controlled. As Figure 5.3 shows, the autonomic
nervous system is made up of the sympathetic and
parasympathetic nervous systems, which control a
number of organs within the body. The sympathetic
nervous systemsets off the “fight-or-flight” reaction
that prepares the body to deal with an immediate
threat. When this system is stimulated, heart rate
and breathing rate increase. Also, blood sugar is
released from the liver to provide the energy
required to deal with the perceived threat.
Many perceived threats today do not require the
fight-or-flight response. In fact, this response may
worsen a situation. For example, some students
suffer from test anxiety. In these individuals, the
stress of a test produces rapid heart and breathing

rates that may interfere with higher levels of brain

activity, such as concentration and memory.

Learning relaxation techniques can often help

people deal with stressful situations. (You will

learn more about the “fight-or-flight” response in

Chapter 6.)

The parasympathetic nervous systemhas an

effect opposite to that of the sympathetic nervous

system. When a threat has passed, the nerves of

this system slow heart rate and breathing rate and

reverse the effects of the sympathetic nervous

system response.

The somatic nervous system is made up of

sensory nervesthat carry impulses from the body’s

sense organs to the central nervous system. This

system also consists of motor nervesthat transmit

Figure 5.3The ganglia of sympathetic nerves are located
near the spine. The ganglia of parasympathetic nerves are
located far from the spine, near the organs they affect. As

you can see, a nerve pathway runs from both of the systems

to every organ indicated, except the adrenal gland.

bladder

Parasympathetic

Nervous System

constrict bronchioles

slow down heartbeat

increase secretion

large intestine

increase motility

empty colon

empty bladder

Sympathetic

Nervous System

spinal

cord

sympathetic

ganglion

chain

constrict

dilate

secrete saliva

stop secretion

dilate bronchioles

speed up heartbeat

secrete adrenaline

decrease secretion

decrease motility

(ability to move

spontaneously)

small intestine

retain colon contents

delay emptying

adrenal

gland stomach