Comparison of somatic and autonomic nervous systems Table 7.1
Property
Somatic
Autonomic
Effectors
Skeletal muscles
Cardiac muscle, smooth muscle, and glands
Type of control
Mainly voluntary
Mainly involuntary
Neural pathway
One motor neuron extends from CNS
and synapses directly with a skeletal
muscle fiber
One motor neuron extends from the CNS and usually syn
apses with another motor neuron in a ganglion; the second
motor neuron synapses with an autonomic effector
Neurotransmitter
Acetylcholine
Acetylcholine or norepinephrine
Action of neurotransmitter
on effector
Always excitatory (causing contraction
of skeletal muscle)
May be excitatory (causing contraction of smooth muscle,
increased heart rate, increased force of heart contraction,
or increased secretion from glands) or inhibitory (causing
relaxation of smooth muscle, decreased heart rate, or
decreased secretions from glands)
The Sympathetic Division of the ANS:
Fight-or-Flight Responses
The sympathetic division of the ANS is also called the
thoracolum bar
divisio n
because
the
sympathetic
nerves
emerge from the thoracic and lumbar segments of the spi-
nal cord. The ganglia lie on either side of the vertebral
column and near large abdominal arteries. A single sym-
pathetic preganglionic axon may synapse with 20 or more
postganglionic neurons and can therefore elicit a number
of effects (Figure 7.15).
The neurons of the sympathetic division use two neu-
rotransmitters. Like cells of the somatic nervous system,
preganglionic
sympathetic
neurons
release
acetylcho-
line (ACh) at their synapses in the sympathetic ganglia.
ACh gets destroyed in the synaptic cleft by the enzyme
acethylcholinesterase (AChE), so synaptic transmission is
short-lived. The sympathetic postganglionic cells release
norepinephrine (NE) at their synapses with target organs.
NE is inactivated more slowly than ACh, so its effects last
longer. The adrenal medulla releases both NE and epi-
nephrine into the blood so they circulate throughout the
body and have an even longer-lasting effect.
The sympathetic division responds to physical or emo-
tional stressors such as exercise, emergency, excitement,
or embarrassment. Your body’s reaction to sympathetic
stimulation is often called the “fight-or-flight” response
because it prepares your body for the physical exertion re-
quired to fight or run from danger (Figure 7.15). Remem-
ber the bus incident described at the beginning of this
section? The reactions that your body experienced were
due to activation of the sympathetic nervous system:
Increased heart rate, force of contraction, and blood
pressure, as well as vasodilation, increase blood flow
to organs and muscles. This helps deliver oxygen and
nutrients to working muscles.
Dilating
the
airways
and
increasing
the
rate
of
breathing increase air flow and bring more oxygen into
the body.
Breaking down glycogen and fatty acids and releasing
glucose into the blood provide energy for working
muscles.
Inhibiting digestion keeps blood from being diverted
from muscles.
210 CHAPTER 7
The Nervous System
previous page 245 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online next page 247 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online Home Toggle text on/off