njit-etd2003-081 - New Jersey Institute of Technology

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and the target cell is the same as the pre- and post- ganglionic neurotransmitter,
acetylcholine. Although acetylcholine generally has an excitatory effect, it is also known
to have inhibitory effects as well, such as the slowing of the heart by the vagus nerve. In
Figure 2.5, it is important to realize that some organs, such as the heart, eyes, and
stomach, receive autonomic activity from both the sympathetic and parasympathetic
nervous systems. This is often called "dual innervation". Usually, but not always,
whatever affects the sympathetic nervous system has on the effector cells; the
parasympathetic nervous system has the opposite effect [5]. In general, the sympathetic
nervous system increases its response under conditions of stress. It is responsible for
what is known as the fight-or-flight response. On the other hand, activity of the
parasympathetic nervous system is associated with relaxing and the storing of energy.
For example, heart rate increases with sympathetic activity and decreases with
parasympathetic activity. Table 2.2 summarizes the effects of the autonomic nervous
system on selected organs.
Dual innervation by nerve fibers that cause opposite responses provides a very
fine degree of control over the effector organ- it is like equipping a car with both an
accelerator and a brake [5]. One can slow the car simply by decreasing the pressure on
the accelerator; however, the combined effects of releasing the accelerator and applying
the brake provide faster and more accurate control. Analogously, the sympathetic and
parasympathetic divisions are usually activated reciprocally; that is, as the activity of one
division is increased, the activity of the other is decreased.
In addition to dual innervation, another important physiological characteristic is
that the sympathetic and parasympathetic nervous systems are continually active.