iology of the system. The nervous system is comprised of the brain, spinal cord,
nerves, and ganglia. Collectively, they receive stimuli and transmit information.
There are two nervous systems. These are the central nervous system (CNS)
and the peripheral nervous system (PNS). The central nervous system consists
of the brain and spinal cord, which are responsible for regulating body function.
The central nervous system receives information from the peripheral nervous
system, which is interpreted, and then the central nervous system sends an
appropriate signal to the peripheral nervous system to stimulate cellular activity.
Depending on the signal, the stimulation either increases or blocks nerve cells,
which are called neurons.
The peripheral nervous system is organized into two divisions. These are the
somatic nervous system (SNS) and the autonomic nervous system (ANS). The
somatic nervous system acts on skeletal muscles to produce voluntary move-
ment. The autonomic nervous system, known as the visceral system, is respon-
sible for involuntary movement and controls the heart, respiratory system,
gastrointestinal system, and the endocrine system (glands).
The autonomic nervous system is further divided into the sympathetic and
parasympathetic nervous systems (see Autonomic Nervous System).
The sympathetic nervous system is called the adrenergic system and uses
the norephinephrine neurotransmitter to send information. The parasympathetic
system, called the cholinergic system, uses the acetylcholine neurotransmitter to
transmit information.
Both the sympathetic and parasympathetic nervous systems innervate organs
within the body. The sympathetic system excites the organ while the parasympa-
thetic system inhibits the organ. For example, the sympathetic system increases
the heart rate while the parasympathetic system decreases the heart rate.NEUROLOGICAL PATHWAYS
Neurological pathways extend from locations in the spinal cord to various areas
of the body. These pathways contain two types of nerve fibers. These pregan-
glionic and postganglionic fibers are connected together by a ganglion. The pre-
ganglionic nerve fiber carries messages from the central nervous system to the
ganglion. The postganglionic nerve fiber transmits that message to specific tis-
sues and organs from the ganglion.
Neurological pathways in the sympathetic nervous system originate from the
thoracic (T1 to T12) and the upper lumbar segments (L1 and L2) of the spinal
cord. This is why the sympathetic nervous system is also referred to as the tho-
racolumbar division of the autonomic nervous system.(^270) CHAPTER 15 Nervous System Drugs