27. Autonomic nervous system: neurotransmitters, receptors, effects.
Classification of ANS drugs.
The nervous system is conventionally divided into the central nervous system (CNS; the
brain and spinal cord) and the peripheral nervous system (PNS; neuronal tissues outside
the CNS). The motor (efferent) portion of the nervous system can be divided into two major
subdivisions: autonomic and somatic.
The autonomic nervous system (ANS) is largely independent (autonomous) in that its
activities are not under direct conscious control. It is concerned primarily with visceral
functions such as cardiac output, blood flow to various organs, and digestion, which are
necessary for life.
The nervous system has several properties in common with the endocrine system, which is
the other major system for control of body function. These include high-level integration in
the brain, the ability to influence processes in distant regions of the body, and extensive use
of negative feedback.
In the nervous system, chemical transmission occurs between nerve cells and between
nerve cells and their effector cells. Chemical transmission takes place through the release of
small amounts of transmitter substances from the nerve terminals into the synaptic cleft.
The transmitter crosses the cleft by diffusion and activates or inhibits the postsynaptic cell
by binding to a specialized receptor molecule.
By using drugs that mimic or block the actions of chemical transmitters, we can selectively
modify many autonomic functions. These functions involve a variety of effector tissues,
including cardiac muscle, smooth muscle, vascular endothelium, exocrine glands, and
presynaptic nerve terminals.
The ANS lends itself to division on anatomic grounds into two major portions: the
sympathetic (thoracolumbar) division and the parasympathetic (craniosacral) division. The
sympathetic preganglionic fibers leave the CNS through the thoracic and lumbar spinal
nerves. The parasympathetic preganglionic fibers leave the CNS through the cranial nerves
and the third and fourth sacral spinal nerve roots.
A large number of peripheral ANS fibers synthesize and release acetylcholine; they are
cholinergic fibers; that is, they work by releasing acetylcholine. Most postganglionic
sympathetic fibers release norepinephrine (also known as noradrenaline); they are
noradrenergic fibers; that is, they work by releasing norepinephrine.
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Classification of ANS drugs.
The nervous system is conventionally divided into the central nervous system (CNS; the
brain and spinal cord) and the peripheral nervous system (PNS; neuronal tissues outside
the CNS). The motor (efferent) portion of the nervous system can be divided into two major
subdivisions: autonomic and somatic.
The autonomic nervous system (ANS) is largely independent (autonomous) in that its
activities are not under direct conscious control. It is concerned primarily with visceral
functions such as cardiac output, blood flow to various organs, and digestion, which are
necessary for life.
The nervous system has several properties in common with the endocrine system, which is
the other major system for control of body function. These include high-level integration in
the brain, the ability to influence processes in distant regions of the body, and extensive use
of negative feedback.
In the nervous system, chemical transmission occurs between nerve cells and between
nerve cells and their effector cells. Chemical transmission takes place through the release of
small amounts of transmitter substances from the nerve terminals into the synaptic cleft.
The transmitter crosses the cleft by diffusion and activates or inhibits the postsynaptic cell
by binding to a specialized receptor molecule.
By using drugs that mimic or block the actions of chemical transmitters, we can selectively
modify many autonomic functions. These functions involve a variety of effector tissues,
including cardiac muscle, smooth muscle, vascular endothelium, exocrine glands, and
presynaptic nerve terminals.
The ANS lends itself to division on anatomic grounds into two major portions: the
sympathetic (thoracolumbar) division and the parasympathetic (craniosacral) division. The
sympathetic preganglionic fibers leave the CNS through the thoracic and lumbar spinal
nerves. The parasympathetic preganglionic fibers leave the CNS through the cranial nerves
and the third and fourth sacral spinal nerve roots.
A large number of peripheral ANS fibers synthesize and release acetylcholine; they are
cholinergic fibers; that is, they work by releasing acetylcholine. Most postganglionic
sympathetic fibers release norepinephrine (also known as noradrenaline); they are
noradrenergic fibers; that is, they work by releasing norepinephrine.
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