Læknaneminn - 01.10.1996, Page 90

Overdoses and poisonings: physiological responses
The autonomic nervous system is responsible for
maintaining homeostasis in the body with respect to:
• Respiration
• Metabolism
• Temperature
• Circulation
• Secretions
It supplies involuntary neural input to the viscera via
two different divisions:
• Sympathetic or thoracolumbar division
• Parasympathetic or craniosacral division
The inputs from these two divisions tend to produce
opposite or balancing effects on a given end organ.
Both divisions utilise ganglia outside the spinal cord.
• Those of the sympathetic division are near the
spinal cord allowing a more intense response.
• Those of the parasympathetic division are near the
end organs limiting response intensity.
A. Neurotransmitters
Acetylcholine is the neurotransmitter for:
• preganglionic fibers of both sympathetic and para-
sympathetic divisions
• postganglionic fibers of the parasympathetic divi-
sion
• neuromuscular junction in the somatic or volunt-
ary nervous system
Norepinephrine is the neurotransmitter for the post-
ganglionic fibers of the sympathetic division.
Each division of the autonomic nervous system can
produce a variety of clinical responses by virtue of the
existence of different types of receptors. These respons-
es may occur as a result of neurotransmitter released at
nerve terminals or carried to the end organ via the cir-
culation.
Which receptor is stimulated determines both the
target end organs to be affected and the nature of the
physiologic response. Further complexity in the sy-
stem is achieved through the binding of ne-
urotransmitter to more than one receptor with differ-
ent affinides.
B. The Sympathetic (Thoracolumbar)
Division
The sympathetic nervous system had three different
neurotransmitters:
• Norepinephrine - found in postganglionic fibers
innervating the skin, eyes,
heart, lungs, gastrointestinal
tract, and exocrine glands
• Epinephrine- found in the adrenal medulla
and certain neurones of the
central nervous system
• Dopamine- found in the extrapyramidal
tracts of the thalamus.
There are two basic types of receptors, each of which
has two subtypes. The two basic receptors are defined
functionally by their response to specific catechola-
mines and adrenoreceptor blocking agents.
1. Alpha receptors
a-1 receptors:
• Predominate in the peripheral nervous system
• Are postsynaptic
• When stimulated cause vasoconstriction, pupillary
dilatation, coronary artery dilatation, decreased
bowel motility, and bladder contraction.
a-2 receptors:
• Are more prevalent in the central nervous system
• Mediate a decrease in sympathetic tone and en-
hancement of vagal tone
There are also several types of a -2 receptors in the
peripheral nervous system:
• Some are presynaptic and mediate negative feed-
back
• Some are extrasynaptic and cause vasoconstriction
when stimulated (main mechanism by which intra-
venous pressors work)
• Some prevent acetylcholine release at muscarinic
(see below) synapses.
2. Beta receptors
-1 receptors:
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2. tbl. 1996, 49. árg.