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Autonomic Nervous System |
| Comparison of the autonomic and somatic (voluntary) nervous
systems: See Figure 14.2.
Both systems involve reflexes and connections to the CNS utilizing unipolar and multipolar sensory neurons, interneurons, and motor neurons. But there are some differences. Functional differences: The autonomic system is often considered a motor system for control of autonomic effectors. These effectors include smooth muscle, glands, and the heart. It also utilizes sensory inputs as part of visceral reflexes and independently as part of broader control mechanisms. Structural differences: While the motor neuron in a somatic pathway travels directly to the skeletal muscle effector, in the autonomic nervous system there are two motor neurons which synapse at an autonomic ganglion. The neurotransmitter also may vary with the parasympathetic division mostly using acetylcholine at both pre and post ganglionic fibers, while the sympathetic division utilizes mostly norepinephrine at the post ganglionic fibers. See below. |
| The Two Autonomic Divisions: Sympathetic and Parasympathetic
These two divisions innervate most of the same effectors, but their actions differ, and they differ structurally as well. See Table 14.1 The Parasympathetic Division - See Figure 14.4. This division originates (has its CNS connection) in either the cranial or sacral portions of the CNS. That means it utilizes cranial nerves (remember III, VII, IX, X) or sacral spinal nerves (the pelvic splanchnic nerves). Its autonomic ganglia are located on or near the effector organ. It utilizes acetylcholine as both the pre-ganglionic and the post-ganglionic neurotransmitter ( we call that cholinergic). The post-ganglionic neurotransmitter is most important because this is the transmitter actually released at the effector organ and which stimulates its receptors. The parasympathetic division has localized and specific effects which can mostly be described as producing the "normal" responses, i.e. non-stress responses, of its effectors. The Sympathetic Division - See Figure 14.5. This division has its origin in the thoracolumbar portion of the spinal cord. Its autonomic ganglia are located in a chain of ganglia (called the lateral chain ganglia) located near the spinal cord. There are also plexuses which allow multiple connections between different components of the division. The sympathetic division uses ACH at its autonomic ganglia, but mostly uses norepinephrine at the effector organ (adrenergic). This permits an entirely different effect on many of the same effectors as the parasympathetic division. In addition the sympathetic division often exhibits a general activation rather than local effect. The sympathetic division often acts as a "stress response" system. The alliteration "Fight or Flight" is sometimes used to describe the way in which the sympathetic division becomes activated to mobilize the body's resources. See Table 14.4, [Figure 14.4 modified] showing the parasympathetic division and [Figure 14.5 modified] showing the sympathetic division for functions of each at the various effectors. |
| Neurotransmitters and Receptors: (See Table 14.3)
The term cholinergic refers to those receptors which respond to the transmitter acetylcholine. Mostly these are parasympathetic. There are two types of cholinergic receptors, classified according to whether they are stimulated by the drug nicotine or by the drug muscarine. The significance is that certain clinical drugs can be used to stimulate or inhibit one type of cholinergic receptor without affecting other types. 1) nicotinic receptors are found at all autonomic ganglia, and at the neuromuscular junctions of skeletal muscles. 2) muscarinic receptors are found at parasympathetic target organs and at certain sympathetic targets - the eccrine sweat glands (which produces copious secretion in thermoregulation to release heat), and blood vessels in skeletal muscles (which are dilated). |
| Adrenergic receptors come in two basic types and each has subtypes. These are found
at sympathetic target organs.
Alpha receptors are mostly excitatory. They cause vasoconstriction of most blood vessels to raise the blood pressure, dilation of the pupils of the eyes, constriction of sphincters in the GI tract as part of its reduced function with sympathetic stimulation. Beta receptors are generally inhibitory, except those to the heart. Beta receptors on the heart increase heart rate and force. But others dilate blood vessels to the heart and lungs, and dilate the bronchi. It is this effect used in anti-bronchitis drugs which are sympathomimetic. Beta receptors inhibit the muscles and glands of the GI tract. |
| Exceptions to general rules (some of which have already been mentioned):
The sweat glands are only innervated by sympathetic fibers. The eccrine gland are innervated by cholinergic fibers which cause them to secrete copiously for thermoregulation. The apocrine glands secrete a viscous fluid containing pheromones in response to stress and sexual arousal. These are controlled by adrenergic fibers. Blood vessels to the sweat glands are adrenergic and dilate in response to sympathetic stimulation. Most blood vessels are innervated by sympathetic fibers only. (The exception is vessels to the genitalia which are dilated by parasympathetic stimulation) Mostly the sympathetic stimulation causes vasoconstriction to raise blood pressure, but to the vessels of the skeletal muscles, heart, and lungs vasodilation occurs as part of the need for more blood and oxygen during "Fight or Flight". |
Revised: April 20, 2001
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