14.3: Autonomic Synapses and Effects
- Page ID
- 63455
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- Determine the effect of the autonomic nervous system on the regulation of the various organ systems
- Explain dual innervation
Autonomic Neurotransmitters
The main signaling molecules of the autonomic nervous system are acetylcholine (ACh) and norepinephrine (NE, also called noradrenaline). The adrenal medulla, which is a modified sympathetic ganglion, releases epinephrine (or adrenaline). Cells that release acetylcholine are called cholinergic, while cells that release norepinephrine or epinephrine are called adrenergic. These molecules will bind to specific receptors on the target cells. Acetylcholine can bind to both nicotinic receptors and muscarinic receptors. Norepinephrine and epinephrine can bind to adrenergic receptors. For each of these classes of receptors, there are multiple subtypes (which we will not discuss in this book) that make the cells respond differently to the binding of the same molecule. Depending on the receptor type and subtype, the molecules released will cause either stimulation or inhibition. Thus, the effects of the autonomic divisions depend on the type of signaling molecule and receptor involved.
- All preganglionic fibers, both sympathetic and parasympathetic, are cholinergic and release ACh (Table \(\PageIndex{1}\)).
- All ganglionic neurons have nicotinic receptors in their cell membranes, which leads to the stimulation and firing of an action potential from the ganglionic fibers in both sympathetic and parasympathetic divisions.
- Most sympathetic postganglionic fibers are adrenergic and release norepinephrine.
- The adrenal medulla which is part of the sympathetic division is also adrenergic and releases epinephrine.
- Parasympathetic postganglionic fibers are cholinergic and release ACh.
This pattern assures that effector organs under the control of both sympathetic and parasympathetic systems can respond differently. Target cells can contain various types and subtypes of receptors and their response will vary depending on the type of receptor and on the neurotransmitter released on them.
Autonomic targets that are innervated by both divisions of the autonomic system respond based on which neurotransmitter is released and what receptor is present. For example, regions of the heart that establish heart rate are contacted by postganglionic fibers from both systems. If norepinephrine is released, it binds to an adrenergic receptor which causes the heart rate to increase. If ACh is released, it binds to a muscarinic receptor that causes the heart rate to slow. Without this parasympathetic input, the heart would work at a rate of approximately 100 beats per minute (bpm). The sympathetic system speeds that up, as it would during exercise, to 120–140 bpm, for example. The parasympathetic system slows it down to the resting heart rate of 60–80 bpm.
| Feature | Sympathetic Division | Parasympathetic Division |
|---|---|---|
| Preganglionic axons | release ACh (cholinergic) | release ACh (cholinergic) |
| Ganglionic neurons | contain nicotinic (ACh) receptors | contain nicotinic (ACh) receptors |
| Postganglionic axons |
release NE (adrenergic); release ACh (cholinergic) only in blood vessels of skeletal muscle and sweat glands |
release ACh (cholinergic) |
| Adrenal medulla | contain muscarinic (ACh) receptors; releases epinephrine (adrenergic) |
Autonomic Effects
Organ systems are balanced between the input from the sympathetic and parasympathetic divisions. When something upsets that balance, the homeostatic mechanisms strive to return it to its regular state. For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state. For example, the resting heart rate is the result of the parasympathetic system slowing the heart down from its intrinsic rate of 100 bpm, consequently the heart can be said to be in parasympathetic tone.
Many effector organs of the autonomic nervous system have dual innervation, meaning that they receive competing inputs from the sympathetic and parasympathetic divisions. These divisions each play a role in effecting change, usually in competing directions. At the level of the target effector, the signal of which system is sending the message is strictly chemical and depending on the division involved and neurotransmitter released, the effects would be diverse. For example, the heart and the eye are examples of organs with dual innervation. The sympathetic system increases heart rate, whereas the parasympathetic system decreases heart rate. The sympathetic system dilates the pupil of the eye, whereas the parasympathetic system constricts the pupil.
In some organs, opposing effects are achieved without dual innervation. For example, the arrector pili muscles, sweat glands, and blood vessels to skeletal muscles and skin are primarily under sympathetic control. Blood pressure is partially determined by the contraction of smooth muscle in the walls of blood vessels. The parasympathetic system has no significant input to the systemic blood vessels, so the sympathetic system determines their tone. The sympathetic system causes vasoconstriction of blood vessels. However, the increasing metabolic activity of muscles causes (paracrine) vasodilation. This allows for blood flow to increase for those skeletal muscles that will be active in the fight-or-flight response.
Not always the sympathetic and parasympathetic divisions have opposite effects and, in a few cases, the two systems cooperate. The best example of cooperative effects occurs in the male sexual function. In reproductive organs, blood vessels of erectile tissues are innervate with parasympathetic projection, making them an exception. Acetylcholine released by these postganglionic parasympathetic fibers cause the vessels to dilate, leading to the engorgement of the erectile tissue and penile erection. At the same time, the sympathetic nervous system stimulates ejaculation by causing the contractions of the seminal vesicle and prostate gland. This synergistic effect facilitates reproduction. Table \(\PageIndex{2}\) summarizes the effects on different organs of the sympathetic and parasympathetic divisions.
| Target Effector | Sympathetic Effect | Parasympathetic Effect |
|---|---|---|
| Arrector pili muscles | Contraction to cause hair erection | None |
| Sweat glands | Secretion | None |
| Salivary glands | Inhibits | Stimulates |
| Pupils | Dilation | Constriction |
| Ciliary muscle | None | Contraction for near vision |
| Heart | Increases heart rate | Decreases heart rate |
| Blood vessels of the heart (coronary) | Vasoconstriction or vasodilation | Vasodilation |
| Blood vessels to skeletal muscles | Vasodilation | None |
| Blood vessels to skin and other organs | Vasoconstriction to increase blood pressure | None |
| Blood vessels to gastrointestinal (GI) tract | Vasoconstriction | Vasodilation |
| Bronchi of lungs | Dilation | Constriction |
| Gastrointestinal (GI) tract gland secretion | Inhibits | Stimulates |
| Gallbladder | Inhibits | Stimulates |
| Peristalsis (motility) | Inhibits | Stimulates |
| Sphincters | Contraction (close) | Relaxation (open) |
| Urinary bladder | Relaxation | Contraction |
| Internal urethral sphincter | Contraction (close) | Relaxation (open) |
| Penis | Stimulates ejaculation | Stimulates erection |
| Clitoris | None | Stimulates erection |
Nervous System: Kehr’s Sign
Kehr’s sign is the presentation of pain in the left shoulder, chest, and neck regions following rupture of the spleen. The spleen is in the upper-left abdominopelvic quadrant, but the pain is more in the shoulder and neck. How can this be? The sympathetic fibers connected to the spleen are from the celiac ganglion, which would be from the mid-thoracic to lower thoracic region whereas parasympathetic fibers are found in the vagus nerve, which connects in the medulla of the brainstem. However, the neck and shoulder would connect to the spinal cord at the mid-cervical level of the spinal cord. These connections do not fit with the expected correspondence of visceral and somatosensory fibers entering at the same level of the spinal cord.
The incorrect assumption would be that the visceral sensations are coming from the spleen directly. In fact, the visceral fibers are coming from the diaphragm. The nerve connecting to the diaphragm takes a special route. The phrenic nerve is connected to the spinal cord at cervical levels 3 to 5. The motor fibers that make up this nerve are responsible for the muscle contractions that drive ventilation. These fibers have left the spinal cord to enter the phrenic nerve, meaning that spinal cord damage below the mid-cervical level is not fatal by making ventilation impossible. Therefore, the visceral fibers from the diaphragm enter the spinal cord at the same level as the somatosensory fibers from the neck and shoulder.
The diaphragm plays a role in Kehr’s sign because the spleen is just inferior to the diaphragm in the upper-left quadrant of the abdominopelvic cavity. When the spleen ruptures, blood spills into this region. The accumulating hemorrhage then puts pressure on the diaphragm. The visceral sensation is actually in the diaphragm, so the referred pain is in a region of the body that corresponds to the diaphragm, not the spleen
Concept Review
All preganglionic fibers are cholinergic and release acetylcholine (ACh). All ganglionic neurons (the targets of these preganglionic fibers) have nicotinic receptors in their cell membranes. Most sympathetic postganglionic fibers are adrenergic and release norepinephrine. The adrenal medulla which is part of the sympathetic division is also adrenergic and releases epinephrine. Parasympathetic postganglionic fibers are cholinergic and release ACh. Target cells can contain various types and subtypes of receptors and their response will vary depending on the type of receptor and on the neurotransmitter released on them. The sympathetic postganglionic fibers that contact the blood vessels within skeletal muscle and sweat glands in the integument release ACh instead of norepinephrine. This does not create any problem because there is no parasympathetic input to these organs.
For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state, which is known as the autonomic tone of the system. Many effector organs of the autonomic nervous system have dual innervation, meaning that they receive competing inputs from the sympathetic and parasympathetic divisions. The sympathetic system increases heart rate, whereas the parasympathetic system decreases heart rate. The sympathetic system dilates the pupil of the eye, whereas the parasympathetic system constricts the pupil. The competing inputs can contribute to the resting tone of the organ system. Heart rate is normally under parasympathetic tone, whereas blood pressure is normally under sympathetic tone. The heart rate is slowed by the autonomic system at rest, whereas blood vessels retain a slight constriction at rest. In a few systems of the body, the competing input from the two divisions is not the norm. The sympathetic tone of blood vessels is caused by the lack of parasympathetic input to the systemic circulatory system. Only certain regions receive parasympathetic input that relaxes the smooth muscle wall of the blood vessels. Sweat glands are another example of organs that only receive input from the sympathetic system. Not always the sympathetic and parasympathetic divisions have opposite effects and, in a few cases, the two systems cooperate.
Critical Thinking Questions
Query \(\PageIndex{1}\)
Glossary
Query \(\PageIndex{2}\)
Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology