The effects of sympathetic and parasympathetic stimulation on the diameter of the pupil of the eye are analogous to the reciprocal innervation of flexor and extensor skeletal muscles by somatic motor neurons see chapter This is because the iris contains antagonistic muscle layers. Contraction of the radial muscles, which are innervated by sympathetic nerves, causes dilation;. The effects of sympathetic and parasympathetic nerves are generally antagonistic; in a few cases, however, they can be complementary or cooperative.
The effects are complementary when sympathetic and parasympathetic stimulation produce similar effects. The effects are cooperative, or synergistic, when sympathetic and parasympathetic stimulation produce different effects that work together to promote a single action. The effects of sympathetic and parasympathetic stimulation on salivary gland secretion are complementary. The secretion of watery saliva is stimulated by parasympathetic nerves, which also stimulate the secretion of other exocrine glands in the digestive tract.
Sympathetic nerves stimulate the constriction of blood vessels throughout the digestive tract. The resultant decrease in blood flow to the salivary glands causes the production of a thicker, more viscous saliva. The effects of sympathetic and parasympathetic stimulation on the reproductive and urinary systems are cooperative. Erection of the penis, for example, is due to vasodilation resulting from parasympathetic nerve stimulation; ejaculation is due to stimulation through sympathetic nerves.
The two divisions of the autonomic system thus cooperate to enable sexual function in the male. They also cooperate in the female; clitoral erection and vaginal secretions are stimulated by parasympathetic nerves, whereas orgasm is a sympathetic nerve response, as it is in the male.
There is also cooperation between the two divisions in the micturition urination reflex. Although the contraction of the urinary bladder is largely independent of nerve stimulation, it is promoted in part by the action of parasympathetic nerves. This reflex is also enhanced by sympathetic nerve activity, which increases the tone of the bladder muscles. Emotional states that are accompanied by high sympathetic nerve activity such as extreme fear may thus result in reflex urination at bladder volumes that are normally too low to trigger this reflex.
Continue reading here: Organs without Dual Innervation. Natural Treatment for Erectile Dysfunction. Mental Impotence Healer Program. The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions.
In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division.
The sympathetic system is associated with the fight-or-flight response , and parasympathetic activity is referred to by the epithet of rest and digest. Homeostasis is the balance between the two systems. At each target effector, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions.
One causes heart rate to increase, whereas the other causes heart rate to decrease. Watch this video to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind.
In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. What two changes does adrenaline bring about to help the skeletal muscle response? To respond to a threat—to fight or to run away—the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together.
Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles.
To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system CNS to a wide array of ganglia that project to the many effector organs simultaneously.
The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change. The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the thoracolumbar system to reflect this anatomical basis.
A central neuron in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges.
There are typically 23 ganglia in the chain on either side of the spinal column. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots, but through ascending or descending connections through the bridges within the chain.
A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. In Figure The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the target effector the organ, tissue, or gland to be innervated.
An example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The axon from the central neuron the preganglionic fiber shown as a solid line synapses with the ganglionic neuron with the postganglionic fiber shown as a dashed line.
This neuron then projects to a target effector—in this case, the trachea—via gray rami communicantes , which are unmyelinated axons. In some cases, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fiber emerges.
An example of this is spinal nerve T1 that innervates the eye. The spinal nerve tracks up through the chain until it reaches the superior cervical ganglion , where it synapses with the postganglionic neuron see Figure The cervical ganglia are referred to as paravertebral ganglia , given their location adjacent to prevertebral ganglia in the sympathetic chain.
Not all axons from the central neurons terminate in the chain ganglia. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve at the level of T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach see Figure Collateral ganglia , also called prevertebral ganglia , are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as central sympathetic neurons.
They are associated with controlling organs in the abdominal cavity, and are also considered part of the enteric nervous system. The three collateral ganglia are the celiac ganglion , the superior mesenteric ganglion , and the inferior mesenteric ganglion see Figure An axon from the central neuron that projects to a sympathetic ganglion is referred to as a preganglionic fiber or neuron, and represents the output from the CNS to the ganglion.
Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and they are myelinated. A postganglionic fiber —the axon from a ganglionic neuron that projects to the target effector—represents the output of a ganglion that directly influences the organ.
Compared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector. These fibers are unmyelinated. The problem with that usage is that the cell body is in the ganglion, and only the fiber is postganglionic. Typically, the term neuron applies to the entire cell. One type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the adrenal medulla , the interior portion of the adrenal gland Fig.
These axons are still referred to as preganglionic fibers, but the target is not a ganglion. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures.
The cells in the adrenal medulla that are contacted by the preganglionic fibers are called chromaffin cells. These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion. Figure The adrenal gland sits on top of the kidneys in the abdominal cavity. When the adrenal gland is coronally sectioned, the inner adrenal medulla is visible. The adrenal medulla contains chromaffin cells that are innervated by preganglionic sympathetic fibers.
These cells release epinephrine directly into the bloodstream. The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response.
All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons—a single preganglionic sympathetic neuron may have 10—20 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets.
At the level of the spinal cord at which the preganglionic sympathetic fiber exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia.
Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Thus, vasoconstriction and bronchodilation in the lungs are a result of sympathetic stimulation, whereas vasodilation and bronchoconstriction are caused by parasympathetic stimulation.
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Context and Applications Related Concepts. What is Dual Innervation? Organs with Dual Innervation Most visceral organs have dual innervation from the autonomic nervous system: Antagonistic effects:The sympathetic and parasympathetic fibers innervate the same cells and the actions are opposite to each other.
For example, the heart rate is increased by the sympathetic division and slowed down by the parasympathetic division. Likewise, digestion is inhibited by the sympathetic division and stimulated by the parasympathetic division.
The dilation and constriction of the pupil are also regulated by the sympathetic and parasympathetic divisions respectively. Some organs, such as the heart are exerted through dual innervations of the same effector cells.
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