Time to run. Similar physiological responses would occur in preparation for fighting off the threat. This response should sound a bit familiar. The autonomic nervous system is tied into emotional responses as well, and the fight-or-flight response probably sounds like a panic attack.
In the modern world, these sorts of reactions are associated with anxiety as much as with response to a threat. It is engrained in the nervous system to respond like this. In fact, the adaptations of the autonomic nervous system probably predate the human species and are likely to be common to all mammals, and perhaps shared by many animals.
That lioness might herself be threatened in some other situation. However, the autonomic nervous system is not just about responding to threats. Her heart rate will slow. Breathing will return to normal. The digestive system has a big job to do. Much of the function of the autonomic system is based on the connections within an autonomic, or visceral, reflex. The nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system.
The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles.
The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses though many can happen without conscious awareness, like breathing , and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis. 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. 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 1. Sympathetic Connections and Chain Ganglia. 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 2 b. 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 2 c.
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 1.
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. 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.
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.
Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs.
The parasympathetic system can also be referred to as the craniosacral system or outflow because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord. The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves.
The targets of these fibers are terminal ganglia , which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ.
Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors. The cranial component of the parasympathetic system is based in particular nuclei of the brain stem.
In the midbrain, the Edinger—Westphal nucleus is part of the oculomotor complex, and axons from those neurons travel with the fibers in the oculomotor nerve cranial nerve III that innervate the extraocular muscles. The preganglionic parasympathetic fibers within cranial nerve III terminate in the ciliary ganglion , which is located in the posterior orbit. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size. In the upper medulla, the salivatory nuclei contain neurons with axons that project through the facial and glossopharyngeal nerves to ganglia that control salivary glands.
Tear production is influenced by parasympathetic fibers in the facial nerve, which activate a ganglion, and ultimately the lacrimal tear gland. Neurons in the dorsal nucleus of the vagus nerve and the nucleus ambiguus project through the vagus nerve cranial nerve X to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gall bladder, and small intestine of the abdominal cavity.
The postganglionic fibers from the ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such as the mesenteric plexus of the digestive tract organs and the intramural ganglia.
Where an autonomic neuron connects with a target, there is a synapse. The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic , meaning that acetylcholine ACh is released, or adrenergic , meaning that norepinephrine is released.
The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds. The cholinergic system includes two classes of receptor: the nicotinic receptor and the muscarinic receptor. Both receptor types bind to ACh and cause changes in the target cell.
The nicotinic receptor is a ligand-gated cation channel and the muscarinic receptor is a G protein—coupled receptor. The receptors are named for, and differentiated by, other molecules that bind to them. Whereas nicotine will bind to the nicotinic receptor, and muscarine will bind to the muscarinic receptor, there is no cross-reactivity between the receptors. The situation is similar to locks and keys.
Imagine two locks—one for a classroom and the other for an office—that are opened by two separate keys. The classroom key will not open the office door and the office key will not open the classroom door.
This is similar to the specificity of nicotine and muscarine for their receptors. However, a master key can open multiple locks, such as a master key for the Biology Department that opens both the classroom and the office doors.
This is similar to ACh that binds to both types of receptors. The molecules that define these receptors are not crucial—they are simply tools for researchers to use in the laboratory. These molecules are exogenous , meaning that they are made outside of the human body, so a researcher can use them without any confounding endogenous results results caused by the molecules produced in the body.
Unlike cholinergic receptors, these receptor types are not classified by which drugs can bind to them. All of them are G protein—coupled receptors. An additional aspect of the adrenergic system is that there is a second signaling molecule called epinephrine. The chemical difference between norepinephrine and epinephrine is the addition of a methyl group CH 3 in epinephrine.
The term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described at the beginning of the chapter.
Adrenaline and epinephrine are two names for the same molecule. Though the drug is no longer sold, the convention of referring to this molecule by the two different names persists. Similarly, norepinephrine and noradrenaline are two names for the same molecule. Having understood the cholinergic and adrenergic systems, their role in the autonomic system is relatively simple to understand.
All preganglionic fibers, both sympathetic and parasympathetic, release ACh. All ganglionic neurons—the targets of these preganglionic fibers—have nicotinic receptors in their cell membranes. The nicotinic receptor is a ligand-gated cation channel that results in depolarization of the postsynaptic membrane.
The postganglionic parasympathetic fibers also release ACh, but the receptors on their targets are muscarinic receptors, which are G protein—coupled receptors and do not exclusively cause depolarization of the postsynaptic membrane. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh Table Autonomic System Signaling Molecules.
Signaling molecules can belong to two broad groups. Neurotransmitters are released at synapses, whereas hormones are released into the bloodstream. These are simplistic definitions, but they can help to clarify this point. Acetylcholine can be considered a neurotransmitter because it is released by axons at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibers release norepinephrine, which can be considered a neurotransmitter.
But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered hormones. What are referred to here as synapses may not fit the strictest definition of synapse.
Some sources will refer to the connection between a postganglionic fiber and a target effector as neuroeffector junctions; neurotransmitters, as defined above, would be called neuromodulators.
The structure of postganglionic connections are not the typical synaptic end bulb that is found at the neuromuscular junction, but rather are chains of swellings along the length of a postganglionic fiber called a varicosity Figure 4. Autonomic Varicosities. What can you do to try to minimize the negative consequences of these particular stressors in your life? Chronic stress can lead to increased susceptibility to bacterial and viral infections, and potentially an increased risk of cancer.
Ultimately, this could be a vicious cycle with stress leading to increased risk of disease, disease states leading to increased stress and so on. Most of these effects directly impact energy availability and redistribution of key resources and heightened sensory capacity. The individual experiencing these effects would be better prepared to fight or flee.
Skip to main content. Search for:. Parts of the Nervous System Learning Objectives By the end of this section, you will be able to: Describe the difference between the central and peripheral nervous systems Explain the difference between the somatic and autonomic nervous systems Differentiate between the sympathetic and parasympathetic divisions of the autonomic nervous system.
Answers 1. Glossary autonomic nervous system controls our internal organs and glands. Licenses and Attributions. The Frontal Lobe controls emotions in its prefrontal cortex. When the cerebellum is depressed such as from alcohol use , the ability most typically impaired would be the one to walk in a straight line because the cerebellum controls and coordinates muscle skills.
The ability to think abstractly is controlled by the frontal lobe, the ability to process visual information is controlled by the occipital lobe, ability to incoporate new memories into a web of old knowledge is controlled by the hippocampus, and the ability to feel emotion is controlled by the amygdala. Which type of medical test would show the brain structures and therefore detect any tumors that may be present but not any brain waves or activity?
CAT or CT scans use the densities of tissues in order to produce a 3D image of the brain's structures but not waves or activity , which can then be used to find tumors. EEGs detect electrical activity in the brain and are often used in sleep research. PET scans detect brain activity by showing how much of a certain substance is being used in a certain part of the brain.
Ultrasounds are not generally used for the brain and are most commonly used for fetal imaging. The hindbrain is the most basic and essential part of the brain because it controls the simplest biological functions that sustain our lives, such as respiration and heartbeat. The hindbrain is made up of structures such as the medulla and the cerebellum.
The other two major areas of the brain are the midbrain, which controls simple movements in conjunction with sensory input, and the forebrain, which controls complicated emotional and cognitive functions. If you've found an issue with this question, please let us know. With the help of the community we can continue to improve our educational resources. If Varsity Tutors takes action in response to an Infringement Notice, it will make a good faith attempt to contact the party that made such content available by means of the most recent email address, if any, provided by such party to Varsity Tutors.
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Correct answer: Central Nervous System. The hypothalamus projects to this area, which includes the parasympathetic vagal nuclei, and also to a group of cells which lead to the sympathetic system in the spinal cord. By interacting with these systems, the hypothalamus controls digestion, heart rate, sweating and other functions.
Brain stem- the brainstem acts as the link between the spinal cord and the cerebrum. Sensory and motor neurons travel through the brainstem, conveying messages between the brain and spinal cord.
The brainstem controls many autonomic functions of the PNS, including respiration, heart rate and blood pressure. Spinal cord- two chains of ganglia are located on either side of the spinal cord. The outer chains form the parasympathetic nervous system, while the chains closest to the spinal cord form the sympathetic element. What are some receptors of the autonomic nervous system? Sensory neuron dendrites are sensory receptors that are highly specialized, receiving specific types of stimuli.
We do not consciously sense impulses from these receptors except perhaps pain. There are numerous sensory receptors: Photoreceptors- respond to light Thermoreceptors- respond to alterations in temperature Mechanoreceptors- respond to stretch and pressure blood pressure or touch Chemoreceptors- respond to changes in internal body chemistry i. In this way, visceral motor neurons can be said to indirectly innervate smooth muscles of arteries and cardiac muscle.
In addition, autonomic motor neurons can continue to function even if their nerve supply is damaged, albeit to a lesser extent. Where are the autonomic nervous system neurons located? The ANS is essentially comprised of two types of neurons connected in a series. The nucleus of the first neuron is located in the central nervous system. SNS neurons begin at the thoracic and lumbar areas of the spinal cord, PNS neurons begin at the cranial nerves and sacral spinal cord. The first neuron's axons are located in the autonomic ganglia.
In terms of the second neuron, its nucleus is located in the autonomic ganglia, while the axons of the second neuron are located in the target tissue. The two types of giant neurons communicate using acetylcholine.
Sympathetic Parasympathetic Function To defend the body against attack Healing, regeneration and nourishing the body Overall Effect Catabolic breaks down the body Anabolic builds up the body Organs and Glands It Activates The brain, muscles, the insulin pancreas, and the thyroid and adrenal glands The liver, kidneys, enzyme pancreas, spleen, stomach, small intestines and colon Hormones and Substances It Increases Insulin, cortisol and the thyroid hormones Parathyroid hormone, pancreatic enzymes, bile and other digestive enzymes Body Functions It Activates Raises blood pressure and blood sugar, and increases heat production Activates digestion, elimination and the immune system Psychological Qualities Fear, guilt, sadness, anger, willfulness, and aggressiveness.
The sympathetic branch mediates this expenditure while the parasympathetic branch serves a restorative function. In general: The sympathetic nervous system causes a speeding up of bodily functions i. The ANS affects changes in the body that are meant to be temporary; in other words, the body should return to its baseline state. It is natural that there should be brief excursions from the homeostatic baseline, but the return to baseline should occur in a timely manner.
When one system is persistently activated increased tone , health may be adversely affected. The branches of the autonomic system are designed to oppose and thus balance each other. For example, as the sympathetic nervous system begins to work, the parasympathetic nervous system goes into action to return the sympathetic nervous system back to its baseline. Therefore, it is not difficult to understand that persistent action by one branch may cause a persistently decreased tone in the other, which can lead to ill health.
A balance between the two is both necessary and healthy. The parasympathetic nervous system has a quicker ability to respond to change than the sympathetic nervous system. Why are we designed this way? Imagine if we weren't: exposure to a stressor causes tachycardia; if the parasympathetic system did not immediately begin to counter the increased heart rate, the heart rate could continue to increase until a dangerous rhythm, such as ventricular fibrillation, developed.
Because the parasympathetics are able to respond so quickly, dangerous situations like the one described cannot occur. The parasympathetic nervous system is the first to indicate a change in health condition in the body.
The parasympathetics are the main influencing factor on respiratory activity. As for the heart, parasympathetic nerve fibers synapse deep within the heart muscle, while sympathetic nerve fibers synapse on the surface of the heart. Thus, parasympathetics are more sensitive to heart damage. Transmission of Autonomic Stimuli Neurons generate and propagate action potentials along their axons.
They then transmit signals across a synapse through the release of chemicals called neurotransmitters, which stimulate a reaction in another effector cell or neuron. This process may cause either stimulation or inhibition of the receiving cell, depending which neurotransmitters and receptors are involved. Individual neurons generate the same potential after receiving each stimulus and conduct the axon potential at a fixed rate of velocity along the axon.
Velocity is dependent upon the diameter of the axon and how heavily it is myelinated- speed is faster in myelinated fibers because the axon is exposed at regular intervals nodes of Ranvier. The impulse "jumps" from one node to the next, skipping myelinated sections. Transmission- transmission is chemical, resulting from the release of specific neurotransmitters from the terminal nerve ending.
These neurotransmitters diffuse across the cleft of the synapse and bind to specific receptors attached to the effector cell or adjoining neuron. Response may be excitatory or inhibitory depending on the receptor. Neurotransmitter-receptor interaction must occur and terminate quickly. This allows for repeated and rapid activation of the receptors. Neurotransmitters can be "reused" in one of three ways: Reuptake- neurotransmitters are quickly pumped back into presynaptic nerve terminals Destruction- neurotransmitters are destroyed by enzymes located near the receptors Diffusion- neurotransmitters may diffuse into the surrounding area and eventually be removed Receptors- receptors are protein complexes that cover the membrane of the cell.
Most interact primarily with postsynaptic receptors; some are located on presynaptic neurons, which allows for finer control of the release of the neurotransmitter. There are two major neurotransmitters in the autonomic nervous system: Acetylcholine- the major neurotransmitter of autonomic presynaptic fibers, postsynaptic parasympathetic fibers.
Erections are controlled by the parasympathetic system through excitatory pathways. Excitatory signals originate in the brain, through thought, sight or direct stimulation. Regardless of the origin of the excitatory signal, penile nerves respond by releasing acetylcholine and nitric oxide, which in turn signal the smooth muscles of the arteries of the penis to relax and fill with blood.
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