Departments of the Autonomic Nervous System - Anatomy and Physiology (2023)

learning goals

At the end of this section you can:

• Name the components that produce the sympathetic and parasympathetic responses of the autonomic nervous system.
• Explain the differences in the starting compounds within the two areas of the autonomic nervous system.
• Describe the signaling molecules and receptor proteins involved in communication within the two areas of the autonomic nervous system.

look at thisVideofor more information on adrenaline and the fight-or-flight response. When you say someone has an adrenaline rush, images of bungee jumpers or skydivers usually come to mind. But adrenaline, also known as epinephrine, is a key chemical in coordinating the body's fight-or-flight response. Watch the physiology of the fight-or-flight response as expected of a firefighter in this video. Your body's response is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme reactions. What two changes does adrenaline cause to make skeletal muscle respond?

In response to a threat, fight, or flight, the sympathetic system causes diverse effects as many different effector organs are activated together for a common purpose. More oxygen has to be inhaled and delivered to the skeletal muscle. The respiratory, cardiovascular and musculoskeletal systems are all activated together. In addition, sweat prevents excess heat from causing muscle contraction to overheat the body. The digestive system stops working, so the blood doesn't absorb nutrients when it should be delivering oxygen to the skeletal muscles. To coordinate all of these responses, connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide variety of ganglia that project to many effector organs simultaneously. The complex structures that make up the output of the sympathetic system make it possible for these diverse effectors to come together in a coordinated systemic change.

The sympathetic nervous system influences the various organ systems of the body through connections emanating from the thoracic and upper lumbar spinal cord. He is known as thethorakolumbales Systemto reflect this anatomical basis. FORNeurone zentralin the lateral horn of each of these spinal regions, it projects via the ventral spinal roots to the ganglia adjacent to the spine. Most of the ganglia of the sympathetic system belong to a network ofsympathetic chain gangliathat runs along the spine. Ganglia appear as a series of clusters of neurons connected by axonal bridges. There are normally 23 ganglia in the chain on each side of the spine. Three correspond to the cervical region, 12 to the thoracic region, four to the lumbar region and four to the sacral region. The cervical and sacral levels are not directly connected to the spinal cord through the spinal roots, but through ascending or descending connections through bridges within the chain.

A diagram showing the connections of the sympathetic system is something like a wiring diagram showing the electrical connections between different outlets and devices. In[Shortcut], the "circuitry" of the sympathetic system is intentionally simplified.

To continue the schematic analogy, there are three different types of "connections" that operate within the sympathetic system ([Shortcut]). The first type is the most direct: the sympathetic nerve projects to the chain ganglion at the same level as the sympathetic nerve.effector target(the organ, tissue or gland to be innervated). An example is the T1 spinal nerve, which synapses with the ganglion of the T1 chain to innervate the trachea. The fibers of this branch are calledshare white branches(singular = communicating branch); are myelinated and are therefore referred to as white (cf[Shortcut]A). The axon of the central neuron (the preganglionic fiber shown as a solid line) is synaptically connected to theGanglion Neuron(with the postganglionic fiber shown as a dashed line). This neuron then projects via to a target effector, in this case the tracheashare gray branches, which are unmyelinated axons.

In some cases, the target effectors are located above or below the spinal segment from which the preganglionic fiber exits. In terms of the "wiring" involved, the synapse with the ganglionic neuron occurs in the ganglia of the chain upstream or downstream of the location of the central neuron. An example of this is the spinal nerve T1, which innervates the eye. The spinal nerve ascends through the chain until it reaches theupper cervical ganglion, where it synapses with the postganglionic neuron (cf[Shortcut]B). Cervical ganglia are calledParavertebrale Ganglien, due to its location adjacent to the prevertebral ganglia in the sympathetic chain.

Not all axons of central neurons end in chain ganglia. Additional branches from the ventral nerve root continue through the chain and into one of the collateral gangliaSplanchnic nerve largerÖsmall splanchnic nerve. For example, at the T5 level, the greater splanchnic nerve is synchronized with a lateral ganglion outside the chain before connecting to the postganglionic nerves supplying the stomach (cf[Shortcut]C).

The collateral knot, calls tooprevertebral gangliaThey are located in front of the spine and receive input from the splanchnic nerves and central sympathetic neurons. They are assigned to the control organs in the abdomen and are also considered part of the enteric nervous system. The three collateral ganglia are theCeliac ganglion, IsGanglion mesenterica superior, it's himGanglion mesenterica inferior(ver[Shortcut]). The word celiac disease derives from the Latin word "celom" which refers to a body cavity (in this case the abdominal cavity) and the word mesenteric refers to the digestive system.

An axon from the central neuron that projects to a sympathetic ganglion is called an axon.preganglionic fiberor neuron, and represents the output from the CNS to the ganglion. Since the sympathetic ganglia are adjacent to the spine, the preganglionic sympathetic fibers are relatively short and myelinated. FORpostganglionic fiber– the axon of a ganglion neuron projecting to the target effector – represents the output of a ganglion affecting the organ directly. Compared to preganglionic fibers, postganglionic sympathetic fibers are long due to the relatively greater distance between the ganglion and the target effector. These fibers are unmyelinated. (Note that the term "postganglionic neuron" can be used to describe the projection from a ganglion to the target. The problem with this usage is that the cell body is in the ganglion and only the fiber is postganglionic. The term Neuron refers to the whole cell.)

A type of preganglionic sympathetic fiber does not end in a ganglion. These are the axons of the central sympathetic neurons that project to theadrenal medulla, the inner part of the adrenal gland. These axons are still known as preganglionic fibers, but the target is not a ganglion. The adrenal medulla releases signaling molecules into the bloodstream instead of communicating with target structures via axons. The cells of the adrenal medulla that are in contact with the preganglionic fibers are calledchromaffin 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 part of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. In response to a threat, the sympathetic system would increase heart and respiratory rates, causing blood flow to 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 these physiological changes must occur together to elude the chasing lioness, or the modern equivalent. This divergence is evident in the branching patterns of the preganglionic sympathetic neurons: a single preganglionic sympathetic neuron may have 10 to 20 targets. An axon emerging from a central lateral horn neuron in the thoracolumbar spinal cord passes through the white communicating branch and enters the sympathetic chain, where it branches to a variety of destinations. At the level of the spinal cord, where the preganglionic sympathetic fiber exits the spinal cord, a branch synapses with a neuron in the adjacent chain ganglion. Some branches extend up or down at a different level than the nodes in the chain. Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Finally, some branches may protrude through the splanchnic nerves to the adrenal medulla. All of this branching means that a preganglionic neuron can affect very different regions of the sympathetic system, affecting widely distributed organs.

The parasympathetic nervous system gets its name because its central neurons lie on either side of the thoracolumbar region of the spinal cord (para- = "next to" or "near"). The parasympathetic system can also be calledCraniosacrales System(or exit) as preganglionic neurons are found in the brainstem nuclei and lateral horn of the sacral spinal cord.

The connections or "circuitry" of the parasympathetic part are similar to the general structure of the sympathetic part with some specific differences ([Shortcut]). Preganglionic fibers from the cranial region migrate to cranial nerves, while preganglionic fibers from the sacral region migrate to spinal nerves. The purposes of these fibers areGanglion terminalsthat are near or even inside the target effector. These nodes are normally calledintramural nodeswhen they are within the walls of the target organ. The postganglionic fiber projects a short distance from the terminal ganglia to the target effector or specific target tissue within the organ. When 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 inside, the target effectors.

The cranial component of the parasympathetic system is based on specific nuclei in the brainstem. In the midbrain, theNúcleo de Edinger-WestphalIt is part of the oculomotor complex, and the axons of these neurons travel with oculomotor (cranial nerve III) fibers that innervate the extraocular muscles. Preganglionic parasympathetic fibers in cranial nerve III terminate in theCiliary ganglion, which is located in the posterior orbit. Postganglionic parasympathetic fibers project to iris smooth muscle to control pupil size. In the upper part of the medulla oblongata, the salivary nuclei contain neurons with axons that project through the facial and glossopharyngeal nerves to the ganglia that control the salivary glands. Tear production is influenced by parasympathetic fibers in the facial nerve that activate a ganglion and eventually the lacrimal gland. neurons in thedorsals Kern des Vagus nervesit's himan ambiguous corethey project via the vagus nerve (cranial nerve X) to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers mainly affect the heart, bronchi and esophagus in the thoracic cavity and the stomach, liver, pancreas, gallbladder and small intestine in the abdominal cavity. Postganglionic fibers from ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such asPlexus mesentericaDigestive tract organs and intramural ganglia.

When an autonomic neuron connects to a target, a synapse occurs. The action potential's electrical signal causes the release of a signaling molecule that binds to receptor proteins on the target cell. Autonomous system synapses are classified ascholinerg, that's what it meansAcetylcholine (ACh)Break free, ohadrenergic, that's what it meansNorepinephrinein the start. The terms cholinergic and adrenergic refer not only to the signaling molecule released, but also to the class of receptors to which each binds.

The cholinergic system includes two classes of receptors: theNicotine Receptorit's himMuskarinrezeptor. Both types of receptors bind to ACh and cause changes in the target cell. The nicotinic receptor is aBinder-gated cation channeland the muscarinic receptor is aG protein-coupled receptor. Receptors are named and distinguished by other molecules that bind to them. Although nicotine binds to the nicotinic receptor and muscarine to the muscarinic receptor, there is no cross-reactivity between the receptors. The situation is similar to lock and key. Imagine two locks, one for a classroom and one for an office, opened with two separate keys. The classroom key does not open the office door and the office key does not open the classroom door. This is similar to the specificity of nicotine and muscarine for their receptors. However, a master key can open many locks, such as B. the master key in the biology department, which opens classroom and office doors. This is similar to how ACh binds to both types of receptors. The molecules that define these receptors are not critical, they are simply tools that researchers can use in the lab. These molecules areexogenous, meaning they are made outside of the human body, allowing a researcher to use them without fussendogenousResults (results caused by the molecules produced in the body).

The adrenergic system also has two types of receptors, calledalpha (α)-adrenergic receptorjbeta (β)-adrenergic receptor. Unlike cholinergic receptors, these types of receptors are not classified based on what drugs can bind to them. They are all G protein-coupled receptors. There are three types of α-adrenergic receptors, called α₁, A₂, sim₃, and there are two types of β-adrenergic receptors, called β₁y b₂. Another aspect of the adrenergic system is that there is a second signaling molecule called the signaling moleculeAdrenalin. The chemical difference between norepinephrine and epinephrine is the addition of a methyl group (CH₃) into epinephrine. The prefix "nor-" actually refers to this chemical difference, which is missing a methyl group.

The term adrenergic should remind you of the word adrenaline associated with the fight-or-flight response described earlier in this chapter. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland (Latin ad- = "on"; renal = "kidney") releases adrenaline. The "-ine" suffix refers to the chemical derived from or extracted from the adrenal gland. A similar construction from Greek, rather than Latin, gives the word epinephrine (epi- = "above"; nephr- = "kidney"). In scientific use, epinephrine is preferred in the United States, while adrenaline is preferred in Britain, since "adrenaline" was once a patented and registered drug name in the United States. Although the drug is no longer sold, the convention of calling this molecule by two different names remains. Likewise, norepinephrine and norepinephrine are two names for the same molecule.

Once the cholinergic and adrenergic systems are understood, their role in the autonomic system is relatively easy to understand. All preganglionic fibers, both sympathetic and parasympathetic, release ACh. All ganglion neurons, the targets of these preganglionic fibers, have nicotinic receptors on their cell membranes. The nicotinic receptor is a ligand-gated cation channel that causes depolarization of the postsynaptic membrane. 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 postsynaptic membrane depolarization. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and blood vessels associated with skeletal muscle that release ACh.[Shortcut]).

Signaling molecules can belong to two large groups. Neurotransmitters are released at synapses while hormones are released into the bloodstream. These are simplified definitions, but they may help clarify the point. Acetylcholine can be considered a neurotransmitter as it is released by axons at synapses. However, the adrenergic system presents a challenge. Postganglionic sympathetic fibers release norepinephrine, which can be viewed as a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into the circulation, so they must be thought of as hormones.

What is referred to herein as a synapse may not meet the strictest definition of a synapse. Some sources refer to the connection between a postganglionic fiber and an effector target as neuroeffector connections; Neurotransmitters as defined above would be referred to as neuromodulators. The structure of postganglionic junctions is not the typical synaptic terminal bulb found at the neuromuscular junction, but rather chains of bumps along a postganglionic fiber called the postganglionic fiber.varicose veins([Shortcut]).

The main job of the autonomic nervous system is to regulate the body's homeostatic mechanisms, which is also part of what the endocrine system does. The key to understanding the autonomic system lies in studying the pathways of response - the performance of the nervous system. The way we respond to the world around us to manage the internal environment based on the external environment is divided between two parts of the autonomic nervous system. The sympathetic part responds to threats and creates a threat-or-flight disposition: the fight-or-flight response. The opposite role is played by the parasympathetic nervous system. When the external environment does not pose an immediate threat, a dormant mode descends on the body and the digestive system becomes more active.

The sympathetic output of the nervous system originates from the lateral horn of the thoracolumbar spinal cord. An axon from one of these central neurons projects through the ventral spinal nerve root and spinal nerve to a sympathetic ganglion, either in the ganglia of the sympathetic chain or at one of the collateral sites where it synapses with a ganglionic neuron. These preganglionic fibers release ACh, which excites the ganglionic neuron through the nicotinic receptor. The axon of the ganglionic neuron, the postganglionic fiber, then projects to an effector target where it releases norepinephrine to bind to an adrenergic receptor, causing a change in the physiology of that organ in accordance with the wildly aberrant sympathetic response. Exceptions, however, are the postganglionic connections with the sweat glands in the skin and the blood vessels that supply skeletal muscle; these fibers release ACh at muscarinic receptors. The sympathetic system has a specialized preganglionic connection to the adrenal medulla that causes epinephrine and norepinephrine to be released into the bloodstream rather than exciting a neuron that comes in direct contact with an organ. This hormonal component means that the sympathetic chemical signal can spread very quickly throughout the body, affecting many organ systems at once.

The parasympathetic output is based on the brainstem and sacral spinal cord. Neurons from specific nuclei in the brainstem or lateral horn of the sacral spinal cord (preganglionic neurons) project to terminal (intramural) ganglia located near or within the wall of target effectors. These preganglionic fibers also release ACh at nicotinic receptors to excite ganglionic neurons. Postganglionic fibers contact target tissues within the organ to release ACh, which binds to muscarinic receptors to induce resting and digestive responses.

Signaling molecules used by the autonomic nervous system are released by axons and can be thought of as neurotransmitters (when they interact directly with the effector) or hormones (when they are released into the bloodstream). The same molecule as norepinephrine can be considered a neurotransmitter or a hormone depending on whether it is released from a postganglionic sympathetic axon or from the adrenal gland. Synapses in the autonomic system are not always the typical type of connection first described at the neuromuscular junction. Instead of synaptic terminal bulbs at the end of an axonal fiber, they may have varicose veins along a fiber so that they form a network of connections within the target tissue.

glossary

alpha (α)-adrenergic receptor

one of the receptors to which epinephrine and norepinephrine bind, which comes in three subtypes: α₁, A₂, sim₃

Acetylcholine (ACh)

Neurotransmitter that binds to a motor endplate to induce depolarization

adrenal medulla

inner part of the adrenal gland (or adrenal gland) that releases epinephrine and norepinephrine into the bloodstream as hormones

adrenergic

Synapse where norepinephrine is released, which binds to α- or β-adrenergic receptors

beta (β)-adrenergic receptor

one of the receptors to which epinephrine and norepinephrine bind, which comes in two subtypes: β₁y b₂

Celiac ganglion

one of the collateral ganglia of the sympathetic system that projects to the digestive system

Neurone zentral

refers specifically to the cell body of a neuron in the autonomic system found in the central nervous system, specifically the lateral horn of the spinal cord or a brainstem nucleus

cholinerg

Synapse where acetylcholine is released and binds to the nicotinic or muscarinic receptor

chromaffin cells

Neuroendocrine cells of the adrenal medulla that release epinephrine and norepinephrine into the bloodstream as part of the activity of the sympathetic system.

Ciliary ganglion

one of the terminal ganglia of the parasympathetic system, located in the posterior orbit and whose axons project to the iris

The collateral knot

Ganglia outside the sympathetic chain conducted by sympathetic preganglionic fibers which are celiac, inferior mesenteric and superior mesenteric ganglia

Craniosacrales System

alternative name for the parasympathetic division of the autonomic nervous system, based on the anatomical location of the central neurons in the brainstem and the lateral horny nuclei of the sacral spinal cord; also called craniosacral outlet

dorsals Kern des Vagus nerves

Location of the parasympathetic neurons that project via the vagus nerve to terminal ganglia in the thoracic and abdominal cavities

Nucleo de Eddinger-Westphal

Location of the parasympathetic neurons that project to the ciliary ganglion

endogenous

describes the substance produced in the human body

Adrenalin

Signaling molecule released from the adrenal medulla into the bloodstream as part of the sympathetic response

exogenous

describes the substance that is manufactured outside of the human body

fight or flight response

a set of responses elicited by sympathetic activity leading to escape or coping with a threat, which in the modern world is often associated with feelings of anxiety

G protein-coupled receptor

Membrane protein complex consisting of a receptor protein that binds to a signaling molecule, a G protein, which is activated by this binding and in turn activates an effector protein (enzyme) that generates a second messenger molecule in the cell's cytoplasm - target

Ganglion Neuron

refers specifically to the cell body of a neuron in the autonomic system located in a ganglion

share gray branches

(singular = communicating branch) myelinated structures that provide a brief connection from a sympathetic chain ganglion to the spinal nerve, which contains the postganglionic sympathetic fiber

Splanchnic nerve larger

Nerve-containing fibers from central sympathetic neurons that do not synapse in chain ganglia but project to the celiac ganglion

Ganglion mesenterica inferior

one of the collateral ganglia of the sympathetic system that projects to the digestive system

intramural nodes

terminal ganglia of the parasympathetic system, lying within the target effector walls

small splanchnic nerve

Nerve-containing fibers from central sympathetic neurons that do not synapse in chain ganglia but project to the inferior mesenteric ganglion

Binder-gated cation channel

Ion channel, like the nicotinic receptor, which is specific for positively charged ions and opens when a molecule, like a neurotransmitter, binds to it

Plexus mesenterica

Nervous tissue within the wall of the digestive tract that contains neurons directed by preganglionic autonomic fibers that project to smooth muscle and glandular tissue in the digestive organ

Muskarinrezeptor

Type of acetylcholine receptor protein characterized in that it also binds to muscarinic and is a metabotropic receptor

Nicotine Receptor

Type of acetylcholine receptor protein characterized in that it also binds nicotine and is an ionotropic receptor

Norepinephrine

Signaling molecule released as a neurotransmitter by most postganglionic sympathetic fibers as part of the sympathetic response or as a hormone in the bloodstream from the adrenal medulla

an ambiguous core

Brainstem nucleus, which contains neurons that project via the vagus nerve to terminal ganglia in the thoracic cavity; specially connected to the heart

parasympathetic division

Department of the autonomic nervous system responsible for resting and digestive functions

Paravertebrale Ganglien

autonomic ganglia superior to the ganglia of the sympathetic chain

postganglionic fiber

Axon of a ganglionic neuron in the autonomic nervous system that projects to and synapses with the target effector; sometimes referred to as the postganglionic neuron

preganglionic fiber

Axon of a central neuron in the autonomic nervous system that projects to and synapses with a ganglionic neuron; sometimes referred to as a preganglionic neuron

prevertebral ganglia

autonomic ganglia that lie anterior to the spine and are functionally related to the sympathetic ganglia

rest and digest

Set of functions associated with the parasympathetic system leading to rest and digestion processes

upper cervical ganglion

one of the paravertebral ganglia of the sympathetic system which projects to the head

Ganglion mesenterica superior

one of the collateral ganglia of the sympathetic system that projects to the digestive system

sympathetic chain ganglia

Set of ganglia adjacent to the spine that receive information from central sympathetic neurons

Sympathetic division

Division of the autonomic nervous system associated with the fight-or-flight response

effector target

Organ, tissue, or gland that responds to the control of an autonomic, somatic, or endocrine signal

Ganglion terminals

Ganglia of the parasympathetic part of the autonomic system, located near or inside the target effector, the latter also known as the intramural ganglion

thorakolumbales System

Alternative name for the sympathetic part of the autonomic nervous system, based on the anatomical location of the central neurons in the lateral horn of the thoracic and superior lumbar cord

varicose veins

Structure of few autonomous connections that is not a typical synaptic terminal bulb but a chain of protuberances along a fiber that forms a network of connections with the target effector

share white branches

(singular = communicating branch) myelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve containing the preganglionic sympathetic fiber