Descending pathways of the autonomic nervous system
What would happen if we constantly had to plan to blink, sweat, breathe and make our heart beat? It would be a very tough and exhausting task, to say the least. Lucky for us, our nervous system has an independent autonomous compartment that maintains our vital functions. Our biological life-support machine is called the autonomic nervous system (ANS) and it will be the subject of this lesson. Most commonly divided into the sympathetic, parasympathetic, and enteric arms, the ANS has some crucial descending pathways that allow the brain control and fine tune not only the functions mentioned above, but also many others.
This article will describe everything you need to know about the autonomic nervous system, including its anatomical elements, as well as the clinical aspects related to the ANS.
Sympathetic: promotes “fight or flight” response, corresponds with arousal and energy generation, inhibits digestion
Parasympathetic: promotes “rest and digest” response, corresponds with calming of the body and enhances digestion
Cervical and upper thoracic segments: cardiac plexus, pulmonary plexus (stimulation of heart function and enhancing respiration)
Lower thoracic and abdominal: greater, lesser, least splanchnic nerves (stimulation of adrenal gland to produce catecholamines)
Lumbar segment: fibers for internal and external genitals (vasoconstriction, ejaculation, inhibition of sigmoid colon and rectum peristalsis)
|Parasympathetic nerves||Vagus nerve (lowers the heart rate, stimulates digestion), oculomotor nerve (miosis of the pupil), facial nerve (increasing the production of saliva in the submandibular and sublingual glands), glossopharyngeal nerve (stimulates saliva production on the parotid gland)|
|Clinical relations||Enophthalmos, Raynaud’s disease, Hirschsprung's disease, ileus, cardiac arrest, arrhythmia|
- Functional organization
- Parasympathetic nervous system
- Sympathetic nervous system
- Clinical aspects
The autonomic nervous system is generally everything about the central nervous system (CNS) that we do not control consciously. The cells that constitute this system are located both within the CNS and the peripheral nervous system (PNS); therefore, we can speak about the central autonomic system or the peripheral autonomic system. The nerves of this system innervate glands, smooth muscles and cardiac muscle, which is why the system is also referred to as the visceral system (lat. viscera – internal organ). The functionally dominant component of this system is efferent, even though the afferent channel is also present. In this manner, the system consists of the visceromotor (efferent) and the viscerosensory (afferent) components.
The most important classification within the autonomic nervous system is based on the role and function of each sub-type:
- Sympathetic nervous system - “fight or flight”
- Parasympathetic nervous system - “rest and digest”
- Enteric system - “overseeing the digestive process”; distinguished as its own category since peristalsis and other spontaneous movements persist after its isolation from all nervous inputs.
It is now clear that this system controls all the necessary mechanisms for basic survival.
As previously mentioned, there is a central and peripheral autonomic nervous system. The fibers of the sympathetic and the parasympathetic components each have a specific location in both systems. Central parasympathetic neurons are found within the brainstem:
- The Edinger-Westphal nucleus
- The salivatory nuclei
- The dorsal nuclei of the vagus nerve
Also, parasympathetic neurons are located in the spinal cord, specifically within sacral segments S2-S4. In contrast, central sympathetic neurons are found within all of the thoracic (T1-T12) and the lumbar segments (L1-L2). The organization of the parasympathetic nuclei is therefore craniosacral, while that of the sympathetic nuclei is thoracolumbar.
The highest integrational organ of the autonomic nervous system is the hypothalamus. It also regulates the endocrine glands through its connection to the hypophysis, and coordinates the autonomic nervous system and the endocrine system. Cell groups in the reticular formation of the brainstem also participate in the central regulation of organ functions.
Like every other nervous pathway, the pathways of the autonomic nervous system consist of the hierarchically ordered neurons. In the case of the ANS, every pathway is a chain of two efferent neurons:
- The first order neuron; body of the central neuron that we discussed above
- The second order neuron; ganglion in the peripheral nervous system
Because the body of neuron II resides as an autonomic ganglion, the nerve cell body is referred to as the postganglionic neuron (postsynaptic neuron), whereas neuron I is known as the preganglionic neuron (presynaptic neuron). In other words, the autonomic ganglion is the structure where the axon of the preganglionic neuron synapses with the body of the postganglionic neuron. The postganglionic neuron is located within the peripheral nervous system, and its fibers end in the specifically targeted tissues. These tissues can be a gland, smooth muscle, or cardiac muscle.
Another classification used for autonomic nerves is based on the type of the neurotransmitter involved in the synaptic communication between two neurons. In this manner, fibers are classified as the adrenergic or the cholinergic, where the adrenergic neurons are responsive to epinephrine (adrenaline), whereas the cholinergic neurons are responsive to acetylcholine.
There are few morphological differences between preganglionic and postganglionic neurons:
- The axon of the preganglionic neuron is longer and myelinated
- The axon of the postganglionic neuron is shorter and is unmyelinated
When it comes to the functional differences between the synapses, the facts are:
- The synapse of the preganglionic and postganglionic neuron, as well as the synapse of the postganglionic neuron and its target cell, uses the neurotransmitter acetylcholine.
- Neuron I and Neuron II of the sympathetic nervous system also synapse and communicate using acetylcholine, but the synapse between neuron II and its target cell is achieved using norepinephrine.
There is only one exception to these two rules of neurotransmission: postganglionic sympathetic fibers serving the eccrine sweat glands release acetylcholine instead of norepinephrine.
When it comes to the sensory component of the ANS, these fibers travel with blood vessels, cranial nerves, and visceral motor fibers. They frequently pass through, but do not synapse in the autonomic ganglia. Instead, their cell bodies (unipolar/pseudounipolar) are located in the sensory ganglia of the cranial and spinal nerves.
Most of the vital organs are under antagonistic control, where one control unit is excitatory, and the other is inhibitory. I.e. heart rate is increased by activation of the sympathetic nervous system, whereas it is decreased with the activation of the parasympathetic nervous system. This means that heart rate is regulated by changes in sympathetic and parasympathetic activity.
Sweat glands and smooth muscles of most blood vessels are not dually innervated; rather, they are only supplied with sympathetic nerves that control the intensity of their activity by increasing or decreasing neural stimulation.
Distribution of neurotransmitter receptors in target tissues defines the responses of those tissues. Most blood vessels have one type of adrenergic receptor (alpha) that, when activated, causes the blood vessels to constrict. On the other hand, arterioles of vital organs have different types of receptors (beta) that, when activated, cause the smooth muscle of that vessel to relax. Norepinephrine activates both of these receptors, which means that the response is determined by the type of receptor, rather than the neurotransmitter.
Enteric nervous system
This system refers to the autonomic neurons scattered throughout the digestive tract (esophagus, stomach, intestines, and rectum). These neurons are grouped into organized networks called plexuses. There are two major plexuses included in the enteric nervous system:
- Meissner’s plexus: lies in the connective tissue between the longitudinal muscle and circular muscle layers in the digestive tract – In other words, it can be found under the mucosa layer, which is why it is commonly referred to as the submucosal plexus.
- Auerbach's plexus: also referred to as the myenteric plexus, lies in the connective tissue between the circular muscle layer and the muscularis mucosa.
Each plexus consists of multipolar, bipolar, and unipolar neurons. The system also includes glial astrocytes that have a supportive role for the neurons. Astrocytes enwrap the neurons and their processes and form a barrier between the blood and the enteric nervous system.
The enteric nervous system provides projections onto the pancreas, gallbladder, and elsewhere. The unipolar and bipolar neurons appear to function as sensory neurons with multipolar cells forming many of the connections.
Parasympathetic nervous system
As previously mentioned, central neurons (preganglionic) of the parasympathetic nervous system have craniosacral localization within the CNS. Fibers of the preganglionic parasympathetic neurons run within various cranial nerves to the parasympathetic ganglia in the region of the head where they synapse. The postganglionic fibers then extend to the effector organs.
The main nerve of the parasympathetic nervous system is the vagus nerve (CN X), but also includes the oculomotor nerve (CN III), the facial nerve (CN VII) and the glossopharyngeal nerve (CN IX), representing the cranial portion of the CNS involvement.
The parasympathetic component of the vagus nerve arises from its dorsal motor nucleus in the medulla. It descends through the carotid sheath of the neck that contains the common carotid artery, internal jugular vein, and the deep cervical lymph nodes. In this sheath, the artery is medial and the vein is lateral, whereas the vagus nerve is positioned posteriorly to these two structures.
After passing through the superior thoracic aperture, the vagus nerve branches to contribute to plexuses in the thoracic and abdominal viscera. These fibers provide preganglionic parasympathetic innervation to the pharynx, larynx, esophagus, lungs, heart, stomach, and intestines down to the transverse colon. The postganglionic fibers originate from ganglia in the mentioned organs. The bodies of these postganglionic neurons are located within the submucosal plexus of Meissner, and in the myenteric plexus of Auerbach in the enteric nervous system.
The parasympathetic fibers arise from the Edinger-Westphal nucleus in the midbrain and innervate the constrictors of the pupil via the parasympathetic ciliary ganglion which contains the postganglionic parasympathetic neurons.
The axons arise from the inferior salivatory nucleus in the medulla. The axons exit with CN IX and end in the otic ganglion, which innervates the parotid gland. The sacral portion of the preganglionic parasympathetic neurons contains the neurons lying in the intermediolateral nucleus and intermediomedial nucleus of the sacral spinal cord. These neurons send their axons through the third and fourth sacral root to form the somatic pudendal nerve and the parasympathetic pelvic splanchnic nerves (S2-S4).
Fibers arising from the latter contribute to the inferior hypogastric plexus and pelvic organs (urinary bladder, rectum, and genitals). Synapses with postganglionic neurons are formed in the inferior hypogastric plexus or in small ganglia of the various organ plexuses.
Sympathetic nervous system
The bodies of preganglionic neurons of the sympathetic nervous system are found in the lateral cell column of spinal cord levels T1 to L2. The axons of these neurons exit the vertebral canal via white rami communicantes at the levels of their origin; they are also referred to as the thoracolumbar outflow. This outflow, as a mass of preganglionic fibers, leaves the spinal cord with the root of the spinal nerve, and then enters the paravertebral sympathetic ganglionic chain where the bodies of the postganglionic sympathetic neurons are found. In addition to this, sympathetic ganglia are also the pelvic, preaortic and small renal ganglia.
After they reach the peripheral ganglia, preganglionic fibers may have three different destinations:
- They may synapse with the postganglionic neurons that send the non-myelinated fibers in the form of gray rami communicantes back to the spinal nerve.
- Other fibers project via the autonomic nerves to various organs.
- Some preganglionic fibers simply pass through the chain without synapsing in any of its ganglia, joining the other preganglionic fibers to form the splanchnic nerves and terminate in the prevertebral ganglia which lie on both sides of the aorta, pelvic or renal ganglia.
To make the studying process as effective as possible, this sympathetic saga will continue just after we learn the topographic classification of the sympathetic chain ganglia. The sympathetic chain ganglia have the following topographic regions:
- Cervical segment
- Upper thoracic segment
- Lower thoracic segment
- Abdominal segment
- Lumbar segment
Cervical and upper thoracic segments
The cervical portion consists of three ganglia:
- The superior cervical ganglion lies below the base of the skull. It receives fibers from the upper thoracic segment via the interganglionic branches, whereas its postganglionic fibers form plexuses around the internal carotid artery and external carotid artery. Branches from the internal carotid plexus extend to:
- The middle cervical ganglion is the smallest and may be absent.
- The inferior cervical ganglion typically fuses with the first thoracic ganglion to form the stellate ganglion. Its postganglionic fibers form plexuses around the subclavian artery and around the vertebral artery.
Nerves from the cervical and upper thoracic ganglia send fibers to the heart and lungs, where they participate with the parasympathetic fibers of the vagus nerve in the formation of the cardiac plexus and the pulmonary plexus.
Lower thoracic and abdominal segments
This group of ganglia is the location where preganglionic fibers pass the sympathetic chain without synapsing. After passing through the ganglia, these nerves extend to the prevertebral ganglia of the abdominal aortic plexus.
The most important nerves belonging to these segments are the splanchnic nerves. They are named according to their length: the greater, the lesser, and the least splanchnic nerves.
- The greater splanchnic nerve leaves the CNS through the roots of the spinal nerves from T5 to T10. The nerve courses through the thoracic ganglia without synapsing to end within the celiac ganglion.
- The lesser and the least splanchnic nerves arise from the preganglionic neurons that are located within the segments T11 and T12 of the spinal cord. Similar to the greater splanchnic nerve, they run through the paravertebral ganglion to reach the ganglia of their final synapse. For the lesser splanchnic nerve, the place of ending is the celiac ganglion, whereas for the least splanchnic nerve, it’s the superior mesenteric ganglion.
The postganglionic fibers arising from these ganglia form branches to supply the aorta, duodenum, pancreas, liver, spleen, etc.
This portion consists of small ganglia that lie along the anterior border of the psoas muscle. The preganglionic fibers originate from the segments L1 and L2 of the spinal cord. They pass through the lumbar ganglia, and end in the inferior mesenteric ganglion, where they innervate:
Innervation in the pelvis and perineum is dual, meaning that these organs have both parasympathetic and sympathetic innervations. The parasympathetic fibers supply the uterus, vagina, testes, erectile tissue (penis and clitoris), sigmoid colon, rectum, and bladder. They cause contraction and emptying of the bladder and erection of the clitoris or penis. The sympathetic fibers cause vasoconstriction and ejaculation of semen and inhibit peristalsis in the sigmoid colon and rectum.
Evidently, the autonomic nervous system controls many things in our body. Speaking grossly, lesions within it can cause disorders related to the function of smooth muscles, the heart, peristalsis, and gland excretion. Since the spectrum of functions of the autonomic nervous system is enormous, it means that the possible disorders associated with it would make quite a list. So, let’s highlight some of the most frequent and important clinical conditions related to the autonomic nervous system disorders:
- Injury to the superior cervical ganglion leads to drooping of the upper eyelid (ptosis) and to a backward displacement of the eyeball which is called enophthalmos.
- Raynaud’s disease is a condition where the sympathetic fibers that innervate the smooth muscles of the peripheral blood vessels are damaged in a way that they cause severe vasoconstriction. This causes the affected body parts to appear pale and cold, but it may progress to local ischemia and cause gangrene of the affected portion.
- Congenital lack of parasympathetic fibers that innervate the colon cause Hirschsprung's disease; also referred to as a congenital megacolon. In this state, the colon is unable to constrict, which causes enormous dilatation and dysfunction of the colon, which quickly progresses to ileus.
- Lesions within the sympathetic fibers from the ciliary ganglion that innervate the dilatator pupillae muscle can cause inability of pupil to dilate.
- The most dangerous conditions are the ones related to lesions of the cardiac plexus. Any form of damage within this sympathetic portion will cause parasympathetic function to take dominance and therefore decreases the heart rate. A lower than normal heart rate may cause hypotension and possibly lead to cardiac shock, one of the most important conditions you will see in the emergency room.
- Analogical to the previous fact, an excessive promotion of sympathetic fibers of the cardiac plexus can cause an increase in heart rate. In turn, this can lead to tachycardia where it can develop further into serious heart conditions such as arrhythmia and ultimately fibrillation that leads to cardiac arrest.
The autonomic nervous system is one of the most essential nervous components of the human body. Sadly, lesions within it may cause great troubles for the affected individual, which is why it is crucial to know the implications.