Definition Merriam Webster:
Sympathetic nervous system (SNS): The part of the autonomic nervous system (ANS) that contains chiefly adrenergic fibers and tends to depress secretion, decrease the tone and contractility of smooth muscle, and increase heart rate — compare

Parasympathetic nervous system (PNS): The part of the involuntary nervous system that serves to slow the heart rate, increase intestinal and glandular activity, and relax the sphincter muscles. The parasympathetic nervous system, together with the sympathetic nervous system, constitutes the autonomic nervous system (ANS). The key to the PNS is that your body works to conserve energy, your digestive system works due to the smooth muscles working properly.
Autonomic nervous system (ANS): The part of the nervous system responsible for control of the bodily functions not consciously directed, such as breathing, the heartbeat, and digestive processes. It is responsible for the regulation of muscles, glands and depends on the environment and conditions. It is responsible for the fight or flight response too. It is involuntary so it is not easy to control when it is triggered.

Enteric Nervous System (ENS): Helps pancreas, gall bladder functions, filters toxins, helps digestion. If this part of the system does not work properly it affects smooth muscle contraction and release and will throw your enitre body off balance.

There are 3 divisions of the ANS:  They are the SNS, PNS and the enteric nervous system (ENS). All work together to keep the body safe. The “normal” or resting state is that of the parasympathetic nervous system and there is often misleading information regarding either this or the SNS being in control. They act in relationship to one another in order to help the body meet its needs on a regulatory basis. This means that you can digest, rest and even learn. When stress occurs your body prepares you for this and this is when your SNS is starting to become more active and the physiological effects are designed to increase your advantage towards survival.

SNS Endocrine System (hormones)

Both of these hormones act in a synergistic manner

Norepinephrine (Noradrenaline) Or NE

  • Released in response to emergency situations giving the body the required mechanisms to function at optimum levels.
  • NE is released before adrenaline to get the body ready and helps the body return quickly to normal if the behaviour is not needed.

Epinephrine (Adrenaline) 

  • Released in response to emergency situations giving the body the required mechanisms to function at optimum levels.
  • Released after NE and the role is to increase the physiological functions which prime the body to act.

Body Responses to activation of SNS.

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Types of adrenergic receptors (those which are activated by the adrenaline and noradrenaline) are shown below. Remember when we were talking about the upregulation of BDNF, we highlight the role of cAMP from adenylate cyclase and how it activates the factors that contribute to learning in the brain. Please keep that in mind here.

The sympathetic nervous system uses norepinephrine (NE) as a neurotransmitter to put your body on full alert.

Types of receptors are:

  • α1 and α2 Coupled to G protein receptors
  • β1, β2 and βare G protein receptor linked and adenylate cyclase dependant and then intracellular effects of PKA activity explains some of the hormone actions that take place.
  • There is then a cascade of events within the cell such as is seen below.



Stimulatory Effects

  • Blood flow increases to skeletal muscles so that they can be used to escape from a situation
  • The heart pumps harder and faster through the action of NE and adrenaline
  • More oxygen is needed so that ATP is produced
  • Breathing rates increase
  • Glycogen which is stored in the liver from glucose is broken down to glucose so it can be used.
  • Pupils dilate to improve the way the eyes will see distant objects
  • The skin sweats to keep the body cool
  • Hearing decreases at vision increases (to improve attention on the object) and heighten visual sensory input

Inhibitory Effects

  • The gastrointestinal tract shuts down
  • Blood flow to genitals is reduced
  • Relaxation of the bladder
  • Inhibition of saliva production in the mouth and tears in the eyes
How The Cardiovascular System Is Regulated
We have already seen that the heart is controlled by the PNS and SNS branches of the ANS. The SNS increases blood pressure and the PNS reduces it. These systems work together constantly and adjustments are made on a minute to minute basis (they are not switched on or off). This is achieved by something called baroreceptors.
Baroreceptors: Are specialised receptors that are found in the in the region of the heart system called the aortic arch  (AA) and carotid sinus (CS). Their job is to convey changes in blood pressure to the CNS (central nervous system).
  • In the Medulla oblongata there are preganglionic neurons. They help adjust the contractility of the heart & the resistance of vessels
  • Parasympathetic preganglionic fibres travel in the vagus nerve and synapse onto local ganglia which subsequently release acetylcholine (ACh), reducing heart rate and contractility
  • The detection of changes in blood pressure occurs in the AA and CS
  • The CS receptors carries via sensory fibers to the glossopharyngeal nerve (Cranial Nerve IX or 9) to the brain stem.
  • The AA sensory fibers are carried by the vagus nerve which is a conduit for the parasympathetic efferent fibers to the heart.
  • The changes in pressure of the arteries are detected and managed by the medulla oblongata and the activities of the PNS and SNS are changed according to this information.
  • Sympathetic nerve fibres reach the ganglia in the sympathetic chain in spinal cord
  • The post sympathetic nerve fibres release noradrenaline and this increases the heart rate and contractility
  • Other post ganglionic fibers innervate blood vessels increasing contraction and thus peripheral resistance
  • The blood pressure results from the effects of these systems on the heart and vascular system.

Figure to show he afferent v efferent nerve fibers directions

(Image from wikipedia)


See figure below showing the reflex control of cardiac output and the role of NE and Adrenaline
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cardiovascular reflexes

 What is the baroreflex?

The most sensitive receptors are in the AA and CS. the CS axons travel within the CN IX and the AA oines within the Vagus Nerve CN X. They travel directly into the the CNS and contact the nucleus of the solitary tract NTS in the brain stem.

I am here::::::::::

Baroreceptor information flows from these NTS neurons to both parasympathetic and sympathetic neurons within the brainstem.

The NTS neurons send excitatory fibers (glutamatergic) to the caudal ventrolateral medulla (CVLM), activating the CVLM. The activated CVLM then sends inhibitory fibers (GABAergic) to the rostral ventrolateral medulla (RVLM), thus inhibiting the RVLM. The RVLM is the primary regulator of the sympathetic nervous system, sending excitatory fibers (glutamatergic) to the sympathetic preganglionic neurons located in the intermediolateral nucleus of the spinal cord. Hence, when the baroreceptors are activated (by an increased blood pressure), the NTS activates the CVLM, which in turn inhibits the RVLM, thus decreasing the activity of thesympathetic branch of the autonomic nervous system, leading to a relative decrease in blood pressure. Likewise, low blood pressure activates baroreceptors less and causes an increase in sympathetic tone via “disinhibition” (less inhibition, hence activation) of the RVLM. Cardiovascular targets of the sympathetic nervous system includes both blood vessels and the heart.

Even at resting levels of blood pressure, arterial baroreceptor discharge activates NTS neurons. Some of these NTS neurons are tonically activated by this resting blood pressure and thus activate excitatory fibers to the nucleus ambiguus and Dorsal nucleus of vagus nerve to regulate the parasympathetic nervous system. These parasympathetic neurons send axons to the heart and parasympathetic activity slows cardiac pacemaking and thus heart rate. This parasympathetic activity is further increased during conditions of elevated blood pressure. Note that the parasympathetic nervous system is primarily directed toward the heart.

Autonomic innervation of the heart

Most textbooks describe the parasympathetic innervation of the heart as restricted to the sinoatrial (SA) node (see cardiac action potentials) and hence only influences heart rate, not the force of contraction of the heart. This is probably an oversimplification; parasympathetic input can influence the speed of conduction of action potentials through the bundle of His. However, the sympathetic innervation of the heart is certainly more extensive and has more influence beyond the pacemaker cells in the SA node. Furthermore, adrenergic receptors in the heart (mainly β1) can respond to circulating adrenaline as well as neuronally-released noradrenaline. By contrast, acetylcholine has a very short half-life in blood (imagine the consequences if it didn’t) and can only act as a discrete messenger where it is released.
Autonomic innervation of the vasculature

Again, the influence of the parasympathetic nervous system is weaker than that of the sympathetic nervous system when it comes to the vasculature. Nearly every blood vessel receives some kind of sympathetic innervation, whereas the parasympathetic nervous system innervates only a few vascular beds (e.g. cerebral and urogenital). It’s important to dispel the myth that the sympathetic and parasympathetic control of the cardiovascular system are equal and opposite, being turned on and off as required. They serve different roles and are both active most of the time. The “on-off” myth is probably perpetuated by the limited number of examples of blood vessels being innervated by both branches of the autonomic nervous system. Where this does occur, each has opposite effects. However, most vessels only receive a sympathetic innervation.

Veins are innervated by sympathetic nerves and have a thin wall of smooth muscle. In their position on the “wrong side” of the capillaries it might at first appear that contracting veins would have little impact on the cardiovascular system since venous blood pressure is so low. However, by changing the volume of the veins it is possible to change the volume of venous blood in a given organ or tissue. This is because most blood volume is stored in veins, rather than arteries. Furthermore, when veins contract, their volume is forced towards the heart, increasing the volume in the right atrium and increasing contractility via the Frank-Starling relationship. This will increase cardiac output and hence blood pressure, by shifting blood volume from the venous pool to the arterial side of the circulation.

Sympathetic neurohumoral control

The sympathetic nerve system can also control levels of adrenaline released by the adrenal gland and renin production by the kidneys. Its roles here are again unopposed by the parasympathetic nervous system.

The chromaffin cells in the medulla of the adrenal gland are essentially adrenergic neurons with no axons. Instead, when activated by sympathetic preganglionic fibres from the spinal cord, these cells release adrenaline into the blood. This is the famous “adrenaline rush” that we associate with the “flight or fight” reflex. Circulating adrenaline differs in its effects on the vasculature since it can bind to receptors on cells that are not “innervated”. Noradrenaline released by sympathetic nerves acts near the point of release from each of the varicosities along the nerve where release occurs. Reuptake mechanisms (and degradation) limit the overflow of noradrenaline at these sites so that predominantly receptors nearby are activated (Figure 3). By contrast, circulating adrenaline has access to all receptors on the each smooth muscle cell. What the overall result of sympathetic nerve activation and adrenaline release will be (contraction or relaxation) will depend on how many receptors of each type are present. In muscle for example, adrenaline tends to overpower sympathetic neurotransmission, whilst in the splanchnic circulation the opposite occurs. This makes some physiological sense, since during a “flight or fight” response, blood flow to the muscles needs to be maximised, while blood flow to organs of digestion can be minimised.

adrenergic varicosity

Figure 2: Differences in the effects of sympathetic nerve-derived noradrenaline and circulating adrenaline. Noradrenaline acts near the site of release, generally by activation of α adrenoceptors, and is transported back into the nerve varicosity or metabolised. By contrast, adrenaline has access to the entire cell surface, activating both β and β adrenoceptors. In this way, adrenaline can overpower sympathetic neurotransmission and have the opposite effect on myocyte contractility and hence blood flow.

Consider the following situation. You are sitting watching television and gently tapping your foot. Most of the muscles in your leg require minimal blood flow and the sympathetic nervous system will limit flow to them. By contrast the muscles performing the foot-tapping manoeuvre will require a bit more blood and the sympathetic drive to blood vessels supplying them will be less. This sort of fine control is constantly occurring. Suddenly, there is an almighty explosion nearby. The surge of adrenaline in blood overpowers the sympathetic control of blood to your legs and dilates all blood vessels ready for you to run in whatever direction is required.

The renin-angiotensin system (RAS) provides endocrine (so more sluggish) control of blood vessel contractility and kidney function to maintain blood pressure. The sympathetic nervous system and the RAS interact. Circulating adrenaline activates juxtaglomerular cells in the kidney (via β1 receptors) to release renin, triggering the activation of the RAS. The final result of this is the production of angiotensin II which exerts several effects to increase blood volume and blood pressure, including directly contracting vascular myocytes. Angiotensin II also causes increased release of noradrenaline from sympathetic nerves innervating blood vessels. So, the two systems – working over different time scales – interact constantly as a team.

CNS & Peripheral NS
Your sensory neurons and your motor neurons make up your peripheral nervous system. Your central nervous system is like the coach and your peripheral nervous system is like the team. The coach calls the plays, but the team is on the front line of action, and sometimes players have to react on their own.Peripheral Nervous System

Your peripheral nervous system is made up ofsensory neurons (which take in information about what you see, hear, taste, touch and smell) andmotor neurons (which send information to your muscles and glands so you can react). The sensory neurons are the ones that tell your brain there’s a mosquito biting you, and then your brain tells your motor neurons to swat it with your hand.

Just as your central nervous system has both conscious and unconscious reflexes (remember the knee jerk?), your peripheral nervous system has some functions that you’re aware of while they’re happening, and others (like digesting food) that you do without thinking. The things you’re conscious of involve your somatic nervous system. When you swat a mosquito, it’s your somatic nervous system kicking in. It controls your external muscles and skin. But, other internal activities happen more or less automatically, like your mouth watering or sweating. While it is possible to regulate how fast your heart is beating or how fast you’re breathing, these are primarily the realm of the autonomic nervous system; they happen whether you’re paying attention to them or not. Autonomic responses are automatic.

Okay, so we can break down these automatic responses further, into sympathetic and parasympatheticresponses. Your sympathetic nervous system gets you ready to fight or flee when you’re faced with a crisis, and your parasympathetic nervous system calms you down and allows you to relax. So, when you hear the fire alarm go off in your building, your sympathetic nervous system gets your heart racing to ready you to act. But, when you realize it’s a false alarm, your parasympathetic nervous system slows your heart back down and allows you to relax.


When it all works together, your nervous system allows you to react to any situation. Your central nervous system is the command center, and your peripheral nervous system is the front lines. The front lines have both automatic, or autonomic, internal reflexes like sweat and fear, and conscious reactions that involve yoursomatic nervous system in your muscles.

Types of Feedback


In your body, very similar events occur between body systems, locations and hormones. All of these feedback mechanisms serve to keep your body’s internal mechanisms running smoothly. If a feedback mechanism were to go into overdrive, or be impaired, the team (akin to your body) would either never try to tie the game up or would fail due to the exhaustion of trying too hard.

When a mother gives birth to a child, she will undergo a process by which milk is secreted from the mammary glands. We call this process lactation. This process is regulated by quite a few hormones. One of the hormones responsible for the production of milk is called prolactin. As the newborn child suckles on the mother’s nipple, milk is released from the mammary glands and, therefore, must be replenished.Positive Feedback

In order to replenish the released milk, more milk must be produced. The suckling reflex of the child actually sends signals to the brain to release more prolactin in order to produce more milk. The more the child suckles, the more the nerves signal for the release of more prolactin. This positive output allows for the child to suckle more milk in the future, which, in turn, causes more prolactin to be released yet again, so the child can continue to suckle milk.

Hence, the process in which X produces Y, which, in turn, stimulates more of X to be produced, is termedpositive feedback. One process feeds off of and enhances the other. This is another way of looking at positive feedback. In our example, the suckling of an infant depends on prolactin, and this suckling enhances prolactin’s production so the baby has something to feed on.

Negative Feedback

On the flipside, there are examples of when a process’s outputs reduce the processes ability for further output – something we call negative feedback. For example, if your blood pressure were to increase due to increased sympathetic nervous system activity, causing vasoconstriction, receptors in certain parts of your great vessels would send signals up to your brain to tell you to shut off your sympathetic nervous system. Once the sympathetic nervous system’s effect on your vasculature has been turned off, the blood vessels will be able to dilate, or expand, in order to decrease the blood pressure.

High blood pressure caused by the sympathetic nervous system leads to a negative feedback loop
Negative Feedback

Therefore, a scenario whereby the original output (increased sympathetic nervous system activity) causes an output that must be stopped (the high blood pressure), there exists a mechanism by which the output (high blood pressure) triggers a feedback to shut off the original process that caused the problem in the first place.

Intermediary Steps in Positive and Negative Feedback

One thing you must keep in mind is that in both negative and positive feedback loops, there are intermediary steps that may actually inhibit or excite a certain pathway. But the end result is either positive or negative feedback, regardless of any excitatory or inhibitory intermediary steps.

Another thing you should also know is that inhibition or stimulation of a positive or negative feedback loop does not have to occur by the final output, influencing what started the entire process in the first place. The final output, or intermediary steps of a process, may inhibit or excite any one or more intermediary steps in a process and, therefore, cause positive or negative feedback.

Fight or Flight

The final output may inhibit or excite intermediary steps, causing positive or negative feedback
Intermediary Steps

Let’s tie what we’ve learned to a real world example I hope no one has to experience. If you are walking around the woods and see a massive bear coming towards you, it’s highly unlikely you’ll want to cuddle with it. That grizzly may look cute, but to the bear, you’re just a tasty snack. The bear is almost certain to actually stress you out and leave you with a choice of either running away or trying to fight off the bear. The concept of our body’s physiological reaction to a stressor, which causes us to fight it or run away from it, is called the fight or flight response. One of the most critical things that occurs during a fight or flight response is a massive increase in your heart rate.

Under normal resting conditions, your sympathetic nervous system’s actions on your heart are not as significant as your parasympathetic nervous system‘s effects. When everything is calm and cool in your life, the parasympathetic nervous system exerts more control over things like the heart rate. In very simple terms, the parasympathetic nervous system lowers your heart rate.

However, when placed under stressful conditions, your sympathetic nervous system actually begins to gain the upper hand over the parasympathetic nervous system’s control of the heart. In this case, the sympathetic nervous system stimulates the heart to beat faster, much faster. All of this happens to help you increase blood flow to either fight the bear or run away from it. It’s your choice in the end, but at least your body prepares you for either scenario through the fight or flight response.

Lesson Summary

Let’s recap all of our important points. The process in which X produces more of Y, which in turn stimulates more of X to be produced, is termed positive feedback. Conversely, when a process’s outputs reduce the processes ability for further output we term this negative feedback. Finally, our body’s physiological reaction to a stressor, which causes us to fight it or run away from it, is called the fight or flight response.

Just remember, positive and negative feedback loops can have one or multiple outputs that may influence one or more steps in the entire process in order to increase or decrease the output of a final or intermediary step. Basically, anything can happen along the way.