“Fight or Flight”

“Fight or Flight”

When you are startled or scared all of the sudden do you feel a rush of adrenalin or feel your heart jump? Now think about how you feel when you are just about to fall asleep and feeling very relaxed. Both of these reactions are caused by the same division of the nervous system called the Autonomic Nervous System or ANS.

The autonomic nervous system, oversees and directs how your body will respond to a stimulus. The ANS does not give you a choice on how you will respond to a stimulus. You can’t consciously control the responses of the ANS, and is also referred to as the involuntary nervous system.

The ANS targets cardiac muscle, smooth muscles, and glands. That means breathing, heartbeats, blood vessel constriction, urination, and everything else that you do not consciously control is taken care of for you by the Autonomic Nervous System. The ANS accomplishes this with two divisions, the parasympathetic division and the sympathetic division. These divisions generally target the same organs, but with opposite effects. So if one division activates something, the other inhibits it. It is a system of checks and balances. Constantly monitoring the body to make sure we are in a state of stability or homeostasis.  

Let’s discuss each division separately and then discuss the results of interactions between the two systems. Anatomically, the parasympathetic is the simpler of the two divisions, so let's start there.

Parasympathetic Nervous system

The parasympathetic division is sometimes called the craniosacral division because its preganglionic fibers emerge from either the brain stem or the sacral region of the spinal cord. The preganglionic fibers are those fibers carrying the message from the CNS to the target organs.

The parasympathetic division is also referred to as the "rest and digest" system. Its role is to conserve body energy by reducing consumption and recycling up to ninety-nine percent of your body's water and metabolic by-products. But while conserving energy and recycling, the parasympathetic division also directs the essential "housekeeping" processes like digestion and urination. If you think about it, this makes sense because by directing these housekeeping sorts of processes, the parasympathetic can directly monitor what and how much is being used, and make adjustments.

So, exactly what type of effect does parasympathetic innervation have on the target organs? So lets us use eating a meal with the family for example. After eating the parasympathetic division activates the digestive organs for digestion and directs blood flow there to more effectively pick up nutrients. By decreasing blood flow to your skeletal muscles, decreasing your heart rate and respiratory rate, and increasing gland and smooth muscle activity in the digestive system, the parasympathetic system makes you feel relaxed and sleepy. This is the system that takes care of eliminating solid and liquid waste, which is more effectively generated during a resting state. In fact, studies have shown that eating on the run or eating very quickly causes digestion to compromised. This can lead to problems with defecation.

Furthermore, if you are not calm and relaxed, it impacts other systems under parasympathetic control, like the reproductive system. Sexual arousal is more likely to occur under conditions free from distress. Therefore, when conditions are calm and organ systems are at minimal levels of energy consumption, this system dominates.

Sympathetic Nervous System

Now let's discuss the opposing side of the parasympathetic division of the ANS -- the sympathetic division. Think of this system as an “energy hog”. The sympathetic system is not concerned about saving energy because it is the system responsible with saving your life. For this reason, the sympathetic division is also referred to as the "fight or flight" system and is more complex than the parasympathetic division.

One reason it is more complex is because it innervates more organs. It innervates all the same organs as the parasympathetic division plus the sweat glands, the arrector pili muscles that give you goose bumps, and all the arteries and veins of the body. Basically, the sympathetic division will use anything and everything available in your body to potentially save your life.

The results of sympathetic innervation include pupil dilation, decreased salivary gland production, and increased heart rate. It is also responsible for innervating tissues and organs in the abdominopelvic cavity, and can inhibit reproductive and digestive system functions and causes the bladder to be emptied and the large intestine to be dumped. That is why when we get nervous we sometimes feel like we need to suddenly need to use the restroom.

There is a direct connection between the spinal cord and the medullary cells of the adrenal gland. These cells secrete norepinephrine and epinephrine into the blood. Norepinephrine and epinephrine are responsible for the "adrenalin rush" you get when you suddenly get scared or angry.

What happens if we stay in a state of stress and our Sympathetic nervous system is in the “fight or flight” state for an extended period of time? This could be the case if we experienced a loss of a loved one, a divorce, or our child getting hurt. When someone experiences a stressful event, the amygdala, an area of the brain that contributes to emotional processing, sends a distress signal to the hypothalamus.

The heart then beats faster than normal, pushing blood to the muscles, heart, and other vital organs. Pulse rate, blood pressures go up and they start to breathe more rapidly. Meanwhile, epinephrine triggers the release of blood sugar (glucose) and fats from temporary storage sites in the body. These nutrients flood into the bloodstream, supplying energy to all parts of the body.

The hypothalamus then activates the second component of the stress response system — known as the HPA axis. This network consists of the hypothalamus, the pituitary gland, and the adrenal glands.

The HPA axis relies on a series of hormonal signals to keep the sympathetic nervous system the "gas pedal" pressed down. If the brain continues to perceive something as dangerous or upsetting, the hypothalamus releases hormones. This hormone travels to the adrenal glands, prompting them to release cortisol. The body thus stays revved up and on high alert.

In this state we might experience loss of sleep or disrupted sleeping patterns. We are not be able to go to sleep for hours when resting in bed or we wake up at 4 in the morning and can’t go back to sleep even though we are exhausted.

Continuing in this state, digestive issues are suddenly appearing. The sympathetic nervous system has reserved its energy and puts the brakes on the digestive processes. We find ourselves with reduced stomach acids and unable to digest food properly. If undigested food passes through the digestive tract for a prolonged period of time, this will cause new digestive complications and serious problems for our health.

In summary, we learned that the dual innervation of the two systems are used to keep us in homeostasis (relatively stable equilibrium). However, some are living in a constant state of sympathetic dominance causing a terrible toll on the body, and its energy is used without regard to the reserves. We should then ask ourselves how are we going to keep unnecessary prolonged stresses out of our daily walk and our bodies out of a constant state of “Fight or Flight”.



Martini, Nath, Anatomy and Physiology, Ninth Ed, 2010