Which part of the nervous system responds automatically during emergencies?

Notice in the picture on the left that the sympathetic nervous system originates in the spinal cord. Specifically, the cell bodies of the first neuron (the preganglionic neuron) are located in the thoracic and lumbar spinal cord. Axons from these neurons project to a chain of ganglia located near the spinal cord. In most cases, this neuron makes a synapse with another neuron (post-ganglionic neuron) in the ganglion. A few preganglionic neurons go to other ganglia outside of the sympathetic chain and synapse there. The post-ganglionic neuron then projects to the "target" - either a muscle or a gland.

Two more facts about the sympathetic nervous system: the synapse in the sympathetic ganglion uses acetylcholine as a neurotransmitter; the synapse of the post-ganglionic neuron with the target organ uses the neurotransmitter called norepinephrine. (Of course, there is one exception: the sympathetic post-ganglionic neuron that terminates on the sweat glands uses acetylcholine.)

The Parasympathetic Nervous System

It is a nice, sunny day...you are taking a nice walk in the park.

This time, however, you decide to relax in comfortable chair that you have brought along. This calls for "Rest and Digest" responses. Now is the time for the parasympathetic nervous to work to save energy. This is when blood pressure can decrease, pulse rate can slow, and digestion can start.

Notice in the picture on the left, that the cell bodies of the parasympathetic nervous system are located in the spinal cord (sacral region) and in the medulla. In the medulla, the cranial nerves III, VII, IX and X form the preganglionic parasympathetic fibers. The preganglionic fiber from the medulla or spinal cord projects to ganglia very close to the target organ and makes a synapse. This synapse uses the neurotransmitter called acetylcholine. From this ganglion, the post-ganglionic neuron projects to the target organ and uses acetylcholine again at its terminal.

Autonomic Nervous System

StructureSympathetic StimulationParasympathetic StimulationIris (eye muscle)Pupil dilationPupil constrictionSalivary GlandsSaliva production reducedSaliva production increasedOral/Nasal MucosaMucus production reducedMucus production increasedHeartHeart rate and force increasedHeart rate and force decreasedLungBronchial muscle relaxedBronchial muscle contractedStomachPeristalsis reducedGastric juice secreted; motility increasedSmall IntestineMotility reducedDigestion increasedLarge IntestineMotility reducedSecretions and motility increasedLiverIncreased conversion of
glycogen to glucoseKidneyDecreased urine secretionIncreased urine secretionAdrenal medullaNorepinephrine and
epinephrine secretedBladderWall relaxed
Sphincter closedWall contracted
Sphincter relaxed

It should be noted that the autonomic nervous system is always working. It is NOT only active during "fight or flight" or "rest and digest" situations. Rather, the autonomic nervous system acts to maintain normal internal functions and works with the somatic nervous system.

The enteric nervous system is a third division of the autonomic nervous system that you do not hear much about. The enteric nervous system is a meshwork of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, and gall bladder).

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The nervous system is typically divided into the central nervous system, which includes not only the brain, but also the spinal cord, and the peripheral nervous system that includes all of the nerves outside of the brain and spinal cord. It is the spinal cord, however, which is the core of the entire nervous system, central and peripheral.

The spinal cord carries information from the entire body into and out of the brain. (Image: Magic mine/Shutterstock)

Reflexes

The spinal cord is really a long tube containing bundles of nerve fibers and has two main functions. The first of this is initiating certain automatic behaviors on its own. These involuntary behaviors are called reflexes, as the response to the incoming stimuli is automatically sent to the spinal cord and then reflected back to the relevant muscles.

Hence, reflexes are basically a shortcut that allows our body to immediately react since the signals never pass through the brain. We’re all born with numerous reflexes—such as blinking in response to a puff of air delivered to the eyes, or the gag reflex in response to an object touching the back of the throat.

When the doctor taps our knee just below the kneecap, our leg automatically kicks out. (Image: Studio 72/Shutterstock)

When the doctor taps our knee just below the kneecap, our leg automatically kicks out. This knee jerk reflex is actually an example of what is known as a deep tendon reflex. The doctor is tapping on a tendon, which then stretches the tendon and the muscle in the thigh that connects to it.

If the nervous system is working properly, that stretch sends a message to the spinal cord and the spinal cord very quickly sends a message back to the muscle telling it to contract, which is what causes the lower leg to suddenly kick out.

Carrier of Information

Another important function which the spinal cord performs is that it carries information from the entire body into and out of the brain. The spinal cord works together with the peripheral nervous system which carries information to and from the central nervous system of the brain and spinal cord to the body’s sense receptors, muscles, and glands.

The peripheral nervous system includes all the nerves outside the brain and the spinal cord. It is subdivided into two distinct parts that control different types of processes: the somatic nervous system and the autonomic nervous system.

Somatic Nervous System

The somatic nervous system carries sensory information to the brain and spinal cord and then carries messages from the brain and spinal cord to muscles.

We can better understand the working of our somatic nervous system through an example. Suppose we are outside, running on a cold winter morning and we see ice in front of us. Our visual system recognizes ice and sends this information to our brain. Our brain then sends signals to our muscles telling them to take action, to shift our body to a different part of the path so that we can avoid the ice, and a potentially dangerous fall. This is all voluntary and is done by the somatic nervous system.

Autonomic Nervous System

The other part of the peripheral nervous system is the autonomic nervous system, which controls involuntary tasks such as heart rate, digestion, breathing. This system (as its name implies) operates at an automatic level. We don’t have to regularly remind ourselves to breathe or digest food.

The autonomic nervous system also further divides into two distinct branches: the sympathetic and the parasympathetic, which work in opposition to each other.

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Sympathetic and the Parasympathetic Nervous Systems

These two branches regulate multiple activities within the body, including breathing, heart rate, digestion, and sexual arousal. These two systems balance each other out.

The sympathetic nervous system helps the body respond quickly to mobilize resources during times of stress; this emergency response is often described as the fight-or-flight response.

The parasympathetic nervous system works to calm the body and conserve energy, including the freeze response. So, the two systems together are sometimes called flight, fight, or freeze.

Handling a Crisis

To understand how crucial a role they play in handling a crisis, let’s take a hypothetical situation. Suppose one is walking in the woods and sees a rattlesnake, coiled and ready to strike. in such a situation, our sympathetic nervous system would immediately kick in and prepare us to run or fight back if running away was not possible.

Our heart rate and breathing would increase, our digestive system would stop working, and cortisol (a stress hormone) would be released into the bloodstream. All of these physiological responses would help get more blood and energy to our muscles, allowing us to take quick action.

Flight, Fight, or Freeze

If we froze, or after we had successfully escaped from the threat, the parasympathetic nervous system would kick in to help return our body to a calmer state. Our heart rate would slow down, our blood pressure would lower, and our digestive process would return to normal.

Hence, although they work in opposition and have opposite effect on our body—whether causing us to flight, fight, or freeze—together they help bail us out of immediate danger.

What part of the nervous system responds to crisis?

What part of the nervous system works automatically?

Which portion of the nervous system controls our response to an emergency situation?

What part of the nervous system prepares the body for emergencies?