Lesson Explainer: The Autonomic Nervous System Biology • Second Year of Secondary School

In this explainer, we will learn how to describe the structure and function of the sympathetic and parasympathetic nervous systems.

A network of billions of nerve cells linked together forms our highly organized nervous system. This is the control center of the body, directing all its functions, even those that we do not think about. The autonomic nervous system is responsible for these subconscious activities, and it keeps working even when we are asleep. It is also the system that helps us not only to escape dangerous situations, but also to digest our food!

There are two main divisions in the nervous system: the central nervous system (CNS) and the peripheral nervous system (PNS). These are shown in Figure 1. The CNS is made up of the brain and spinal cord. The PNS consists of a network of spinal nerves and the cranial nerves outside of the brain that carry information between the CNS and the rest of the body.

There are several more divisions of the peripheral nervous system, which are shown in Figure 2.

While the sensory division of the PNS conducts impulses from receptors to the CNS, the motor division conducts impulses from the CNS to effector organs. Effector organs, such as muscles and glands, are those that carry out a response. Within the motor division, the somatic nervous system usually conducts impulses to skeletal muscles. It is responsible for voluntary movements that we can control.

The autonomic nervous system is the other division of the motor system, and it will be the focus of this explainer. The purpose of the autonomic nervous system is to regulate involuntary and subconscious activities. It involves sending nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands.

Example 1: Describing the Primary Role of the Autonomic Nervous System

What is the primary purpose of the autonomic nervous system?

1. To control voluntary movements and coordination
2. To transmit information from the brain to the rest of the body
3. To regulate involuntary activities
4. To initiate the fight-or-flight response

Answer

There are two main divisions in the nervous system: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain and spinal cord. The PNS consists of a network of spinal and cranial nerves that carry information between the CNS and the rest of the body.

There are several more divisions of the peripheral nervous system, shown in the figure below.

While the sensory division of the PNS conducts impulses from receptors to the CNS, the motor division conducts impulses from the CNS to effector organs. Effector organs, such as muscles and glands, are those that cause a response. Within the motor division, the somatic nervous system usually conducts impulses to skeletal muscles. It is responsible for voluntary movements that we can control.

The purpose of the autonomic nervous system is to regulate involuntary and subconscious activities. It involves sending nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. The autonomic nervous system is not the only system that transmits information from the brain to the rest of the body, as the somatic system does this too.

The autonomic system conducts information from the CNS, which includes both the brain and the spinal cord and not just the brain as one of the choices suggests.

Though the fight-or-flight response is one of the roles of the autonomic nervous system, it also has another contrasting response that causes the body to “rest and digest.”

Therefore, the primary purpose of the autonomic nervous system is to regulate involuntary activities.

Let’s compare and contrast the two divisions of the autonomic nervous system: the sympathetic and the parasympathetic nervous systems.

These two systems are sometimes called antagonistic, as they are in balance with each other to keep the body in a state of equilibrium. They are considered antagonistic as they generally cause opposite responses, such as increasing or decreasing the heart rate. This helps to maintain a constant internal environment in a process called homeostasis. While the sympathetic nervous system prepares the body for a coherent and coordinated action, the parasympathetic nervous system returns it to a harmonious resting state.

A summary of the two systems and the effects they have on the different organs of the body is shown in Figure 3.

Let’s look at the parasympathetic nervous system first.

The nerve fibers of the parasympathetic nervous system start in the CNS, specifically in the brain stem and sacral, or bottom, regions of the spinal cord. You can see this in Figure 3, as the blue lines represent nerves traveling from the bases of the brain and spinal cord to the different organs on the left.

The parasympathetic nervous system is responsible for the rest-and-digest response in the human body. Generally, if the outcome of a nervous response decreases activity, it involves the parasympathetic nervous system. It does this by sending nerve impulses to many different organs and tissues, as outlined in Figure 4. Parasympathetic nerves use a neurotransmitter called acetylcholine that is released at each synapse. Synapses form junctions between neurons. Acetylcholine ensures that the signal travels in the right direction.

The digestive system contains many target organs of the parasympathetic nervous system. For example, nerve impulses are sent to the stomach to stimulate it to secrete gastric juices. Signals are also sent to the small intestine to increase its production of digestive enzymes, to the salivary glands to increase secretion of saliva, and to the liver to increase bile production. All of these responses aid digestion.

The parasympathetic nervous system is also involved in responses of the circulatory system. For example, if your blood pressure and heart rate are too high, the parasympathetic system can decrease them. Baroreceptors, located in the carotid artery and the aorta, detect that the blood pressure is too high and send nerve impulses to the CNS. The CNS sends electrical impulses through the nerves of the parasympathetic nervous system, which acts to lower the heart rate and therefore the blood pressure.

The body aims to conserve energy wherever possible in these resting conditions so that if a dangerous situation arises, it can expend energy. For example, the responses it carries out can lead to a decrease in blood glucose level by increasing storage of glucose as glycogen in liver cells. This means that less glucose will be used in cellular respiration, and more energy will be conserved.

There is smooth muscle in the walls of blood vessels. In the blood vessels leading to skeletal muscles, this smooth muscle contracts, which means that the blood vessels constrict. Therefore, less blood is delivered to voluntary muscles. A lot of blood is instead diverted to blood vessels that lead to the digestive system. The smooth muscle in these digestive system vessels relaxes, which causes the blood vessels to dilate. This means that more blood can access the digestive organs to increase their supply of oxygen and glucose for cellular respiration.

Bronchi, the air passages to the lungs, relax to reduce ventilation of the lungs. This means less energy is expended on frequent inhalation and exhalation, which are not needed in the rest-and-digest state. The parasympathetic nervous system can even cause the circular muscles in the iris to contract, which makes the pupils constrict so that less light enters the eye! This contributes to making us more lethargic and helps us avoid waking up induced by light when we sleep.

In antiquity, women applied eye drops made with the berries from a poisonous plant called Atropa belladonna (meaning “beautiful lady” in Italian). The atropine contained in the juice of the berries is a chemical that blocks the effect of acetylcholine on the iris, causing the pupils to remain more open. This was believed to make them look more beautiful!

Let’s now look at the sympathetic nervous system.

The nerve fibers of the sympathetic nervous system originate in the CNS, specifically in the thoracic and lumbar regions of the spinal cord. You can see this in Figure 3, as the red lines represent nerves traveling from the middle regions of the spinal cord to the different organs on the right.

The sympathetic nervous system is responsible for the fight-or-flight response in the human body. It aims to help us either escape a dangerous situation or battle our way out of it. These responses likely originated long ago in our human ancestors as a method of escaping from predators. They are still in play today, however, as you might experience fight-or-flight responses in scary situations like performing on stage, standing at the top of a very high building, or even seeing a spider! Generally, if the outcome of a nervous response increases activity, it involves the sympathetic nervous system. It does this by sending nerve impulses to many different organs and tissues, which you can see in Figure 5.

The sympathetic nervous system acts antagonistically to the parasympathetic nervous system.

The main neurotransmitters in the sympathetic nervous system are adrenaline (epinephrine) and noradrenaline (norepinephrine). These also act as hormones that are released in higher quantities by the adrenal glands when a person finds themselves in high-risk situations. The sympathetic nervous system activates the adrenal medulla of the adrenal glands to increase its production of adrenaline and noradrenaline. These hormones bind to target tissues and bring about responses that enable the body to respond quickly.

In such cases, heart rate increases, as the faster your heart beats and your blood is oxygenated, the faster the rate of respiration in cells becomes. This means that your cells, especially muscle cells, can release more energy to carry out emergency action. Blood glucose also increases, as more glycogen storage molecules are broken down in the liver to supply glucose to cells. Most blood vessels of the body constrict. This increases the blood flow toward the heart, skeletal muscles, and vital organs to ensure glucose and oxygen can get there faster. The blood vessels of the skeletal muscles, heart, lungs, and muscles dilate under the effect of the adrenaline released by the adrenal gland.

A similar response to increase the heart rate occurs when there is a high concentration of carbon dioxide in the blood, for example, when someone is running a race and their cells are respiring at a fast rate. A high concentration of carbon dioxide results in a relatively low blood pH, as carbon dioxide dissolves in the blood to form carbonic acid. This is detected by chemoreceptors that send a signal to the CNS. The CNS sends an electrical impulse via the sympathetic nervous system to increase the heart rate. This involves the respiratory system too, as more blood is pumped to the lungs to remove carbon dioxide and oxygenate the blood, thereby increasing its pH to a normal level.

There are many other target tissues of adrenaline and noradrenaline that cause various responses. For example, when these hormones target the smooth muscle in the bronchi, the passages to the lungs, they relax. This allows ventilation of the lungs to increase, so more oxygen is supplied to the blood vessels surrounding them.

Other responses of the sympathetic nervous system include circular muscles in the iris relaxing to dilate the pupils so that more light can enter the eyes. This improves vision and alertness and allows someone to respond better to sudden changes in the environment.

The sympathetic nervous system typically aims to reduce energy expenditure on actions that do not aid a person in their fight or flight, such as digestion. Therefore, some responses it inhibits include saliva production and peristalsis in the stomach and small intestine. It also decreases the secretion of digestive enzymes from the stomach, small intestine, and pancreas. Blood vessels to the digestive system also constrict, so less blood is delivered to organs that are not helpful in the fight-or-flight response.

Let’s recap some of the key points we have covered in this explainer.