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Lesson Video: The Autonomic Nervous System Biology

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

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Video Transcript

In this video, we’ll learn how to describe the structure and function of the autonomic nervous system. We’ll also learn to describe the different functions of the subdivisions of the autonomic nervous system. Then, we’ll compare and contrast the effects of the parasympathetic and the sympathetic nervous system on the human body.

The human nervous system is a complex network of nerve cells. All of our functions, even those that we do not think about, are controlled by this electrical wiring in our bodies. And although the nervous system is highly complex, it is also highly organized and can be subdivided based on the characteristics of the nerve cells. There are two main divisions of the human nervous system: the central nervous system and the peripheral nervous system. The central nervous system is further subdivided into the brain and the spinal cord. These two components of the central nervous system work together to act as the central processing unit for the human nervous system.

In contrast, the peripheral nervous system, which consists of all of the nerves outside of the brain and spinal cord, can be subdivided three more times. The peripheral nervous system is first subdivided into the sensory and motor divisions. The sensory division carries impulses from receptors in the periphery towards the central nervous system. The motor division, in contrast, carries information away from the central nervous system back out to the periphery, where the muscles, glands, and organs can carry out a specific response. The motor division is further subdivided based on the type of motor response that is to be carried out. If the motor response is voluntary, then it falls under the control of the somatic nervous system.

Impulses from the somatic nervous system stimulates skeletal muscles and require our conscious control to be performed. If the motor response is involuntary, then it falls under the control of the autonomic nervous system. The purpose of the autonomic nervous system is to conduct impulses for all of our involuntary and subconscious activities. This can include functions like changing the size of our blood vessels, digesting our food, or escaping danger. Since these involuntary functions need to be activated at very different times, the autonomic nervous system can be further subdivided into the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system is responsible for the body’s fight-or-flight response, which is active when the body is preparing for action. The parasympathetic nervous system is responsible for the body’s rest-and-digest response, which is active when the body is relaxed, resting, or feeding.

Given the wide range of involuntary actions in these subdivisions, let’s compare and contrast the effects of the parasympathetic and sympathetic nervous systems. The parasympathetic and sympathetic nervous systems work together to keep the body in a state of homeostatic balance. They do this by acting in opposition to one another. One key example of these opposite effects is one subdivision increases the heart rate, while the other is responsible for decreasing the heart rate. This energetic balancing act helps to maintain a constant internal environment which supports the homeostatic balance within the body.

The parasympathetic nervous system is activated when the human body needs to rest and digest, which means if the goal of the nervous response is to decrease activity, it involves the parasympathetic nervous system. The parasympathetic nervous system is mainly composed of the cranial nerves and the sacral spinal nerves. Cranial nerves are nerves at the peripheral nervous system that emerge directly from the brain, including the brain stem. Spinal nerves are nerves at the peripheral nervous system that emerge from segments of the spinal cord. The word sacral indicates that these nerves are from the sacral region of the spinal cord, which is located at the bottom of the spinal cord.

The cranial and sacral spinal nerves use the neurotransmitter, or chemical messenger, acetylcholine to communicate with each effector organ. To achieve the rest phase of “rest and digest,” the cranial nerves of the parasympathetic nervous system stimulate the pupils, the heart, and our airways. With our eyes, the parasympathetic nervous system promotes rest through the pupils which constrict, decreasing the amount of light entering through the eye. This action helps make rest easier to achieve and maintain. With the heart, the parasympathetic nervous system controls the heart rate and the blood pressure. When baroreceptors in the carotid artery and the aorta detect that blood pressure is too high, the parasympathetic nervous system acts to lower the heart rate and therefore the blood pressure.

Finally, in our lungs, the parasympathetic nervous system promotes rest through the constriction of our bronchi or air passageways in our lungs. In a rest state, the bronchi relax to reduce ventilation to the lungs. This means that less energy is expended on frequent inhalations and exhalations.

To achieve the digest phase of “rest and digest,” the cranial and sacral spinal nerves stimulate the liver, digestive system, and urinary system. In the liver, the parasympathetic nervous system promotes digestion by stimulating the production and release of bile. In the digestive system, the parasympathetic nervous system stimulates the secretion of gastric juices and increases the production of digestive enzymes. Finally, since absorption and excretion are also important parts of digestion, the nerves of the parasympathetic nervous system also help to stimulate an increase in urinary output. All of these actions, from the constriction of the pupils to the secretion of gastric juices and the increase in urinary output, all aid in the resting and digesting actions of the parasympathetic nervous system.

Now that we know how the parasympathetic nervous system controls the rest-and-digest response, let’s compare the actions of the sympathetic nervous system. The sympathetic nervous system is responsible for the human body’s fight-or-flight response. It aims to help us either escape a dangerous situation or battle our way out of it. So if the goal of the nervous response is to increase activity, it involves the sympathetic nervous system. The sympathetic nervous system is mainly composed of the thoracic and lumbar spinal nerves, which originate in the thoracic and lumbar regions of the spinal cord.

The main neurotransmitters in the sympathetic nervous system are adrenaline, which is also called epinephrine, and noradrenaline, which is also called norepinephrine. Adrenaline increases the rate of blood circulation, breathing, and carbohydrate metabolism and prepares the muscles for exertion. Noradrenaline functions to increase heart rate and the force of skeletal muscle contractions. In the sympathetic nervous system, these neurotransmitters work through the thoracic and lumbar spinal nerves to stimulate the actions of fight or flight. The thoracic spinal nerves control the action of the heart, the airways, sweat glands, the liver, and the digestive system.

In the heart, the sympathetic nervous system increases the heart rate as well as increasing the force of heart contractions. This helps to ensure that all of the skeletal muscles throughout the body have enough blood to support our fight-or-flight actions. In the lungs, the air passageways called bronchi dilate, which increases airflow throughout the lungs. This supports an increased supply of oxygenated blood to the skeletal muscles found throughout the body. The sympathetic nervous system also stimulates the sweat glands, which causes an increase in sweat production. This helps to cool down the body as more energy is expended during the fight-or-flight response. Without the sweat glands, during a fight or an escape, our bodies would overheat and be unable to keep up with the increased physical exertion.

A key function of the sympathetic nervous system is to halt the many components of digestion. This reduces energy expenditure on actions that are not needed in a fight or during an escape. So the liver will increase the production of glucose to make more energy available throughout the body, while the stomach and the intestines decrease activity to slow down digestion. These activities help to ensure that the energy available in the body can be efficiently used by the parts of the body most involved in the fight-or-flight response.

The lumbar spinal nerves control the action of the adrenal glands and the urinary system. The adrenal glands support the production of neurotransmitters important to the sympathetic nervous system response. In the urinary system, the bladder is relaxed. This decreases urine output during a fight-or-flight response, which is important since during an escape or combat, there is very little time for a restroom break. All of these actions from the increase in heart rate to the decrease in urinary output aid in the fight-or-flight response of the sympathetic nervous system. Now that we know how the functions of the parasympathetic and sympathetic nervous system compare and contrast, we’re ready to try a practice problem.

What is the primary purpose of the autonomic nervous system? (A) To regulate involuntary activities. (B) To control voluntary movements and coordination. (C) To transmit information from the brain to the rest of the body. (D) To initiate the fight-or-flight response.

This question is asking us to recall the primary purpose or main goal of a specific part of the human nervous system. So to answer this question, let’s review how the human nervous system is organized, paying special attention to the structural organization of the autonomic nervous system. The human nervous system is a very complex network of nerve cells. And although it is very complex, the human nervous system is so highly organized that it can be subdivided based on the primary purpose of the nerve cells.

The first main subdivision of the nervous system is based on the location of the nerves. This creates two subdivisions, the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord. These two components work together to act as the central processing unit of the human nervous system. They handle the most important actions of integrating and processing the information carried by nerve cells. In contrast, the peripheral nervous system consists of all of the nerves outside of the brain and spinal cord.

Because of the wide diversity of nerves found within the peripheral nervous system, it can be subdivided three more times. The peripheral nervous system is first subdivided based on the flow of information. The sensory division carries impulses from the receptors in the periphery towards the central nervous system. The motor division carries information away from the central nervous system to the periphery, where the muscles, glands, and organs can carry out a specific response. Since a wide variety of responses can be carried out by the motor division, the motor division is further subdivided based on the type of motor response that is to be carried out.

If the motor response is voluntary, then it falls under the control of the somatic nervous system. Impulses from the somatic nervous system stimulate skeletal muscles and require our conscious control to be performed. If the motor response is involuntary, then it falls under the control of the autonomic nervous system. The primary purpose of the autonomic nervous system is to conduct impulses for all of our involuntary and subconscious activities. This can include functions like increasing the heart rate, digesting our food, and escaping from danger.

Given the range of functions that fall under the autonomic nervous system, it can be subdivided based on the activity level of the motor response. When an increase in activity of involuntary actions is needed, they are governed by the sympathetic nervous system. These involuntary actions generally support our fight-or-flight responses. When a decrease in activity of involuntary actions is needed, it is governed by the parasympathetic nervous system. These actions generally support our rest-and-digest responses.

Now that we’ve reviewed the organization of the human nervous system and understand the function of the autonomic nervous system, we are prepared to answer the question. The primary purpose of the autonomic nervous system is to regulate involuntary activities.

Let’s recap some of the key points we have covered in this video. The purpose of the autonomic nervous system is to regulate involuntary activities. The autonomic nervous system is subdivided into the parasympathetic and sympathetic nervous systems. The parasympathetic nervous system is involved in rest-and-digest responses. The sympathetic nervous system is involved in fight-or-flight responses.

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