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.
