Lesson Explainer: Organization of the Nervous System Biology

In this explainer, we will learn how to describe the organization of the human nervous system.

A fundamental part of being human is the ability to feel, think, sense, and respond to the world around us. The system that makes this possible in the human body is the nervous system. From basic reflexes to more complicated behaviors like finding food and courting potential mates, the nervous system is the electrical wiring that runs throughout the human body to make all these behaviors possible.

Overall, the human nervous system has three main functions: gathering sensory input, information processing, and responding via motor output. The nervous system receives information about conditions both within and around the body. It processes and integrates this information on a variety of levels and directs the body to respond appropriately.

To efficiently collect, process, and respond to information, the nervous system has a highly organized structure. The human nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). These two main divisions, which are shown in Figure 1, help ensure that the three broad functions of the nervous system are carried out efficiently.

As you can see from the diagram, the CNS consists of the brain and the spinal cord. In contrast, the PNS refers to all of the nerves outside of the CNS that extend from the spinal cord to the external limbs and other organs. This includes the cranial nerves, which extend from the brain to the face muscles and sensory organs (eyes, olfactory bulb, tongue, and ears).

Definition: Central Nervous System (CNS)

The central nervous system is made up of the brain and spinal cord.

Definition: Peripheral Nervous System (PNS)

The peripheral nervous system is made up of nerves that are outside of the CNS and extend into the external limbs and other organs.

Example 1: Describing the Main Divisions of the Human Nervous System

What part of the nervous system is made up of the nerves that extend from the brain and spinal cord into the external limbs and other organs?

  1. Central nervous system
  2. Peripheral nervous system

Answer

A fundamental part of being human is the ability to feel, think, sense, and respond to the world around us. The system that makes this possible in the human body is the nervous system. From basic reflexes to more complicated behaviors like finding food and courting potential mates, the nervous system is the electrical wiring that runs throughout the human body to make all these behaviors possible.

Overall, the human nervous system has three main functions: gathering sensory input, information processing, and responding via motor output. To efficiently collect, process, and respond to information, the nervous system has a highly organized structure. The human nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

The CNS is often called the central processing unit of the body. This is because it is responsible for integrating sensory information and responding accordingly. In the diagram below, the central nervous system, highlighted in pink, consists of the brain and the spinal cord.

These two main divisions help ensure that the three broad functions of the nervous system are carried out efficiently. The peripheral nervous system, which is shown in blue, refers to all of the nerves outside of the CNS that extend from the spinal cord to the external limbs and other organs.

Therefore, the part of the nervous system that is made up of the nerves that extend from the brain and spinal cord into the external limbs and other organs is the peripheral nervous system.

Let’s take a closer look at the function and organization of CNS and PNS.

The CNS is often called the central processing unit of the body. This is because it is responsible for integrating sensory information and responding accordingly. The CNS consists of two main components, the brain and the spinal cord, shown in Figure 2.

The brain is responsible for processing and interpreting sensory information and coordinating body functions, both consciously and unconsciously. Complex functions, such as thinking and emotional feeling as well as maintaining a constant internal environment in the body (homeostasis), are attributed to different parts of the brain. After the information is processed by the brain, the brain will determine the appropriate response.

Key Term: Homeostasis

Homeostasis is the maintenance of a constant, normal internal environment within an organism.

The brain will then command and control the functions of the body in response to the interpreted information by sending signals through the neurons of the spinal cord. The spinal cord serves as an information superhighway. It is responsible for passing messages from the body to the brain and for transmitting the signals from the brain to the body.

Although the brain and spinal cord are responsible for processing and transmitting information to the CNS, it does not collect the information about the external environment nor does it carry out the commands of the brain. Instead, these functions are the responsibility of the PNS. Let’s take a closer look at the PNS.

The PNS includes all other nervous system structures that are outside the CNS. The primary purpose of the PNS is to transmit information toward and away from the CNS. This is because in nerve cells, information can travel only in one direction. So, there is one set of neurons that carries the information toward the CNS and a set of nerves that carries information away from the CNS. This flow of information in the PNS can be seen in the two subdivisions: the sensory and motor divisions, as shown in Figure 3.

Information that moves from the periphery to the CNS is sensory information and is regulated by the sensory division of the PNS. Within the sensory division, sensory nerves collect the information about the physical stimuli in our environment or changes in our internal organs and send this information to the CNS to relay the state of our body and the external environment.

The collection of sensory information from our body and the external environment is performed by sensory nerves. Sometimes these nerves are also described by the word afferent. This refers to the flow of sensory information into the CNS as the word afferent is Latin for “bringing toward.”

Key Term: Sensory (Afferent) Division

The sensory division of the PNS conducts sensory information from the sensory nerves to the CNS.

The collection of sensory information is performed by sensory receptors of the sensory nerves, which then convey the sensory information to the PNS. Sensory nerves are activated by physical or chemical input from the environment like lights, sounds, or the molecules in food. These inputs correspond to the five major senses, such as touch, smell, taste, hearing, and sight (illustrated in Figure 4). Additionally, sensory neurons collect information about pain, body position (sometimes called proprioception), and sensory information from our internal organs.

Let’s make sure we understand the use of the words neuron and nerve. Neurons exist in both the CNS and the PNS. However, in the PNS, where extremely long nerve fibers are most frequently observed, the term nerve (or nerve fibers) refers to a collection or bundle of neurons. When discussing the two nervous systems, you will commonly see “neuron” used in reference to the CNS, but “nerves” when discussing the PNS.

Once the sensory information, or sensory input, has been gathered, it is transmitted by the sensory division of the PNS to the CNS for integration and processing by the brain. As the command center, the brain will formulate a coordinated response to the collected sensory information, typically in the form of motor output.

Key Term: Sensory Input

Sensory input is information gathered from sensory receptors of the sensory nerves which is then transmitted to the CNS.

Key Term: Motor Output

Motor output is a command from the CNS in response to sensory information and typically takes the form of a motor movement.

While the brain formulates the coordinated response, it is the motor division of the PNS that carries the signal from the brain to the muscles, glands, or organs to execute the command.

Within the motor division, it is the job of motor nerves to pass instructions from the CNS to other parts of the body, such as muscles or glands. Motor nerves send signals in the opposite direction to sensory ones, transmitting commands from the brain to control the contraction of smooth, skeletal, and cardiac muscles. Motor nerves are also sometimes referred to as efferent nerves. The word efferent is Latin for “carrying away.” Thus, the term efferent nerves helps to describe the action of motor nerves carrying the commands away from the CNS to the muscles, organs, and glands of the PNS. The difference in the flow of information between sensory (afferent) nerves and motor (efferent) nerves is shown in Figure 5.

Key Term: Motor (Efferent) Division

A motor division of the PNS carries nerve impulses away from the CNS to the effectors.

As you move through the day, you may realize that some motor responses are conscious decisions that require your awareness, like turning on a light when you are unable to see in a dark room. Other motor behaviors occur automatically without the need to constantly relay a message to the brain. An example of unconscious automatic behaviors would be breathing or digestion. While we know we need to perform these actions, they occur without our awareness or intentional thought. These two types of motor responses are handled by one of the two main subdivisions of the motor division: the somatic or the autonomic nervous system, which is illustrated in Figure 6.

Key Term: Conscious Thought

Conscious thought refers to behaviors that are made with your individual awareness.

Conscious motor behaviors like opening a door or turning on a light switch are initiated by the brain and executed by the skeletal or voluntary muscles. Skeletal muscles are generally attached to the skeleton and are controlled through conscious thought and awareness. This means the movement of voluntary (skeletal) muscles is a result of intentional thought, usually to achieve a specific goal.

The somatic nervous system controls the voluntary muscles in the body by transmitting commands from the CNS to skeletal muscles in the periphery. To carry these motor commands to the skeletal muscle, the somatic system uses somatic nerves. The somatic nerves connect the brain and spinal cord with skeletal muscles and sensory receptors in the skin.

Key Term: Voluntary Muscles

Voluntary muscles are skeletal muscles whose activity is under conscious control and awareness.

Key Term: Somatic Nervous System

The somatic nervous system controls the voluntary actions in the body by transmitting impulses from the CNS to skeletal muscles.

In addition to controlling voluntary muscle movements, the somatic system is responsible for a specific type of involuntary skeletal muscle response known as reflexes. During reflex action, muscles move involuntarily without input from the brain. For a reflex action to occur, a somatic nerve connects directly to the spinal cord.

The reflex is an important ability. It allows for the muscles to respond quickly with no input from the brain, which helps safeguard the body from harm in certain situations. For example, if you accidentally touch something very hot, your body needs to react very fast. In this case, a reflex skips the voluntary control of the muscles and automatically commands the muscle to move away from the hot surface. It is possible to get a voluntary control of a reflex, which may be necessary if, for example, we need to touch or hold something very hot for a short time.

Example 2: Defining the Function of the Somatic Nervous System

The diagram provided shows how the peripheral nervous system and its motor division are divided into their component parts. What is the primary function of the somatic nervous system?

  1. To transmit information to muscles and glands that are not under voluntary or conscious control
  2. To transmit information to muscles that are under voluntary or conscious control and to mediate reflex actions.

Answer

Within the motor division, it is the job of motor nerves to pass instructions from the CNS to other parts of your body, such as muscles or glands. Motor nerves send signals in the opposite direction from sensory ones, transmitting commands from the brain to control the contraction of smooth, skeletal, and cardiac muscles.

Conscious motor behaviors like opening a door or turning on a light switch are initiated by the brain and executed by the skeletal or voluntary muscles. Skeletal muscles are generally attached to the skeleton and are controlled through conscious thought and awareness. This means the movement of voluntary (skeletal) muscles is a result of intentional thought, usually to achieve a specific goal.

The somatic nervous system controls the voluntary muscles in the body by transmitting commands from CNS to skeletal muscles in the periphery. To carry these motor commands to the skeletal muscle, the somatic system uses somatic nerves. The somatic nerves connect the brain and spinal cord with skeletal muscles and sensory receptors in the skin.

In addition to controlling voluntary muscle movements, the somatic system is responsible for a specific type of involuntary skeletal muscle response known as reflexes. During reflex action, muscles move involuntarily without input from the brain. For a reflex action to occur, a somatic nerve connects directly to the spinal cord.

The reflex is an important ability. It allows for the muscles to respond quickly with no input from the brain, which helps safeguard the body from harm in certain situations. For example, if you accidentally touch something very hot, your body needs to react very fast. In this case, a reflex skips the voluntary control of the muscles and automatically commands the muscle to move away from the hot surface. It is possible to get a voluntary control of a reflex, which may be necessary if, for example, we need to touch or hold something very hot for a short time.

Therefore, the way to use the primary function of the somatic nervous system is to transmit information to muscles that are under voluntary or conscious control and to mediate reflex actions.

Many of the behaviors that keep you alive and well are not performed because of conscious, voluntary action by the skeletal muscles. Actions like breathing or digestion occur with unconscious thought but are incredibly important if we hope to stay alive! The part of the motor division that controls muscles of internal organs, such as the pupils, blood vessels, lungs, stomach, and intestines, is the autonomic nervous system.

Key Term: Unconscious Thought

Unconscious thought refers to behaviors that are exhibited without individual awareness.

The autonomic nervous system helps to control involuntary actions like our heart rate, digestion, respiratory rate, salivation, perspiration, pupil diameter, urination, and arousal. This occurs through control of the involuntary muscles. Involuntary or smooth muscles are found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. Unlike skeletal or voluntary muscles, smooth muscles can never be under voluntary control.

Since involuntary muscles can never be under our control, the autonomic nervous system is incredibly important as a control system to regulate our ability to perform the most basic survival functions of being human!

Key Term: Involuntary Muscles

Involuntary muscles are smooth muscles whose activity is not under conscious control.

Definition: Autonomic Nervous System

The autonomic nervous system controls the involuntary actions in the body, such as digestion, respiration, and salivation, by transmitting nerve impulses from the CNS to the organs and glands.

Example 3: Defining the Functions of the Autonomic Nervous System

Consider the following activities and processes:

  1. Dilation and constriction of pupils
  2. Kicking a ball
  3. Breathing
  4. Digestion
  5. Scratching an ear

Which of the following choices shows the activities or processes controlled by the autonomic nervous system?

  1. 2, 3, and 4
  2. 3 only
  3. 4 only
  4. 2 and 5
  5. 1, 3, and 4

Answer

Many of the behaviors that keep you alive and well are not performed because of conscious, voluntary action by the skeletal muscles. Actions like breathing or digestion occur with unconscious thought but are incredibly important if we hope to stay alive! The part of the motor division that controls muscles of internal organs, such as the pupils, blood vessels, lungs, stomach and intestines, is the autonomic nervous system.

The autonomic nervous system helps to control involuntary actions like our heart rate, digestion, respiratory rate, salivation, perspiration, pupil diameter, urination, and arousal. This occurs through control of the involuntary muscles. Involuntary or smooth muscles are found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. Voluntary, or skeletal, muscles are generally attached to the skeleton and are controlled through conscious thought and awareness. Unlike skeletal muscles, smooth muscles can never be under voluntary control.

Let’s take a look at the activities listed and determine if they occur with voluntary or involuntary muscles. The question is asking us to determine which activities are involuntary and controlled by the autonomic nervous system.

Actions like dilation and constriction of pupils happen in response to the amount of light our eyes are exposed to and are involuntary. Therefore, this motor response would be governed by the autonomic nervous system.

The action of kicking a ball requires the skeletal or voluntary muscles of the leg. Therefore, this motor response would be governed by the somatic nervous system.

Although we can control how deep or shallow we take a breath, the action of breathing occurs even if we forget to think about it or when we are not conscious, like when we sleep. Therefore, breathing would be governed by the autonomic nervous system.

After eating a big meal, as much as we would hope to speed our digestion through our conscious efforts, we are unable to exert control over the smooth muscles found in the stomach or intestines. The control of such motor functions is not voluntary. Therefore, digestion is governed by the autonomic nervous system.

Finally, scratching an ear requires use of the skeletal or voluntary muscles of the finger and hand. Therefore, this motor response would be governed by the somatic nervous system.

Therefore, the activities or processes controlled by the autonomic nervous system are dilation and constriction of pupils (1), breathing (3), and digestion (4).

To review, the flow of information within the nervous system proceeds in a somewhat linear fashion. First, sensory information is collected by the sensory or afferent nerves of the PNS, then the information is taken to the CNS for integration and processing. Within the CNS, the brain determines a coordinated motor response. Depending on the type of response needed, the coordinated motor output is handled by the somatic or autonomic nervous system. If the motor response is voluntary, such as taping a key on a keyboard, the somatic nervous system will be used. If the motor response is involuntary, such as coughing or breathing, the autonomic nervous system will be used. The coordination of these motor functions in response to sensory information helps us to maintain homeostasis to keep our internal environment in a fairly constant and stable state. This process is summarized in the relationship box below.

Process: Functional Flow of Information in the Human Nervous System

Receiveinformationprocessinformationcoordinateresponsemaintainhomeostasis

Example 4: Understanding the Functional Flow of Information in the Human Nervous System

Which of the following diagrams correctly outlines the functioning of the human nervous system?

Answer

The flow of information within the nervous system proceeds in a somewhat linear fashion. First, sensory information is collected by the sensory or afferent nerves of the PNS; then, the information is taken to the CNS for integration and processing. Within the CNS, the brain determines a coordinated motor response. Depending on the type of response needed, the coordinated motor output is handled by the somatic or autonomic nervous system. If the motor response is voluntary, such as taping a key on a keyboard, the somatic nervous system will be used. If the motor response is involuntary, such as coughing or breathing, the autonomic nervous system will be used. The coordination of these motor functions in response to sensory information helps us to maintain homeostasis to keep our internal environment in a fairly constant and stable state. This process is summarized as follows: receiveinformationprocessinformationcoordinateresponsemaintainhomeostasis

Therefore, to answer the question, we are looking for a set of boxes that detail this same linear flow of information. The first box should describe the sensory division of the PNS receiving information from the internal or external environment. The second box should describe how the sensory information gathered from the PNS is then sent to the CNS to be processed by the brain. The third box should describe the brain sending out the coordinated response to the motor division of the PNS. And the final box should describe how this motor response helps to keep our bodies in homeostasis or maintain a constant, normal internal environment within the human body.

Answer choices A and D all have the correct first box that describes receiving information from the internal or external environment. Therefore, we can pair down our choices to one of these two answers. To further eliminate one of these possible answers, we must look for the answer that contains the correct information in the second box, the processing of the sensory information. Only one answer has the correct information in the second box. To ensure we have chosen the correct answer, we will want to make sure that the third and fourth boxes describe coordinating a response and maintaining homeostasis.

Therefore, the diagram that correctly outlines the functioning of the human nervous system is D.

Let’s summarize what we have learned in this explainer.

Key Points

  • The nervous system is divided into two main components, the central and peripheral nervous systems.
  • The central nervous system includes the brain and spinal cord.
  • The peripheral nervous system is further subdivided into the sensory and motor divisions.
  • The sensory division of the PNS conducts sensory information from the sensory receptors toward the CNS.
  • The motor division sends signals in the opposite direction from sensory ones, transmitting commands from the CNS to control the contraction of smooth, skeletal, and cardiac muscles.
  • The motor division of the PNS is further split into the somatic and autonomic nervous systems.
  • The somatic nervous system controls the voluntary muscles in the body by transmitting commands from the CNS to skeletal muscles in the periphery.
  • The autonomic nervous system regulates body functions, such as blood pressure and the rate of breathing, that occur involuntarily.

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