Nervous System Divisions

The human nervous system is a complex communication network that allows you to respond to stimuli in your external environment and move about in it. With around 86 billion neurons existing in the brain alone, when we factor in the rest of the nervous system, the complexity of the human nervous grows exponentially. So, what are the nervous system divisions? How can we categorise the elaborate structure of the nervous system? Let's explore nervous system divisions further to find out.

Nervous System Divisions Nervous System Divisions

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Table of contents
    • First, we are going to outline the human nervous system divisions.
    • We will delve into the central nervous system as well as the peripheral nervous system divisions.
    • Following this, we will explore the autonomic nervous system divisions, covering both the sympathetic division of the nervous system and the parasympathetic division of the nervous system.
    • We will also cover the somatic nervous system.
    • To illustrate our points, we will provide a nervous system division diagram.

    Nervous System Divisions, see through fake human body showing skeleton and nerves, StudySmarterFig. 1 - The human nervous system allows you to respond and communicate with stimuli from the environment.


    Nervous System Divisions

    The nervous system is a network in the body that is in charge of communication. All activity in the body is controlled by passing on information via its specialised cells, the neurons.

    Nerves are bundles of neurons grouped together.

    The two main functions of the nervous system are:

    1. To receive sensory input through receptors.
    2. Coordinate all the different elements in the body to produce responses through effectors (cells, glands, etc.).

    The nervous system can be subdivided into the peripheral nervous system and the central nervous system, and further subdivided into more systems.

    Nervous System Divisions Diagram

    The nervous system is comprised of the central nervous system (CNS) and the peripheral nervous system. The CNS consists of the brain and spinal cord, and the peripheral nervous system is made up of the autonomic nervous system and the somatic nervous system.

    The autonomic nervous system is divided into the sympathetic nervous system and the parasympathetic nervous system.

    Nervous System Divisions, Diagram of the human nervous system, StudySmarterFig. 2 - The human nervous system is comprised of multiple systems.

    We can explore each division of the nervous system to find out what it is they specialise in doing.

    The nervous system is quite complex, and the divisions aren’t always clear-cut, so there is some disagreement between researchers on the exact boundaries of the subdivisions of the nervous system.

    Central Nervous System Divisions and Peripheral Nervous System Divisions

    The nervous system can be placed into divisions, and the two main divisions are the central nervous system and the peripheral nervous system.

    • Central nervous system (CNS) - The central nervous system includes the brain and spinal cord. The CNS is the centre of control for the entire organism. It’s responsible for conscious decisions as well as automatic reactions (reflexes) to stimuli.
    • Peripheral nervous system (PNS) -The peripheral nervous system connects the CNS to the body, the nervous system sends impulses from the peripherals to and from the CNS. The peripheral nervous system then is subdivided by function into the autonomic nervous system and the somatic nervous system. The autonomic nervous system can either be aroused or calmed. Depending on the response, it is overseen by the sympathetic nervous system (fight-or-flight response) and the parasympathetic nervous system (rest and digest response).

    In Biopsychology texts, acronyms of the names of the nervous system divisions are often used because the full names are so long. You can remember the different functions for the acronyms of the nervous system division like this: C, as in Control in the Central Nervous system. A, as in automatic in the autonomic nervous system.

    Central Nervous System Divisions

    The central nervous system includes the brain and the spinal cord. This subsystem has physiological measures in place that prevent harmful toxins from entering the central nervous system. A specific, plasma-like fluid circulates in and around the central nervous system called cerebrospinal fluid (CSF).

    It has several molecular structures and membranes functioning as security gates, preventing toxins from entering the brain even if they’re already circulating in the body in substances such as blood.

    This means that although the brain and spinal cord connect to the other nerves, the central nervous system is a closed system in itself.

    The Brain

    If you compare the size of other mammals' brains to human brains, the human brain-to-body ratio is the same as that of a mouse or monkey. Therefore, if a rat or mouse were as tall as a human, their brains would be the same size as the human brain. Brains are very different from organism to organism - some animals don’t have a brain - such as a jellyfish. On the other hand, some animals, such as octopuses, have much larger brain-to-body ratios than humans.

    However, the primary structural difference between humans and other animals is that the brain’s surface area, called the cerebral cortex, is much larger than that of other mammals.

    The human cortex is folded up, which is different from a rat’s smooth brain. The cerebral cortex’s increased surface area makes humans better at integrating information and planning than other animals.

    Conscious and unconscious decisions are made in the brain. The brain stem connects the brain to the spinal cord.

    The Spinal Cord

    The spinal cord is a tubular structure of nerves that extend from the brain into the peripheral nervous system. It reaches from the base of the brain called the hindbrain to the second lumbar vertebra in the lower back, about five centimetres above the pelvis.

    To enable the body to react quickly, specialised neurons, called relay neurons, carry out unconscious reactions to stimuli known as reflexes.

    Pulling your hand away from a hot plate, jumping when startled, and a knee-jerking up when a doctor hits it are all examples of reflexes.

    The spinal cord includes the nerve endings that serve as connectors to the peripheral nervous system.

    Peripheral Nervous System Divisions

    In the peripheral nervous system, information gets passed to the CNS and from the CNS to muscles and organs, known as effectors. Information taken in by the senses (smell, taste, sight) and receptors (touch, heat, pain) is passed to the CNS for integration.

    The peripheral nervous system is subdivided into the somatic nervous system and the autonomic nervous system. These two divisions of the nervous system run parallel to each other (they are not divided by location).

    • Somatic nervous system: This part of the peripheral nervous system communicates with your senses (“soma”). It also is responsible for the voluntary control of your muscles. Any activity that you consciously control, such as moving fingers or speaking, falls under the banner of the somatic nervous system.

    • Autonomic nervous system: This is the part of the peripheral nervous system in charge of the involuntary and unconscious control of processes of the body such as heart rate, blinking, digestion, relaxation and arousal. It works autonomously and is influenced by a specific part of the brain called the hypothalamus. The autonomic nervous system can again be divided into two functional units.

    Autonomic Nervous System Divisions

    The autonomic nervous system, as we discussed briefly before, controls the unconscious decisions your body makes.

    Examples include heart rate and digestion, processes you typically do not have voluntary control over.

    The nervous system, sympathetic and parasympathetic nervous systems, StudySmarterFig. 3 - The parasympathetic and sympathetic nervous systems have different effects on the body.

    The sympathetic and parasympathetic nervous systems are functional divisions of the autonomic nervous system that are automatically activated in response to stimuli.

    Sympathetic Nervous System Division

    The sympathetic nervous system is known more commonly as the fight-or-flight division of the nervous system and prepares the body to move if necessary.

    • The sympathetic nervous system (responsible for “fight, flight or freeze”): the part of the autonomic nervous system that is also called the fight-or-flight response (or in more modern textbooks, fight, flight or freeze response). It mobilises the organism in response to stimuli perceived as dangerous to be able to fight the danger or flee from it.

      • When activated, the sympathetic nervous system causes pupils to dilate, allowing for a better perception of light. It makes the body release stress hormones into the bloodstream, which mobilises carbohydrates in the body for energy. The heart rate increases to get more energy to all parts of the body quickly to carry out fast movements.

        So if you hear a bump in the night and your heart starts racing, and your breathing is rapid, the sympathetic nervous system is the nervous system division responsible.

    Parasympathetic Nervous System Division

    The parasympathetic nervous system calms the system down and is commonly referred to as the rest-and-digest division of the nervous system.

    • The parasympathetic nervous system (responsible for “rest and digest”): the part of the autonomic nervous system that returns the body to its homeostasis (biological balance) by counteracting the sympathetic nervous system.

      • It slows the heart rate and breathing and blocks stress hormones. This is the body's response when the organism knows it’s safe and can now eat and sleep in peace and safety without danger. So when you’ve just had a massage or have just finished working out, this is the nervous system division responsible for that feeling of deep relaxation you feel afterwards.

    Freezing in light of danger is widely acknowledged in the medical community, but it hasn't worked its way into the A-Level syllabus yet.


    Nervous System Divisions - Key takeaways

    • The nervous system is comprised of the central nervous system (CNS) and the peripheral nervous system, and can subdivided by function.
    • The CNS consists of the brain and spinal cord (the control centre), and the peripheral nervous system connects the CNS to the body.
    • Th peripheral nervous system is made up of the autonomic nervous system (unconscious and involuntary actions, i.e., heart rate and digestion) and the somatic nervous system (conscious control of activities and the senses).
    • The autonomic nervous system can be divided into the sympathetic nervous system and the parasympathetic nervous system.
    • The sympathetic nervous system is also known as the fight-or-flight division, and prepares the body for action, whilst the parasympathetic nervous system is known as the rest-and-digest division, and it calms the body down.
    Frequently Asked Questions about Nervous System Divisions

    Which division of the nervous system has short preganglionic neurones?

    The sympathetic division of the autonomic nervous system has short preganglionic neurones.

    What are the two divisions of the autonomic nervous system?

    The autonomic nervous system is divided into the sympathetic and the parasympathetic nervous systems.

    What are the functional divisions of the nervous system?

    The nervous system has three functions: sensing, processing and reacting. When divided by functions, the central nervous system (CNS) acts as the command centre, and the peripheral nervous system connects the CNS to the body to both detect stimuli and enact commands to effectors. Functionally, the peripheral nervous system can be subdivided further into the somatic nervous system (senses and conscious control) and the autonomic nervous system (unconscious actions, the sympathetic nervous system, and the parasympathetic nervous system).

    What are the main divisions of the central nervous system?

    The main divisions of the central nervous system are the brain and the spinal cord.

    What is the division of the peripheral nervous system?

    The peripheral nervous system is a division of the nervous system that includes all parts of the nervous system except the brain and spinal cord.


    It includes:


    • The somatic nervous system (conscious control and senses).
    • The autonomic nervous system (unconscious control, i.e., heart rate).
      • The sympathetic nervous system (fight-or-flight).
      • The parasympathetic nervous system (rest-and-digest).

    Test your knowledge with multiple choice flashcards

    True or False: The nervous system's job is to receive, process and react to sensory input.

    True or False: A specific, plasma-like fluid circulates in and around the central nervous system called cerebrospinal fluid (CSF). 

    True or False: Reflexes bypass conscious control. 

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