AQA Syllabus focus:
'The divisions of the nervous system: central and peripheral nervous systems.'
The nervous system is the body’s communication network. For AQA, you need to understand its two main divisions, what each includes, and how they coordinate activity across the body.
The nervous system as a communication network
The nervous system allows the body to detect changes, process information, and produce responses. It is essential for sensation, movement, attention, and coordinated action.
Nervous system: The body’s rapid communication system, made up of the central nervous system and the peripheral nervous system.
One major division is the central nervous system (CNS). It acts as the main control center, receiving information, integrating it, and helping the body decide what to do next.
Central nervous system (CNS): The division of the nervous system consisting of the brain and spinal cord.
The CNS depends on communication links that connect it to the rest of the body.
This diagram contrasts the central nervous system (brain and spinal cord) with the peripheral nervous system (nerves and ganglia outside the CNS). It visually reinforces that the PNS forms the communication links that carry information between the CNS and the rest of the body. Source
These links make up the peripheral nervous system (PNS).
Peripheral nervous system (PNS): The division of the nervous system made up of all the nerves outside the brain and spinal cord.
Although the CNS and PNS are separate divisions, they are constantly working together.
The central nervous system
The brain is the most complex part of the CNS. It receives incoming information from the body and the environment, interprets it, and organizes responses. In psychology, this matters because thought, emotion, memory, and voluntary action all depend on activity in the brain.
The spinal cord is the second part of the CNS. It is a long bundle of nerve tissue extending from the brain down the back. Its main role is to carry messages between the brain and the rest of the body. Information traveling upward can be passed to the brain for processing, while information traveling downward carries instructions from the brain to body systems.
Because the CNS is the body’s command center, damage to it can have widespread effects. If the brain or spinal cord is injured, communication may be disrupted, leading to difficulties in movement, sensation, or coordinated responses. This helps show why the CNS is central to both biological functioning and behavior.
A useful way to think about the CNS is as the place where information is:
received
organized
interpreted
used to guide responses
The CNS does not work in isolation. It relies on signals arriving from outside itself and on pathways that allow its instructions to reach muscles and organs. Those pathways belong to the PNS.
The peripheral nervous system
The peripheral nervous system includes all nerves outside the brain and spinal cord. Its main function is to act as a communication network linking the CNS to the body.
Information from different parts of the body travels through the PNS to the CNS. This means the CNS can be informed about changes both inside the body and in the outside environment. After the CNS has processed this information, messages can be sent back through the PNS to produce a response.
The PNS is therefore essential in two directions:
carrying incoming information to the CNS
carrying outgoing instructions from the CNS
Without the PNS, the brain and spinal cord would not be able to communicate effectively with the body.
This diagram labels the CNS (brain and spinal cord) alongside key PNS structures (cranial nerves and spinal nerves). It helps you visualize how peripheral nerves provide the physical routes for incoming sensory information and outgoing motor instructions. Source
The CNS may be the control center, but the PNS is the system of connections that makes control possible.
The PNS allows communication with:
sensory receptors, which detect stimulation
muscles, which produce movement
glands and organs, which carry out bodily responses
This makes the PNS crucial for everyday functioning. Basic activities such as detecting temperature, moving a hand, or adjusting bodily states all depend on signals passing between the body and the CNS. In exam answers, it is important to emphasize that the PNS is not the decision-making center. Its role is mainly transmission and communication.
How the CNS and PNS work together
The clearest way to understand these divisions is to see them as parts of one larger system.
A typical flow of information is:
This diagram summarizes the nervous system’s organization by separating the CNS from the PNS and showing how the PNS branches into functional divisions. It supports exam explanations by making the structural hierarchy behind information flow easier to recall. Source
a change is detected somewhere in the body or environment
information travels through the PNS toward the CNS
the CNS processes and interprets the information
instructions are sent back through the PNS
the body carries out a response
This interaction explains why psychologists study the nervous system when explaining behavior. Even simple actions depend on rapid coordination between the CNS and PNS. More complex behavior, such as making decisions or responding to emotional situations, still relies on the same basic pattern of communication.
When revising, keep the distinction clear:
CNS = brain and spinal cord
PNS = all nerves outside the brain and spinal cord
CNS role = control, processing, and coordination
PNS role = carrying messages to and from the CNS
A common mistake is to treat the whole nervous system as if it were only the brain. For AQA, you need to show that the nervous system has divisions, and that each division has a distinct role. The CNS is the central control system, while the PNS links that control system to the rest of the body.
Practice Questions
Identify two structures that make up the central nervous system. (2 marks)
1 mark for brain
1 mark for spinal cord
Outline the roles of the central nervous system and the peripheral nervous system. Refer to how they work together. (6 marks)
Award 1 mark for each of the following, up to 6 marks:
The CNS is made up of the brain and spinal cord
The PNS is made up of nerves outside the brain and spinal cord
The CNS acts as the main control or processing center
The PNS carries information from the body to the CNS
The PNS carries instructions from the CNS to the body
The PNS links the CNS to the rest of the body
The two systems work together to coordinate responses
FAQ
The spinal cord is classed as part of the CNS because it is directly connected to the brain and forms part of the body’s main processing and relay system.
It does more than just carry messages. It also helps organize and route information, which is a central feature of the CNS rather than the PNS.
A nerve is a bundle of fibers in the PNS.
A tract is a bundle of fibers in the CNS.
This distinction helps you tell the two divisions apart:
Nerves connect the CNS to the body
Tracts carry information within the brain and spinal cord
Several structures protect the CNS:
The skull protects the brain
The vertebrae protect the spinal cord
The meninges are protective membranes around the CNS
Cerebrospinal fluid cushions the brain and spinal cord
These protections reduce injury risk, although serious damage can still occur.
Cells in the PNS usually have a better environment for repair, so some damaged fibers can regrow more easily.
In the CNS, regeneration is much more limited. Damage to brain or spinal cord tissue is therefore often more permanent and can disrupt many functions at once.
These are two major groups of nerves in the PNS.
Cranial nerves connect mainly with the brain
Spinal nerves connect with the spinal cord
Even though cranial nerves are attached to the brain, they are usually classed as part of the PNS because they extend outside the CNS and help link it to the rest of the body.
