Divisions of the Nervous System

The nervous system is the body's specialised network for collecting, processing and responding to information. It is built from neurons (nerve cells) that carry electrical and chemical signals at high speed, letting different parts of the body communicate almost instantly.

It has two main jobs:

• To collect, process and respond to information in the environment.
• To coordinate the activity of different organs and cells in the body.

To do this, the nervous system is organised into distinct divisions, each with its own role. The whole structure branches out from one root into increasingly specialised parts, so the clearest way to learn it is as a hierarchy. The two divisions at the very top are the central nervous system (CNS) and the peripheral nervous system (PNS).

The nervous system is a communication network made of neurons. Its two jobs are to gather and respond to information from the environment and to coordinate the activity of the body's organs and cells.

Getting the structure right is the point of this topic. Exam questions often give you a function and ask which division is responsible, so you need the branches clear in your head before anything else.

The central nervous system (CNS) is made of exactly two structures: the brain and the spinal cord. It is the control centre of the whole nervous system, where information is processed and responses are coordinated.

The brain is the centre of conscious awareness. Its outer layer, the cerebral cortex, is only a few millimetres thick but is highly developed in humans and is what distinguishes our higher mental functions from those of other animals. The brain also regulates vital physiological functions to keep the body alive.

The spinal cord is an extension of the brain that runs down the back inside the vertebral column. It has two roles: it relays messages between the brain and the rest of the body, and it is responsible for reflex actions, which happen without input from the brain.

Structure	Main role
Brain	Centre of conscious awareness; cerebral cortex governs higher functions; regulates vital functions
Spinal cord	Relays messages between brain and body; controls reflex actions

The CNS is only the brain and the spinal cord. The spinal cord is part of the CNS, even though nerves branch off it to reach the rest of the body.

The peripheral nervous system (PNS) is everything else: all the nerves that lie outside the CNS. Its job is to transmit messages, via millions of neurons, to and from the central nervous system. It is the communication line that connects the CNS to the body's receptors, muscles and organs.

The PNS itself divides into two further systems:

• The somatic nervous system (SNS).
• The autonomic nervous system (ANS).

These two subdivisions split the workload by the kind of control they provide. The somatic system handles the parts of the body you can consciously control, such as moving your arm. The autonomic system handles the parts that run automatically, such as your heartbeat and digestion, which you cannot easily control at will.

System	Type of control	Examples
Somatic (SNS)	Voluntary	Moving a limb, catching a ball, speaking
Autonomic (ANS)	Involuntary	Heart rate, breathing, digestion, sweating

The PNS is all the nerves outside the CNS. It splits into the somatic system (voluntary movement) and the autonomic system (involuntary vital functions).

The somatic nervous system (SNS) governs voluntary movement. It does two things: it carries sensory information from the body's receptors (in the skin, eyes, ears and so on) inward to the CNS, and it carries motor commands outward from the CNS to the skeletal muscles. Because it works in both directions and connects to muscles you control on purpose, it is the system behind every deliberate action.

The autonomic nervous system (ANS) governs involuntary, vital functions that keep you alive without any conscious thought: heart rate, breathing, digestion, and the body's response to stress. It largely carries motor information to the internal organs and glands, working in the background whether you are awake or asleep.

The ANS is itself split into two opposing branches, the sympathetic and the parasympathetic, which are covered on the next slide. Keeping the somatic and autonomic systems apart is one of the most common places students lose marks, so anchor it now:

Somatic = voluntary movement of skeletal muscles, carrying both sensory and motor signals. Autonomic = involuntary control of internal organs. If you can choose to do it, it is somatic; if it happens automatically, it is autonomic.

The autonomic nervous system has two branches that work in opposition to each other. One prepares the body for action; the other returns it to calm. Together they keep the body's internal state balanced.

The sympathetic branch is arousing. It prepares the body for action, the response often summarised as "fight or flight". When it activates, it increases heart rate and breathing rate, dilates the pupils, and inhibits digestion, diverting energy to where it is needed for a rapid physical response.

The parasympathetic branch is calming. It conserves energy and returns the body to a resting state, summarised as "rest and digest". It slows the heart rate, constricts the pupils and stimulates digestion, undoing the changes the sympathetic branch produced.

Effect on the body	Sympathetic ("fight or flight")	Parasympathetic ("rest and digest")
Heart rate	Increases	Decreases
Pupils	Dilate	Constrict
Digestion	Inhibited	Stimulated
Overall state	Arousal, energy mobilised	Calm, energy conserved

The two branches are antagonistic: sympathetic arouses, parasympathetic calms. After a stressor passes, the parasympathetic branch brings the body back to its resting baseline.

Because this topic is all about structure, the clearest way to hold it in memory is as a single branching tree. Every division you have met slots into one place, from the whole nervous system at the top down to the two branches of the ANS at the bottom.

Read it top to bottom: the nervous system splits into CNS and PNS; the CNS is brain plus spinal cord; the PNS splits into somatic and autonomic; and the autonomic splits into sympathetic and parasympathetic. If you can reproduce this tree, you can answer almost any structural question on this part of the spec.

1. Putting the spinal cord in the peripheral nervous system

The spinal cord is part of the central nervous system, alongside the brain. Nerves branch off it to reach the body, but the cord itself is CNS. Placing it in the PNS is one of the most frequent errors on this topic.

2. Confusing the somatic and autonomic systems

The somatic system controls voluntary movement of skeletal muscles; the autonomic system controls involuntary functions such as heart rate and digestion. If an action is under conscious control it is somatic; if it runs automatically it is autonomic.

3. Mixing up sympathetic and parasympathetic

The sympathetic branch is arousing (fight or flight, heart rate up); the parasympathetic branch is calming (rest and digest, heart rate down). A quick check: sympathetic gets you ready for stress, parasympathetic settles you afterwards.

4. Thinking the autonomic system controls voluntary movement

The autonomic system runs involuntary internal functions only. Voluntary, deliberate movement of the skeletal muscles is the job of the somatic system. Do not credit the ANS with controlling actions you choose to make.

5. Treating the CNS as more than two structures

The CNS is only the brain and the spinal cord. Neurons and nerves running to and from it are part of the peripheral nervous system, not the CNS itself.