aqa  The divisions of the nervous system: central and peripheral (somatic and autonomic)

edexcel The central nervous system (CNS)

The Divisions of the Nervous System


The nervous system is a complex system of nerves, which carry messages around the body via electrical systems. It is comprised of two main divisions: the central nervous system and the peripheral nervous system. The central nervous system is made up of the brain, spinal cord and nerves. The peripheral nervous system consists of sensory neurons, ganglia (clusters of neurons) and nerves that connect to one another and to the central nervous system.

The two systems work together to collect information from inside the body and from the environment outside it. The systems process the collected information and then dispatch instructions to the rest of the body, facilitating an appropriate response.

Functionally, the nervous system has two main subdivisions: the somatic, or voluntary, component; and the autonomic, or involuntary, component. The autonomic nervous system regulates certain body processes, such as blood pressure and the rate of breathing, that work without conscious effort. The somatic system consists of nerves that connect the brain and spinal cord with muscles and sensory receptors in the skin.

Here’s how the nervous system works:


Receptors detect a stimulus and send impulses along sensory neurons to the CNS. The CNS coordinates the information and sends impulses along motor neurons to the effectors, which bring about a response. The sequence is as follows:

  1. Stimulus
  2. Receptor
  3. Sensory neuron
  4. Central nervous system
  5. Motor neuron
  6. Effector
  7. Response

The Central Nervous SystemCNS

The central nervous system (CNS) consists of two main parts: the brain and the spinal cord. Nerves in our body send information via the spinal cord to the brain, which the brain then processes, sending a message to the body through the spinal cord. One example of this is our eyes sending a message about a car coming towards us; the brain processes how far away it is and sends a message back telling us to cross the road. Neurons in the brain pass messages along through electrical impulses. Neurotransmitters are then released and cross the synaptic gap to be picked up by receptor sites.

The Peripheral Nervous System

The peripheral nervous system (PNS) is the division of the nervous system containing all the nerves that lie outside of the central nervous system (CNS). The primary role of the PNS is to connect the CNS to the organs, limbs and skin. These nerves extend from the central nervous system to the outermost areas of the body.

The nerves that make up the peripheral nervous system are actually the axons or bundles of axons from neuron cells. In some cases, these nerves are very small but some nerve bundles are so large that they can be easily seen by the human eye.

The peripheral nervous system is divided into two parts:

  • The somatic nervous system
  • The autonomic nervous system

The Somatic Nervous System

The somatic system is the part of the peripheral nervous system responsible for carrying sensory and motor information to and from the central nervous system. The somatic nervous system derives its name from the Greek word soma, which means “body.”

The somatic system is responsible for transmitting sensory information as well as for voluntary movement. This system contains two major types of neurons:

  1. Sensory neurons (or afferent neurons) that carry information from the nerves to the central nervous system
  2. Motor neurons (or efferent neurons) that carry information from the brain and spinal cord to muscle fibres throughout the body.

The Autonomic Nervous System

The autonomic system is the part of the peripheral nervous system responsible for regulating involuntary body functions, such as blood flow, heartbeat, digestion and breathing. This system is further divided into two branches:

  1. The sympathetic system regulates the flight-or-fight responses. This system prepares the body to expend energy and deal with potential threats in the environment. When action is needed, the sympathetic system will trigger a response by speeding up the heart rate, increasing breathing rate, increasing blood flow to muscles, activating sweat secretion and dilating the pupils.
  2. The parasympathetic system helps maintain normal body functions and conserve physical resources. Once a threat has passed, this system will slow the heart rate, slow breathing, reduce blood flow to muscles and constrict the pupils.


Go Further

  • The BBC with Prof Susan Greenfield produced a documentary called ‘Brain Story’ which is an excellent introduction to the brain and it’s functions. See it all here.
  • V. Ramachandran (Ted Talk below) has also written a fantastic book called ‘The Tell Tale Brain’.
  • For a more detailed look at the nervous system, watch the Crash Course video below: