Reference guide
What is the central nervous system?
The central nervous system, or CNS, is made up of the brain, spinal cord, and retina. It receives signals, processes information, and sends instructions that help the body sense, move, learn, and maintain basic functions.
The CNS works closely with the peripheral nervous system. Peripheral nerves bring information toward the CNS and carry its instructions back to muscles, organs, and glands. This communication supports both conscious actions and automatic processes.
Neurons communicate across a vast network of fibres and junctions. Supporting glial cells help maintain the environment in which these signals travel. The system continues to adapt throughout life as connections are strengthened, changed, or reduced through development and experience.
The brain
The brain interprets incoming information and helps organise thought, memory, emotion, movement, language, and perception. Its outer surface has many folds, increasing the available area for networks of nerve cells.
Brain size alone is not a dependable measure of intelligence. Abilities depend on many interacting factors, including the organisation of neural circuits, development, learning, and the demands placed on different systems.
Structures and functions of the brain
The brain contains grey matter, where many neuron cell bodies and connections are located, and white matter, which contains many insulated axons carrying signals between regions.
The cerebrum is divided into two hemispheres connected by the corpus callosum. Its cerebral cortex is organised into areas commonly described as lobes:
- Frontal lobe: contributes to planning, decision-making, voluntary movement, language, and aspects of memory.
- Parietal lobe: combines touch and body information and supports spatial awareness.
- Temporal lobe: helps process sound and language and includes structures involved in memory and emotion.
- Occipital lobe: processes visual information.
These descriptions are useful starting points, but brain functions are distributed across connected networks. The popular idea that a person is simply “left-brained” or “right-brained” is not a good summary of how the two hemispheres work.
The brainstem links the brain with the spinal cord. It includes the midbrain, pons, and medulla and helps relay signals while supporting essential functions such as breathing, heart regulation, alertness, and consciousness. The thalamus helps route information, while the hypothalamus contributes to sleep, temperature, appetite, and hormonal regulation. The cerebellum supports coordination, balance, and the refinement of movement.
Protection and tissue types
The brain and spinal cord need strong physical protection. The skull surrounds the brain, the vertebral column surrounds the spinal cord, and three membrane layers called the meninges enclose both.
Cerebrospinal fluid circulates around the brain and spinal cord. It cushions the nervous tissue, helps maintain a stable environment, and assists with the movement of nutrients and waste products.
Grey matter contains many neuron cell bodies, dendrites, supporting cells, and small blood vessels. White matter contains many axons, often wrapped in myelin, which helps signals travel efficiently. Their arrangement differs between the brain and spinal cord.
The spinal cord
The spinal cord is a long bundle of nervous tissue extending from the brainstem through the vertebral column. It carries information in both directions and can organise rapid reflexes before signals reach the brain.
Spinal nerves emerge in pairs from different regions. These regions are named cervical, thoracic, lumbar, sacral, and coccygeal. In cross-section, grey matter forms a central shape surrounded by white matter.
Reflex pathways demonstrate why the spinal cord is more than a simple cable. Sensory information can trigger a quick protective response through local circuits, while the brain receives information and helps interpret what happened.
The retina
The retina is the light-sensitive neural tissue at the back of the eye. It develops as an extension of the forebrain and contains several layers of specialised cells.
Rods and cones detect light. Signals then pass through retinal circuits, including bipolar, ganglion, horizontal, and amacrine cells. Ganglion-cell axons form the optic nerve, carrying visual information toward the brain.
The retina therefore illustrates how closely sensory organs and the CNS are connected: light is detected in the eye, transformed into neural signals, and interpreted through networks in the brain.