Brain

The brain is the central organ of the nervous system (the network of sensory pathways in the body) in most animals. It consists of units called neurons (nerve cells that transmit electrical and chemical signals) and supporting units called glia. The brain receives information from sensory receptors, processes it, and then sends commands to muscles and glands.

In humans, the brain is located inside the skull and is protected by three membranes called meninges and fluid known as cerebrospinal fluid. Its weight is approximately 1.3–1.4 kilograms in an adult, and it uses a large portion of the energy the body consumes, though this is small relative to overall body weight.

Major divisions

The human brain is divided into three major parts. The cerebrum is the largest part, and it splits into two hemispheres (left and right) connected by the corpus callosum. The cerebellum (smaller brain) is located at the lower back and is involved in coordinating movement and balance. The brainstem connects the brain to the spinal cord and controls vital functions such as breathing and heart rate.

Cortex and functional organization

The upper surface of the cerebrum is called the cortex, and it has ridges and grooves that increase its area. The cortex is divided into four lobes: frontal, parietal, temporal, and occipital. The occipital lobe is mainly associated with vision, the temporal lobe with hearing and memory, while the frontal lobe is involved in planning, decision-making, and behavior control. This division is approximate; many functions depend on networks distributed across different regions that work together.

How it works

A neuron transmits information via an electrical signal that travels along the axon (the long projection of the cell). Where two neurons meet, called a synapse, the signal converts into a chemical one called a neurotransmitter, which either activates or inhibits the next neuron. The strength of synaptic connections can change when new experiences occur — a process called plasticity, which underlies learning and memory.

Variation across animals and research

All vertebrate animals have brains, but their size and structure vary considerably. Many animals without backbones, such as insects, have clusters of neurons resembling a brain but simpler in form. Brain research uses imaging tools such as MRI and EEG, which measure changes in blood flow or electrical activity in the brain, as well as studies at the cellular level.