The brain

The adult human brain weighs about 1.4 kilograms and contains roughly 86 billion neurons, along with a similar number of supporting glial cells. Although it makes up only about 2 percent of body weight, it uses around 20 percent of the body's energy at rest.

The brain, spinal cord and nerves together form the nervous system. The brain receives information from the senses, controls movement, regulates vital functions like breathing and heart rate, and produces thought, memory, language and emotion.

A neuron has branching dendrites that receive signals, a cell body and a long axon that sends signals onward. Signals travel along the axon as electrical impulses. Many axons are wrapped in a fatty myelin sheath, which greatly speeds up transmission.

Neurons do not quite touch. At junctions called synapses, an electrical signal triggers the release of chemical messengers called neurotransmitters, such as dopamine, serotonin and glutamate, which cross the tiny gap and influence the next neuron. A single neuron may form thousands of synapses, giving the brain an estimated trillions of connections.

• Cerebrum: the largest part, divided into left and right hemispheres, responsible for thought, perception and voluntary movement.
• Cerebellum: at the back, coordinating balance, posture and fine movement.
• Brainstem: connecting to the spinal cord and controlling breathing, heart rate and sleep cycles.
• Thalamus: a relay station routing sensory information to the cortex.
• Hypothalamus: regulating hunger, thirst, temperature and hormones.
• Hippocampus: essential for forming new memories.
• Amygdala: involved in processing emotions, especially fear.

The wrinkled outer layer of the cerebrum, the cortex, is folded to fit a large surface area inside the skull. Each hemisphere has four lobes. The frontal lobe is involved in planning, decision-making and movement; the parietal lobe processes touch and spatial awareness; the temporal lobe handles hearing, language and memory; and the occipital lobe processes vision.

Functions are not rigidly confined to single spots, and complex abilities rely on networks across many regions. Much of what we know came from studying people with brain injuries. In 1861 Paul Broca linked damage to a region of the left frontal lobe with loss of the ability to produce speech.

The brain is not fixed. Neuroplasticity is its ability to change connections in response to experience. Learning a skill strengthens the synapses involved, and people recovering from strokes can sometimes regain abilities as other areas take over lost functions.

The brain develops rapidly in childhood, and the prefrontal cortex, important for self-control and long-term planning, continues maturing into a person's twenties. Some cognitive abilities, such as processing speed, tend to decline gradually with age, while knowledge and vocabulary often keep growing.

Research links several habits to better brain health over time: regular physical activity, sufficient sleep, which helps consolidate memories and clear waste products, a healthy diet, staying socially connected and continuing to learn. Controlling blood pressure, avoiding smoking and limiting alcohol reduce the risk of stroke and dementia.

Protecting the head matters too. Helmets and seat belts reduce the risk of traumatic brain injury. Anyone experiencing sudden symptoms of a stroke, such as face drooping, arm weakness or slurred speech, should seek emergency help immediately, because rapid treatment can limit brain damage.