Brain

The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is made of nervous tissue and is usually found in the head, close to organs that help with vision, hearing, and olfaction. As the most specialized organ, the brain receives information from the body's senses, processes that information through thought, cognition, and intelligence, and coordinates motor control like moving muscle and regulating the endocrine system.

In humans, the brain contains billions of tiny cells called neurons that connect to one another through structures known as synapses. These connections allow the brain to send quick signals through axons, helping the body react fast to changes in the environment. The brain also controls other parts of the body by telling muscle cells to move and by managing the release of hormones, which are chemicals that affect many body functions.

Brains provide centralized control, allowing animals to respond quickly and work together as a whole. While simple reactions can happen without the brain, more complicated actions need the brain's ability to bring together many pieces of information. Scientists study the brain to understand how these cells work together, comparing brains across different animal species and focusing especially on vertebrates.

Structure

Cross section of the olfactory bulb of a rat, stained in two different ways at the same time: one stain shows neuronal cell bodies, the other shows receptors for the neurotransmitter GABA.

The brain comes in many shapes and sizes across different animals, but it always has some basic features in common. Scientists study the brain using many tools. They can look at it with their eyes, but they also use special methods like cutting thin slices and using stains to see tiny details.

All brains are made up of special cells called neurons and glial cells. Neurons are the star players — they send electrical signals through long arms called axons to talk to other cells. These signals help the brain think, learn, and control the body. The brain also has trillions of connections called synapses, which help it remember and adapt over time.

Evolution

The brain has evolved in animals to help them process information and control their movements. In very simple animals like sponges, there is no brain at all, but in animals with bodies that have left and right sides (called bilaterians), a small structure called a brain begins to form. This brain gets larger and more complex in animals with backbones (vertebrates) like fish, amphibians, reptiles, birds, and mammals.

Some animals without backbones, like insects and octopuses, also have complex brains. Scientists study these animals to learn more about how brains work. In vertebrates, the brain develops from three main parts early in an animal's growth. Over time, especially in mammals, the front part of the brain (forebrain) becomes much larger and more complex, helping with advanced thinking and problem-solving.

Encephalization Quotient
Species	EQ
Human	7.4–7.8
Common chimpanzee	2.2–2.5
Rhesus monkey	2.1
Bottlenose dolphin	4.14
Elephant	1.13–2.36
Dog	1.2
Horse	0.9
Rat	0.4

Development

Main article: Neural development

Brain of a human embryo in the sixth week of development

The brain grows in a carefully planned way. It starts as a simple bump at the front of a tiny cord and becomes more complex as it develops. Special cells called neurons are made in certain areas and then move to their correct places. Once in place, they stretch out and connect to each other, forming pathways that help the brain work.

In animals with a backbone, the early steps of brain development are similar. The brain begins as a small plate that folds into a tube, then splits into different sections that will become the forebrain, midbrain, and hindbrain. These sections further divide to form the parts of the brain we know today. The connections between neurons are not fully set from the start; they are refined over time, with some extra connections being removed to make the brain more efficient. In humans and many other animals, most brain cells are formed before birth, but a few areas continue to produce new cells throughout life. Both genes and experiences shape the brain, with early experiences playing a big role in how the brain develops.

Physiology

Brain electrical activity recorded from a human patient during an epileptic seizure

The brain works by using special cells called neurons that send and receive signals. These signals are created by chemicals called neurotransmitters that connect at points known as synapses. Each neuron usually uses the same neurotransmitter, which helps it talk to other neurons.

The brain also creates tiny electric fields when it’s active. We can sometimes see these fields from outside the head using tools like EEG. The brain needs a lot of energy to work, and it gets this mostly from sugar in our blood. Special cells around neurons help keep everything running smoothly.

Function

Model of a neural circuit in the cerebellum, as proposed by James S. Albus

Information from our senses is collected by the brain. The brain processes this information to help us understand our surroundings and decide what actions to take. It combines what it senses with our needs and past experiences to guide our movements and responses.

The brain controls all our actions, allowing muscles to work together in complex ways. It helps different parts of the body work together instead of against each other. The brain also manages our sleep patterns, keeping our body’s balance and helping us remember and learn new things.

Major areas involved in controlling movement
Area	Location	Function
Ventral horn	Spinal cord	Contains motor neurons that directly activate muscles
Oculomotor nuclei	Midbrain	Contains motor neurons that directly activate the eye muscles
Cerebellum	Hindbrain	Calibrates precision and timing of movements
Basal ganglia	Forebrain	Action selection on the basis of motivation
Motor cortex	Frontal lobe	Direct cortical activation of spinal motor circuits
Premotor cortex	Frontal lobe	Groups elementary movements into coordinated patterns
Supplementary motor area	Frontal lobe	Sequences movements into temporal patterns
Prefrontal cortex	Frontal lobe	Planning and other executive functions

Research

Main article: Neuroscience

"Brain research" redirects here. For the scientific journal, see Brain Research.

The field of neuroscience studies the brain and the nervous system. Psychology looks at how the mind and behavior work, while neurology is a medical field that treats nervous system diseases. The brain is also a key focus in psychiatry, which deals with mental disorders. Cognitive science combines neuroscience with other areas like computer science and philosophy to understand the brain better.

Scientists study the brain in many ways. One way is by looking at its structure using special stains and microscopes. They also use tools like EEG to measure brain activity and fMRI to see which parts of the brain are active during different tasks. By studying damage to the brain, such as from strokes, researchers can learn about how different parts of the brain work. Computers help scientists simulate how groups of brain cells might work together. Recent advances include studying genes and using advanced imaging to see how brain cells communicate and change over time.

Society and culture

As food

Animal brains are eaten in many different foods around the world.

In rituals

Some old clues from archaeological studies show that European Neanderthals may have eaten brains as part of their traditions.

The Fore people of Papua New Guinea have a tradition where they eat human brains during special ceremonies for people who have passed away. This was done to feel connected to the person who died. This practice led to a disease called kuru, which was later understood to be caused by a special kind of germ called a prion.