A Three-Pound Organ With Many Parts
The adult human brain weighs roughly three pounds and contains tens of billions of neurons, along with other support cells and a dense network of blood vessels. Its folded outer surface makes it look uniform, but it is organized into regions that tend to specialize. This tour moves from the outside in, and from the top of the head down toward the spinal cord.
A caution applies throughout. Textbook maps assign each region a job, which is a useful simplification. In practice, almost every behavior involves many regions working together, and the same region contributes to many different tasks.
The Cerebral Cortex
The cortex is the wrinkled outer layer of the two cerebral hemispheres. The folds let a large surface area fit inside the skull. It is usually divided into four lobes, each named for the skull bone above it.
- Frontal lobe. The largest lobe, involved in planning, decision making, attention, voluntary movement and aspects of personality and social behavior. The part nearest the forehead, the prefrontal cortex, is closely linked with self-control and weighing options.
- Parietal lobe. Sits toward the top and back. It combines touch and body position information and helps with spatial awareness and directing attention.
- Temporal lobe. Located near the ears. It handles hearing, parts of language comprehension and the formation of certain memories, and it contains structures central to recognizing faces and objects.
- Occipital lobe. At the back of the head, devoted largely to processing vision.
The left and right hemispheres are connected by a thick band of fibers called the corpus callosum. Popular claims that people are either left-brained or right-brained are not supported by the evidence. Some functions, such as language in most right-handed people, lean toward one side, but healthy brains use both hemispheres constantly.
Deeper Structures
Beneath the cortex lie structures that act as relay stations and regulators.
- Thalamus. Often described as a relay hub, it passes most sensory signals on to the appropriate part of the cortex and takes part in attention and alertness.
- Hypothalamus. A small region that helps regulate body temperature, hunger, thirst, sleep cycles and hormone release through its connection to the pituitary gland.
- Basal ganglia. A group of structures involved in starting and smoothing movement, and in forming habits and learning from reward. Disorders affecting this area, such as Parkinson’s disease, show how closely movement and these circuits are tied.
- Hippocampus. A seahorse-shaped structure that is important for forming new memories of facts and events. Damage to it can make new memories hard to form, which is one reason it features so prominently in research on memory formation.
- Amygdala. An almond-shaped cluster involved in emotional processing, particularly in responding to possible threats. It is just one part of a wider network, and calling it the fear center is an oversimplification. The wider network is discussed under emotion circuits.
Together the hippocampus, amygdala and some neighboring areas are often grouped as the limbic system, though neuroscientists debate how well that grouping reflects the brain’s real organization.
The Cerebellum and Brainstem
Tucked beneath the back of the cortex is the cerebellum, whose name means little brain. It contains a surprisingly large share of the brain’s neurons. It is best known for coordinating movement, balance and timing, and evidence suggests it also contributes to some thinking and language tasks.
The brainstem connects the brain to the spinal cord. It includes the midbrain, pons and medulla. These regions control basic functions such as breathing, heart rate and wakefulness, and nearly all signals traveling between the brain and body pass through it. Damage here can be life threatening, which reflects how much depends on this small stretch of tissue.
Cells, Wiring and Chemistry
Regions are built from neurons that communicate through electrical impulses and chemical messengers called neurotransmitters. Supporting cells known as glia help nourish neurons, insulate their fibers and clear away waste. White matter, mostly insulated fibers, links regions, while gray matter is where cell bodies cluster. A fuller account appears in the discussion of neurons and glia.
Communication is the key idea. The brain is better understood as a set of networks than a set of boxes. Researchers describe large-scale networks, for example one active when the mind wanders and others engaged during focused tasks. Which networks are in play depends on what you are doing.
How the Parts Work Together
Consider a simple moment, such as catching a ball. The occipital lobe processes the visual image, the parietal lobe tracks where the ball is relative to your body, the frontal lobe plans the movement, the cerebellum fine-tunes timing and coordination, the basal ganglia help start the action, and the brainstem keeps your breathing and balance going in the background. No single region deserves the credit.
The same is true of language, memory and emotion. Each draws on several areas and many connections. This is why damage to one place often has surprising effects elsewhere, and why the same symptom can have different causes in different people.
What the Maps Leave Out
Several caveats are worth keeping in mind:
- Individual brains vary in size, shape and the exact location of functional areas.
- The brain is adaptable. Regions can take on new roles after injury or with experience, especially in the young, a quality often described as brain plasticity.
- Brain imaging shows activity patterns, not thoughts. Interpreting colorful scans requires care, and many early claims about single regions have been revised.
- Neuroscience is still unsettled on many questions, from how memories are stored to how conscious experience arises.
A tour of regions is a starting point. The real insight is that the brain’s abilities come from how its parts interact, which is why researchers increasingly study connections as well as locations.