How the Human Brain Works: Its Major Structures Explained

About 86 billion neurons sit inside a structure weighing barely more than a bag of sugar. It is folded, wrapped in fluid, and bolted to the top of the spinal cord. That structure runs everything you would call yourself. It handles what you think, what you feel and what you remember. It also runs the automatic jobs that keep you breathing.

This guide walks through the brain’s major regions. It covers the deep structures that relay and record information. It describes the physical setup that protects the tissue. And it ends with the surprising fact that the brain keeps rebuilding itself as you read.

The Brain at a Glance

An adult human brain weighs about 1.3 to 1.5 kilograms. That is roughly 2 percent of body weight, though the exact share varies from person to person.

Inside sit about 86 billion neurons. Alongside them are roughly 85 billion non-neuronal cells. Some of those are glial cells, and some are other types. The neurons are not spread out evenly. About 16 billion sit in the cerebral cortex. About 69 billion sit in the cerebellum. So a structure that is only about a tenth of the brain by mass holds four-fifths of its neurons.

The folded surface is the cerebral cortex. Folding packs a very large sheet of tissue into a small space. Image: Frank Gaillard, CC BY-SA 4.0, via Wikimedia Commons

The Three Major Regions

Anatomically the brain divides into three parts. They are the cerebrum, the cerebellum and the brainstem. Each has a very different shape, a very different cell count, and a very different job.

The Cerebrum: Where Thinking Happens

The cerebrum is the largest part. It is made of two hemispheres. Each one has an inner core of white matter wrapped in an outer cerebral cortex of gray matter. The cortex is thin, between about 1 and 4.5 millimeters, averaging around 2.5 millimeters. Most of it is the neocortex, which has six layers of neurons. The older part, the allocortex, has three or four. Between 14 and 16 billion neurons live in the cortex. It is folded into ridges called gyri and grooves called sulci. That folding is what lets so much sheet fit inside the skull.

Each hemisphere is divided into four lobes. The frontal lobe handles executive function: self-control, planning, reasoning and abstract thought. It also contains the region that produces speech. The parietal lobe processes touch, temperature, pain and spatial awareness. The temporal lobe manages hearing, language comprehension and memory. The occipital lobe is dedicated to vision. The anatomist Korbinian Brodmann went further. He mapped the cortex into dozens of distinct areas, around 48 in his final human map. Modern neuroscience still refers to those areas.

The two hemispheres stay in contact through the corpus callosum. It is the largest white matter structure in the human brain. It spans about 10 centimeters at the midline and carries about 300 million axons between the hemispheres.

The Cerebellum: The Little Brain

The cerebellum means "little brain" in Latin. It sits underneath the cerebral hemispheres, at the back of the skull. It looks nothing like the cerebrum. The cerebral cortex has broad, irregular folds. The cerebellar cortex, by contrast, is covered in finely spaced parallel grooves.

A distinctive feature of the cerebellum is the tightly packed Purkinje cell layer. Image: Herizora, CC BY-SA 3.0, via Wikimedia Commons

Here is the detail worth pausing on. The cerebellum contains about 69 billion neurons. That is roughly 80 percent of all the neurons in the brain, more than the rest of the brain put together. Yet it is only about a tenth of the brain by mass. Density like that usually signals something important.

What it does is narrower than most people assume. The cerebellum does not start movement. Instead it receives input from the spinal cord and other brain regions, and it refines that input. It contributes to coordination, precision and accurate timing. Damage disrupts fine movement, posture, balance and motor learning. It does not usually cause paralysis. The cerebellum also contributes to attention, language and emotional regulation. Its motor role is the best established of those.

The Brainstem: Keeping You Alive

The brainstem connects the brain to the spinal cord. It is small, and it is the part you cannot afford to lose. It helps regulate heart rate, breathing rate and blood pressure. It also supplies the main motor and sensory nerves to the face and neck, through the cranial nerves.

Midbrain, pons and medulla: a small structure running the body’s vital settings. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons

It has three parts. The midbrain takes part in vision and hearing reflexes, eye movement and body movement. The pons connects to the cerebellum and helps control breathing. The medulla oblongata helps regulate heart function and blood flow. It also runs reflexes such as swallowing and coughing. A small organ doing essential work, with almost no margin for error.

The Inner Structures: Relays, Memory and Emotion

Underneath the cortex sit several structures. The popular "left brain / right brain" picture tends to leave them out. They are where much of the actual routing happens.

The Thalamus: The Relay Station

The thalamus means "chamber" in Greek. It is a large mass of gray matter between the cerebral cortex and the midbrain. Nerve fibers radiate out of it to the cortex in all directions. It relays sensory and motor signals. It also helps regulate consciousness, sleep and alertness. In effect it is a switchboard that decides where a signal goes.

The thalamus relays sensory and motor signals to the cortex and helps set alertness. Image: Madhero88, CC BY-SA 3.0, via Wikimedia Commons

The Hippocampus and Amygdala: Memory and Emotion

The hippocampus helps form new memories. It also helps stabilize and reorganize them, working with other brain regions. And it supports the spatial memory that lets you navigate. It is one of the first regions affected in Alzheimer’s disease. That is why short-term memory loss and disorientation are among the earliest symptoms. Damage to both hippocampi can leave a person unable to form new memories at all.

The amygdala sits within the temporal lobes. It handles the processing of memory, decision-making and emotional responses, including fear, anxiety and aggression. The name comes from the almond shape, the Latin for which is amygdala. Karl Friedrich Burdach used the term in the early nineteenth century, commonly dated to 1822. Both structures belong to the limbic system. That is the older ring of brain tissue handling emotion, memory and motivation.

The limbic system ties emotion, memory and motivation together. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons

The Basal Ganglia: Starting and Sticking

The basal ganglia are groups of nuclei at the base of the forebrain. They have strong connections to the cortex, the thalamus and the brainstem. They help regulate voluntary movement. They are also tied to procedural learning, habit formation, eye movements, cognition and emotion. The main parts are the striatum, the globus pallidus, the substantia nigra and the subthalamic nucleus. Habit lives here more than anywhere else. That is why basal ganglia disorders look less like clumsiness and more like a loss of automatic movement.

The basal ganglia shape which movements get started, and which habits persist. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons

Structure	Where it sits	Primary role
Cerebral cortex	Outer folded layer, 1–4.5 mm thick	Thinking, senses, voluntary action
Corpus callosum	Between the two hemispheres	Lets the hemispheres talk to each other
Cerebellum	Under the hemispheres, back of skull	Fine-tunes movement, timing and balance
Brainstem	Between brain and spinal cord	Breathing, heart rate, blood pressure
Thalamus	Between cortex and midbrain	Relays signals; regulates sleep and alertness
Hippocampus	Medial temporal lobe	Forms and stabilizes new memories; navigation
Amygdala	Medial temporal lobe	Fear, emotion, decision-making
Basal ganglia	Base of the forebrain	Movement initiation, habits, procedural learning

How the Brain Is Protected

Soft tissue inside a rigid box is a dangerous arrangement. So the brain comes with its own protection. Three membranes, the meninges, wrap it. The outer one is the dura mater. Then comes the arachnoid mater. The inner one, the pia mater, follows every fold. Together they act as a physical and immunological barrier. They anchor the tissue and carry the blood vessels that supply it.

Dura, arachnoid and pia wrap the brain between the skull and the tissue. Image: Scientific Animations, CC BY-SA 4.0, via Wikimedia Commons

Between the arachnoid and the pia lies the subarachnoid space. It is filled with cerebrospinal fluid. That fluid acts as a shock absorber and a buffer. A person carries about 125 to 150 milliliters of it at any moment, while producing about 500 milliliters a day. The fluid also helps the brain manage its own blood flow.

Fluid also fills four interconnected cavities called the ventricles. Inside each is a region of tissue called the choroid plexus. That is where most of the cerebrospinal fluid is made. The barrier between blood and this fluid is formed by tight junctions between the choroid plexus epithelial cells. The fourth ventricle runs straight into the central canal of the spinal cord. So the whole nervous system is bathed by one circulating fluid.

The ventricular system: four cavities that produce and circulate cerebrospinal fluid. Image: Matt Skalski, CC BY-SA 4.0, via Wikimedia Commons

A Brain That Never Finishes Building

The strongest fact about the brain is that it is not fixed. Neuroplasticity is the brain’s ability to reorganize and rewire its connections. It happens in response to learning, environmental change, injury and practice. It was once assumed to be a childhood-only process. Research from the late twentieth century onward showed that many aspects of it continue into adulthood.

The counterweight is synaptic pruning. This is the removal of synapses that are not being used. It begins near birth and continues at least through adolescence. During pruning, an axon withdraws or dies back and the dendrite decays. This helps explain a surprising number. A newborn brain holds roughly 100 billion neurons, about 15 percent more than an adult brain has. So the brain does not grow by adding cells. It refines itself by deciding which connections to keep.

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means here
Neuron	A cell that fires electrical signals to carry information.
Cerebrum	The largest part of the brain, made of two hemispheres.
Cerebral cortex	The thin folded outer layer of gray matter.
Neocortex	The six-layered, evolutionarily newer part of the cortex.
Gyrus	A ridge on the folded surface of the cortex.
Sulcus	A groove between two ridges of the cortex.
Corpus callosum	The thick band of axons joining the two hemispheres.
Cerebellum	The "little brain" that fine-tunes movement, posture and balance.
Brainstem	The stalk connecting the brain to the spinal cord.
Thalamus	The relay station that routes signals to the cortex.
Hippocampus	The structure that helps form and stabilize new memories.
Amygdala	The almond-shaped structure central to fear and emotion.
Basal ganglia	Nuclei involved in starting movement and forming habits.
Limbic system	The ring of tissue handling emotion, memory and motivation.
Meninges	The three membranes that wrap the brain.
Cerebrospinal fluid	The clear fluid that cushions and bathes the brain.
Ventricle	One of four fluid-filled cavities in the brain.
Choroid plexus	The tissue that makes most of the cerebrospinal fluid.
Neuroplasticity	The brain’s ability to rewire its own connections.
Synaptic pruning	The removal of unused synapses during development.

The words in this article that would otherwise send you to a dictionary.

Key Takeaways

• The adult brain weighs about 1.3 to 1.5 kilograms and holds roughly 86 billion neurons plus about 85 billion non-neuronal cells.
• The cerebrum, cerebellum and brainstem are three distinct regions, with different shapes, cell counts and jobs.
• The cerebral cortex holds about 16 billion neurons; the cerebellum holds about 69 billion, roughly 80 percent of the total, in about a tenth of the mass.
• The cerebellum does not start movement. It refines coordination, precision and timing, and its damage impairs fine movement and balance rather than causing paralysis.
• The brainstem is small but runs breathing, heart rate and blood pressure, with almost no margin for error.
• The thalamus, hippocampus, amygdala and basal ganglia handle relaying, memory, emotion and habit.
• Three meninges, about 125 to 150 milliliters of cerebrospinal fluid and four ventricles protect and nourish the tissue.
• The brain keeps rewiring through life. It also prunes unused connections, and its neuron count falls by about 15 percent from birth to adulthood.

Frequently Asked Questions

Do people really use only 10 percent of their brains?

No. Brain imaging shows activity across essentially the whole brain. Regions that look quiet in a scan are simply not busy at that moment. They have another job. Damage to any substantial region causes specific problems. That is only possible if the region was doing something. The ten percent figure has no basis in anatomy. It may be a distortion of an old claim about the ratio of neurons to other brain cells.

Is one side of the brain more important than the other?

Both hemispheres are broadly similar in shape and function, and they work together through the corpus callosum. Some functions lean more on one side than the other. But that is a tendency, not a split into two separate minds. The familiar "left brain logical, right brain creative" story is a popular simplification. Most tasks people call creative or analytical recruit networks on both sides.

Why does the cerebellum hold so many neurons if it is so small?

Because it does a precise job that needs huge local detail. The cerebellum compares the intended movement against the actual sensory feedback, then corrects the difference in real time. That needs a very large number of small local circuits, not long-range connections. Its densely packed Purkinje cells are the classic example. They are also why damage there disrupts fine movement and balance while leaving strength intact.

Can the brain recover from injury?

Partly. Neuroplasticity lets undamaged regions take over some functions of injured ones. Recovery from stroke or trauma is often better than was once believed. But it is not unlimited. Mature neurons generally do not divide, so cells that die are not replaced by copies. The brain can rewire around a loss. It cannot simply rebuild the lost cells. That is why rehabilitation, which pushes surviving networks to reorganize, works, while waiting for the lost cells to return does not.

Sources: neuron and non-neuron counts follow Herculano-Houzel and von Bartheld et al. on cell numbers in the human brain. Regional figures follow Van Essen et al. on the cerebral cortex and the cerebellum. Cortical thickness follows Fischl and Dale. Brodmann's areas follow Zilles. The corpus callosum follows the Queensland Brain Institute. Brainstem and cerebellum function follow NCBI Bookshelf (StatPearls). Cerebrospinal fluid follows NCBI Bookshelf. The amygdala follows Pabba. Synaptic pruning follows Sakai. Figures are from Wikimedia Commons, with the author and license named in each caption.