The human brain has three main parts: the cerebrum, the cerebellum, and the brainstem. Together, they weigh about 1,300 to 1,400 grams and contain roughly 86 billion nerve cells. But within those three divisions are dozens of specialized structures, each responsible for different aspects of thinking, feeling, moving, and staying alive.
The Three Major Divisions
The cerebrum is the largest part of your brain, making up the bulk of what you see from the outside. It handles everything you consciously experience: interpreting your senses, controlling speech, forming memories, reasoning through problems, and shaping your personality and behavior. It’s split into two halves, the left and right hemispheres, which are connected by a thick bundle of more than 200 million nerve fibers called the corpus callosum. This bridge lets the two sides share information constantly.
The cerebellum sits at the back of your head, below the cerebrum and behind the brainstem. It’s roughly the size of a fist. Despite being much smaller than the cerebrum, it plays a critical role in coordinating movement, maintaining balance, and fine-tuning posture. When you catch a ball, play an instrument, or simply walk without stumbling, your cerebellum is doing the behind-the-scenes work.
The brainstem connects the rest of the brain to the spinal cord and controls the functions you never have to think about: heart rate, breathing, blood pressure, sleep-wake cycles, and swallowing. It’s made up of three segments stacked on top of each other. The midbrain, at the top, helps with hearing, movement, and responding to environmental changes. The pons, in the middle, controls chewing, blinking, facial expressions, balance, and tear production. The medulla, at the bottom where the brain meets the spinal cord, regulates heart rhythm, breathing, and blood flow. It also triggers reflexes like coughing, sneezing, vomiting, and swallowing. Damage to the medulla is life-threatening because it controls so many survival functions.
The Four Lobes of the Cerebrum
Each hemisphere of the cerebrum is divided into four lobes, named after the skull bones that cover them.
The frontal lobe, at the front of the brain, is where higher-level thinking happens: planning, reasoning, problem-solving, and emotional regulation. It also contains the primary motor cortex, the strip of tissue that sends signals to your muscles to produce voluntary movement. Two specialized areas within the frontal lobe deserve mention. One, located on the left side, is responsible for speech production and articulation, helping you form words accurately in both spoken and written language.
The parietal lobe sits behind the frontal lobe, toward the top of the head. It integrates sensory information from across your body, processing touch, temperature, pressure, and pain. When you reach into a bag and identify an object by feel alone, your parietal lobe is piecing together those sensations into something meaningful.
The temporal lobe runs along each side of the brain, roughly behind the ears. It processes sound, turning raw auditory signals into recognizable words, music, or a baby’s cry. It also helps you recognize complex visual information like faces and scenes. Deep inside the temporal lobe is the hippocampus, a structure essential for forming new memories. A separate area in the upper rear portion of the temporal lobe handles language comprehension, allowing you to understand the meaning of words you hear or read.
The occipital lobe, at the very back of the head, is the brain’s visual processing center. It receives raw information from the eyes and interprets it into depth, distance, location, and the identity of objects. Even though your eyes collect the light, you actually “see” with your occipital lobe.
Deep Structures: The Limbic System
Buried beneath the cerebrum’s surface are a set of interconnected structures collectively called the limbic system. This network is the emotional core of the brain, managing feelings, motivation, behavior, and certain automatic body functions.
The hypothalamus is a small but powerful structure that produces hormones and regulates hunger, thirst, body temperature, sleep, sexual arousal, mood, blood pressure, and heart rate. It acts as a thermostat and control center for many of the body’s internal conditions.
The amygdala shapes how you experience emotions, particularly anxiety, anger, and fear. It also plays a role in memory and in reading social situations, helping you interpret other people’s intentions and expressions. The hippocampus, mentioned earlier as part of the temporal lobe, is the structure most directly responsible for your ability to form new memories and learn new information. The thalamus works as a relay station, processing incoming sensory information (hearing, taste, sight, and touch) and routing it to the right parts of the cerebrum for further interpretation. It also contributes to memory and planning.
The Basal Ganglia and Movement
Another group of deep structures, the basal ganglia, plays a key role in controlling movement and building habits. The basal ganglia include several interconnected clusters of nerve cells, the most prominent being the caudate nucleus and the putamen (together called the striatum), plus the globus pallidus.
Rather than directly generating movement, the basal ganglia act like a gatekeeper. They normally suppress inappropriate motor programs and, when the right moment comes, release the correct one so you can execute a practiced action smoothly. Think of reaching for a cup of coffee without consciously planning every muscle contraction. The basal ganglia also appear to be involved in learning which actions lead to rewards, essentially helping the brain build motor habits over time. When these structures malfunction, the result can be involuntary movements or difficulty initiating movement at all.
Gray Matter, White Matter, and How Signals Travel
Brain tissue comes in two types, and they do very different jobs. Gray matter sits on the brain’s outer surface and in certain deep clusters. It contains the cell bodies of nerve cells along with the short branches (dendrites) that receive messages from neighboring cells. Gray matter is where actual processing happens: thinking, remembering, making decisions, interpreting senses, and controlling movement.
White matter fills the brain’s interior. It’s made of long nerve fibers coated in a fatty insulation called myelin, which helps electrical signals travel faster. White matter carries messages between different regions of the brain and between the brain and the rest of the body. You can think of gray matter as the computers and white matter as the cables connecting them. The brain contains roughly equal numbers of nerve cells and supporting cells (called glial cells), with about 86 billion of each.
Protective Layers Around the Brain
The brain doesn’t sit directly against the skull. It’s wrapped in three layers of membranes called the meninges. The outermost layer, the dura mater, is a tough, durable sheet that sits closest to the skull. The middle layer, the arachnoid mater, is a web-like membrane. The innermost layer, the pia mater, clings directly to the brain’s surface, following every fold and groove. Between the arachnoid and pia layers, cerebrospinal fluid circulates, cushioning the brain against impact and helping to remove waste products. These layers, together with the skull itself, form a multi-layered defense system for the most complex organ in the body.