The human brain can be broken into a handful of major structures, each with distinct jobs, and understanding those parts turns what looks like a wrinkled mass of tissue into something that makes intuitive sense. At the broadest level, you are looking at the cerebrum on top, the cerebellum tucked behind and below, and the brainstem connecting everything to the spinal cord. Nested inside are smaller but critical structures like the thalamus, hypothalamus, hippocampus, and amygdala. Knowing what each region does gives you a working map that holds up whether you are reading a medical article, watching a documentary, or trying to make sense of a diagnosis.
The Cerebrum and Its Four Lobes
The cerebrum is the largest part of the brain, making up roughly 80 percent of its total mass. It is the deeply folded outer structure you see in every brain image, and it is divided down the middle into a left hemisphere and a right hemisphere. Each hemisphere is further divided into four lobes, named after the skull bones that sit over them.
- Frontal lobe: Sits behind the forehead. Handles planning, decision-making, personality, voluntary movement, and speech production (in a region called Broca’s area on the left side in most people).
- Parietal lobe: Sits behind the frontal lobe, toward the top of the head. Processes touch, temperature, pain, and spatial awareness.
- Temporal lobe: Sits along the sides, roughly behind the ears. Manages hearing, language comprehension (Wernicke’s area, usually on the left), and parts of memory.
- Occipital lobe: Sits at the very back of the head. Dedicated almost entirely to processing vision.
These lobes do not work in isolation. A conversation, for instance, involves the temporal lobe for hearing, the frontal lobe for forming a reply, and parietal regions for understanding spatial context and gestures. The boundaries on a diagram are neat, but actual brain activity flows across them constantly.
The Motor and Sensory Maps
Running along the border between the frontal and parietal lobes is a pair of strips that show up on many brain diagrams as a cartoon figure stretched across the surface. This figure is the homunculus, a distorted map of the body drawn onto the brain’s surface to show which patch of cortex controls which body part. Wilder Penfield famously mapped it during brain surgery in the mid-twentieth century, and updates based on modern brain-stimulation techniques continue to refine the picture.
On the frontal side of the border sits the motor strip, which sends commands to muscles. On the parietal side sits the sensory strip, which receives touch and pressure information. In both strips, the body is mapped from top to bottom: feet and legs sit near the top of the brain (toward the midline), while the face and tongue sit near the bottom (along the side). The hands and lips get a disproportionately large share of cortical real estate, reflecting how much fine control and sensitivity those areas require. Research confirms that this somatotopic layout is present even before birth, with distinct clusters of activation for the ankle, wrist, and mouth already identifiable in the preterm brain.
1Cerebral Cortex. Somatotopic Mapping of the Developing Sensorimotor Cortex in the Preterm Human BrainThe Cerebellum
Tucked beneath the occipital lobe at the back of the skull, the cerebellum looks like a miniature brain with its own tightly packed folds. Though it accounts for only about 10 percent of the brain’s weight, it contains more than half of the brain’s neurons. Its primary job is coordinating movement: it fine-tunes the timing and force of muscle contractions so that actions are smooth rather than jerky. It plays a particularly crucial role in balance and locomotion, and damage to the cerebellum produces a characteristic unsteady walk known as ataxia.
2PubMed. Cerebellar control of balance and locomotionBeyond movement, the cerebellum also contributes to motor learning, like the process of getting better at a musical instrument through practice. Newer research links it to aspects of language and attention as well, though those roles are less well understood. On a brain diagram, the cerebellum is easy to spot: it sits below and behind the cerebrum, separated from it by a dense fold of tissue.
The Brainstem
The brainstem is the stalk-like structure connecting the cerebrum to the spinal cord. It is made up of three parts stacked vertically: the midbrain on top, the pons in the middle, and the medulla oblongata at the bottom. Together they serve as the brain’s highway, carrying nerve fiber tracts that relay sensory information upward and motor commands downward to move the limbs and trunk.
3Anaesthesia & Intensive Care Medicine. The brain: functional divisionsBut the brainstem is far more than a relay. The medulla and pons house the circuits that generate your breathing rhythm. Normal, quiet breathing is produced by neural circuits in this region, though higher brain areas can override that rhythm for voluntary actions like speaking, singing, or holding your breath.
4PubMed. Forebrain projection neurons target functionally diverse respiratory control areas in the midbrain, pons, and medulla oblongataThe brainstem also regulates heart rate, blood pressure, and consciousness. Because it controls so many life-sustaining functions, injuries here tend to be more immediately dangerous than comparable damage in most other brain regions.
The Thalamus and Hypothalamus
Deep inside the brain, sitting roughly at its center, lies the diencephalon, a group of structures usually highlighted on diagrams as a small cluster between the cerebrum and the brainstem. The two most important members for a general overview are the thalamus and the hypothalamus.
The thalamus acts as the brain’s relay station. Almost all sensory information (sight, sound, touch) passes through it on the way to the cortex, where it gets processed consciously. The thalamus is extensively connected to the cortex, and it does not just passively forward signals; it helps filter and prioritize incoming data, playing a role in attention and awareness.
3Anaesthesia & Intensive Care Medicine. The brain: functional divisionsThe hypothalamus, sitting just below the thalamus, is tiny but enormously influential. It bridges the nervous system and the hormonal system, maintaining the body’s internal balance. It regulates body temperature, hunger, thirst, sleep-wake cycles, and the release of hormones from the pituitary gland, which dangles just beneath it. By processing both external cues (like ambient temperature) and internal signals (like blood sugar levels), the hypothalamus keeps your physiology within livable ranges.
5PubMed Central. Integrative Functions of the Hypothalamus: Linking Cognition, Emotion and Physiology for Well-being and AdaptabilityThe Limbic System
Diagrams often label a ring of structures near the center of the brain as the “limbic system.” Two structures in this ring get the most attention: the hippocampus and the amygdala. Both sit within the temporal lobe, and both are paired, one per hemisphere.
The hippocampus is critical for forming new memories, especially the kind tied to personal experience and facts about the world. The amygdala is most associated with processing emotions, particularly fear and threat detection. These two structures interact closely: the amygdala can strengthen or weaken the storage of hippocampal memories depending on how emotionally charged an event is, while the hippocampus can shape the amygdala’s response by providing context about whether a stimulus has been dangerous in the past.
6PubMed. Human emotion and memory: interactions of the amygdala and hippocampal complexThe limbic system also includes the cingulate gyrus, a curved band of cortex that sits above the corpus callosum. It participates in emotion regulation, pain perception, and linking memories with emotional responses. These limbic regions connect extensively to the mammillary bodies and the anterior thalamic nuclei, forming circuits that are strongly implicated in memory functions.
7PubMed Central. Hippocampal – diencephalic – cingulate networks for memory and emotion: An anatomical guideThe Basal Ganglia
Buried deep within the cerebrum, the basal ganglia are a cluster of structures that many simple diagrams skip, even though they are essential. They include the striatum, pallidum, subthalamic nucleus, and substantia nigra. Their best-known job is helping you select and execute the right movement at the right time. When you reach for a coffee cup instead of swatting it off the table, that is partly the basal ganglia selecting the appropriate motor program and suppressing competing ones.
8PubMed Central. The basal ganglia and motor controlBut these nuclei do more than choreograph movement. They participate in parallel circuits supporting cognition, motivation, and habitual behavior. The loss of dopamine-producing neurons in the substantia nigra, for instance, is the hallmark of Parkinson’s disease, which produces both motor symptoms like tremor and non-motor symptoms like difficulty with planning and motivation.
9PubMed Central. Functional neuroanatomy of the basal gangliaGrey Matter and White Matter
If you were to slice the brain open, you would notice two distinct tissue colors. The outer surface and certain deep structures appear darker (grey matter), while the interior bulk appears lighter (white matter). This distinction shows up on brain scans and diagrams alike, and it reflects a real functional difference.
Grey matter is packed with neuron cell bodies, the parts of nerve cells that do the computing. It forms the cerebral cortex, the cerebellar cortex, and deep nuclei like the thalamus and basal ganglia. White matter consists mainly of long nerve fibers (axons) coated in a fatty insulation called myelin, which gives the tissue its pale color. These myelinated fibers are the cables that carry signals between grey matter regions, sometimes across great distances within the brain. The chemical profiles of the two tissues are measurably different: grey matter has higher concentrations of certain metabolites related to neuronal activity, while white matter is richer in compounds associated with myelin maintenance.
10PubMed Central. Analysis of the brain proton magnetic resonance spectroscopy – differences between normal grey and white matterThe Corpus Callosum
On a sagittal diagram (the brain viewed from the side, as though you sliced it down the middle), you will see a broad, arching band of white fibers connecting the two hemispheres. This is the corpus callosum, the brain’s largest fiber bundle and the main communication bridge between left and right. Without it, the hemispheres have difficulty sharing information.
Research on patients who have had the corpus callosum surgically cut to treat severe epilepsy illustrates just how important it is. After the surgery, communication between the hemispheres drops markedly, with the biggest losses in the frontal and parietal association areas that handle higher-level thinking. Some communication does persist through smaller alternative pathways, suggesting that the corpus callosum is not the only route between hemispheres, but it is by far the dominant one.
11PubMed Central. On the role of the corpus callosum in interhemispheric functional connectivity in humansClassic studies of people with callosal damage confirmed that sensory information and learned skills processed in one hemisphere often fail to transfer to the other when this bridge is disrupted.
12PubMed. Classical disconnection studies of the corpus callosumProtective Layers and Fluid Systems
The brain itself is soft and needs significant protection. Three layers of tissue called the meninges wrap around it: the tough outer dura mater, the web-like arachnoid mater in the middle, and the delicate pia mater clinging directly to the brain’s surface. Between the arachnoid and pia layers, cerebrospinal fluid (CSF) circulates, cushioning the brain against sudden impacts.
CSF is produced inside the brain in a set of interconnected chambers called ventricles. There are four ventricles: two large lateral ventricles (one in each hemisphere), a narrow third ventricle in the midline, and a fourth ventricle between the brainstem and the cerebellum. These chambers are linked by thin channels, forming an intricate series of CSF-filled cavities.
13Neuroimaging Clinics of North America. The Ventricular System: Anatomy, Variants, and PathologyCSF does more than cushion. It removes metabolic waste, delivers nutrients, and helps maintain a stable chemical environment around neurons. The fluid eventually drains into the bloodstream through structures in the meninges. If drainage is blocked and fluid builds up, the resulting increase in pressure can compress brain tissue, a condition known as hydrocephalus.
Blood Supply and the Circle of Willis
Despite weighing only about 1.4 kilograms, the brain consumes roughly 20 percent of the body’s oxygen and glucose. That appetite demands a reliable blood supply, and the brain has a built-in safety system to protect it: the circle of Willis. This ring of arteries at the base of the brain connects the major vessels coming from the front (internal carotid arteries) with those coming from the back (vertebral and basilar arteries). If one feeding artery narrows or becomes blocked, blood can reroute through the ring to keep downstream tissue supplied. The communicating arteries within the circle also protect the blood-brain barrier from sudden swings in pressure.
14PubMed Central. Function of circle of WillisWhen blood supply is interrupted and the circle of Willis cannot compensate, the result is a stroke. Brain tissue starved of oxygen begins to die within minutes, which is why speed of treatment matters so much in stroke care.
The Cranial Nerves
Not every nerve connection passes through the spinal cord. Twelve pairs of cranial nerves emerge directly from the brain (mostly from the brainstem) and exit the skull through small openings. They handle an enormous range of functions for everyday life, including vision, eye movement, facial sensation and expression, hearing, taste, swallowing, and control of many internal organs.
15PubMed Central. On the Cranial NervesThe vagus nerve, the tenth cranial nerve, is the longest and arguably the most famous. It wanders from the brainstem all the way down into the abdomen, regulating heart rate, gut motility, and aspects of the immune response. When diagrams label the brainstem, the cranial nerve exit points are often shown as small nubs along its surface.
Why the “Reptilian Brain” Model Is Wrong
You may have seen diagrams that divide the brain into three layers: a “reptilian” brainstem, a “mammalian” limbic system, and a “rational” neocortex. This triune brain model, popularized in the 1960s, is intuitive and still appears in pop-psychology books, but modern neuroscience has moved well past it. The model implies that emotion and reason live in separate compartments, with the cortex overriding the primitive layers below. In reality, emotion and cognition are interdependent processes that rely on overlapping networks; there are no purely emotional circuits and no purely cognitive circuits.
16PubMed Central. The Brain Is Adaptive Not Triune: How the Brain Responds to Threat, Challenge, and ChangeA more accurate picture sees the brain as an adaptive organ whose regions cooperate fluidly. The brainstem is not a leftover from lizard ancestors in any meaningful functional sense; it evolved alongside the cortex and communicates with it constantly. Treating the limbic system as a self-contained “emotion center” can also be misleading, because structures like the amygdala and hippocampus participate in memory, perception, and decision-making, not just feeling. When looking at a brain diagram, it is better to think of the labeled regions as members of overlapping networks rather than as a hierarchy of old and new parts.
The Default Mode Network and Resting Brain Activity
Traditional brain diagrams show anatomy: physical structures with borders and labels. But the brain also organizes itself into functional networks, groups of regions that activate together even when you are not doing any particular task. The most studied of these is the default mode network, which includes the medial prefrontal cortex, medial temporal lobes, and posterior cingulate cortex.
17PubMed Central. Resting-state functional connectivity reflects structural connectivity in the default mode networkThe default mode network becomes active when your mind is wandering, daydreaming, recalling the past, or imagining the future. It quiets down when you focus on an external task. Disruptions to this network have been linked to conditions ranging from Alzheimer’s disease to depression. The existence of functional networks is a good reminder that brain diagrams, while useful, only capture the static anatomy. The dynamic patterns of activity that define thought, mood, and consciousness are a layer on top that no single diagram can fully show.
Glial Cells and the Supporting Cast
Neurons get most of the attention, but they make up only part of the brain’s cellular population. Glial cells, often called the brain’s support staff, perform functions without which neurons could not survive. Astrocytes are star-shaped cells that maintain the chemical environment around synapses, regulate concentrations of ions, and provide metabolic fuel to neurons. Oligodendrocytes produce the myelin insulation that wraps around axons in the brain, enabling electrical signals to travel quickly over long distances. Microglia act as the brain’s immune cells, clearing debris and responding to infection or injury.
Glial cells do not fire electrical signals the way neurons do, which is why they were historically seen as passive filler. That view has changed dramatically. Astrocytes, for example, actively influence how signals pass between neurons at the synapse, and disruption of glial function is now recognized as a contributor to conditions like multiple sclerosis (where myelin breaks down) and certain brain tumors (gliomas arise from glial cells).
Neuroplasticity and Why the Map Keeps Changing
A brain diagram is a snapshot, but the real brain is a moving target. Neuroplasticity refers to the brain’s ability to reorganize its structure and function in response to experience, learning, and injury. At the cellular level, this involves synaptic remodeling, changes in gene expression driven by neural activity, and shifts in how strongly different neurons are connected.
18PubMed Central. Neuroplasticity and Nervous System Recovery: Cellular Mechanisms, Therapeutic Advances, and Future ProspectsPlasticity is why stroke patients can sometimes recover lost abilities: neighboring brain regions gradually take over functions that the damaged area once handled. It is also why practicing a skill, whether it is playing piano or learning a second language, physically alters the brain’s wiring over time. Children’s brains are especially plastic, but the capacity never disappears entirely in adults. The implication for anyone looking at a brain diagram is that the borders and assignments of each labeled area are averages across many people at one moment. Your own brain’s functional map has been reshaped by everything you have ever learned and every injury you have ever recovered from.