Every part, explained

The human brain

About three pounds of you: 86 billion neurons behind every thought, feeling and move.

Your brain is the soft, wrinkled organ inside your skull that runs everything you do, from breathing and walking to falling in love and solving puzzles. It is made of about 86 billion nerve cells, called neurons, that constantly send tiny electrical and chemical signals to each other. Different areas specialize in different jobs, but almost nothing happens in just one spot: every thought is teamwork across many regions.

The cortex

Frontal lobe Your brain's CEO: planning, deciding, speaking and moving, right behind your forehead
- Prefrontal cortex The front-most part of your brain, where you plan, focus and keep yourself in check
  - Superior frontal gyrus The top ridge of the frontal lobe: a hub for self-control, focus and starting movement
  - Dorsolateral prefrontal cortex Your brain's mental whiteboard: holding, juggling and using information in the moment
  - Caudal middle frontal gyrus Where you aim your eyes and ready your hands: home of the frontal eye field
  - Frontal pole The very tip of your brain: juggling goals and thinking about your own thinking
- Broca's area Where thoughts become speech: the famous language region discovered in 1861
  - Pars opercularis The back half of Broca's area: planning the sounds and structure of speech
  - Pars triangularis The triangle-shaped front of Broca's area: fetching words and their meanings
- Orbitofrontal cortex Right above your eyes: the region that decides what things are worth to you
  - Lateral orbitofrontal cortex Notices when a choice stops paying off and helps you change course
  - Medial orbitofrontal cortex The inner underside of the frontal lobe: where pleasure and feelings guide choices
  - Pars orbitalis The lowest front corner of the language area: meaning, rhythm and emotion in words
- Motor strip A strip across the top of your brain that sends the "go" signal to your muscles
  - Primary motor cortex Your brain's movement keyboard: a body map that commands every muscle you move on purpose
  - Paracentral lobule Where the body map tucks inside: moving and feeling your legs, feet and bladder
Parietal lobe Your body map and inner GPS: it knows where you are and where everything is.
- Primary somatosensory cortex Your touch map: every patch of skin has its own spot on this strip.
- Superior parietal lobule Where your body meets space: it guides your reach and tracks your limbs.
- Supramarginal gyrus Helps you sound out words, use tools, and tell your feelings from others'.
- Angular gyrus region A crossroads where sight, sound and touch meet: reading, math and meaning.
- Precuneus The brain's hidden inner eye for memories, imagination and your sense of self.
Temporal lobe Hearing, language, faces and memory: the brain's sound and story center.
- Superior temporal gyrus Where sound turns into words: home of Wernicke's area.
- Primary auditory cortex The first cortical stop for sound, laid out by pitch like piano keys.
- Superior temporal sulcus Your social radar: it reads gazes, voices and body language.
- Middle temporal gyrus A storehouse of word meanings, and near its back, a hub for seeing motion.
- Inferior temporal gyrus Names what you see: the end of the brain's "what is that?" pathway.
- Fusiform gyrus Your face and word detector, on the underside of the brain.
- Temporal pole The front tip of the temporal lobe, where facts about people meet feelings.
- Medial temporal cortex The memory gateway on the inner temporal lobe, wrapped around the hippocampus.
  - Entorhinal cortex The hippocampus's front door, and home of the brain's built-in grid map.
  - Parahippocampal gyrus Your scene recognizer: it knows a kitchen from a beach at a glance.
Occipital lobe The brain's vision center, at the very back of your head.
- Primary visual cortex V1: the first cortical stop for sight, a precise map of what your eyes see.
- Cuneus A wedge of visual cortex that sees the lower half of your view.
- Lingual gyrus Below the calcarine groove: sees the upper half of your view and helps with color.
- Lateral occipital cortex Where lines become objects, and where the brain sees motion.
Insula The hidden island of the brain: sensing your heartbeat, gut feelings, taste and disgust
Cingulate cortex A belt of cortex hugging the brain's center: emotion, pain, errors and memory
- Rostral anterior cingulate The emotional front of the cingulate: calming fear and steadying your mood
- Caudal anterior cingulate Your inner "oops" detector: spotting mistakes, conflicts and the sting of pain
- Posterior cingulate cortex The hub of your wandering mind: memories, daydreams and thinking about yourself
- Isthmus of the cingulate The narrow bend of the cingulate: helping you find your way and place your memories

Deep inside

Basal ganglia Your action selector: says "go" to one move and "no-go" to all the rest
- Caudate nucleus A C-shaped nucleus with a long tail that helps you learn, plan and chase goals
- Putamen The shell-shaped hub where practiced movements become smooth, automatic habits
- Globus pallidus The "pale globe": the gatekeeper that holds movements in check until it is time
  - External segment (GPe) A busy middle station on the "no-go" pathway that helps hold back unwanted moves
  - Internal segment (GPi) The basal ganglia's main exit door, and a key target for brain pacemakers
- Nucleus accumbens The brain's reward hotspot, turning "I like that" into "I want more"
- Subthalamic nucleus A small brake that can shout "wait!", and a top target for Parkinson's therapy
Diencephalon The "in-between brain": home of the thalamus, hypothalamus and a few tiny glands
- Thalamus The brain's grand central station: nearly every sense stops here on its way up
  - Anterior nucleus A memory relay on the famous Papez loop that also helps you find your way
  - Mediodorsal nucleus The prefrontal cortex's close partner, linking thinking and planning with feelings
  - Ventral anterior nucleus A motor relay that carries the basal ganglia's "go" decisions back to the cortex
  - Ventral lateral nucleus Where the cerebellum's timing and the basal ganglia's choices meet on the way to motion
  - Ventral posterior nucleus The body's touch and pain relay, passing every tap, ache and taste to the cortex
  - Lateral posterior nucleus A neighbor of the pulvinar that links what you see to where things are
  - Pulvinar The thalamus's big back cushion, a spotlight that helps pick what you look at
  - Lateral geniculate nucleus Vision's relay station: a six-layered bent knee where signals from your eyes arrive
  - Medial geniculate nucleus The hearing relay: the sounds you hear pass through this little bent knee
  - Midline & intralaminar nuclei The thalamus's wake-up crew, helping switch on alertness and attention
- Hypothalamus An almond-sized control center for temperature, hunger, thirst, sleep and hormones
- Pituitary gland The pea-sized "master gland" that sends hormone orders to the whole body
- Pineal gland A tiny pine cone deep in the brain that releases melatonin when darkness falls
- Habenula The brain's disappointment detector, a tiny "anti-reward" center linked to depression
Limbic system The brain's emotion and memory team, wrapped around its deep core
- Hippocampus The seahorse-shaped memory maker that turns your day into lasting memories
  - Cornu Ammonis (CA fields) Ammon's horn: the curled band of the hippocampus where memories take shape
  - Dentate gyrus The hippocampus's front door, and one of the few spots where new neurons may form
  - Subiculum The hippocampus's main exit, sending its processed signals out to the rest of the brain
- Amygdala The almond-shaped alarm that spots danger and gives memories their emotional punch
  - Basolateral complex The amygdala's learning hub: works out what is dangerous and what is rewarding
  - Centromedial nuclei The amygdala's alarm switch: triggers the racing heart and frozen body of fear
  - Superficial (cortical) nuclei The amygdala's smell-and-social corner, where scents meet feelings
- Fornix The arching cable that carries the hippocampus's messages out
- Mammillary bodies Two tiny round bumps under the brain that are key stops on the memory loop
Ventricular system A hidden network of fluid-filled chambers that cushions and cleans the brain
- Lateral ventricles Two big C-shaped fluid chambers, one curling through each half of the brain
- Third ventricle A thin, slit-like fluid chamber squeezed between the two halves of the thalamus
- Cerebral aqueduct A narrow canal through the midbrain that links the third and fourth ventricles
- Fourth ventricle A tent-shaped chamber between brainstem and cerebellum where fluid flows out
- Choroid plexus The brain's fluid factory: frilly tissue that makes cerebrospinal fluid
White matter pathways The brain's wiring: bundles of fat-wrapped cables that let distant regions talk fast
- Corpus callosum The giant bridge of about 200 to 300 million fibers that lets your left and right brain talk
- Corticospinal tract The main highway for movement, running from your brain all the way down the spinal cord
- Arcuate fasciculus The curved cable linking the brain's areas for speaking and understanding words
- Optic radiation A fan of fibers carrying vision from the thalamus to the back of the brain
- Acoustic radiation The last stretch of the hearing pathway, from the thalamus to the hearing cortex
- Cingulum A belt of fibers looping around the brain's middle, linking emotion, focus, and memory
- Uncinate fasciculus A hook-shaped cable linking memory and emotion to decision-making
- Inferior fronto-occipital fasciculus One of the brain's longest cables, running from the back of your head to your forehead
- Optic chiasm The X-shaped crossing where half of each eye's fibers swap sides

Brainstem and cerebellum

Brainstem The stalk connecting brain and body, running breathing, heartbeat and wakefulness
- Midbrain The top of the brainstem: eye reflexes, hearing relays and dopamine factories
  - Substantia nigra The "black substance": dark dopamine cells whose loss causes Parkinson's disease
    - Pars compacta Tightly packed dopamine cells that power smooth movement, lost in Parkinson's
    - Pars reticulata A gatekeeper that keeps movements and glances on hold until the right moment
  - Red nucleus A rosy, iron-rich hub in the midbrain linking the cerebellum to movement
  - Ventral tegmental area A dopamine hub for reward and motivation, firing when life beats your expectations
  - Superior colliculus The midbrain's "look over there!" center, snapping your eyes toward things
  - Inferior colliculus The hearing hub of the midbrain, a key relay for sound signals on their way up
- Pons The "bridge" linking cortex to cerebellum, and a key player in dreams and breathing
  - Locus coeruleus The "blue spot": a tiny cluster that sets how alert and ready for action you are
  - Raphe nuclei A chain of cell clusters down the brainstem's middle that makes most brain serotonin
- Medulla oblongata Your life-support center: breathing, heartbeat, blood pressure and swallowing on autopilot
- Cranial nerves Twelve pairs of nerves wired straight from the brain to your eyes, face, throat, and gut
  - Olfactory nerve & bulb (I) Your smell line: the only sense that skips the thalamus and heads straight to emotion
  - Optic nerve (II) The cable from each eye to the brain, and really a piece of brain grown out to the eye
  - Oculomotor nerve (III) The main eye mover: it aims your eye, lifts your eyelid, and shrinks your pupil
  - Trochlear nerve (IV) The thinnest cranial nerve, running one eye muscle that loops through a tiny pulley
  - Trigeminal nerve (V) The face's feeling nerve: touch, pain, and heat from your whole face, plus chewing
  - Abducens nerve (VI) Turns each eye outward, so you can glance to the side without moving your head
  - Facial nerve (VII) The nerve behind every smile, frown, and wink, and it carries taste from your tongue
  - Vestibulocochlear nerve (VIII) Two nerves in one: hearing from the snail-shaped cochlea and balance from the inner ear
  - Glossopharyngeal nerve (IX) Tastes the back of your tongue, starts your gag reflex, and checks your blood pressure
  - Vagus nerve (X) The "wandering" nerve that runs from your brainstem to your heart, lungs, and gut
  - Accessory nerve (XI) Lets you shrug your shoulders and turn your head, and takes a strange U-turn to get there
  - Hypoglossal nerve (XII) The tongue's movement nerve, for talking, chewing, swallowing, and sticking it out
- Spinal cord The body's main information highway, and home to lightning-fast reflexes
Cerebellum The "little brain" that holds most of your neurons and makes every move smooth
- Anterior lobe The top front part of the cerebellum, keeping your walk and posture smooth
- Posterior lobe The cerebellum's large back part, polishing skilled moves and even helping you think
- Flocculonodular lobe The oldest part of the cerebellum, keeping you balanced and your eyes steady
- Vermis The worm-shaped middle strip of the cerebellum, keeping your body balanced and upright
- Arbor vitae The "tree of life": branching white wiring hidden inside the cerebellum
- Deep cerebellar nuclei The cerebellum's hidden exit doors, sending out its final polished messages
  - Dentate nucleus The biggest cerebellar exit, linking movement planning with thinking
  - Interposed nucleus The middle cerebellar nucleus, correcting arm and leg movements as they happen
  - Fastigial nucleus The innermost cerebellar nucleus, keeping your body balanced and your gaze on target

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