A Tour Through The Brain

Welcome to a guided tour of the human brain!

Our expedition will take us from the vast expanses of the brain’s major regions down to the cities of neurons and their bustling communication hubs. Along the way, we’ll learn what each part is, what it does, and how it connects to the whole. We shall begin the tour at the highest level.

The brain’s three divisions

Our map shows three major divisions: the cerebrum, the cerebellum, and the brainstem. Think of these as the main territories of the brain, each with a distinct culture and function, but all united as one nation.

The cerebrum is the largest part of the brain, the iconic, wrinkled portion filling most of the skull. It is divided into a left and right hemisphere connected by the corpus callosum. Its surface, the cerebral cortex, is our headquarters for higher functions, such as conscious thought, language, and planning.

The brainstem connects the brain to the spinal cord. It is composed of the midbrain at the top, pons in the middle, and medulla at the bottom. It is an ancient structure that is the brain’s life-support system, governing automatic functions like breathing, heart rate, and sleep cycles. The midbrain orientates us towards surprises, the pons stabilizes sleep/wake cycles, and the medulla silently governs survival. A single lesion here can halt breathing.

The cerebellum sits behind the brainstem. It acts as a coordinator. Cerebellum means “little brain” though that name is deceptive. It contains 69 billion neurons, roughly 80% of the brain’s total amount, packed in just one-tenth of its volume. This densely packed structure is the master of coordination, precision, and timing, ensuring our movements are smooth and automatic. Damage to the cerebellum leads to poor muscle control (a.k.a. ataxia).

Preparations

Before we tour the lobes of the cortex, it is essential to understand the two types of tissue that make up the brain and to know a few anatomical terms.

Grey vs white matter

Grey matter is composed of neuron cell bodies, dendrites, and their short-range connections. This is where the processing happens. It forms the brain’s wrinkled outer layer (cortex) and deep clusters called nuclei.

White matter is made of long nerve fibres (axons) wrapped in a fatty insulating coating called myelin. This is the brain’s wiring. It transmits signals at high speed between different grey-matter regions, connecting them into functional networks, which we shall see later in the tour. This distinction is important: grey matter processes information, while white matter transmits it. Think of grey matter as the data centres and the white matter as interconnects.

Anatomical terms

In what follows, it may be useful to be aware of a few anatomical terms of location. The brain or cross-sections of the brain are typically shown from the side in a sagittal plane, which divides left/right, or a transverse plane, which is parallel to the ground, and in which we look down upon the brain from above. The sagittal plane gets its name from the sagittal suture on the top of the skull.

A couple of key anatomical terms to remember include:

• dorsal/ventral: towards the spine/belly
• medial/lateral: towards/away from the midline of the body (i.e. middle/side)
• anterior/posterior: towards the front/back
• superior/inferior: towards the top/bottom

Note that in humans (and bipeds) the designations ventral/dorsal match the anterior/posterior, but that is not the case for four-legged animals, such as dogs and horses, for whom the ventral/dorsal equates to superior/inferior. In neuroscience, dorsal/ventral tends to mean superior/anterior, though.

Let’s return to the tour.

The cerebrum’s four lobes

The cerebrum’s wrinkled grey-matter surface is divided into four lobes.

Frontal lobe

The frontal lobe is located behind the forehead. It is the brain’s command centre, responsible for planning, decision-making, and controlling voluntary movements. The front of the frontal lobe, the prefrontal cortex (PFC) is a collection of highly specialized sub-regions that work in concert. The dorsolateral prefrontal cortex (DLPFC) is the ultimate thinking cap, as it manages working memory, logical reasoning, and top-down planning. It is the analytical part of the brain.

The PFC is not fully wired up until the age of 25. This explains why teenagers have trouble with impulse control: the DLPFC’s brake pedal is still under construction.

The ventromedial prefrontal cortex (vmPFC) is located a bit below and more centrally. It acts as the valuation engine and moral compass. It integrates emotion and memory to help you make value-based decisions, regulate your feelings, and derive meaning. It is the intuitive part of the brain.

The anterior cingulate cortex (ACC) is a conflict monitor and attentional switchboard. It detects errors (that “oops” feeling when you make a mistake) and signals when to shift your attention. It is also activated by social pain, such as the sting of exclusion.

Parietal lobe

The parietal lobe sits behind the frontal lobe. It is the sensor and mapper: it integrates sensory information to help you understand your body’s position in space. The temporoparietal junction (TPJ) is where it meets the temporal lobe. It is crucial for empathy and seeing things from another person’s perspective.

Temporal lobe

The temporal lobe is on the sides of the brain. It handles hearing, language comprehension, and the formation of memories. Essentially, it is the library and media centre.

Folded within the fissure that separates the temporal lobe from the parietal and frontal lobes lies the insula or insular cortex. It is the brain’s centre for interoception, the ability to sense the internal state of the body. This is where bodily sensations are translated into social emotions such as empathy and disgust.

Occipital lobe

At the very back of the brain we find the occipital lobe or visual department. This lobe is dedicated to vision: it translates signals from the eyes into meaningful images.

Subcortical structures and nuclei

Let’s now dive deeper into the subcortical structures. A key term here is nucleus: a dense cluster of neuron cell bodies that works as a functional unit. Many structures, such as the basal ganglia, are actually composites of several distinct nuclei.

The thalamus is the master relay station, as it directs almost all incoming sensory information to the correct cortical areas. Almost? Yes, smell (a.k.a. olfaction) bypasses the thalamus and goes straight to the cortex.

The hypothalamus is a tiny structure that acts as a control centre, regulating hunger, thirst, and body temperature. It also initiates our stress response by activating the HPA (hypothalamic-pituitary-adrenal) axis.

Basal ganglia are a group of nuclei that include the caudate, putamen, and globus pallidus, which help initiate voluntary movements and inhibit unwanted ones. They are also crucial components for habit formation. Its reward-processing hub is the nucleus accumbens (NAcc), which receives dopamine from the VTA when you anticipate or experience rewards. We’ll meet the VTA a bit later in the tour.

Beyond movement, the basal ganglia’s ventral striatum, including the NAcc, evaluates rewards. Dysfunction here links to addiction and depression.

The hippocampus is the structure that acts as an archivist. It is essential for forming new memories of facts and events. To boost memory, it is best to space study sessions rather than cram, because it reactivates knowledge at intervals and leverages LTP/LTD cycles better. We shall discuss these cycles when we reach the brain’s cellular structures in our tour. The hippocampus also supports adult neurogenesis, that is, the formation of new neurons throughout life.

The almond-shaped cluster named amygdala is the brain’s emotional processing hub. It is basically an emotional alarm system, as it is best known for fear and threat detection. The amygdala’s basolateral nucleus learns reward associations, while the central nucleus triggers bodily stress responses. Interestingly, labelling feelings can help regulate emotions, because it activates the PFC, which in turn weakens the amygdala activation.

The brain’s functional networks

So, how do all these parts cooperate? The answer lies in functional networks: separate brain regions form alliances, synchronizing their activity to perform complex tasks.

The default mode network (DMN) becomes active during introspection, daydreaming, and recalling memories. Daydreaming is not laziness; it is essential to have lucid moments in which light bulbs flash inside your head, because it activates the DMN. So, after focused work, take a walk without your phone to let solutions pop up naturally. A hyperconnected DMN leads to rumination, which is a symptom of depression, whereas a disrupted DMN causes spatial disorientation, commonly found in patients with Alzheimer’s.

The frontoparietal control network is known as the central executive network (CEN). Driven by the DLPFC, it engages whenever you focus on a mentally demanding task such as solving a problem. The CEN recruits the mentalizing network for social decision-making.

The mentalizing network is the social brain. To understand other people’s thoughts and feelings, the brain uses a network including the medial prefrontal cortex (mPFC), the TPJ, and other structures. This is the neural basis of empathy.

The salience network, anchored by the ACC and insula, acts as a switchboard, detecting important internal and external events to which it then directs our attention. It acts as a neural traffic police officer between the DMN and CEN. Overactivity in this network can indicate a predisposition to burnout. The salience network calms down when the vagus nerve is stimulated, so humming a tune can actually improve stress resilience.

The reward pathway is the brain’s motivation engine. It is a circuit critical for driving goal-oriented behaviour. It is not a single spot, but a neural path that begins in the brainstem at the ventral tegmental area (VTA), the dopamine factory we met earlier. When you anticipate or experience something rewarding, VTA neurons fire, which releases a pulse of dopamine along two tracks. One track goes to the NAcc, which creates the feeling of desire, anticipation, and motivation. Another track goes to the PFC, which helps you focus your attention and plan the actions needed to get the reward. This VTA–NAcc–PFC loop is what drives us to seek, learn, and achieve.

The brain’s cells

We have thus arrived at the lowest level: the cellular level.

The brain is built from an estimated 86 billion neurons and a nearly 1:1 ratio of glial cells. These information processors communicate across tiny gaps called synapses using chemical messengers called neurotransmitters. Synapses can strengthen and weaken with experience, a phenomenon called synaptic plasticity. This is the physical basis for all learning, and it is driven by molecular mechanisms such as long-term potentiation (LTP), which strengthens connections, and long-term depression (LTD), which weakens them. Spaced repetition in learning leverages LTP/LTD by reactivating synapses at optimal intervals.

Glia are active partners that regulate the communication (astrocytes), speed up signals (oligodendrocytes), and maintain brain health (microglia). Microglia clean up waste and prune unused connections.

Key neurotransmitters include:

• Glutamate: the most abundant neurotransmitter in the brain that is important for many cognitive functions.
• Gamma-aminobutyric acid (GABA) regulates brain activity to prevent problems with anxiety, concentration, and sleep.
• Dopamine: the molecule of motivation and reward, which is released by the VTA–NAcc pathway when you anticipate or achieve a goal.
• Serotonin: the molecule of mood and social status, which influences feelings of well-being, confidence, and pride. It also regulates mood, sleep, and digestion.
• Oxytocin: the molecule of bonding and trust, which is released during positive social interactions, fostering psychological safety and group cohesion. Its effects vary by context.
• Cortisol and norepinephrine: the molecules of stress and focus, which are released by the HPA axis during threats. They sharpen focus in the short term but impair higher-order thinking when chronically elevated, because chronic elevation shrinks the hippocampus. When under threat, norepinephrine narrows attention, which can lead to tunnel vision. Cortisol suppresses “non-essential systems” such as digestion and immunity.
• Epinephrine (a.k.a. adrenaline) is responsible for the fight-or-flight response.

The brain’s traffic patterns

So far, our tour has taken us past the landmarks of the brain. Our last stop is where we visit the brain at rush hour to see the neural “vehicles” zip along white-matter roads.

At the centre lies a massive roundabout with two main exits: one leading to threat avenue and the other to reward boulevard. Threat avenue is controlled by the amygdala, which upon detecting danger, immediately diverts traffic towards the hypothalamus, triggering a brain-wide stress alarm through cortisol release. Reward boulevard runs through the VTA and NAcc plaza, where unexpected joyful events release dopamine, which encourages more traffic along this path.

Nearby, we see neuroplasticity roadworks, where crews are constantly repaving old roads and building new bridges. Such roadworks happen all through life. Repeated use of a road (i.e. practice) strengthens the pavement through LTP in hippocampal and cortical lanes, whereas underused backstreets weaken via LTD. Astrocytes and oligodendrocytes are glial engineers who lay down new insulation and tweak roadside signals, thereby shaping traffic flow and creating detours for learning and memory.

Overlooking this magnificent area is the PFC control tower, where traffic controllers manage three key patterns:

• Holding patterns, in which controllers manage working memory by checking incoming flights (thoughts) on DLPFC screens.
• No-entry zones for inhibitory control: as soon as distractions arise, the ventrolateral PFC (VLPFC) and ACC close off side streets to prevent errant vehicles (impulses) from entering the main thoroughfare.
• Route switching to maintain cognitive flexibility: when the main roads have traffic jams, controllers reroute traffic with the help of the medial PFC (mPFC) corridors.

At the busy social signal intersection, sensors and cameras help read drivers’ intentions. Cameras at the TPJ and mPFC read licence plates to infer destinations to aid in understanding why cars are slowing down or turning. Such mentalizing monitors offer a theory of mind, which is the ability to comprehend other people’s thoughts, emotions, desires, and beliefs. And whenever we move or watch someone else move, mirror neurons fire, which fosters empathy and learning by observation. These cameras are in the premotor and inferior parietal zones.

There are many heuristic shortcuts: lanes that save time but sometimes lead to dead ends. Under high congestion (stress), the PFC tollbooth fails, and traffic defaults to subcortical backroads in the amygdala and striatum, which are part of the subcortical basal ganglia. These routes are fast but error-prone. Common biases are akin to taking shortcuts: they may be fast, but not necessarily optimal.

Closing thoughts

And that concludes the tour, where we have gone from the brain’s three divisions all the way down to individual cells, covering the lobes and nuclear complexes of the subcortical structures, too. The brain is a masterpiece of specialization and integration: while lobes specialize, most behaviours and skills emerge from cross-regional networks. In understanding this intricate architecture, we get a glimpse of what makes us human.