Brain Anatomy and Function

Linas Juozenas

Brain Anatomy & Function:
From Neurons to Complex Networks

Every thought you form, memory you store, or emotion you feel emerges from the concerted activity of roughly 86 billion neurons woven into what is arguably the most intricate structure in the known universe—the human brain.¹ Understanding how its individual parts operate and communicate not only illuminates the biological roots of consciousness, but also guides breakthroughs in medicine, education, and artificial intelligence. This article explores the roles of key brain structures and explains how neurons link together to form dynamic networks that support behavior, learning, and health.

Table of Contents

Introduction
Anatomical Overview of the Central Nervous System
Key Brain Structures & Their Functions
Neurons: Building Blocks of Signaling
Neural Networks & Plasticity
How We Study Brain Structure & Connectivity
Implications for Health & Disease
Conclusion

1. Introduction

In ancient Egypt, embalmers discarded the brain during mummification, believing the heart housed intellect. Modern neuroscience leaves no such doubt: cognition, emotion, and vital autonomic functions all emerge from the central nervous system (CNS)—the brain and spinal cord—while peripheral nerves convey information to and from the body.² Because dysfunction at any hierarchical level can produce profound clinical symptoms, mapping form to function remains a cornerstone of biomedical research.

2. Anatomical Overview of the CNS

The adult human brain weighs about 1.3–1.4 kg (≈ 3 lb) yet consumes 20–25 % of the body’s resting metabolic energy.³ During embryonic development it differentiates into three primary vesicles—prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain)—that fold into the following adult structures:

Forebrain: cerebrum (cortex & subcortical nuclei), thalamus, hypothalamus.
Midbrain: tectum & tegmentum, part of the brainstem.
Hindbrain: cerebellum, pons, medulla oblongata.

These subdivisions orchestrate sensory processing, motor control, homeostasis, memory, and higher‑order cognition through a finely tuned hierarchy of networks.

3. Key Brain Structures & Their Functions

3.1 Cerebral Cortex

The cerebral cortex is the brain’s outer sheet—2–4 mm thin yet folded into sulci (grooves) and gyri (ridges), expanding surface area to ≈ 2,500 cm². Histologically it contains six horizontal layers populated by pyramidal projection neurons and a rich diversity of interneurons, all arranged vertically in cortical columns that process specific inputs.⁴ Evolutionarily, the neocortex grew dramatically in primates, supporting language, abstract reasoning, and social cognition.

Lobes & Specializations

Frontal lobe (front): executive functions, voluntary movement via primary motor cortex (M1), speech production (Broca’s area), impulse control, and working memory.⁵
Parietal lobe (top): bodily sensation (primary somatosensory cortex, S1), spatial attention, numerical cognition, and mental rotation.
Temporal lobe (side): auditory processing, language comprehension (Wernicke’s area), semantic memory, and face recognition (fusiform face area).
Occipital lobe (back): primary (V1) and secondary visual cortices that transform edges and contrast into shapes, color, motion, and eventually object identity.
Insula (hidden): interoception (sense of internal bodily state), gustatory taste cortex, pain integration, and emotional awareness.

Although localization is evident—damage to left inferior frontal gyrus disrupts speech—most abilities arise from distributed networks linking multiple lobes, illustrating the brain’s cooperative architecture.

3.2 Hippocampus

Resembling a seahorse in coronal section, the hippocampus sits in the medial temporal lobe. It converts transient experiences into declarative (long‑term) memories, encodes spatial maps through “place cells,” and supports contextual fear learning.⁶ Lesions famously produced anterograde amnesia in patient H.M., demonstrating its indispensable role in memory consolidation.⁷ Chronic stress or elevated cortisol shrinks hippocampal volume, linking emotional health to memory performance.

3.3 Amygdala

Nestled anterior to the hippocampus, the amygdala comprises multiple nuclei that tag stimuli with emotional meaning—especially fear, disgust, and reward.⁸ It modulates autonomic responses via the hypothalamus, strengthens memory of emotional events via noradrenergic signaling to the hippocampus, and influences social decision‑making and aggression.

3.4 Thalamus

Acting as the brain’s “Grand Central Station,” the thalamus relays nearly all sensory information (except olfaction) to the cortex through topographically organized nuclei.⁹ It also participates in motor loops and consciousness; deep brain stimulation of intralaminar nuclei can restore arousal in minimally conscious patients. The pulvinar modulates visual attention, while the ventral posterior nucleus handles somatic sensation.

3.5 Basal Ganglia

This set of subcortical nuclei—the caudate, putamen, globus pallidus, substantia nigra, and subthalamic nucleus—forms feedback loops with motor and prefrontal cortex to initiate or inhibit movement, select actions, and encode reward prediction errors.¹⁰ Dopaminergic degeneration in the substantia nigra causes Parkinson’s disease; conversely, striatal dopamine overactivity contributes to compulsive behaviors and addiction.

3.6 Cerebellum

Long regarded solely as a motor coordinator, the cerebellum fine‑tunes movement timing, balance, and posture by comparing intended commands with sensory feedback. Modern imaging reveals its contributions to language, emotion, and working memory via closed loops with prefrontal and parietal cortex.¹¹ Pediatric cerebellar injury can impair social cognition, underscoring its broader role beyond gait and reflexes.

3.7 Brainstem

The midbrain, pons, and medulla house nuclei controlling eye movements, sleep–wake cycles, cardiovascular and respiratory centers, and cranial nerves mediating facial sensation and swallowing.¹² The reticular formation running through the brainstem modulates arousal, filtering incoming stimuli so that only salient information reaches the cortex—a prerequisite for attention.

3.8 Hypothalamus

Despite its modest size, the hypothalamus maintains homeostasis—regulating temperature, hunger, thirst, circadian rhythms, and endocrine output via the pituitary gland.¹³ Neurons here sense blood osmolarity, glucose, and even immune signals, coordinating autonomic, hormonal, and behavioral responses essential for survival and reproduction.

3.9 Corpus Callosum & Commissures

The corpus callosum—over 190 million axons—connects the left and right cerebral hemispheres, allowing rapid interhemispheric communication. Other commissures (anterior, posterior, hippocampal) link temporal lobes and optic tracts.¹⁴ Surgical severing (for severe epilepsy) produces “split‑brain” phenomena: patients can verbally name objects viewed in the right visual field but only draw those in the left, revealing lateralized processing.

3.10 Ventricular System & Cerebrospinal Fluid (CSF)

Four interconnected ventricles produce and circulate CSF, cushioning the brain, removing waste, and distributing neuroactive compounds. Blockage of CSF flow causes hydrocephalus, while reduced CSF turnover is implicated in Alzheimer’s pathology.¹⁵

4. Neurons: Building Blocks of Signaling

4.1 Cellular Anatomy

A stereotypical neuron consists of:

Soma (cell body): contains the nucleus and metabolic machinery.
Dendrites: branched receivers gathering synaptic input.
Axon: a singular projection, often myelinated, conducting action potentials to distant targets.
Synapse: specialized junction where an axon terminal communicates with another neuron or effector cell.¹⁴

4.2 Excitatory, Inhibitory & Modulatory Neurons

In cortex ≈ 80 % of neurons are glutamatergic excitatory pyramidal cells projecting long distances, whereas ≈ 20 % are GABAergic interneurons that inhibit local circuits, sharpening timing and preventing runaway excitation.¹⁶ Neuromodulatory cells—dopaminergic (midbrain), serotonergic (raphe nuclei), noradrenergic (locus coeruleus), and cholinergic (basal forebrain)—broadcast diffuse signals that alter global network gain and learning rules.

4.3 Electrical Communication

Neurons maintain a resting membrane potential (~ –70 mV). When depolarization reaches threshold, voltage‑gated Na⁺ channels open, generating an action potential that propagates along the axon without decrement.¹⁷ Myelin sheaths from oligodendrocytes (CNS) or Schwann cells (PNS) insulate axons, enabling saltatory conduction between Nodes of Ranvier and boosting velocity up to 120 m/s. Demyelination in multiple sclerosis slows or blocks conduction, causing sensory and motor deficits.

4.4 Chemical Synaptic Transmission

Action potential invades presynaptic terminal.
Voltage‑gated Ca²⁺ channels open; influx triggers vesicle fusion.
Neurotransmitter (e.g., glutamate, GABA, acetylcholine, dopamine) diffuses across the synaptic cleft.
Binding to postsynaptic receptors opens ion channels or activates G‑protein cascades, changing membrane potential or gene transcription.

Synapses are plastic: repeated activation strengthens some connections (long‑term potentiation) and weakens others (long‑term depression), the cellular basis of learning.

4.5 Glial Support Cells

Glia outnumber neurons roughly 1.5 : 1 and include:

Astrocytes: maintain extracellular ion balance, recycle neurotransmitters, modulate synapses, and form the blood–brain barrier.
Oligodendrocytes / Schwann cells: generate myelin in CNS and PNS.
Microglia: immune sentinels clearing debris, pruning synapses, releasing cytokines.
Ependymal cells: line ventricles, produce CSF, and drive its flow.

Far from passive, glia actively regulate synaptic strength and neurovascular coupling, and astrocytic calcium waves can influence local blood flow during neural activity.

5. Neural Networks & Plasticity

5.1 Microcircuits

Within a cubic millimeter of cortex reside ≈ 100,000 neurons wired into canonical motifs such as feed‑forward excitation, feedback inhibition, lateral competition, and recurrent loops that underlie feature detection, contrast enhancement, and working memory.¹⁸ These motifs appear across species, suggesting conserved computational primitives.

5.2 Oscillations & Brain Rhythms

Populations of neurons synchronize into oscillations—delta (0.5–4 Hz), theta (4–8 Hz), alpha (8–12 Hz), beta (13–30 Hz), and gamma (30–100 Hz) bands—observable in EEG and MEG. Theta rhythms coordinate hippocampal encoding during navigation; alpha rhythms gate visual attention; gamma bursts bind features into coherent percepts.¹⁹ Abnormal oscillations are linked to epilepsy (hyper‑synchronous discharges) and schizophrenia (reduced gamma power).

5.3 Large‑Scale Functional Networks

Resting‑state fMRI and diffusion tensor imaging reveal that distant brain regions synchronize into intrinsic networks:

Default Mode Network (DMN): medial prefrontal, posterior cingulate, and angular gyri—active during mind‑wandering and self‑referential thought.²⁰
Salience Network: anterior insula and dorsal anterior cingulate— detects behaviorally relevant stimuli and switches between DMN and executive networks.
Central Executive Network: dorsolateral prefrontal and parietal regions—maintains working memory and goal‑directed behavior.

Disruption of network connectivity is implicated in Alzheimer’s disease, major depression, ADHD, and chronic pain syndromes.

5.4 Neuroplasticity: Adapting Connections

Experience, learning, and injury reshape neural circuits through:

Synaptic plasticity: LTP/LTD adjusting connection strength.
Structural plasticity: dendritic spine growth or pruning, axonal sprouting.
Neurogenesis: birth of new neurons in adult hippocampus and olfactory bulb, supporting pattern separation and mood regulation.

Plasticity peaks during critical periods (e.g., language acquisition) but persists across life, enabling rehabilitation after stroke or sensory loss.²¹

6. How We Study Brain Structure & Connectivity

MRI: reveals anatomy with millimeter resolution; diffusion MRI traces white‑matter tracts (connectome).
fMRI: detects blood‑oxygen‑level‑dependent (BOLD) signals reflecting population activity.
EEG & MEG: capture millisecond electrical/magnetic fields, crucial for studying oscillations.
Optogenetics & Calcium Imaging: enable cell‑type‑specific control and visualization in animals.²²
Transcranial Magnetic Stimulation (TMS): non‑invasively perturbs cortical circuits, offering causal inference in humans.
Single‑cell & Spatial Transcriptomics: catalog molecularly defined cell types and their spatial arrangement.
Brain Organoids: stem‑cell‑derived 3‑D cultures recapitulate early cortical development and model genetic diseases.

7. Implications for Health & Disease

Neurological and psychiatric disorders often reflect circuit dysfunction: dopaminergic depletion in basal ganglia (Parkinson’s), hippocampal degeneration (Alzheimer’s), amygdala hyper‑reactivity (PTSD), or dysregulated prefrontal networks (ADHD). Demyelination causes multiple sclerosis; aberrant electrical discharges drive epilepsy. Advances in deep brain stimulation, neurofeedback, targeted pharmacology, gene editing, and brain‑computer interfaces aim to restore network balance or bypass damaged nodes.²³ Lifestyle factors—exercise, sleep, social engagement, and balanced nutrition—can bolster neuroplasticity and cognitive reserve, mitigating age‑related decline.

8. Conclusion

The human brain’s elegant architecture—layered cortex, memory‑crafting hippocampus, emotion‑gating amygdala, homeostatic hypothalamus, and more— works only because billions of neurons exchange rapid electrical spikes and versatile chemical signals, supported by equally vital glial cells. These elements self‑organize into networks whose rhythms and strengths shift as we learn, age, or heal. By studying anatomy hand‑in‑hand with physiology and emerging molecular tools, scientists inch closer to decoding consciousness and developing therapies for brain disorders. For students, clinicians, and curious readers alike, appreciating the dance between structure and connectivity offers a profound window into what makes us human.

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Disclaimer: This article is for educational purposes only and does not constitute medical advice. Readers with health concerns should consult licensed healthcare professionals.

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· Definitions and Perspectives on Intelligence

· Brain Anatomy and Function

· Types of Intelligence

· Theories of Intelligence

· Neuroplasticity and Lifelong Learning

· Cognitive Development Across the Lifespan

· Genetics and Environment in Intelligence

· Measuring Intelligence

· Brain Waves and States of Consciousness

· Cognitive Functions

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