The nervous system is the body’s information network. It detects what is happening inside and outside the organism, transforms that information into electrical and chemical signals, integrates the signals across vast distributed circuits, and produces output — movement, perception, thought, emotion, autonomic regulation. The system that does all this weighs about 1.4 kilograms in the average adult, consumes about 20 percent of the body’s energy at rest despite being only 2 percent of its mass, and is the most metabolically demanding tissue in the body per gram. It is also the only organ system whose loss is irrecoverable: bones heal, livers regenerate, kidneys can be replaced, but a destroyed neuron is gone.

This page is the orientation map for the neuroanatomy section. It places the major divisions of the nervous system in relation to one another, explains the organizational logic that recurs throughout the system, and points to the dedicated pages that cover each piece in detail. The point is to give the reader a mental skeleton onto which the rest of the neuroanatomy curriculum can hang.

The Central and Peripheral Divisions

The nervous system divides into two parts based on where the cell bodies sit:

Central nervous system (CNS): the brain and the spinal cord. Encased in bone (skull and vertebral column), wrapped in meninges, bathed in cerebrospinal fluid, and supplied by a vasculature with a tight blood-brain barrier. Composed of neurons, glia, and intricate networks of fibers.
Peripheral nervous system (PNS): everything else — the cranial and spinal nerves, the ganglia (clusters of nerve cell bodies outside the CNS), and the autonomic ganglia. The PNS connects the CNS to muscles, skin, viscera, and sense organs.

The boundary between them is the point at which an axon leaves the brainstem or spinal cord. Once a fiber crosses that boundary, it is myelinated by Schwann cells (PNS), not oligodendrocytes (CNS), and it occupies a different immunologic environment. The two divisions react differently to injury: CNS axons regenerate poorly because of the inhibitory environment created by myelin and astroglia, while PNS axons regenerate, sometimes substantially.

The Functional Divisions: Somatic and Autonomic

Another organizing division concerns what the nervous system controls:

Somatic nervous system: voluntary motor control of skeletal muscle and conscious perception of the body and the outside world. Includes the corticospinal tract, the lower motor neurons, and the sensory pathways from skin, joints, and special senses.
Autonomic nervous system: involuntary control of viscera, blood vessels, glands, smooth muscle. Divides further into sympathetic (thoracolumbar outflow), parasympathetic (craniosacral outflow), and enteric (the intrinsic nervous system of the gut).

The two systems are anatomically intermingled but functionally distinct. Most peripheral nerves carry both somatic and autonomic fibers; most central pathways integrate both. The autonomic nervous system is covered in detail on its own page.

The Major Subdivisions of the CNS

The CNS is conventionally divided from rostral to caudal:

Telencephalon: the cerebral hemispheres, including the cerebral cortex, the underlying white matter, the basal ganglia, and the limbic structures.
Diencephalon: the thalamus, hypothalamus, epithalamus (pineal gland), and subthalamus.
Mesencephalon (midbrain): the rostral brainstem, housing CN III and IV nuclei, the substantia nigra, the red nucleus, and the cerebral peduncles.
Metencephalon: the pons (housing CN V, VI, VII, VIII nuclei) and the cerebellum.
Myelencephalon (medulla): the caudal brainstem, housing CN IX, X, XI, XII nuclei and the cardiovascular and respiratory centers.
Spinal cord: the caudal-most division, exiting the cranium through the foramen magnum.

Together, the midbrain, pons, and medulla constitute the brainstem — the most evolutionarily conserved portion of the CNS, carrying functions essential to life (breathing, cardiovascular control, arousal). The diencephalon and telencephalon together are sometimes called the forebrain; the metencephalon and myelencephalon together are the hindbrain; the mesencephalon alone is the midbrain.

Gray Matter and White Matter

Two visible tissues compose the CNS:

Gray matter: regions where neuronal cell bodies, dendrites, and synapses are concentrated. The cerebral cortex (a sheet of gray matter on the surface of the hemispheres), the deep gray nuclei (basal ganglia, thalamus), the cerebellar cortex, and the central core of the spinal cord. Appears gray in fresh tissue.
White matter: regions composed of myelinated axons connecting gray matter areas. The corona radiata, internal capsule, corpus callosum, association fibers, and spinal cord columns. Appears white because of the lipid content of myelin sheaths.

The relative position of gray and white matter differs between brain and cord. In the cerebrum and cerebellum, gray matter is on the surface (cortex) with white matter beneath; in the spinal cord, gray matter is central (forming the H-shaped central core) with white matter on the outside.

Symmetry, Asymmetry, and Lateralization

The nervous system is approximately bilaterally symmetric — left and right hemispheres, left and right brainstem halves, left and right limbs of the cord. But the symmetry breaks down at the functional level: most cortical functions are lateralized. Language is dominant in the left hemisphere in about 95% of right-handers and 70% of left-handers. Spatial attention favors the right hemisphere. Emotional processing has lateralized components. The right and left thalami have slightly different connection patterns. The left and right amygdalae participate differently in emotion. The functional asymmetry coexists with anatomical near-symmetry, and recognizing both is essential to clinical neurology.

The Recurring Organizational Principles

Several principles recur throughout the nervous system, and recognizing them helps make sense of any new structure:

Topographic mapping: nearby points in the periphery (skin, retina, cochlea) map to nearby points in the relevant cortex. The body’s surface is laid out on the postcentral gyrus (sensory homunculus), the visual field on the calcarine cortex (retinotopy), the cochlea on Heschl’s gyrus (tonotopy). The maps are distorted — high-resolution areas (hand, fovea, voice frequencies) take disproportionate cortical territory.
Decussation: most pathways cross the midline at some point in their course, so that the right brain controls the left body and vice versa. The corticospinal tract decussates at the pyramidal decussation in the medulla. The dorsal column–medial lemniscus pathway crosses at the medulla. The spinothalamic tract crosses at the cord level of entry. The cerebellar outflow crosses in the brainstem and then back again at lower levels. The crossing locations and conventions are clinically essential.
Convergent and divergent connections: a single neuron receives input from many others (convergence) and sends output to many others (divergence). The result is a highly distributed, redundant network in which most functions are not localized to a single point.
Excitation and inhibition: every signal in the nervous system is processed by a balance of excitatory and inhibitory inputs. Glutamate is the primary excitatory neurotransmitter; GABA is the primary inhibitory. The balance determines whether a neuron fires.
Neural circuits as loops: most brain regions participate in recurrent loops with other regions. Basal ganglia–thalamus–cortex loops control voluntary movement and cognition. Cerebellum–thalamus–cortex loops coordinate movement and procedural learning. Hippocampus–cortex loops underlie memory consolidation. The loop architecture is the substrate of feedback and modulation.
Hierarchy: processing flows from primary cortical areas (handling raw sensory input or basic motor output) through association areas (integrating multiple modalities and abstract concepts) to higher-order cortices (subserving language, executive function, social cognition). Damage at different levels produces different patterns of deficit.
Modularity with integration: specific functions can often be localized to specific regions (Broca’s area for speech production, hippocampus for episodic memory), but the function as a whole always depends on the integration of multiple regions. A pure modular deficit is rare; partial network deficits are the rule.

The Cellular Building Blocks

The nervous system is built from two principal cell types:

Neurons: the signaling cells. About 86 billion in the human brain (about 16 billion in the cerebral cortex, the rest spread across the rest of the CNS and PNS). Each neuron has a cell body (soma), dendrites (receiving inputs), and an axon (sending output). Neurons communicate at synapses, where neurotransmitter release crosses a gap of about 20 nanometers to bind receptors on the postsynaptic cell.
Glia: support cells. About as numerous as neurons in the human brain, though the ratio varies by region. Five main types: astrocytes (structural and metabolic support, blood-brain barrier), oligodendrocytes (CNS myelin), microglia (immune surveillance), ependymal cells (lining the ventricles and central canal), Schwann cells (PNS myelin and support).

Each of these cell types has its own anatomy, its own diseases, and its own clinical relevance — covered in detail on the cellular neuroanatomy pages.

Energy and the Vascular Supply

The nervous system has no significant energy stores. It cannot extract energy from anything other than glucose and small amounts of ketone bodies. It consumes oxygen at roughly ten times the rate of resting muscle. Any interruption of blood flow produces neurological dysfunction within seconds and irreversible injury within minutes. The system that supplies this critical organ is described in detail on the cerebrovascular anatomy pages, but a few facts are foundational:

Two pairs of arteries (the carotids and the vertebrals) supply the entire CNS.
The Circle of Willis at the base of the brain provides anastomotic connections between these arteries.
The blood-brain barrier — a specialized vascular endothelium — restricts what crosses from blood to brain.
Venous drainage occurs through a system of dural venous sinuses to the internal jugular veins.
The arterial territories are clinically essential — stroke syndromes are named for the territory they affect.

The Coverings and the Fluid System

The CNS is bathed in cerebrospinal fluid (CSF) and wrapped in three layers of meninges (dura, arachnoid, pia). CSF is produced by the choroid plexus inside the ventricles, circulates through a series of communicating cavities, exits into the subarachnoid space surrounding the brain and cord, and is absorbed back into the venous system through arachnoid granulations. Approximately 150 mL of CSF is present at any time; about 500 mL is produced per day, so the volume turns over about three times daily.

The meninges, the ventricular system, and the CSF dynamics are covered in detail on the Coverings page.

Clinical Relevance of the Overview

The clinical neurologist uses this overview every day:

A patient with hemiparesis and contralateral facial weakness localizes to one hemisphere or the contralateral brainstem — the decussation of pathways tells you which.
A patient with bilateral leg weakness, a sensory level, and bladder dysfunction localizes to the spinal cord, not the brain — the topographic organization tells you why.
A patient with sudden-onset aphasia and right hemiparesis from a middle cerebral artery stroke localizes to the left frontal lobe — the vascular territory tells you the artery.
A patient with rapidly progressive dementia, ataxia, and myoclonus localizes broadly throughout the cortex, basal ganglia, and cerebellum — the multifocal pattern points to prion disease, paraneoplastic syndromes, or autoimmune encephalitis.

Each of these inferences is a small application of the principles above, projected onto the actual anatomy. The rest of the neuroanatomy curriculum exists to make those projections faster and more accurate.

🔍 Did You Know?

The human cerebral cortex contains an estimated 16 billion neurons with approximately 10¹⁴ (one hundred trillion) synapses. If each synapse were a person, the total would exceed the human population of Earth by a factor of more than 10,000. The cortex is folded into gyri and sulci because the surface area required to accommodate all those neurons exceeds what could fit smoothly inside the skull — only about a third of the cortical surface is visible from the outside; the rest is buried in the depths of the sulci.

How to Use the Neuroanatomy Pages

The pages that follow are organized to be readable in order from beginning to end as a curriculum, or as standalone reference pages on individual topics. Most include cross-references to relevant pages elsewhere on the wiki — to the examination pages for clinical correlates, to other neuroanatomy pages for shared structures, to disease pages for pathology. A few suggestions:

For trainees: read in order. Start with this overview, then the cellular pages, then work through cerebral hemispheres, deep structures, brainstem, spinal cord, and finally the functional systems and vascular anatomy.
For clinical reference: jump to the relevant page for the structure or pathway you need.
For exam preparation: the vascular territories, the brainstem cross-sections, the major tracts, and the cranial nerve nuclei are the most commonly tested. Focus there.

References

Standring S, ed. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier; 2021.
Kandel ER, Schwartz JH, Jessell TM, et al, eds. Principles of Neural Science. 5th ed. McGraw-Hill; 2013.
Mtui E, Gruener G, Dockery P. FitzGerald’s Clinical Neuroanatomy and Neuroscience. 8th ed. Elsevier; 2020.
Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. 5th ed. Elsevier; 2018.
Brodal P. The Central Nervous System: Structure and Function. 5th ed. Oxford University Press; 2016.
Herculano-Houzel S. The remarkable, yet not extraordinary, human brain as a scaled-up primate brain. Proc Natl Acad Sci USA. 2012;109(Suppl 1):10661-10668.