← Biology / the living world

THE BRAIN & NERVOUS SYSTEM / AN INTERACTIVE FIELD GUIDE

A world inside
your head.

A thought, a touch, a movement. Each involves cells communicating across a living network. Get to know its structures, follow its signals, and experiment with the rules that make it work.

LEARN BY EXPLORING

Rotate.
Trace.
Experiment.

10 structures · 4 playgrounds
From the whole body to a single neuron
01 / ANATOMY EXPLORER

A living landscape.

CEREBRUM · CEREBELLUM · BRAINSTEM3D STUDY MODEL
RUpFront

Preparing the brain model…

Original, stylized geometry. Folds, borders, deep-structure positions, and relative sizes are approximate; this is not a scan-derived anatomical atlas. The cutaway hides the left cerebral hemisphere to reveal internal structures.

MAKE IT STICK

Can you find it?

Use the model and region buttons to answer five short anatomy challenges.

02 / THE BIGGER SYSTEM

One network.
Central and peripheral.

The brain belongs to a system that extends through the body. Distinguish where a structure is from the direction a signal travels.

THE WHOLE ARCHITECTURE

One brain. Two metres of wiring.

SKELETON + NERVOUS SYSTEM3D STUDY MODEL

Building the body model…

Central nervous system
Cranial nerves
Trunk & autonomic
Plexuses
Nerves of the arm
Nerves of the leg

Original, stylized geometry built from anatomical proportions. Bone shapes, nerve routes and branch points are schematic teaching approximations, not scan-derived anatomy. Nerves are drawn thicker than scale so they stay visible inside the skeleton.

CNS

Brain + spinal cord

The central nervous system integrates information and organizes responses. The spinal cord contains processing circuits of its own, including circuits involved in reflexes.

Find both in the 3D model ↑
PNS

Connections through the body

The peripheral nervous system includes nerves and ganglia outside the brain and spinal cord. Sensory pathways bring information inward; motor pathways carry output to effectors such as muscles and glands.

See the connections below ↓
SENSORY / AFFERENTBody → central nervous system

Information about the outside world and the body’s internal state.

MOTOR / EFFERENTCentral nervous system → body

Output that influences muscles, glands, and other effectors.

Somatic and autonomic

Somatic motor pathways control skeletal muscle, including in reflexes. Autonomic pathways regulate functions such as heart activity, digestion, and gland secretion. Sympathetic and parasympathetic divisions coordinate organ-specific responses; they are not simply universal on/off switches.

Gray matter and white matter

Gray matter contains many cell bodies, dendrites, and synapses. White matter contains many myelinated axons that connect regions. Both contain glial cells. The corpus callosum is a large white-matter connection between the cerebral hemispheres.

Study further: NICHD’s guide to the nervous system and the Society for Neuroscience brain map.

03 / THE SIGNAL PLAYGROUND

Follow a signal.

Choose a touch, a voluntary movement, or a withdrawal reflex. Step through the route and watch where the central and peripheral systems meet.

A simplified route for conscious touch from a hand. This omits several relays and the crossing of many ascending fibers.

CNS / brain + cordPNS / connecting nerves
Step 1 of 5 · symbolic timing

The order highlights a route through a network. Real activity overlaps in time, includes feedback, and follows more branches than this diagram shows.

The reflex example draws on Purves and colleagues’ explanation of spinal circuitry.

04 / THE NEURON PLAYGROUND

Small cells.
Electrical conversations.

Neurons receive inputs, integrate them, and send signals. Select a cell part, then change the inputs below to see when this simplified neuron fires.

inputintegrationpropagation →transmission

Axon. Action potentials propagate along an axon. Voltage-gated channels regenerate the signal; a stronger input does not simply make each spike taller.

EXPERIMENT / WHEN DOES IT FIRE?

Add input. Find the threshold.

Change excitatory and inhibitory current during a 200 ms input window. This simplified neuron leaks toward rest, fires when it reaches threshold, resets, and briefly becomes refractory.

-70-5030Membrane voltage (mV)0100200300 ms
Net drive2.5 nA
Spikes in 200 ms6
Window firing rate30 Hz
What does this model include—and leave out?

We use a leaky integrate-and-fire model: τ dV/dt = −(V − Vrest) + RI. Here τ = 20 ms, R = 10 MΩ, Vrest = −70 mV, threshold = −50 mV, reset = −65 mV, and refractory time = 3 ms. These are illustrative parameters. Each drawn spike is a marker at +30 mV; the model does not simulate sodium/potassium channel dynamics, spike shape, synaptic conductances, or a complete biological cell.

In biological action potentials, ion-channel changes produce a rapid rise and fall in voltage. The sodium–potassium pump helps maintain ion gradients over time; it is not the mechanism that directly produces each spike’s rapid falling phase.

EXPERIMENT / HOW LONG DOES THE SIGNAL TAKE?

Change the journey, not the spike height.

Compare two chosen conduction speeds along the same axon distance. Myelin affects propagation; it does not change the firing threshold in this separate teaching calculation.

Travel time = distance ÷ speed10.0 ms

Real conduction depends on axon diameter, myelination, temperature, and other properties. These chosen speeds illustrate the relationship; the drawing is not to scale and the travel time excludes synapses and other processing.

For the biological basis of conduction, read how myelination supports faster signal propagation.

05 / CONNECT THE IDEAS

More than the sum
of its regions.

The colored areas make anatomy easier to learn. Real behavior emerges from cells, pathways, feedback, and experience working together.

01

Cells support other cells

Glia help maintain the neural environment. Astrocytes have metabolic and signaling roles; microglia participate in immune surveillance. Oligodendrocytes make myelin in the CNS, while Schwann cells make it in the PNS.

02

Synapses connect the conversation

Most synapses communicate chemically through transmitter release and receptors. Electrical synapses use gap junctions to couple cells directly. A synapse’s effect depends on its properties and the state of the receiving cell.

03

Experience can change circuits

Plasticity includes changes in synaptic strength, structure, and cellular excitability. Learning can involve several mechanisms over different timescales. A useful explanation asks what changed, in which circuit, and what evidence shows it.

Pause and explain it to yourself.

Does one colored region do one job?

No. Regions contain different cell populations and participate in multiple networks. The lobe descriptions are starting points for exploring contributions, not a one-to-one map of functions.

Is a reflex outside the central nervous system?

A spinal reflex uses the spinal cord, which is part of the CNS. It can organize a response without waiting for a conscious decision, and descending pathways from the brain can modify it.

Does a stronger input make an action potential taller?

In the model above, every spike has the same drawing height. Stronger sustained input can increase the number of spikes per unit time. Biological action potentials are regenerative events; their amplitude is not a simple proportional measure of input strength.

Are people either “left-brained” or “right-brained”?

Some functions show lateralization, but the hemispheres communicate and cooperate. Dividing people into a logical left-brain type and a creative right-brain type does not describe how cognition works.

KEEP EXPLORING / SOURCES

Go deeper into the science.

The anatomy and explanations follow these educational references. The 3D geometry, pathway sequences, and numerical experiments are simplified teaching models.