What if you could see neurons in action?
The human nervous system is an incredible network that coordinates all our actions and sensory experiences. But how does it actually work? This interactive simulation lets you explore neurons, action potentials, and the divisions of the nervous system.
In this exploration, you'll discover:
- How neurons transmit information through action potentials
- The structure and function of different neuron types
- How the central and peripheral nervous systems work together
- The role of cranial nerves in controlling bodily functions
Start by selecting a tab below to begin your exploration!
Neuron Structure: The Building Block of the Nervous System
Click on any part of the neuron to learn more Tap neuron parts to learn more
Key Neuron Structures
Neurons are specialized cells that transmit information throughout the body. While neurons are the primary signaling units, they work alongside glial cells that actively support and modulate their function. Click on any part in the diagram or cards below to explore in detail!
Dendrites
Signal Reception
Soma (Cell Body)
Metabolic Center
Axon
Signal Transmission
Myelin Sheath
Insulation Layer
Axon Terminals
Neurotransmitter Release
Action Potential: How Neurons Communicate
Membrane Potential Simulation
The action potential is a rapid electrical signal that travels along a neuron's membrane. Adjust the stimulus strength to see how it affects the neuron's response:
The Action Potential Process
1. Resting Potential (-70mV)
Membrane at equilibrium with selective permeability:
- K⁺ leak channels (PK:PNa = 25:1 ratio)
- Na⁺/K⁺-ATPase pump (3 Na⁺ out, 2 K⁺ in)
- Impermeable intracellular anions (A⁻)
- Goldman-Hodgkin-Katz equation determines Em
2. Depolarization (Threshold: -55mV)
Voltage-gated Nav1.1-1.9 channels activate:
- Positive feedback loop: Na⁺ influx → further depolarization
- Peak at +30 to +40mV (ENa = +60mV)
- Duration: 0.5-2ms (temperature dependent)
- All-or-none (regenerative) response
3. Repolarization
Na⁺ inactivation and K⁺ channel opening:
- Fast Na⁺ inactivation (h-gates close, τ = 0.1-0.2ms)
- Delayed rectifier Kv channels (Kv1.1-1.8) activate
- K⁺ efflux restores electronegativity
- Membrane approaches EK (-90 to -100mV)
4. Afterhyperpolarization (AHP)
Prolonged K⁺ conductance creates undershoot:
- Delayed rectifier K⁺ channels remain open
- Ca²⁺-activated K⁺ channels (SK, BK) contribute
- Membrane reaches -75 to -90mV
- Duration: 2-5ms (varies by neuron type)
5. Recovery & Refractory Periods
Channel kinetics determine excitability:
- Absolute refractory: Na⁺ channels inactivated (1-2ms)
- Relative refractory: Higher threshold due to K⁺ conductance
- Na⁺/K⁺-ATPase restores gradients (metabolically expensive)
- Recovery from inactivation: τ = 1-10ms
Clinical Insights
Action potentials follow the "all-or-none" principle - once threshold (-55mV) is reached, the regenerative process is inevitable and stereotyped.
Medical relevance: Local anesthetics (lidocaine, procaine) block voltage-gated Na⁺ channels, preventing action potential propagation and eliminating pain sensation.
Neuron Types: Specialized for Different Functions
Functional Classification
Neurons are classified based on their function in the nervous system:
Sensory (Afferent) Neurons
Convert external stimuli into electrical signals for the brain. Found in sensory organs and the PNS.
Motor (Efferent) Neurons
Transmit signals from the CNS to muscles and glands to initiate actions and movements.
Interneurons
Connect other neurons within the CNS. They process and integrate information between sensory and motor neurons.
Structural Classification
Neurons can also be classified based on their structure:
Multipolar Neurons
Have one axon and multiple dendrites. Most common type in the CNS. Examples include motor neurons and interneurons.
Bipolar Neurons
Have one axon and one dendrite extending from opposite sides of the soma. Found in sensory pathways like the retina, inner ear, and olfactory system.
Unipolar Neurons
Have a single process that divides into two branches - one functions as a dendrite, the other as an axon. Common in sensory neurons of the PNS.
Function vs. Structure
While there's some correlation between a neuron's structure and function, they're not always directly related:
- Sensory neurons are typically unipolar or bipolar
- Motor neurons are generally multipolar
- Interneurons are almost always multipolar
Nervous System Organization
Central vs Peripheral Nervous System
Central Nervous System (CNS)
Brain: Control center for all body functions, containing ~86 billion neurons
Spinal Cord: Information highway between brain and body, 31 segments
- Protected by bone (skull & vertebrae)
- Surrounded by cerebrospinal fluid
- Blood-brain barrier protection
Peripheral Nervous System (PNS)
All neural tissue outside the CNS
- 12 pairs of cranial nerves (from brain)
- 31 pairs of spinal nerves (from spinal cord)
- Ganglia: Clusters of neuron cell bodies
- Sensory receptors throughout the body
Functional Divisions
Somatic Nervous System:
- Voluntary motor control
- Sensory information processing
Autonomic Nervous System:
- Sympathetic (fight-or-flight)
- Parasympathetic (rest-and-digest)
- Enteric (gut nervous system)
Cranial Nerves: Direct Brain Connections
🧠 Your Brain's Direct Hotlines
Think of cranial nerves as dedicated phone lines between your brain and specific body parts. Unlike spinal nerves that take a detour through the spinal cord, these 12 pairs connect directly to your brainstem - like having a direct line to the CEO!
👁️ Special Senses (I, II, VIII)
Smell, vision, hearing, balance - your connection to the world
👀 Eye Movement (III, IV, VI)
Precise eye control for tracking, reading, depth perception
🗣️ Face & Voice (V, VII, IX, X, XI, XII)
Expression, speech, chewing, swallowing, autonomics
Each nerve has a unique personality and job description. Some are pure sensory (like your smell detector), others are pure motor (like your tongue controller), and some are mixed (doing multiple jobs). Click on any cranial nerve below to explore detailed anatomy and clinical correlations!
I. Olfactory
Smell Sensation
II. Optic
Vision
III. Oculomotor
Eye Movement & Pupil
IV. Trochlear
Eye Rotation (Intorsion)
V. Trigeminal
Facial Sensation & Chewing
VI. Abducens
Lateral Eye Movement
VII. Facial
Facial Expression & Taste
VIII. Vestibulocochlear
Hearing & Balance
IX. Glossopharyngeal
Taste & Swallowing
X. Vagus
Parasympathetic Control
XI. Accessory (Spinal)
Neck & Shoulder Muscles
XII. Hypoglossal
Tongue Movement
Clinical Memory Aids
Names: "On Old Olympus' Towering Top, A Finn And German Viewed Some Hops"
Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal
Types: "Some Say Marry Money, But My Brother Says Big Brains Matter More"
S = Sensory, M = Motor, B = Both (Mixed)
🔬 Modern Neuroscience: Beyond the Classical View
Neuroscience has been transformed by discoveries in the last decade. While the fundamentals covered in this chapter remain essential, researchers have uncovered entirely new dimensions of how the brain works. Here's what the latest science reveals.
Glia: The Brain's Hidden Majority
Once dismissed as "glue," glial cells are now recognized as active participants in nearly every brain function
Astrocytes: The Third Synaptic Partner
The "tripartite synapse" model (Araque et al., 1999) established that astrocytes actively wrap around synapses and regulate neurotransmission. Recent 2025-26 studies show astrocytes:
- Release gliotransmitters (ATP, glutamate, D-serine) that modulate synaptic strength
- Control brain state transitions, arousal, attention, and even behavioral persistence (Freeman et al., 2025)
- Synthesize cholesterol needed for synapse formation and maintenance
- Clear extracellular glutamate via transporters, protecting neurons from excitotoxicity
🧈 Oligodendrocytes: Your Brain's Namesake
Oligodendrocytes do far more than insulate axons. Myelin plasticity, meaning experience-dependent changes to myelin, is now recognized as a fundamental learning mechanism:
- Motor learning triggers new myelin formation within days (McKenzie et al., 2014)
- Oligodendrocyte precursor cells (OPCs) remain active throughout life, responding to neuronal activity
- Dynamic myelination tunes conduction speed, synchronizing neural circuits
- Myelin dysfunction is linked to cognitive impairment beyond demyelinating diseases
Microglia
The brain's resident immune cells. They prune synapses during development and continuously surveil for damage. Recent work shows they also shape learning-related synaptic changes.
NG2 Glia (OPCs)
A distinct class of glia that maintain proliferative capacity into adulthood. They receive direct synaptic input from neurons, a remarkable discovery that blurs the line between "neuron" and "glia."
Satellite Glia
In the PNS, satellite glia transfer mitochondria to neurons via tunneling nanotubes. This 2026 discovery suggests that glia literally fuel neurons (Nature, Jan 2026).
Myelin Plasticity: Learning Changes Brain Wiring
Myelin is not just static insulation; it's a dynamic learning mechanism
Classical neuroscience taught that myelin simply speeds up signal transmission. We now know myelin actively adapts with experience:
How Myelin Adapts
- Activity-dependent myelination: Neuronal firing triggers OPC differentiation and new myelin formation
- Conduction tuning: Adjusting myelin thickness and internode length synchronizes signals across circuits
- Motor learning: Mice that learn new motor skills require new myelin; blocking myelination impairs learning
- Continues into adulthood: Myelination peaks in the 20s but adaptive changes persist throughout life
Why It Matters
- Explains why white matter changes are seen in learning disorders and cognitive decline
- Multiple sclerosis is not just a myelin disease; it's a learning and circuit timing disease
- Opens therapeutic avenues: Can we enhance myelination to support cognitive rehabilitation?
- White matter plasticity may be as important as synaptic plasticity for complex learning
Adult Neurogenesis: Your Brain Keeps Making Neurons
A long debate, now resolved: adult humans do grow new neurons
The Resolution
For decades, the prevailing view was that "you are born with all the neurons you'll ever have." This is now known to be incomplete.
Using single-cell transcriptomics, Dumitru et al. (2025, Science) identified proliferating neural progenitor cells in the adult human hippocampus, confirming that new neurons are born throughout life, even into old age. Additional 2025-26 studies show "SuperAgers" maintain higher rates of neurogenesis well into their 80s.
Where It Happens
Primarily in the dentate gyrus of the hippocampus. New neurons integrate into existing circuits and contribute to pattern separation in memory.
What Influences It
Exercise, enriched environments, and learning promote neurogenesis. Chronic stress, aging, and neuroinflammation suppress it. Sleep quality also modulates progenitor cell activity.
Why It Matters
Adult neurogenesis is a form of structural plasticity that contributes to memory formation, mood regulation, and cognitive resilience. Its decline is associated with depression and cognitive aging.
📋 How to Read This Chapter Now
The fundamentals you learned in the earlier tabs, neuron structure, action potentials, and myelin, remain correct and essential. What's changed is the context:
- Neurons don't work alone; glia are active partners in every process
- Myelin is not just insulation; it's a learning mechanism
- Your brain isn't fixed; new neurons can form throughout life
- The story of neural communication is richer and more collaborative than classical models implied