t 0.0 msSpeed - m/sSyn. delay - msStimulus to B - msSpikes A/B/C 0/0/0FPS 0

neuron A (stimulated) B (target) C (receiver) D (2nd input) transmitter. Glow = membrane potential. Click soma A or press Space to fire. Educational model, not medical advice. Glow is local and smooth: no full-screen flashing (at most 1 pulse per 2 s when auto-fire is on).

Neural Signal Relay: From Dendrite to Synapse

Follow one nerve impulse from a dendritic input to the next cell and measure its speed and delay.

Educational model, not medical advice and not a diagnostic tool. Channel sliders are abstract parameters.

Stimulus

Threshold: press the button (it bisects the dendritic conductance that makes the axon hillock fire).

Axon

Synapses (weights)

Time

Measurements

Oscilloscope V(t)

Potential along the axon now

Missions

Threshold: a sub-threshold input fails and one just above threshold fires (use Find threshold).
Myelin: measure the speed with myelin on and off (more than 2 times faster).
Delay: record the synaptic delay at two temperatures 8 C apart.
Refractory: find an interval where a second input is lost and one where it fires again.
AND gate: with weights at 0.6 neither A nor D alone fires B, both together do.

Independent numerical checks

Controls

Click or tap soma A, or press Space: a synaptic conductance opens on the stimulated dendrite. Drag orbits, wheel or pinch zooms, view buttons fly the camera.
Keys: Space fire A, B burst, D fire neuron D, M myelin, P pause, R reset, [ and ] speed.
Stimulus site: 0 % is at the soma, 100 % is 500 um out on the dendrite. Strength is a lumped conductance (many contacts).
Smart time slows the clock while the impulse is travelling and fast-forwards the quiet phases.

The science

Membrane (Hodgkin-Huxley 1952). C_m dV/dt = I - g_Na m³h (V-E_Na) - g_K n⁴ (V-E_K) - g_L (V-E_L), gating dx/dt = φ [α_x(1-x) - β_x x], classic squid values (C_m = 1 uF/cm², g_Na = 120, g_K = 36, g_L = 0.3 mS/cm², E_Na = 50, E_K = -77, E_L = -54.4 mV) and a temperature factor φ = 3^{(T - 6.3)/10}. Above about 32 C these squid parameters stop producing propagating spikes on a bare axon (a known limit of the classic parameter set), so the temperature slider ends there.

Cable. The neuron is a chain of compartments: C dV/dt = ∑ G_ij(V_j - V_i) - I_ion + I_syn, axial conductance G = π a² / (R_i L) (R_i = 35.4 ohm cm). Sub-threshold voltage decays as V(x) = V₀ e^-x/λ, λ = √(a R_m / 2R_i) (376 um for the 2 um dendrite). Solved with backward Euler in V (stable for the stiff axial term), exponential (Rush-Larsen) gate updates, tridiagonal sweep; the textbook point neuron is also integrated with RK4 for the checks.

Myelin. Active channels sit only at the nodes of Ranvier (10x the squid density, same g ratios so the rest potential stays -65 mV). Each node carries the capacitance and leak of its internode divided by the number of myelin lamellae (25 per um of diameter, constant g-ratio). Then speed scales with diameter (about 6 m/s per um in the literature; here roughly 5-6 m/s per um), while a bare axon scales with the square root of the diameter. Both laws are verified numerically below.

Spike initiation. The axon initial segment has a high Na channel density, so the hillock reaches threshold first; a dendritic input only decays toward it (attenuation by e^-x/λ). Inputs are a dual-exponential conductance g_s(t) = g_max(e^-t/τd - e^-t/τr)/norm, τr = 0.5 ms, τd = 3 ms, E_s = 0 mV.

Synapse. The presynaptic spike amplitude sets the calcium signal, P_rel = Ca⁴ / (Ca⁴ + K⁴) (K = 10 uM, Ca = 20 uM x amplitude²), 10 release sites give a binomial number of quanta, and the delay is a calcium/fusion lag (Q10 = 2) plus 0.2 ms of diffusion, about 0.9 ms here (0.5 to 1 ms is typical for warm mammalian synapses; this one is at 24 C). Cells B and C are leaky integrate-and-fire units (tau 10 ms, threshold -50 mV, 2 ms refractory); a negative D weight is an inhibitory input (E = -80 mV).

Honest simplifications. One-dimensional cable, fixed geometry, lumped synaptic input, no calcium-activated K channels, no channel noise, no ion concentration changes, deterministic except for the seeded release. The 3D scene is not to scale: the 5 mm axon is drawn as a path of about 17 scene units, and the foreground axon is decorative. All parameters are model values, check a physiology text before quoting numbers. Models were built with Blender and rendered with Three.js.

Photosensitivity: the glow follows the simulated voltage locally and smoothly; the background sparkle twinkles slower than 1 Hz and auto-fire is limited to one input per 2 s.

Educational model, not medical advice.