# Neural circuit ## Microsim (three.js) <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Neural_circuit.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin"></iframe> </div> *Part of the Wikitube neuroscience collection, this microsim opens up the smallest wiring motif in the brain: one excitatory input that both drives a principal cell and, a synapse later, recruits an interneuron to silence it. It links the biophysics of the Action potential to the circuit logic behind the spinal Reflex and fast Motor control.* > Feedforward inhibition is a circuit in which a single source of excitation reaches a target neuron by two routes: a direct excitatory path and an indirect path through an inhibitory interneuron. Because the inhibitory detour costs one extra synapse, inhibition lands a millisecond or two after the excitation, leaving a brief "window of opportunity" during which the target can fire. In this microsim you drive the input, set how strong the excitation and inhibition are, change how fast the interneuron's axon conducts, and watch whether the principal neuron reaches threshold before inhibition shuts the window. Turning feedforward inhibition off lets you see the same input produce sloppy, poorly-timed firing. ## About this microsim The microsim runs a live two-cell circuit that you steer with five sliders. **Input rate (Hz)** (0.3–6) sets how often the afferent fires; **Excitatory weight (mV)** (2–20) and **Inhibitory weight (mV)** (0–25) set the amplitudes of the EPSP and IPSP delivered to the principal neuron M; **IN axon speed (u/s)** (1–8) sets how fast the interneuron's spike travels, which fixes the inhibitory delay; and **M spike threshold (mV)** (−60 to −35) sets the voltage at which M fires. A **Feedforward Inhibition: ON** toggle removes the inhibitory arm for comparison, the **Pause** and **Step +50 ms** buttons advance the clock by hand, and **Reset** restores the defaults. ## Related microsims - Action potential — each M output is an all-or-none spike; this sim decides when one is allowed to occur. - Reflex — spinal reflex arcs use the same disynaptic inhibition to coordinate opposing muscles. - Motor control — principal neurons like M relay commands whose timing inhibition sharpens. - Neuromodulation — neuromodulators retune the excitation–inhibition balance you set here by hand. - Neural decoding — narrow integration windows improve the temporal code that downstream decoders read. ## Links (Wikipedia order) <!-- injected from _registry/childlinks/Neural_circuit.json (2026-07-30T02:09:12Z) --> `Affective_neuroscience` · `Arithmetic` · `Artificial_neuron` · `Astrocyte` · `Auditory_cortex` · `Autonomic_nervous_system` · `Axon` · `Axon_hillock` · `Basal_ganglia` · `Basic_research` · `Behavioral_epigenetics` · `Behavioral_neurology` · `Behavioral_neuroscience` · `Behavioural_genetics` · `Brain` · `Brain-reading` · `Brain_mapping` · `Brainstem` · `Brain–computer_interface` · `Cellular_neuroscience` · `Central_nervous_system` · `Central_pattern_generator` · `Cerebellum` · `Cerebrum` · `Chemical_synapse` · `Chronobiology` · `Clinical_neurophysiology` · `Clinical_neuroscience` · `Cognitive_neuropsychology` · `Cognitive_neuroscience` · `Computational_neuroscience` · `Concentration_of_measure` · `Connectionism` · [[Connectomics]] · `Consumer_neuroscience` · `Cranial_nerves` · `Cultural_neuroscience` · `Dendrite` · `Depolarization` · [[Detection_theory]] · `Development_of_the_nervous_system` · `Diencephalon` · `Educational_neuroscience` · `Electrical_synapse` · `Electroencephalography` · `Electrophysiology` · `Enteric_nervous_system` · `Evolutionary_neuroscience` · `Excitatory_postsynaptic_potential` · [[Feedback]] · `Forebrain` · `Functional_magnetic_resonance_imaging` · `Functional_neuroimaging` · `Global_neurosurgery` · `Hebbian_theory` · `Herbert_Spencer` · `Hindbrain` · `Hippocampus` · `History_of_neuroscience` · `Hypothalamus` · `Imaging_genetics` · [[Implementation]] · `Integrative_neuroscience` · `Intraoperative_neurophysiological_monitoring` · `Limbic_lobe` · `Limbic_system` · `List_of_regions_in_the_human_brain` · [[Logic]] · `Long-term_depression` · `Long-term_potentiation` · `Medulla_oblongata` · `Memory` · `Meninges` · `Midbrain` · `Mirror_neuron` · `Molecular_cellular_cognition` · `Molecular_neuroscience` · `Motor_control` · `Motor_nerve` · `Motor_neuron` · `Multipolar_neuron` · `Muscles_of_respiration` · `Nerve_tract` · [[Nervous_system]] · [[Network_science]] · `Neural_basis_of_self` · `Neural_coding` · `Neural_decoding` · `Neural_engineering` · `Neural_network_(biology)` · [[Neural_network_(machine_learning)]] · `Neural_oscillation` · `Neural_pathway` · `Neuro-oncology` · `Neuro-ophthalmology` · `Neuroanatomy` · `Neuroanthropology` · `Neurobranding` · `Neurocardiology` · `Neurochemistry` · `Neurochip` · `Neurocinema` · `Neurocriminology` · `Neurodegenerative_disease` · `Neurodevelopmental_disorder` · `Neurodiversity` · `Neuroeconomics` · `Neuroendocrinology` · `Neuroepidemiology` · `Neuroepistemology` · `Neuroesthetics` · `Neuroethics` · `Neuroethology` · `Neurogenesis` · `Neurogenetics` · `Neurohacking` · `Neurohistory` · `Neuroimaging` · `Neuroimmune_system` · `Neuroimmunology` · `Neuroinformatics` · `Neurointensive_care` · `Neurolaw` · `Neuroleadership` · `Neurolinguistics` · `Neurology` · `Neuromanagement` · `Neuromarketing` · `Neurometrics` · `Neuromodulation` · `Neuromorphology` · `Neuromuscular_junction` · `Neuron` · `Neuropathology` · `Neuropharmacology` · `Neurophenomenology` · `Neurophilosophy` · `Neurophysics` · `Neurophysiology` · `Neuroplasticity` · `Neuropolitics` · `Neuroprosthetics` · `Neuropsychiatry` · `Neuropsychology` · `Neuroradiology` · `Neurorobotics` · [[Neuroscience]] · `Neuroscience_of_music` · `Neuroscience_of_religion` · `Neuroscientist` · `Neurosexism` · `Neurosurgery` · `Neurotechnology` · `Neurotology` · `Neurotoxin` · `Neurotransmission` · `Neurovirology` · `Nutritional_neuroscience` · `Optic_nerve` · `Outline_of_neuroscience` · `Paleoneurobiology` · `Parasympathetic_nervous_system` · `Parkinson's_disease` · `Perceptron` · `Peripheral_nervous_system` · `Phosphorylation` · `Pons` · `Psychiatry` · `Psychology` · `Pulse-coupled_networks` · `Reflex` · `Reflex_arc` · `Respiratory_center` · `Retina` · `Santiago_Ramón_y_Cajal` · `Scholarpedia` · `Sensory_nerve` · `Sensory_neuroscience` · `Sigmund_Freud` · `Social_cognitive_neuroscience` · `Social_neuroscience` · `Somatic_nervous_system` · `Spike-timing-dependent_plasticity` · `Spinal_cord` · `Spinal_nerve` · `Stanford_University` · `Striatum` · `Substantia_nigra` · `Summation_(neurophysiology)` · `Symbol` · `Sympathetic_nervous_system` · `Synapse` · `Synaptic_plasticity` · `Synaptogenesis` · [[Systems_neuroscience]] · `Urination` · `Voltage-gated_sodium_channel` · `Walter_Pitts` · `Warren_Sturgis_McCulloch` · [[Wayback_Machine]] · `William_James` ## Overview A neural circuit is a set of neurons connected by synapses to perform a specific function. Feedforward inhibition is one of the most common and best-studied circuit motifs in the nervous system, appearing in the hippocampus, thalamocortical pathways, cerebellum, and sensory relays. In it, an afferent excitatory input diverges: it makes a direct excitatory synapse onto a principal (projection) neuron and simultaneously excites a GABAergic interneuron that inhibits the *same* principal neuron. The consequence is that every excitatory volley is chased, one synapse later, by inhibition. This disynaptic delay narrows the window over which the principal cell integrates its inputs, so the cell responds only to inputs that are strong and well-timed — enforcing temporal precision, controlling gain, and protecting the circuit from runaway excitation. ## The mechanism The motif has two pathways from the same input: | Pathway | Synapses | Sign | Arrival | |---|---|---|---| | Input → M | 1 (monosynaptic) | excitatory (EPSP) | early | | Input → IN → M | 2 (disynaptic) | inhibitory (IPSP) | delayed by Δt | The integration window is roughly $\Delta t = t_{\text{syn}} + \ell / v$, where $\ell/v$ is the interneuron's axon length divided by its conduction speed — the microsim's *IN axon speed*. Faster conduction shortens Δt and tightens the window. Within that window the membrane potential $V_m$ climbs toward threshold; if a large enough **excitatory weight** carries it across before the IPSP arrives, M spikes. A large **inhibitory weight** closes the window sooner and more forcefully, suppressing all but the earliest, strongest inputs. ## Controls -> what each maps to | Control | Maps to | Range / values | Meaning | |---|---|---|---| | Input rate (Hz) | Firing frequency of the afferent input | 0.3–6 Hz | How often the circuit is driven | | Excitatory weight (mV) | EPSP amplitude onto principal neuron M | 2–20 mV | Strength of the direct excitatory synapse | | Inhibitory weight (mV) | IPSP amplitude onto M from interneuron IN | 0–25 mV | Strength of feedforward inhibition | | IN axon speed (u/s) | Conduction velocity of the interneuron axon | 1–8 u/s | Sets the excitation→inhibition delay Δt | | M spike threshold (mV) | Voltage at which M fires | −60 to −35 mV | Excitability of the principal neuron | | Pause | Freeze the simulation clock | button | Hold the current state | | Step +50 ms | Advance the clock 50 ms | button | Move forward frame by frame | | Feedforward Inhibition | Enable/disable the IN→M arm | ON / OFF | Compare with vs. without inhibition | | Reset | Restore default parameters | button | Return to starting conditions | ## Learning objective Understand how the extra synaptic delay in a feedforward inhibitory pathway creates a brief integration window that governs whether — and how precisely — a principal neuron fires. ## Limits and connections This is a deliberately minimal two-cell reduction: EPSPs and IPSPs are fixed-amplitude events, and it ignores dendritic filtering, short-term plasticity, and the feedback inhibition and disinhibition that real interneuron networks add. Its core lesson still generalizes — that a synapse's *timing*, not just its sign, determines its effect, across cortex, hippocampus, and spinal circuits. ## Poster & source <div class="microsim-fallback"> <!-- poster image pending backfill --> <p><em>Live microsim · <a href="https://wikitube-3d-microsims.netlify.app/Neural_circuit.html">open full</a> · source: Microsims for Dissemination/GENOMICS_ThreeJS_Microsims/Neural_circuit.html</em></p> </div> <!-- CRAFT-LINK:START g12 --> *Built to the [[WT!Three_js_Microsim_Master_Class|three.js Master Class]].* <!-- CRAFT-LINK:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Neural_circuit) : [Wikitube](https://en.wikitube.io/wiki/Neural_circuit) ## Previous hub tags Tree parent: [[Self-organization]]. Legacy hubs: `GENOMICS`. --- *Sources: 1 legacy note. Minted wave 1, 2026-07-30 (v1.6 order).*