Ananya Krishnan scored 168/180 in Biology in NEET 2025 (AIR 340). Before she used the neuron simulator, Neural Control and Coordination was her weakest Biology chapter — she was getting just 2 out of 5 questions right in mock tests. Here's how she used the NeetLab neuron propagation simulator to transform it into a near-certain 5/5.
"The action potential graph in NCERT has depolarisation going up and repolarisation coming down. I could copy that graph perfectly. But when a question asked 'during which phase can a second stimulus produce another action potential,' I had no idea — I couldn't connect the graph to what was physically happening inside the neuron."
— Ananya Krishnan, NEET 2025, AIR 340What NEET Actually Tests in Neural Control and Coordination
This chapter (Class 11, Chapter 21) contributes 3-5 questions per NEET paper. The question types break down as:
- Action potential stages: Which ion moves at which phase? What is the membrane potential at rest vs depolarisation vs repolarisation?
- Refractory period: Absolute vs relative refractory period — when can a second stimulus trigger another AP?
- Synapse mechanics: Which neurotransmitter, which receptor type, how is transmission terminated?
- Reflex arc components: The 5-component order and what happens if one is damaged
- Receptor types: Nociceptors, photoreceptors, mechanoreceptors — their structure and function
The Action Potential: What the Simulator Makes Obvious
The resting membrane potential of a neuron is approximately -70 mV (inside negative relative to outside). This is maintained by the sodium-potassium pump (3 Na⁺ out, 2 K⁺ in per cycle) and the differential permeability of the membrane to K⁺.
When a stimulus exceeds the threshold, voltage-gated Na⁺ channels open. The simulator animates this:
Phase 1 — Depolarisation
Na⁺ rushes in (high external concentration + positive charge drawn to negative interior). The membrane potential shoots from -70 mV to approximately +40 mV. The simulator shows Na⁺ ions as red dots flooding inward, the voltage graph rising steeply.
Phase 2 — Repolarisation
At peak (+40 mV), Na⁺ channels close. K⁺ channels open. K⁺ rushes out (high internal concentration + now-positive interior repels positive K⁺). The membrane potential falls back toward -70 mV. You see K⁺ ions (shown in blue) streaming outward.
Phase 3 — Hyperpolarisation (After-hyperpolarisation)
K⁺ channels are slow to close — the potential briefly dips below -70 mV (to approximately -80 mV). This is the absolute refractory period (during depolarisation + early repolarisation — no second AP possible) and relative refractory period (during hyperpolarisation — a second AP is possible but only with a stronger-than-normal stimulus).
The question Ananya couldn't answer before the simulator: During absolute refractory period, Na⁺ channels are inactivated (not just closed — they cannot be opened again by any stimulus). During relative refractory period, K⁺ channels are still open, making the threshold higher. The simulator shows the channel states at each phase — you literally see which channels are open, closed, or inactivated.
Saltatory Conduction — The Speed Question
In myelinated neurons, the myelin sheath (formed by Schwann cells in PNS, oligodendrocytes in CNS) insulates the axon. The action potential "jumps" between nodes of Ranvier — this is saltatory conduction, and it is significantly faster than conduction in unmyelinated fibres.
The simulator shows a myelinated and unmyelinated fibre side by side. You watch the AP propagate as a wave — hopping from node to node in the myelinated fibre, crawling continuously along the unmyelinated one. The speed difference is visually obvious. NEET tests why myelinated fibres are faster and what multiple sclerosis (demyelination) does to signal speed.
NEET NUMBERS
Myelinated fibre conduction velocity: 70-120 m/s. Unmyelinated: 0.5-2 m/s. Resting potential: -70 mV. Threshold potential: ’55 mV. Peak action potential: +40 mV. These numbers appear directly in NEET questions.
Synaptic Transmission: The Process NEET Tests Most Carefully
Synaptic transmission occurs at the synaptic cleft between the pre-synaptic membrane and post-synaptic membrane. The sequence:
- AP arrives at axon terminal → voltage-gated Ca²⁺ channels open → Ca²⁺ enters
- Ca²⁺ triggers fusion of synaptic vesicles with pre-synaptic membrane
- Neurotransmitter (e.g., acetylcholine) released into synaptic cleft by exocytosis
- NT binds to receptors on post-synaptic membrane → ion channels open → EPSP or IPSP generated
- NT is removed: enzymatic degradation (acetylcholinesterase breaks down ACh) or reuptake
Which of the following ions triggers the fusion of synaptic vesicles with the pre-synaptic membrane?
(A) Na⁺ (B) K⁺ (C) Ca²⁺ (D) Cl⁻
Answer: (C) Ca²⁺ — calcium ions trigger vesicle fusion and neurotransmitter release.
The Reflex Arc — 5 Components in Order
NEET tests the reflex arc components in the correct sequence and asks which component is damaged given a described symptom:
- Receptor → detects stimulus
- Afferent neuron → carries impulse to spinal cord (sensory neuron)
- Nerve centre → spinal cord (for spinal reflex) or brain
- Efferent neuron → carries impulse to effector (motor neuron)
- Effector → muscle or gland that responds
"After the simulation I stopped thinking of the action potential as a graph and started thinking of it as a wave of ion movements. That shift made the refractory period question obvious — during depolarisation, Na⁺ channels are busy. During hyperpolarisation, K⁺ channels are still open. It's physically impossible to fire again. I'd never thought about it that way before."
— Ananya Krishnan, AIR 340, NEET 2025Try the Neuron Propagation Simulator Free
Watch the action potential animate in real time, see ion channels open and close, and compare myelinated vs unmyelinated conduction speed. Then answer the built-in NEET MCQs.
Open Neuron Simulator →Quick Revision Checklist
- Can you name the ion responsible for each phase of the action potential?
- Do you know the membrane potential at rest, threshold, peak, and after-hyperpolarisation?
- Can you distinguish absolute and relative refractory periods and explain them in terms of ion channel states?
- Do you know what triggers neurotransmitter release at a synapse?
- Can you name the 5 components of a reflex arc in order?
- Do you know why myelinated fibres conduct faster than unmyelinated ones?