Explain nerve impulse transmission — resting potential to action potential

At rest, a neuron maintains a resting membrane potential of about -70 mV (inside negative relative to outside). This means the inside of the neuron is more negative than the outside.

How is this maintained?

• The sodium-potassium pump (Na⁺/K⁺ ATPase) actively pumps 3 Na⁺ out and 2 K⁺ in per cycle — using ATP. This creates electrochemical gradients: high Na⁺ outside, high K⁺ inside.
• The membrane is much more permeable to K⁺ than Na⁺ at rest. K⁺ leaks out along its concentration gradient, leaving negatively charged proteins behind — making inside negative.

The neuron is polarised at rest: outside positive, inside negative.

When a stimulus reaches the neuron:

1. Voltage-gated Na⁺ channels open — Na⁺ rushes into the cell (down its electrochemical gradient — both concentration and electrical gradients drive it inward)
2. The inside becomes less negative — this is depolarisation
3. If the membrane potential reaches the threshold (about -55 mV), a full action potential fires — this is the “all-or-none” response
4. Na⁺ channels fully open → Na⁺ floods in → membrane potential rises rapidly to +30 to +40 mV

The neuron is now depolarised: inside temporarily positive.

1. Na⁺ channels inactivate (close) after a brief period
2. Voltage-gated K⁺ channels open — K⁺ rushes out (down its concentration gradient)
3. Inside becomes negative again — repolarisation
4. K⁺ channels close slowly → membrane briefly overshoots to more negative than -70 mV — this is hyperpolarisation
5. The Na⁺/K⁺ pump gradually restores the original ion distribution

The action potential at one point creates local currents that depolarise the adjacent membrane, triggering another action potential there. This wave of depolarisation travels along the axon from the cell body to the axon terminal — this is the nerve impulse.

In myelinated neurons: The myelin sheath insulates the axon except at the nodes of Ranvier (gaps in myelin). The action potential “jumps” from node to node — called saltatory conduction. This is much faster (~120 m/s) than in unmyelinated neurons (~1–2 m/s).

Refractory period: Immediately after an action potential, the region cannot fire again (absolute refractory period — Na⁺ channels are inactivated). This ensures the impulse travels in ONE direction only — forward, never backward.

At the axon terminal, the action potential triggers:

1. Voltage-gated Ca²⁺ channels open → Ca²⁺ enters
2. Synaptic vesicles fuse with the presynaptic membrane
3. Neurotransmitters (e.g., acetylcholine) released into the synaptic cleft
4. Neurotransmitters bind to receptors on postsynaptic membrane → open ion channels → new potential in next neuron