CorpusHuman Anatomy

Neuron & Action Potential

Myelinated lower motor neuron, schematic (not to scale)

A multipolar lower motor neuron drawn from left to right: branched dendrites and the cell body (soma) with its nucleus and Nissl bodies, the Nissl-free axon hillock and initial segment (trigger zone), then a myelinated axon whose internodes are each formed by one Schwann cell, separated by nodes of Ranvier, ending in axon terminals on a skeletal muscle fibre. Internodes are really about 100 times longer than the axon is wide, so the drawing is schematic.

The animation runs as one loop. EPSPs travel along the dendrites to the soma, the trigger zone reaches threshold and fires, and the action potential jumps from node to node (saltatory conduction) until the terminals release acetylcholine onto the motor end plate. The graph shows the membrane potential at the trigger zone, and its moving dot keeps time with the impulse. The inset shows the ion channels behind each phase: voltage-gated Na⁺ channels with activation and inactivation gates, delayed voltage-gated K⁺ channels and the Na⁺/K⁺-ATPase.

Step-by-step

  1. 1 · Resting membrane potential

    At rest the inside of the axolemma sits at ≈ −70 mV relative to the extracellular fluid. K⁺ leak channels let K⁺ diffuse out down its steep gradient (≈ 140 mM inside vs ≈ 4 mM outside), leaving negatively charged proteins and phosphates behind. Resting K⁺ permeability is much higher than Na⁺ permeability, so the RMP lies close to the K⁺ equilibrium potential (≈ −90 mV). The Na⁺/K⁺-ATPase (3 Na⁺ out : 2 K⁺ in) maintains the gradients.

  2. 2 · Graded potentials reach threshold

    Excitatory synapses on the dendrites and soma produce EPSPs, which spread passively and fade with distance. They summate in space and time at the trigger zone (axon hillock and initial segment), where voltage-gated Na⁺ channels are densest. IPSPs pull the membrane potential away from threshold. If the summed depolarisation reaches threshold (≈ −55 mV), an action potential fires.

  3. 3 · Depolarisation — Na⁺ rushes in

    Voltage-gated Na⁺ channels open their activation gates. Na⁺ influx depolarises the membrane, which opens more Na⁺ channels (positive feedback). The membrane potential overshoots 0 mV and peaks at ≈ +30 mV within ≈ 0.5 ms, approaching the Na⁺ equilibrium potential (≈ +60 mV).

  4. 4 · Repolarisation — K⁺ flows out

    About 1 ms after opening, the Na⁺ channel inactivation gates swing shut. Meanwhile the slower voltage-gated K⁺ channels open, and K⁺ efflux returns the membrane potential toward rest. Inactivated Na⁺ channels cannot reopen, so the membrane is absolutely refractory.

  5. 5 · Hyperpolarisation & refractory periods

    The K⁺ channels close slowly, so the membrane potential undershoots to ≈ −80 to −90 mV before returning to −70 mV. Absolute refractory period: no second spike is possible because the Na⁺ channels are open or inactivated. Relative refractory period: a stronger-than-normal stimulus is needed. Refractoriness limits firing frequency and keeps propagation one-way.

  6. 6 · Saltatory conduction

    Local currents spread rapidly and passively through the axoplasm under the insulating myelin to the next node of Ranvier, where clustered Na⁺ channels regenerate the action potential. The impulse therefore jumps from node to node. Large myelinated Aα fibres conduct at ≈ 70–120 m/s, compared with ≈ 0.5–2 m/s in unmyelinated C fibres, and use less energy because fewer ions cross the membrane. Each internode is one Schwann cell (PNS) or one oligodendrocyte process (CNS).

  7. 7 · Transmitter release at the terminals

    At the axon terminals, depolarisation opens voltage-gated Ca²⁺ channels. Ca²⁺ influx triggers exocytosis of acetylcholine at the neuromuscular junction. ACh binds nicotinic receptors on the motor end plate and produces an end-plate potential. The end-plate potential always reaches threshold and fires a muscle action potential, so the fibre contracts. One motor neuron together with all the muscle fibres it supplies forms a motor unit.

Labelled parts