H

A

C

B

G

D

C

I

B

The accumulation of positive potassium ions increases the voltage of the synaptic terminal sufficiently to trigger the opening of voltage-gated calcium channels.

The accumulation of positive sodium ions increases the voltage of the synaptic terminal sufficiently to trigger the opening of voltage-gated calcium channels.

Sodium ions binding to intracellular chemically-gated calcium channels.

Calcium ions binding to extracellular chemically-gated calcium channels

an influx of Ca ions into the synaptic terminal from the axon

an influx of Ca ions into the synaptic terminal from the extracellular space

an influx of K+ ions into the synaptic terminal from the axon

an influx of Na+ ions into the synaptic terminal from the axon

1 and 3

3 and 4

1, 2 and 3

1 and 4

2 and 4

Voltage-gated Na⁺ channels

Chemically-gated K⁺ channels

Voltage-gated K⁺ channels

Chemically-gated Na⁺ channels

Leaky sodium channels allow sodium to diffuse into the cell.

The interior of the neuron is at -70mV relative to the outside.

The sodium-potassium pump forces 3 sodium ions outside of the cell and 2 potassium ions inside of the cell, against each ion’s concentration gradient.

There are large negatively charged proteins inside the cytoplasm.

There is a concentration imbalance of potassium with more present outside the cell.