How are signals transmitted across the synaptic cleft?

When the action potential reaches the synaptic knob it causes voltage-gated calcium ion channels to open, allowing calcium ions to diffuse in.

These calcium ions cause synaptic vesicles containing acetylcholine to move to an fuse with the pre-synaptic membrane where they release the acetylcholine into the synaptic cleft by exocytosis.

The acetylcholine moves across the cleft by diffusion and binds to receptors on the post-synaptic membrane.

This causes sodium ion channels to open allowing sodium ions to diffuse into the post-synaptic neurone.

This creates an excitory post-synaptic potential (EPSP) and if this is above threshold potential, an action potential is created in the post-synaptic neurone.