Synaptic transmission

Synapse.jpg

Synaptic transmission occurs when an action potential reaches an axon terminal, depolarising the presynaptic membrane. Voltage-gated Ca²⁺ channels in the presynaptic membrane open in response to the depolarisation, allowing Ca²⁺ ions to enter the axon terminal down their concentration gradient. This causes vesicles containing neurotransmitter molecules (e.g.Acetylcholine molecules) to migrate towards the presynaptic membrane, and then fuse with the presynaptic membrane, releasing neurotransmitter molecules into the synaptic cleft by the process of exocytosis. The molecules of neurotransmitter diffuse across the synaptic cleft and bind to their receptors on the postsynaptic membrane. The binding of neurotransmitter molecules causes ligand-gated Na⁺ channels in the postsynaptic membrane to open. Na⁺ ions subsequetly rush into the postsynaptic cell, and because these channels are non-specifc, K⁺ ions can also leave the postsynpatic cell down their concentration gradient. This particular movement of ions generates a  graded_potential in the postsynaptic membrane. If that graded potential is suprathreshold when it arrives at the axon hillock then an action potential will fire in the postsynaptic neurone, if the graded potential is subthreshold when it arrives at the axon hillock then an action potential will not fire in the postsynaptic neurone.