A synapse is a “connection” or junction between neurons, there are two types of synapse:

  • Electrical synapses use gap junctions

  • Chemical synapses use neurotransmitters

The Synapses Doctrine: Synapses are the basis for memory and learning.

Excitatory and Inhibitory

Synapses can be excitatory or inhibitory. Excitatory synapses tend to increase postsynaptic membrane potential whereas inhibitory synapses tend to decrease postsynaptic membrane potential.

An excitatory synapse works like this

Input spike –> Neurotransmitter release (e.g. Glutamate) –> Binds to ion channel receptors –> Ion channel open –> \(\text{Na}^{+}\) influx –> Depolarization due to EPSP (excitatory postsynaptic potential)

And inhibitory synapse works like this

Input spike –> Neurotransmitter release (e.g. GABA) –> Binds to receptors –> Ion channel open –> \(\text{K}^{+}\) leaves cell –> Hyperpolarization due to IPSP (inhibitory postsynaptic potential)

Synaptic Plasticity

Hebbian Plasticity:

If neuron A repeatedly takes part in firing neuron B, then the synapse from A to B is strengthened.

  • Long Term Potentation (LSP): Experimentally observed increase in synaptic strength that lasts for hours or days.
  • Long Term Depression (LSD): Experimentally observed decrease in synaptic strength that lasts for hours or days.

Synaptic Plasticity depends on spike timing. LTP/LTD depends on relative timing of input and output spikes:

  • LTP: Input spike before output spike
  • LTD: Input spike after output spike