Synaptic transmission

• Synapses connect neurons to form large circuits, there are 10^15 neurons
• They can undergo two types of changes:
•                    Short term - Used for signalling
•                    Long term - Used for learning and memory
  

Electrical synapses

• This is a type of synapse where the current flows from one neuron to another through a gap junction.
• The membranes are only 3.5 nanometers apart.
• They are comprised of two hemi-channels called connexons.
•            Connexons have six subunits called connexins.
•            A connexin has four membrane spanning domains.
• They allow for faithful transmission/high speed transmission and bidirectional signalling, but they have limited flexibility as they are faithful conveyors of signal.

• Otto Loewi showed the importance of chemical synapses by taking synaptic fluid from a frog vagua nerve and putting it into a recipient heart; the fluid caused the heart rate to slow.

The Nicotinic acetylcholine receptor

• It has 2 alpha, 1 beta and 1 gamma subunit.
• A twisting response upon acetylcholine binding opens the channel.

• A postsynaptic depolarisaion is called an excitatory postsynaptic potential (EPSP).
• In muscle, the same thing is called an end plate potential
• We know that the two major players are Na and K through studying the reversal potential.
•                    You can determine this by recording the post synaptic potential against a control                         membrane potential
•                    You start at -70mV and evoke a response and then increase the potential                                   stepwise; the point at which no ion flow occurs is the reversal potential.

• In inhibitory synapses, the result of stimulation is hyperpolarisation, which called an inhibitory postsynaptic potential (IPSP).
• There may also be an influx of Cl-.