I read in one of my textbooks that synapses can be both stimulatory and inhibitory but I don't really understand what this means or how synapses could be inhibitory. Don't synapses always cause depolarisation of the post-synaptic neurone and therefore an action potential? Is it the depolarisation itself that is abe to have an inhibitory effect?
Thank you for your help.

Hi Esmee,
From your question, I suppose that you know that excitatory synapses involve the release of a neurotransmitter that causes depolarisation of the post-synaptic membrane. If that depolarisation is sufficient to reach a threshold voltage of the post-synaptic membrane, then an action potential (often referred to as an 'electrical impulse') is generated in the post-synaptic neurone and thereby transmission has occured.
Some neurotransmitters can be inhibitory and they work by making it less likely for the post-synaptic membrane to reach the threshold voltage required for an action potential to be generated. This inhibitory action may involve hyperpolarising the post-synaptic membrane (i.e. changing the voltage in the direction away from threshold) or simply changing the membrane resistance so that it is less likely for threshold to be reached (a good way of thinking about this is to consider Ohm's Law: a change in voltage will equal the resistance multiplied by the current - a lower resistance will mean that more current is required to achieve the same voltage change).
GABA is an example of an inhibotory neurotransmitter.
I hope this helps.
Best wishes,
Andy.

Here is also a quite good, brief summary that backs up what Andy is saying - http://www.ncbi.nlm.nih.gov/books/NBK11117/

Note that this talks about postsynaptic effects. GABA is the major inhibitory major neurotransmitter in the brain, which it can do by activating GABAA channels (composed of a variety of subunits) on, I think, mostly post-synaptic membranes, but also at pre-synaptic and ‘extra-synaptic’ sites. There is another type of GABA receptor, the ‘metabotropic’ G protein coupled GABAB, which can also mediate pre- and post-synaptic inhibition.

Excitatory and inhibitory neurons can synapse on the same neuron - this is not uncommon. And there is a lot of input, e.g., from neuropeptides and other substances like a major inhibitory transmitter such as endocannabinoids (e.g., activating the cannabinoid CB1 receptor in the brain, one of the most abundant central G protein-coupled receptors), that modulate either excitatory or inhibitory synapses.

Quite often you have to look at synapses in the brain as part of a network, e.g., inhibitory input onto an excitatory neurone (e.g., GABA onto a glutamate neurone), excitatory input onto an inhibitory neurone, inhibitory input onto an inhibitory neurone, etc.

Last edited by Steve Lolait (8th Apr 2016 08:17:32)