Mirtazepine works by increasing the availability of both serotonin and noradrenalin in the brain.
You're right above that it keeps the serotonin and noradrenalin in the brain longer instead of being taken up by receptors. It's explained really well here
http://m.netdoctor.co.uk/depression/...es/zispin.html
Reference for which neurotransmitters this medication used is here
http://whatyoushouldknow.depression-...s/mirtazapine/