The head of a Roman snail, four tentacles well visible. [RN] Observing a snail crawling around, the view is immediately attracted by its head, orienting itself in its surroundings using the tentacles. The snail feels its environment, touches an obstacle and withdraws the tentacle.
Norepinephrine is a neurotransmitter secreted by neurons in the sympathetic nervous system. This chemical is secreted in response to stress, which triggers the 'fight or flight response.' The release of norepinephrine causes several changes that help our body to work more efficiently.
Another relative of norepinephrine and epinephrine is dopamine, discovered to be a neurotransmitter in the 1950s by another Swede, Arvid Carlsson. It is an inhibitory neurotransmitter, meaning that when it finds its way to its receptor sites, it blocks the tendency of that neuron to fire. Dopamine is strongly associated with reward mechanisms in the brain. Drugs like cocaine, opium, heroin, and alcohol increase the levels of dopamine, as does nicotine.