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.
a. Two basic types of epinephrine receptors -- called alpha and beta adrenergic receptors (adrenergic = for adrenaline). The two types are distinguished (primarily) by their relative affinities for epinephrine (adrenaline) and norepinephrine (noradrenaline).n example -- effects of epinephrine (adrenaline) on smooth muscle. Some smooth muscles relax, and some contract in response to epinephrine. In this case, different receptors & 2nd messengers are involved.