Which neurons receive a signal first? (information about the environment like heat or pain)

Dendrites. The first two neuronal functions, receiving and processing incoming information, generally take place in the dendrites and cell body.

by drinking easy

very easy

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products