"If a neurotransmitter that binds with a metabotropic receptor causes an escalating sequence of events, then a(n) has occurred."

Amplification cascade

Explanation:

A neurotransmitter is a biomolecule that allows neurotransmission, that is, the transmission of information from a neuron (a type of cell in the nervous system) to another neuron, a muscle cell or a gland, through the synapse that separates them. The neurotransmitter is released from the synaptic vesicles at the end of the presynaptic neuron, towards the synapse, crosses the synaptic space and acts on the specific cell receptors of the target cell. In biology the term receptors refers to the proteins or glycoproteins that allow the interaction of certain substances with the mechanisms of cell metabolism. They are present in the plasma membrane, in the organelle membranes, in the cellular cytosol or in the cellular nucleus, to which other chemical substances such as hormones and neurotransmitters, specifically bind.

The binding of a signaling molecule to its specific receptors triggers a series of reactions inside the cells (signal transduction), whose final result depends not only on the stimulus received, but also on many other factors, such as the cell stage, the presence of pathogens, the metabolic state of the cell, etc. Usually changes in the receptor trigger changes in the permeability  of the membrane or a cascade of activation of a series of intracellular signaling molecules. A biochemical cascade, also known as a signaling cascade or signaling pathway, is a series of chemical reactions that are initiated by a stimulus (first messenger) acting on a receptor that is translated into the cell through second messengers (which amplify the initial signal) and ultimately to the effector molecules, resulting in a cellular response to the initial stimulus. At each step of the signal cascade, several control factors intervene to regulate cellular actions, responding effectively to signals about their changing internal and external environments. The "amplification cascade" means that, at each step, the process becomes larger and larger.

So, in many signal transduction processes, an increasing number of enzymes, proteins and substances are involved in the event. This is from the beginning of the stimulus, which starts from the adhesion of a ligand to the membrane receptor. Then to the activation at the receptor, which converts the stimulus into a response, and which, within the cell, causes the chain of steps (signaling cascade or second messenger route) whose result is the amplification of the signal as it was explained. This means, a small stimulus causes a large cellular response.

i think the answer is composed of atoms