Answer: Brownian motion determines how much a ligand will bind to a receptor.
Ligands in the synaptic space move around at random following the rules of Brownian motion. This randomness determines ligand binding, and is especially relevant when more than just an agonist is present at the synapse.
It is important to keep in mind that ligand binding is reversible - after a ligand binds to a receptor, it can also unbind. While the ligand is bound, the receptor is active.
Consider the case when only an agonist is in the synapse. For example, consider acetylcholine acting at the nicotinic acetylcholine receptor. Acetylcholine will randomly move about the synapse, binding to the receptor occasionally. It can also unbind as well. Increasing the concentration of acetylcholine in the synapse increases the chances that binding will occur since there are now more particles that it may interact with at random.
This model of molecular motion has implications for antagonists at the synapse, particularly for competitive antagonists.
Competitive antagonists act at the same site as the agonist. They inhibit agonist activation by physically filling in the space where the agonist would bind. The presence of a competitive antagonist will randomly inhibit the binding of the agonist. Increasing the agonist concentration can overcome the presence of the antagonist, as it will “outcompete” the antagonist for binding sites. Brownian motion determines the relationship between agonist activation of the receptor and antagonist-mediated inhibition of agonist action.