An ionotropic receptor is a type of receptor that directly controls the flow of ions across a cell membrane. When a specific molecule, known as a neurotransmitter, binds to the receptor, it causes a change in the receptor's shape. This change opens an ion channel, allowing ions such as sodium, potassium, or calcium to enter or exit the cell, leading to rapid changes in the cell's electrical potential. These receptors play a crucial role in fast synaptic transmission in the nervous system. Examples of ionotropic receptors include the Nicotinic acetylcholine receptor and GABA_A receptor. Their quick response times make them essential for processes like muscle contraction and reflex actions.