An ionotropic receptor is a type of receptor that directly controls the flow of ions across a cell membrane. When a specific neurotransmitter binds to this receptor, it causes a conformational change that opens an ion channel. This allows 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, which are involved in processes like muscle contraction and inhibitory signaling, respectively.