What are small molecules that control the opening or closing of ion channels called?

Small molecules that control the opening or closing of ion channels are indeed known as neurotransmitters. These are chemical messengers that transmit signals across synapses between neurons or from neurons to other cells. When a neurotransmitter binds to specific receptors on the surface of a target cell, it can cause changes in the cell’s membrane potential, influencing whether ion channels open or close. This is crucial for processes such as muscle contraction, action potential propagation in nerves, and a variety of other physiological functions.

The other options provided serve different roles. Proteins are structural and functional components of cells, but they do not function specifically as small molecules initiating the opening or closing of ion channels like neurotransmitters do. Enzymes are proteins that catalyze biochemical reactions but do not typically influence ion channel activity directly. Hormones, while they can affect ion channels, do so mainly through more complex signaling pathways and are not typically categorized as small molecules that directly control ion channels in the same manner as neurotransmitters.