Neurotransmitters are the chemical substances that act to communicate across synaptic cleft. These neurotransmitters helps in transferring of information from one neuron to the next neuron.
The neurotransmitters are classified into three broad classes based in the chemical composition:
1. Small-molecule transmitters
2. Peptide hormones
3. Transmitter gases
The two classes of receptors through which the neurotransmitters act are,
1). Ionotropic receptors (allow the passage of ions)
2). Metabotropic receptors (allow the binding of neurotransmitters)
Thus, the blanks can be filled with small-molecule transmitters, peptide hormones, transmitter gases; ionotropic receptors, metabotropic receptors.