<P> The excitatory neurotransmitters, the most common of which is glutamate, then migrate via diffusion to the dendritic spine of the postsynaptic neuron and bind a specific transmembrane receptor protein that triggers the depolarization of that cell . Depolarization, a deviation from a neuron's resting membrane potential towards its threshold potential, increases the likelihood of an action potential and normally occurs with the influx of positively charged sodium (Na) ions into the postsynaptic cell through ion channels activated by neurotransmitter binding . </P> <Dl> <Dd> There are two different kinds of synapses present within the human brain: chemical and electrical . Chemical synapses are by far the most prevalent and are the main player involved in excitatory synapses . Electrical synapses, the minority, allow direct, passive flow of electric current through special intercellular connections called gap junctions . These gap junctions allow for virtually instantaneous transmission of electrical signals through direct passive flow of ions between neurons (transmission can be bidirectional). The main goal of electrical synapses is to synchronize electrical activity among populations of neurons . The first electrical synapse was discovered in a crayfish nervous system . </Dd> </Dl> <Dd> There are two different kinds of synapses present within the human brain: chemical and electrical . Chemical synapses are by far the most prevalent and are the main player involved in excitatory synapses . Electrical synapses, the minority, allow direct, passive flow of electric current through special intercellular connections called gap junctions . These gap junctions allow for virtually instantaneous transmission of electrical signals through direct passive flow of ions between neurons (transmission can be bidirectional). The main goal of electrical synapses is to synchronize electrical activity among populations of neurons . The first electrical synapse was discovered in a crayfish nervous system . </Dd> <Dl> <Dd> Chemical synaptic transmission is the transfer of neurotransmitters or neuropeptides from a presynaptic axon to a postsynaptic dendrite . Unlike an electrical synapse, the chemical synapses are separated by a space called the synaptic cleft, typically measured between 15 and 25 nm . Transmission of an excitatory signal involves several steps outlined below . </Dd> </Dl>

​which of the following is primarily an excitatory neurotransmitter