The nerve impulse is typically generated at the axon initial segment (AIS) located at the junction between the axon and cell body. This region is often called axon hillock. Once the nerve impulse is generated, it will travel to other regions within the neuron, but not to other neurons because there is a gap between two neurons. This gap is known as the synapse (Figure 2-1). There are two types of synapses: chemical and electrical. We will focus on chemical synapses.
Figure 2-1. Signal transmission through synapses.
In most cases, a chemical synapse is formed between an axon terminal and a dendritic spine (a small protrusion from the dendrite). An axon may have one or more terminals. A dendrite typically contains many spines. Therefore, a single neuron can make thousands of connections with other neurons.
Figure 2-2. Spines on the dendrite of a neuron.
Signal transmission through the synapse is mediated by a class of small molecules called neurotransmitters. At the axon terminal, neurotransmitters are stored in vesicles. When the nerve impulse arrives, membrane depolarization will open voltage-gated calcium channels for the entry of Ca2+ ions, which then induce the release of neurotransmitters from the vesicles. Subsequently, the neurotransmitters diffuse through the synaptic cleft (about 200 - 500 Å wide) to bind with their receptors in the dendritic spine of a postsynaptic neuron (Figure 2-3).
Figure 2-3. Synaptic transmission is mediated by neurotransmitters.
Glutamate is the most important neurotransmitter for memory. Two of its receptors, NMDA receptor (NMDAR) and AMPA receptor (AMPAR) play crucial roles in learning and memory. They belong to ligand-gated ion channels whose opening probability depends on the binding of specific molecules called ligands (e.g., neurotransmitters). Both NMDAR and AMPAR conduct cations (Na+ and Ca2+). Therefore, when these channels open, the cationic influx will make the membrane potential more depolarized, which has excitatory effect on neuronal firing (i.e, generation of nerve impulse). The neurotransmitter-induced depolarization is called excitatory postsynaptic potential (EPSP). Acetylcholine is another neurotransmitter that can induce EPSP by binding with its receptor on the postsynaptic membrane.
In contrast to EPSP, some neurotransmitters (such as GABA) can induce inhibitory postsynaptic potential (IPSP), because their receptors conduct mainly chloride ions (Cl-). The entry of negatively charged Cl- ions into the cytoplasm makes the membrane potential more hyperpolarized, which has inhibitory effect on neuronal firing.
Figure 2-4. Neuronal firing by summation of EPSPs and IPSPs at the axon hillock.
In the postsynaptic neuron, the membrane potential changes at spines will spread to other regions. Since a neuron contains many spines and each spine may generate EPSP or IPSP, the membrane potential at AIS (axon hillock) is determined by the summation of these EPSPs and IPSPs when they spread to AIS. If the membrane potential at AIS is depolarized above the threshold, the neuron will fire. To facilitate the generation of nerve impulses, AIS contains high density of voltage-gated Na+ and K+ channels.
Author: Frank Lee