Synaptic Transmission

A neuron is composed of three parts: dendrites, axon and nerve terminals. The dendrites receive signals from other neurons, the axon conducts action potentials, and the nerve terminals transmit signals to other neurons. At the dendrite, the action potential can be elicited if the membrane potential is depolarized to a threshold. The action potential then travels along the axon to the nerve terminal, where depolarization may open calcium channels for the entry of Ca²⁺ ions, which then induce the release of neurotransmitters stored in the vesicles. In a neural network, the nerve terminal of a neuron may form a synapse with either the dendrite or nerve terminal of another neuron. The synaptic cleft between two neurons is about 200 - 500 Å wide. Neurotransmitters released from the vesicle may diffuse through the synaptic cleft to act on their receptors in the postsynaptic neuron (Figure 11).

Figure 11. The mechanism of synaptic transmission. "T" represents the neurotransmitters which are stored in vesicles (represented by circles) at the presynaptic nerve terminal. The action potential at the presynaptic terminal causes the entry of Ca²⁺ ions through voltage-activated calcium channels, leading to the release of neurotransmitters.

Figure 12. Chemical structures of major neurotransmitters.

The receptors for neurotransmitters can be classified into two categories: G-protein-coupled receptors and ionotropic receptors. G-protein-coupled receptors are involved in signal transduction. Like other agonists, binding of neurotransmitters on G-protein-coupled receptors may trigger a series of signaling processes (more info). The ionotropic receptors form an ion channel which may be activated upon binding of specific neurotransmitters. Once activated, the influx of cations (e.g., Na⁺) may cause the postsynaptic membrane to depolarize. If the depolarization reaches the threshold, an action potential can be generated on the postsynaptic neuron.

The ion channel formed by neurotransmitter receptors is called the synaptic channel. Each synaptic channel consists of five receptors (subunits). Figure 13 shows the general structure of a nicotinic acetylcholine receptor (nAChR) channel.

Figure 13.  Schematic drawing of the nAChR channel which consists of five subunits: a, a, b, g,and d.  (a) Side view.  (b) Top view.  (c) The domain structure for each subunit.  Five M2 segments (one from each subunit) form the channel pore as shown in (b).  Activation (opening) of an nAChR channel requires binding of two acetylcholine molecules, one on each a subunit.

The glutamate receptor (GluR) channel plays a critical role in the Hebbian type of learning. It may be divided into three subtypes: NMDA, kainate and AMPA. All glutamate receptors bind efficiently to the glutamate molecule, but their binding affinity for other agonists varies. The NMDA subtype has a high binding affinity for the NMDA molecule, but low affinity for the kainate or AMPA molecule. Therefore, a channel formed by NMDA receptors can be activated by NMDA or glutamate, but not by kainate or AMPA. Similarly, the channel formed by AMPA receptors can be activated by AMPA or glutamate, but not by NMDA or kainate.