Key Concepts of Section 22.3

• Synapses are the junctions between a presynaptic cell and a postsynaptic cell and are the site of communication between neurons (see Figure 22-3).

• Synapse formation is mediated by interactions between presynaptic axonal compartments and postsynaptic dendritic compartments. Cell-adhesion molecules keep the cells aligned. At the neuromuscular junction, motor neurons induce the accumulation of acetylcholine receptors in the postsynaptic muscle plasma membrane close to the forming axon terminus (see Figure 22-23).

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• In presynaptic cells, low-molecular-weight neurotransmitters (e.g., acetylcholine, dopamine, epinephrine) are imported from the cytosol into synaptic vesicles by H⁺-linked antiporters. V-class proton pumps maintain the low intravesicular pH that drives neurotransmitter import against a concentration gradient.

• Neurotransmitters (see Figure 22-25) are stored in hundreds to thousands of synaptic vesicles in the axon termini of the presynaptic cell (see Figure 22-23). When an action potential arrives there, voltage-sensitive Ca²⁺ channels open and the calcium causes synaptic vesicles to fuse with the plasma membrane, releasing neurotransmitter molecules into the synapse (see Figure 22-26, step 4).

• Neurotransmitters diffuse across the synapse and bind to receptors on the postsynaptic cell, which can be a neuron or a muscle cell. Chemical synapses of this sort are unidirectional (see Figure 22-3).

• Synaptic vesicles fuse with the plasma membrane using cellular machinery that is standard for exocytosis, including SNAREs and SM proteins. Synaptotagmin protein is the calcium sensor that detects the action potential–stimulated rise in calcium that leads to the fusion (see Figure 22-28). RIM and RIM-BP tether voltage-gated Ca²⁺ channels to the release machinery, ensuring fast coupling between action potentials and neurotransmitter release.

• Following neurotransmitter release from the presynaptic cell, vesicles are re-formed by endocytosis and recycled (see Figure 22-26, step 6).

• Dynamin, an endocytosis protein, is critical for the formation of new synaptic vesicles, specifically for the “pinching off” of inbound vesicles.

• Coordinated operation of four gated ion channels at the synapse of a motor neuron and a striated muscle cell leads to release of acetylcholine from the axon terminus, depolarization of the muscle membrane, generation of an action potential, and subsequent muscle contraction (see Figure 22-29).

• The nicotinic acetylcholine receptor, a ligand-gated cation channel, contains five subunits, each of which has a transmembrane α helix (M2) that lines the channel (see Figure 22-30).

• Neurotransmitter receptors fall into two classes: ligand-gated ion channels, which permit ion passage when open, and G protein–coupled receptors, which are linked to separate ion channels.

• A postsynaptic neuron generates an action potential only when the plasma membrane at the axon hillock is depolarized to the threshold potential by the summation of small depolarizations and hyperpolarizations caused by activation of multiple neuronal receptors (see Figure 22-31).

• Electrical synapses are direct gap junction connections between neurons and between glia. Electrical synapses, unlike chemical synapses that employ neurotransmitters, are extremely fast in signal transmission and are usually bidirectional.