Summary

Chemical synapses between neurons in the nervous system are similar to the synapse at the neuromuscular junction in the following ways:

• Neurotransmitter molecules are stored in the synaptic terminal in membrane-bound synaptic vesicles.

• Influx of external calcium ions into the terminal triggers release of neurotransmitter.

• Synaptic vesicles fuse with the plasma membrane of the terminal to release their neurotransmitter content.

• Neurotransmitter molecules combine with specific postsynaptic receptors' molecules and open ion channels in the postsynaptic membrane.

Nervous system synapses differ from the neuromuscular junction in the following ways:

• At most synapses, a single presynaptic action potential produces only a small change in postsynaptic membrane potential. By contrast, a single presynaptic action potential at the neuromuscular junction produces a large depolarization of the muscle cell and triggers a postsynaptic action potential.

• Synapses between neurons can be either excitatory or inhibitory.

• Acetylcholine is the neurotransmitter at the neuromuscular junction, but many different neurotransmitter substances (including ACh) are released at synapses in the nervous system.

• A skeletal muscle cell receives synaptic input from only one neuron a single motor neuron. A neuron in the nervous system may receive synaptic connections from thousands of different neurons. The output of a neuron depends on the integration of all the inhibitory and excitatory inputs active at a given instant.

• At the neuromuscular junction, ACh directly opens channels by combining with postsynaptic binding sites that are part of the channel protein. In other parts of the nervous system, a neurotransmitter may directly bind to an ion channel or may indirectly affect ion channels via an internal second messenger in the postsynaptic cell.

• Synapses in the central nervous system usually undergo short-term or long-term changes in synaptic strength, and this synaptic plasticity plays a role in the complex information-processing capacity of the brain.

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