Key Points

• Synaptic transmission occurs by electrical and chemical synapses; chemical synapses are the predominant type in the nervous system.

• The end-plate potential is due to the release of ACh from the presynaptic terminals of motor axons.

• ACh is removed from the synaptic cleft by diffusion and hydrolysis by AChE.

• ACh opens channels in the postsynaptic membrane that are equally permeable to Na+ and K+.

• Ca2+ is essential for the release of neurotrans-mitter; the presynaptic terminal contains voltage-dependent Ca2+ channels, and Ca2+ influx during an action potential in the presy-naptic terminal promotes the exocytosis of vesicles of transmitter.

• Transmitter release is quantized.

• Synaptic potentials can be either excitatory or inhibitory and are mediated by ionotropic and metabotropic receptors.

• Synaptic responses mediated by ionotropic receptors are generally fast because they involve the direct opening of a channel by the transmitter.

• Synaptic responses mediated by metabotropic receptors are generally slow because they can involve the activation of second messenger pathways.

• Integration of synaptic potentials occurs through temporal and spatial summation.

Essential Medical Physiology, Third Edition

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