Synaptic Transmission Is Usually Chemical but Can Be Electrical

• Electrical synapses transmit signals instantaneously
• Chemical synapses can modify and amplify signals

Synaptic Potentials Control Neuronal Excitability

• Synapses onto a spinal motor neuron exemplify functions of fast synaptic potentials
• Synapses excite or inhibit a neuron by depolarization or hyperpolarization at the site of impulse initiation

Fast Chemical Synaptic Actions Are Exemplified by the Vertebrate Neuromuscular Junction

• Chemical synapses work by releasing and responding to neurotransmitters
• Postsynaptic potentials result from permeability changes that are neurotransmitter-dependent and voltage-independent
• EPSPs between neurons resemble neuromuscular EPSPs but are smaller
• Fast IPSPs can result from an increase in permeability to chloride

Presynaptic Neurons Release Neurotransmitter Molecules in Quantal Packets

• Acetylcholine is synthesized and stored in the presynaptic terminal
• Neurotransmitter release requires voltage-dependent Ca²⁺ influx
• Neurotransmitter release is quantal and vesicular
• Synaptic vesicles are cycled at nerve terminals in distinct steps
• Several proteins play roles in vesicular release and recycling

Neurotransmitters Are of Two General Kinds

• Neurons have one or more characteristic neurotransmitters
• An agent is identified as a neurotransmitter if it meets several criteria
• Vertebrate neurotransmitters have several general modes of action
• Neurotransmitter systems have been conserved in evolution

Postsynaptic Receptors for Fast Ionotropic Actions: Ligand-Gated Channels

• ACh receptors are ligand-gated channels that function as ionotropic receptors
• Many, but not all, ligand-gated channel receptors have evolved from a common ancestor

Postsynaptic Receptors for Slow, Metabotropic Actions: G Protein–Coupled Receptors

• G protein–coupled receptors initiate signal transduction cascades
• Metabotropic receptors act via second messengers
• Other mechanisms of G protein–mediated activity
• G protein–coupled receptors mediate permeability-decrease synaptic potentials and presynaptic inhibition

Synaptic Plasticity: Synapses Change Properties with Time and Activity

• Neurotransmitter metabolism is regulated homeostatically
• Learning and memory may be based on synaptic plasticity
• Habituation and sensitization in Aplysia
• Long-term potentiation in the hippocampus
• BOX 13.1 Synapse Formation: Competing Philosophies, Matthew S. Kayser
• Long-term potentiation is a necessary component of learning

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