Chapter 7: Mechanisms of Synaptic Transmission
Which of the following best describes a basic property of synapses in the central nervous system (CNS)?
A. Synaptic vesicles constitute important features for transmission in both chemical and electrical synapses.
B. A postsynaptic neuron typically receives input from different presynaptic axons that are either excitatory or inhibitory, but it cannot receive inputs from both types.
C. Synaptic delay is approximately the same for both chemical and electrical synapses.
D. Neurotransmitter (NT) receptors can regulate the gating/opening of an ion channel either directly (ionotropic receptor) or indirectly (metabotropic receptor)
E. The mechanism of indirect gating of ions normally does not involve the activation of G proteins.
D. Electrical synapses do not involve neurotransmitter and thus do not require synaptic vesicles. A typical postsynaptic neuron receives thousands of inputs, with synaptic inputs from excitatory, inhibitory, and modulatory neurons. Because there is no neurotransmitter release or receptors, synaptic delay is much shorter at electrical synapses. Ionotropic receptors are neurotransmitter gated ion channels, whereas metabotropic receptors can regulate ion channels indirectly. Indirect gating of ion channels involves G proteins, which can interact with ion channels, or G proteins that produce second messengers, which control protein kinases that phosphorylate and regulate ions channels.
Which of the following events directly determines the release of NTs from the terminal of the presynaptic neuron?
A. Activation of voltage-gated Na+ channels and Na+ influx
B. Activation of voltage-gated Na+ channels and Na+ efflux
C. Activation of voltage-gated K+ channels and K+ influx
D. Activation of voltage-gated K+ channels and K+ efflux
E. Activation of voltage-gated Ca2+ channels and Ca2+ influx
E. Although voltage-gated Na+ and K+ channels are involved in the generation and conduction of the action potential (with Na+ influx and K+ efflux), which is necessary to depolarize the presynaptic membrane potential, it is the influx of Ca2+ through the activation of voltage-gated Ca2+ channels that directly activates the fusion of synaptic vesicles and neurotransmitter release by the presynaptic axon.
The level and duration of NT release are determined predominantly by which of the following?
A. The magnitude and duration of the presynaptic action potential
B. The inactivation of the presynaptic voltage-gated Na+, K+, and Ca2+ channels
C. The frequency and pattern of presynaptic action potentials
D. The rate of recycling and refilling of synaptic vesicles
E. The extent of synaptic delay
B. The action potential has a fairly consistent (stereotypical) magnitude and duration, which does not change. Voltage-gated Na+ channels inactivate, but voltage-gated K+ and Ca2+ channels do not typically inactivate. The frequency and pattern of presynaptic action potentials determine how long the voltage-gated Ca2+ channels will be open and thus how much presynaptic Ca2+ influx there is and how much neurotransmitter is released and for how long. The synaptic delay does not affect the level or extent of neurotransmitter release.
Which of the following best describes NT removal from the synaptic cleft? NTs are removed by
A. astrocytic end feet located in nearby capillaries.
B. channels that open in response to depolarization.
C. transporters or degradative enzymes.
D. endocytosis by the presynaptic plasma membrane.
E. oxidation and diffusion out of the cleft.
C. Astrocytic end feet would not have access to neurotransmitters at the synapse. There are no neurotransmitter channels. Neurotransmitter transporters transport glutamate, GABA, glycine, and monoamines and biogenic amines, whereas acetylcholine is degraded by acetylcholine esterase. Most neurotransmitters are not oxidized at the cleft (monoamine oxidases were previously thought to be present in the synapse but have now been shown to be mainly intracellular).
Following exocytosis, lipids and transmembrane proteins of the synaptic vesicles that fuse with the plasma membrane can be recycled by
A. endocytosis and trafficking to the early/synaptic endosome.
B. release into the synaptic cleft and uptake by nearby astrocytes.
C. secretory trafficking via the rough endoplasmic reticulum and Golgi complex.
D. degradation through the lysosomal pathway.
E. trafficking back to the trans-Golgi network (TGN) in the cell soma.
A. After synaptic vesicles fuse with the presynaptic membrane, they undergo endocytosis and are either refilled or traffic to the early/synaptic endosome. Vesicles are not released into the cleft; only their content, the neurotransmitter, is released. The endocytosed vesicles do not traffic to the secretory pathway. Most endocytic vesicles are not degraded by lysosomes. Endocytic vesicles do not traffic back to the cell body where the TGN (Golgi) is located.