Answers to Quiz 116 · Synaptic Transmission in Central Nervous System
Classified by the parts of the neurons that meet, the commonest synapse in the CNS is:
Answer: Axodendritic
Axodendritic is commonest, axosomatic next; in the cerebral cortex about 80% of synapses are on dendrites and 15% on cell bodies. Axoaxonic is less common and dendrodendritic is the rare one. Revise synapses and synaptic transmission
The width of the synaptic cleft is about:
Answer: 20–50 nm
20–50 nm, filled with extracellular fluid, and the transmitter crosses it by simple diffusion. A gap of 2–5 nm is the order of a gap junction, not a chemical synapse. Revise synapses and synaptic transmission
Tetanus toxin blocks transmitter release by acting on:
Answer: Synaptobrevin
Tetanus toxin attacks synaptobrevin, the v-SNARE on the vesicle, so it can no longer grip syntaxin and the vesicle cannot fuse. Botulinum C is the type that acts on syntaxin. Revise synapses and synaptic transmission
In a spinal motor neuron, graded potentials (EPSPs and IPSPs) are generated mainly at the:
Answer: Dendrites and cell body
Graded potentials arise where the synapses are, on dendrites and soma. The initial segment is where the all-or-none action potential begins, because it has the lowest threshold. Revise synapses and synaptic transmission
Which change in the postsynaptic membrane would produce an IPSP?
Answer: Opening of Cl⁻ channels
Cl⁻ entering down its gradient hyperpolarizes the membrane; K⁺ efflux or closure of Na⁺ or Ca²⁺ channels does the same. Closing K⁺ channels and opening Na⁺ or Ca²⁺ channels are the routes to an EPSP. Revise synapses and synaptic transmission
The Renshaw cell circuit of the spinal cord is an example of:
Answer: Negative feedback inhibition
A collateral of the motor axon excites the Renshaw cell, which then inhibits the same motor neuron: a negative feedback loop. A branch of it also inhibits neighbouring motor neurons, which is lateral inhibition. Revise properties of synapses
The synaptic delay at a single synapse is about:
Answer: 0.5 ms
About 0.5 ms, spent on Ca²⁺ entering the knob, transmitter release, and the transmitter acting on the postsynaptic membrane. Counting these delays is how a pathway is called mono-, di- or polysynaptic. Revise properties of synapses
In presynaptic (indirect) inhibition, transmission fails because:
Answer: Less transmitter is released from the excitatory terminal
An axoaxonic ending on the excitatory terminal cuts its transmitter output, so the postsynaptic neuron's own membrane potential is untouched. Hyperpolarizing the postsynaptic neuron with an IPSP is direct inhibition. Revise properties of synapses
Long-term potentiation differs from post-tetanic potentiation in that it depends on:
Answer: A rise in Ca²⁺ in the postsynaptic neuron through NMDA channels
LTP is a postsynaptic change: Ca²⁺ enters through NMDA channels and intracellular proteins rise. PTP is presynaptic, the shorter-lived one, and works through Ca²⁺ left in the knob after a train of stimuli. Revise properties of synapses
GABA is formed from:
Answer: Glutamate, by glutamate decarboxylase
Decarboxylation of glutamate by glutamate decarboxylase makes GABA; GABA transaminase later turns it into succinate. Hydroxylation and decarboxylation of tryptophan is how serotonin is made. Revise neurotransmitters
Which statement about the GABA-B receptor is correct?
Answer: It is coupled to a G protein and raises K⁺ conductance
GABA-B is metabotropic: through a G protein it raises K⁺ conductance, cuts Ca²⁺ influx and lowers adenylyl cyclase. The five-subunit Cl⁻ channels are GABA-A and GABA-C, GABA-C being the retinal one, and benzodiazepines act on GABA-A. Revise neurotransmitters
Benzodiazepines produce sedation and relieve anxiety by:
Answer: Raising Cl⁻ conductance through GABA-A receptors
They bind the α subunit of the GABA-A receptor and let more Cl⁻ in, which hyperpolarizes the neuron. Blocking peripheral Na⁺ channels is how local anaesthetics work. Revise neurotransmitters
Atropine blocks:
Answer: All five muscarinic subtypes
Selective blockers exist for each muscarinic subtype, but atropine blocks M1 to M5 alike. Mecamylamine is the ganglionic nicotinic blocker and α-bungarotoxin the one that works at the motor end plate. Revise neurotransmitters
Glutamate in high concentration acts as an excitotoxin because:
Answer: It drives a large Ca²⁺ influx into neurons and kills them
Glutamate that astrocytes fail to clear acts on the cell bodies and lets in enough Ca²⁺ to kill them. The same mechanism damages brain tissue in ischemia, hypoxia, hypoglycemia, trauma and stroke. Revise neurotransmitters
During long-term potentiation, which transmitter carries the signal from the postsynaptic neuron back to the presynaptic neuron?
Answer: Nitric oxide
NO is made from arginine by NO synthase in the postsynaptic cell and diffuses back into the terminal, so it is the retrograde messenger of long-term potentiation and depression. Adenosine is a general CNS depressant, not a retrograde signal. Revise neurotransmitters
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