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Chapter 6. Synaptic & Junctional Transmission

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A neurology resident was conducting research in an electrophysiology laboratory. He was recording from the soma of two motor neurons while electrically stimulating the dorsal root afferent fiber very close to its entry into the spinal cord. In motor neuron A he recorded an inward current that had an onset latency of 0.75 ms after the stimulus. In motor neuron B, the same stimulus induced an outward current whose onset latency was 1.45 ms.

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Which of the following could explain the response recorded from motor neuron A?

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A. The stimulus caused the release of GABA that led to the opening of Cl channels and a fast inhibitory postsynaptic potential (IPSP).

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B. The stimulus caused the release of glycine that led to the closing of Cl channels and a fast IPSP.

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C. The stimulus caused the release of glutamate that led to opening of Na+ channels and a fast excitatory postsynaptic potential (EPSP).

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D. The stimulus caused the release of glutamate that led to an increase in Na+ conductance and an endplate potential in the postsynaptic neuron.

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E. The stimulus caused the release of glycine that led the closure of K+ channels and a fast IPSP.

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The correct answer is C. An inward current denotes depolarization (fast EPSP) which would occur due to an action of an excitatory neurotransmitter such as glutamate to increase Na+ conductance. Options A, B, and E can be ruled out since the release of an inhibitory neurotransmitter such as GABA or glycine induces hyperpolarization (outward current, fast IPSP) due to either an increase in Cl or K+ conductance. Option D can be ruled out since an endplate potentials are recorded at the neuromuscular junction and result from release of acetylcholine by a motor neuron.

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A neurology resident was conducting research in an electrophysiology laboratory. He was recording from the soma of two motor neurons while electrically stimulating the dorsal root afferent fiber very close to its entry into the spinal cord. In motor neuron A he recorded an inward current that had an onset latency of 0.75 ms after the stimulus. In motor neuron B, the same stimulus induced an outward current whose onset latency was 1.45 ms.

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Which of the following could explain the response recorded from motor neuron B?

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A. The stimulus caused the release of GABA that led to the opening of Cl channels and a fast inhibitory postsynaptic potential (IPSP).

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B. The stimulus caused the release ...

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