Ch. 11 Introduction to the Nervous System and Nervous Tissue

Amerman - Human Anatomy & Physiology 2nd Edition

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Amerman 2nd Edition

Ch. 11 Introduction to the Nervous System and Nervous Tissue

Problem L3.5

Chapter 11, Problem L3.5

Predict the effect that tetrodotoxin would have on Albert's muscle fiber action potentials (see question 3). Would it affect end-plate potentials at the motor end plate? Why or why not?

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Understand the role of tetrodotoxin (TTX): Tetrodotoxin is a potent neurotoxin that blocks voltage-gated sodium (Na⁺) channels, preventing the influx of Na⁺ ions during depolarization. This is critical for generating action potentials in excitable cells, such as neurons and muscle fibers.

Analyze the effect of TTX on muscle fiber action potentials: Since muscle fiber action potentials depend on the opening of voltage-gated Na⁺ channels to initiate depolarization, TTX would inhibit the generation of action potentials in muscle fibers by blocking these channels.

Consider the role of end-plate potentials (EPPs): End-plate potentials are localized depolarizations at the motor end plate caused by the binding of acetylcholine (ACh) to nicotinic receptors, which opens ligand-gated ion channels. This process is independent of voltage-gated Na⁺ channels.

Determine if TTX affects EPPs: Since TTX specifically blocks voltage-gated Na⁺ channels and not the ligand-gated ion channels involved in EPP generation, it would not directly affect the formation of end-plate potentials at the motor end plate.

Summarize the overall effect: TTX would prevent the propagation of action potentials in muscle fibers by blocking voltage-gated Na⁺ channels, but it would not interfere with the generation of end-plate potentials, as these are mediated by ligand-gated ion channels at the motor end plate.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Tetrodotoxin Mechanism

Tetrodotoxin is a potent neurotoxin that blocks voltage-gated sodium channels in nerve and muscle cells. By inhibiting these channels, tetrodotoxin prevents the influx of sodium ions, which is crucial for the generation and propagation of action potentials. This blockage leads to a failure in the transmission of electrical signals, affecting muscle contraction and nerve communication.

Action Potentials

Action potentials are rapid, temporary changes in the membrane potential of a neuron or muscle fiber, allowing for the transmission of signals. They are initiated when a threshold level of depolarization is reached, primarily through the opening of sodium channels. In muscle fibers, action potentials trigger the release of calcium ions, leading to muscle contraction, making their integrity essential for normal muscle function.

End-Plate Potentials

End-plate potentials (EPPs) are localized depolarizations that occur at the motor end plate, where motor neurons communicate with muscle fibers. EPPs are generated by the binding of neurotransmitters, such as acetylcholine, to receptors on the muscle membrane, leading to the opening of ion channels. While tetrodotoxin primarily affects action potentials in the muscle fiber, it does not directly block EPPs, as these are initiated by neurotransmitter release rather than action potentials.

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Explain what would happen if depolarization of the trigger zone led to a negative feedback loop instead of a positive one.

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During a surgical procedure, an anesthesiologist administers to the patient an inhaled anesthetic agent that opens Cl⁻ channels in the postsynaptic membranes of neurons in the brain. Explain why this would put the patient 'to sleep' for the duration of the surgical procedure.

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The drug neostigmine blocks the actions of acetylcholinesterase in the synaptic cleft. What effect would this have on synaptic transmission? What effects might you expect to see as a result of this drug?

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Textbook Question

Albert accidentally ingests the poison tetrodotoxin from the pufferfish, which you know blocks voltage-gated Na⁺ channels. Predict the symptoms Albert will experience from this poisoning.

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Textbook Question

Mark the following statements as true or false. If a statement is false, correct it to make a true statement.

The resting membrane potential refers to the voltage difference across the membranes of excitable cells at rest.

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Textbook Question

Albert, the patient in question 3, takes the drug lithium, which reduces the permeability of the neuronal axolemma to Na⁺ (that is, it allows fewer Na⁺ to enter the axon). Predict the effect this would normally have on his neuronal action potentials. Do you think this drug would be beneficial or harmful, considering his condition?

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