Ch. 11 Introduction to the Nervous System and Nervous Tissue

Amerman3rd EditionHuman Anatomy & PhysiologyISBN: 9780138247201, 9780138247928, 9780138201814Not the one you use?Change textbook

Amerman 3rd Edition

Ch. 11 Introduction to the Nervous System and Nervous Tissue

Problem L3.6

Chapter 11, Problem L3.6

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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Understand the normal process: In a typical neuron, depolarization of the trigger zone (axon hillock) leads to a positive feedback loop. This means that as voltage-gated sodium (Na⁺) channels open, sodium ions rush into the cell, causing further depolarization and opening of more Na⁺ channels. This amplifies the signal and allows the action potential to propagate down the axon.

Define a negative feedback loop: A negative feedback loop works to counteract a change rather than amplify it. If depolarization triggered a negative feedback loop, the system would act to reduce or stop the depolarization rather than enhance it.

Predict the outcome: If a negative feedback loop occurred, mechanisms would activate to oppose the depolarization. For example, potassium (K⁺) channels might open earlier or more extensively, allowing K⁺ to leave the cell and repolarize the membrane, or chloride (Cl⁻) channels might open to hyperpolarize the membrane.

Explain the physiological impact: The action potential would likely fail to propagate because the depolarization would be counteracted before it could reach the threshold needed to open additional Na⁺ channels. This would disrupt communication between neurons and other cells, impairing processes like muscle contraction, sensory perception, and reflexes.

Summarize the significance: A negative feedback loop at the trigger zone would fundamentally alter the function of the nervous system, preventing the rapid and efficient transmission of electrical signals necessary for normal physiological activities.

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Key Concepts

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

Depolarization

Depolarization refers to the process by which the membrane potential of a neuron becomes less negative (or more positive) than the resting potential. This occurs when sodium channels open, allowing Na+ ions to flow into the cell, which is crucial for the initiation of action potentials. In the context of the trigger zone, depolarization is essential for the propagation of nerve impulses.

Feedback Loops

Feedback loops are processes in which the output of a system influences its own operation. In biological systems, positive feedback amplifies a response, while negative feedback reduces it. A positive feedback loop in neuronal activity would enhance depolarization and action potential generation, whereas a negative feedback loop would inhibit these processes, potentially leading to reduced neuronal excitability.

Neuronal Excitability

Neuronal excitability refers to the ability of a neuron to respond to stimuli and generate action potentials. This property is influenced by ion channel dynamics and the balance of excitatory and inhibitory signals. If depolarization at the trigger zone led to a negative feedback loop, it would decrease neuronal excitability, making it harder for the neuron to fire and transmit signals effectively.

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Textbook Question

Explain how an action potential is propagated down an axon in continuous conduction. Why is saltatory conduction faster than continuous conduction?

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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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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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Explain why cardiac muscle cells and some smooth muscle cells will continue to contract even when their nerve supply has been removed

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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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Textbook Question

A hypothetical poison blocks K⁺ leak channels. How would this affect the resting membrane potential of skeletal muscle fibers? Explain your reasoning.

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