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Ch. 6 The Muscular System
Marieb - Essentials of Human Anatomy & Physiology 13th Edition
Marieb13th EditionEssentials of Human Anatomy & PhysiologyISBN: 9780135624340Not the one you use?Change textbook
All textbooksMarieb 13th EditionCh. 6 The Muscular SystemProblem 2
Chapter 6, Problem 2

After ACh attaches to its receptors at the neuromuscular junction, what is the next step?
a. Sodium channels open
b. Calcium binds to regulatory proteins on the thin filaments
c. Cross bridges attach
d. ATP is hydrolyzed

Verified step by step guidance
1
Step 1: Understand the role of acetylcholine (ACh) at the neuromuscular junction. ACh is a neurotransmitter that binds to receptors on the muscle cell membrane (sarcolemma), initiating a sequence of events leading to muscle contraction.
Step 2: Recognize that after ACh binds to its receptors, it triggers the opening of sodium channels in the sarcolemma. This allows sodium ions (Na⁺) to flow into the muscle cell, leading to depolarization of the membrane.
Step 3: Depolarization of the sarcolemma generates an action potential that travels along the muscle fiber and into the T-tubules, stimulating the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum.
Step 4: Calcium ions bind to regulatory proteins (troponin) on the thin filaments, causing a conformational change that exposes binding sites on actin for the myosin heads to attach, forming cross bridges.
Step 5: ATP is hydrolyzed by the myosin heads to provide energy for the cross-bridge cycling process, which leads to muscle contraction. This step occurs later in the sequence after calcium binding and cross-bridge formation.

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Key Concepts

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

Neuromuscular Junction

The neuromuscular junction is the synapse or connection point between a motor neuron and a muscle fiber. When a motor neuron releases acetylcholine (ACh), it binds to receptors on the muscle cell membrane, initiating a series of events that lead to muscle contraction. Understanding this process is crucial for determining the subsequent steps after ACh binding.
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A. Events at the Neuromuscular Junction

Action Potential and Sodium Channels

An action potential is an electrical signal that travels along the neuron and triggers the release of neurotransmitters like ACh. When ACh binds to its receptors, it causes sodium channels in the muscle cell membrane to open, allowing sodium ions to flow into the cell. This influx of sodium ions is essential for depolarizing the muscle cell and initiating contraction.
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Action Potential

Muscle Contraction Mechanism

Muscle contraction involves a series of biochemical events, including the binding of calcium ions to regulatory proteins on the thin filaments, which allows cross-bridges to form between actin and myosin. This process is powered by ATP hydrolysis, which provides the energy necessary for muscle fibers to contract. Understanding this mechanism is vital for answering questions about the sequence of events following ACh binding.
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Overview of Muscle Contraction
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Which are ways in which muscle names have been derived?

a. Attachments

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c. Function

d. Location

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Compare these images of a relaxed skeletal muscle fiber and a fully contracted muscle fiber. Which would you see only in the relaxed fiber?

a. Z discs

b. Sarcomeres

c. I bands

d. A bands

e. H zones

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