Unit 4 assessment: activation and mechanism of the actin-myosin cross | Anatomy & Pyhsiology 1 | Herzing University–Minneapolis


Muscle cells have the incredible ability to convert the chemical energy of adenosine triphosphate (ATP) into the mechanical energy of muscle contraction. 

There are three main events that need to take place to allow contraction of skeletal muscle fiber:

  1. Electrical excitation of a muscle fiber
  2. Excitation-contraction coupling
  3. Muscle fiber contraction due to the sliding filament mechanism


Read and watch the following animation below to evaluate and interpret the process involved in the activation of the actin-myosin cross bridge (sliding filament mechanism) in skeletal muscle contraction.

  1. Electrical Excitation of a Muscle Fiber

Skeletal muscle fiber cells can be stimulated by a somatic motor neuron resulting in depolarization of the sarcolemma. If this depolarization reaches threshold, an action potential is initiated resulting in muscle contraction.

  • Describe a somatic motor neuron and how it communicates with a skeletal muscle cell?
  • Describe the process of depolarization within the sarcolemma and what is required for a stimulus to reach threshold, resulting in skeletal muscle contraction?
  1. Excitation-Contraction Coupling

An action potential is transmitted along the sarcolemma and down the T tubules (transverse tubules).  This action causes calcium ions to be released from the terminal cisternae of the sarcoplasmic reticulum. Calcium ions couple electrical excitation to muscle fiber contraction by binding to troponin.

  • Two events happen when calcium binds to troponin.
  • Describe these two events in great detail.
  1. Muscle Fiber Contraction due to the Sliding Filament Mechanism
  • Watch the animated interaction between actin and myosin in the sliding filament mechanism during skeletal muscle contraction. https://www.youtube.com/watch?v=hgLSLJ09fZM
  • Evaluate and interpret the process in the actin-myosin cross bridge (sliding filament mechanism) during skeletal muscle contraction and answer the following questions below:
    • What happens to the position of the myosin head when ATP is hydrolyzed and ADP is attached to the myosin head group?
    • What event happens immediately after the myosin head binds to the active site on actin? Describe this process in great detail.
    • What happens to the position of the myosin head when ATP binds to the myosin head group?
    • Why does the actin filament only move in one direction? Describe in great detail.
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