At the foundation of all movement is the humble muscle cell. If you remember from biology, there are quite a few steps to a simple muscle contraction, so we’ll do our best to simplify it in this article.
There are plenty of resources to explain the sliding filament theory and the makeup of a muscle cells and sarcomeres. Like here and here. But if you’re not looking for a textbook style post, keeping reading this summary version.
A little muscle contraction fun 🙂
If you prefer a hands on learning experience, you might be interested in this giant sarcomere model on Amazon for $2,000. There’s only one available, so act fast haha. 😉 In all seriousness, this giant model of a sarcomere is kind of interesting to look at because it makes it easy to see how the components work together.
So let’s do a quick review of muscle contraction physiology:
- An action potential in a motor neuron causes acetylcholine to release in the synaptic cleft.
- Acetylcholine binds with receptors on the cell membrane on the muscle fiber, opening Ca2+ -Na+ channels. Usually referred to as Calcium channels.
- Calcium is released from the terminal cisternae into the muscle fiber.
- Calcium binds to troponin
- Troponin shifts tropomyosin, which was blocking the active site on the actin.
- Myosin heads attach to actin by breaking down ATP to ADP and a phosphate via Myosin-ATPase
- The Myosin head forms a ‘cross-bridge’ on the active site of the actin filament.
- The cross bridge pulls actin, which slides over the myosin – known as the ‘Power Stroke.’
- The release of ADP completes the cross-bridge movement and ATP attaches to myosin, breaking the actin-myosin crossbridge.
- Every time ATP is split into ADP + P, the myosin head ‘cocks’ into place to form another cross bridge with actin.
This entire process shortens the sarcomere, which is functional unit of a muscle cell.
You can read a textbook, or even study the simplified steps above, but watching it in action is very helpful. Here’s a short snippet of a simulated muscle contraction showing the parts of a muscle cell and what happens during a muscle contraction.
How can we explain an increase strength or force production with the sliding filament theory in mind?
Two ways: Frequency and Quantity
The faster a motor unit is stimulated, the greater strength and force it can produce.
Likewise, the more motor units activated, the greater your strength or force can be.
Remember, a muscle unit fires in an ‘all or none’ pattern, so that means no partial contractions. You’re either recruiting more or firing them faster.
Was this a good reminder of the basics? Share your thoughts in the comments below.