Edited by: Danielle Abel

Sliding filament theory of muscle contraction

In order to understand what the sliding filament theory of muscle contraction is, we must start with myofibrils. 

Myofibrils are smaller than a human hair, which means that the actin & myosin filaments are even smaller than this. 

In order to trigger the sliding filament process, the body needs to activate the muscle through the nervous system. The nervous system uses a chemical messenger called a neurotransmitter to send this signal. Acetylcholine is the main neurotransmitter involved in the muscle contraction process. 

The neurotransmitter helps an action potential travel down the length of the nerve to the area on the muscle that the nerve "innervates." Innervation simply means the area where these two structures come very close to one another; they don't actually touch. This "space" is called the neuromuscular junction.

Acetylcholine Release

Acetylcholine crosses the neuromuscular junction to signal the release of calcium. Calcium itself is what regulates the muscle contraction.

The calcium release occurs inside of the muscle cell within the cell membrane called the sarcolemma. The sarcolemma is the plasma-based membrane of the muscle cell. This membrane separates out what is inside the cell and what is outside the cell. Inside is also referred to as "intra" whereas outside is called "extra" cellular.

The cell membrane itself has an area of depression, in a concave shape, called a T-tubule. When the depolarization signal from acetylcholine is transmitted around the sarcolemma, it gets down into the T-tubule. It is when the T-tubule's receptors are activated that the release of calcium from the sarcoplasmic reticulum occurs.

Troponin Binding

The calcium then locates and binds with troponin, which are the receptor sites that pull the tropomyosin out of the way of the binding site on the actin filament. The tropomyosin is like a rope of molecules around the actin. It's easy to associate these two together by thinking "tropo" - "ropo." When the binding site is exposed, the crossbridge cycle can occur. 

Myosin & Actin Crossbridge

The myosin filament heads are able to bind with the actin filaments at the binding site to facilitate the crossbridge cycle. So, in a sequential fashion, calcium binds with troponin, the troponin moves the tropomyosin out of the way of the actin binding site, and when the actin binding site is exposed, the myosin filament head can bind with the actin filament. 

H Zone & I Band Shortening

When you think of actin & myosin, think about it like a series of horizontal rungs on a ladder within the sarcomere (the structural unit of a myofibril consisting of dark and light striations). Every other rung is a row of actin and a row of myosin. Within the width of the ladder, are vertical segments that divide the horizontal rows of actin & myosin into distinct zones & bands. 

• H Zone = the middle of the sarcomere that divides it into left and right halves and only contains myosin filaments that span from Z disc to Z disc 
• I Band = the ends of the sarcomere on both the far left and the far right sides that only contain actin filaments
• A Band = the total length of the myosin within the sarcomere spanning the full distance between the Z discs
• Z Discs = the vertical, left, and right structure of the sarcomere (the sides of the ladder) where the horizontal actin and myosin "rungs" are attached to the sarcomere

The lengthening and shortening of the muscle influence what occurs in the zones & bands. 

• H Zone (myosin filaments) = shortens with muscle contraction
• I Band (actin filaments) = shortens with muscle contraction
• A Band (total length of the myosin) = stays the same length with muscle contraction  

A Band Length

Since myosin spans the length of the Z discs, it's not possible for the A Band length to change because it's fixed to either end of the Z discs. Meaning, it's the actin that is causing the shortening or lengthening of the muscle and therefore shortening and lengthening of the H Zone and the I Bands. 

Practical Application & CSCS Study Questions

If you relate the shortening and lengthening to the concentric and eccentric portions of a muscle contraction or movement, you can better understand what areas within the sarcomere are changing length. 

• H Zone = shortens and lengthens
• I Band = shortens and lengthens
• A Band = stays the same length

1. During an eccentric muscle contraction, which of the following are the same length?

A. H Zone

B. I Band

C. A Band

In this case, we know that the A Band section of the sarcomere stays the same length, so the answer would be C. 

2. When there is no longer an action potential present and the muscle is relaxed, where does the calcium move to?

A. Mitochondria

B. Converted into ATP

C. Sarcoplasmic reticulum 

In this case, the calcium is present inside the sarcoplasmic reticulum before depolarization occurs, so it makes sense that it would be taken back into the sarcoplasmic reticulum. This process occurs when calsequestrin and parvalbumin pull calcium back in so that it's ready for the next muscle contraction. 

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