What are the Functional Properties of Muscle?

The first functional property that all 3 muscle types share is called excitability. Excitability is the ability of a cell to change its inner-membrane charge to a more positive charge (depolarization) in response to a stimulus. As we will discuss shortly in the section about the neuromuscular junction, when a stimulus is received signaling that it is time for the muscle to contract, the polarized cell membrane exchanges charges across the membrane due to an influx of positively charged ions. This makes the inside bi-layer of the cell membrane positive (where it was originally negative) in a process called depolarization (when charges flip from negative to more positive inside and less positive outside). When the stimulus is removed the membrane returns back to its original polarized state. Excitability is the functional property that allows us to turn on certain muscles when they need to be used and turn them back off when we don't need them, saving many resources and giving us control over certain aspects of body function. Additionally, this property is not unique to muscle cells. Neurons are also excitable and can undergo depolarization in response to a stimulus. Often times the stimulus is external and actives depolarization in the nervous system which will cause depolarization in the muscle cells and evoke a response. It might sound complicated but it happens all day and you are often not aware of it occurring.    

Another functional property is that all muscle cells are contractible. Contractibility is the ability of a muscle cell to contract or shorten. The shortening movement causes force production which in turn is used for that particular muscle's function, including locomotion, pumping blood, or changing the size of your pupil. This functional property is made possible because of movement within the cell of the contractile proteins actin and myosin. There is more depth on this topic to come.

A third functional property is that all muscle cells are compliant. Compliance is the ability of a muscle cell to stretch. Compliance is often called extensibility - because the muscle can be stretched or extended. Muscles are capable of contracting over a variety of lengths but the amount of tension produced will vary depending on the length of the muscle cell. This property is often confused with the last functional property, elasticity. Elasticity is the ability of a muscle to return back to its original size during relaxation. Both of these properties are due to the elastic fibers in the skeletal muscle. To visualize these 2 properties, imagine holding a rubberband in your hands (or grab one if it is nearby). When you stretch that rubberband, you are demonstrating compliance. When you release the rubberband and it snaps back to its original length, that is elasticity. These 2 properties have an inverse relationship to one another. If a muscle has a high compliance, than it will have very little elasticity. Meaning it will stretch easily but not recoil back to its original length quickly. The opposite is true as well. Watch the animations below so see an example of this inverse relationship in action.

 

Animation 2. View the Compliance and Elasticity animation on YouTube (opens in new window). 

 

 

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