Voluntary and involuntary muscles Types of muscles Muscle action

Within the human body there are more than 600 muscles which make up about 35% of our body mass. Some of these muscles we can see and feel while others we can't, but each plays a specialised role in helping specific parts of our body to move.

Voluntary and involuntary muscles
Muscles can be classified on the basis of our conscious control:

• Voluntary muscles can be controlled by our conscious thought, e.g. the muscles in our hands as we catch a ball.
  
  

  Bending of an arm

  
  
• Involuntary muscles, like those that control the beating of the heart, do not require conscious thought to make them move even though their action is still controlled by the nervous system.

Types of muscles
Muscles can also be classified into three groups, based on their structure.

Skeletal Muscle: All muscles attached to the bones of the skeleton are called skeletal muscles. They are all voluntary muscles because we can make them work by our conscious thinking. We can either lift our arm by moving our muscles or allow them to relax, which means that our arm will fall to our side. There are four main types of skeletal muscle, named according to their shape - spindle, triangular, flat and circular (ring).

• Smooth Muscle: All smooth muscle is involuntary muscle. Its action is not controlled by our thoughts. Smooth muscle makes up tissues that are vital for continuous, everyday life. In the digestive system, it is found in the stomach walls to mechanically digest our food and in the intestine it passes the food along by wave-like contractions (peristalsis). In the circulatory system, the smooth muscle of the artery walls maintains the blood pressure and helps regulate flow. In the iris of the eye smooth muscle regulates the opening of the pupil. Thankfully all these actions are involuntary. Imagine having to think about all these things all the time!
  
  

  Peristalis

  
  
• Cardiac Muscle: Cardiac muscle, another important involuntary muscle, is found only in the walls of the heart. Without the regular contractions of the heart (about seventy/minute) to pump blood all over our body, life could not be sustained.
  
  

  The heart beat

Muscle action
Voluntary muscles receive an electrical signal from the central nervous system via motor nerve cells that connect with individual muscle fibres to make them contract or relax. As the muscle contracts, it becomes thicker and shorter, thus pulling on the bone. As it relaxes, it lengthens again.

Muscles that work with the skeleton are usually found in pairs with opposing actions. A muscle that bends a joint when it contracts is called a flexor (e.g. biceps). The opposing muscle on the other side of the bone that straightens the joint when it contracts is called an extensor (e.g. triceps). Skeletal muscles are attached to the bones with tendons which are very tough tissue that do not stretch much. An example of this is the Achilles tendon, which you can feel behind your ankle.

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