Structure of the Skeletal Muscle

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Types of muscle:

Muscles are known as effector organs. They respond to signals sent by the Central Nervous System (CNS) which causes them to contract. There are 3 types of muscles in the body:

  • Cardiac muscles: found only in the heart. 
  • Smooth muscles: found in the walls of the gut and blood vessels.
  • Skeletal muscles: found attached to bones and make up the bulk of body muscle.

Skeletal muscles are the only ones in the body to be consciously controlled. Both cardiac and smooth muscles contract automatically.


Muscles are made up of tiny strands called myofibrils. These wrap around each other to form bundles and the bundles together make up the entire muscle. The myofibrils on their own are not very strong, it is the combined force that provides the muscle with its strength.   

Myofibrils are made up of individual cells situated end to end that have been fused together. They share nuclei and sarcoplasm. Inside the sarcoplasm there are is a large concentration of mitochondria and endoplasmic reticulum.

Microscopic structure:


Myofibrils are made up of two types of protein: 

  • Actin - the thinner of the two and consists of two strands twisted around each other. 
  • Myosin - the thicker one and consists of long rod-shaped fibres with bulbous heads that project out to the side. 

Above is a cross section of a myofibril, with the larger dots representing the myosin and the actin being the smaller dots. When looking at this from the side it appears striped due to alternating light and dark coloured bands. 


The light bands are called isotropic bands (I-bands). The seem lighter because the actin and myosin don't overlap. The dark bands are called anisotropic bands (A-bands) and are


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