The Muscle System

This revision guide is meant to be used along side your lecture notes for SMBE1513/SQBS3273).

 

There aeer several functions of muscles, among others are:

  • for movement of skeleton
  • controls of opening e.g. eyelid, mouth, anus
  • stabilize joint (maintain posture)
  • produce heat through exercises, and
  • for showing emotion (facial expression)

 

Nomenclature and gross anatomy of muscles

First, you were introduced that muscles were named through several of their characters. Among these characters are:

  1. Shape e.g. deltoid muscle because it looks like the greek letter ‘delta’
  2. The number of heads e.g. biceps = 2 heads; triceps = 3 heads; quadriceps = 4 heads
  3. The location e.g. biceps brachii is so named because it is located in the arms (Lain: brachii, arm)
  4. Length – either short (brevis) or long (longus)
  5. Function – either for extension, adduction, abduction, etc.

For example, consider the muscle ‘extensor carpus radialus longus‘. You could have guessed by its name that the muscle lies between the wrist (carpus) and along the radius (radialus). It is considered long (longus) as it attaches at the distal end of the humerus (origin) to the metacarpal or palm of the hand (insertion). The name extensor indicates that the muscle’s function is to extends and abducts the wrist (continue below for explanation on muscle origin and insertion).

 

Muscles are needed for movement. For movement to happen, muscle must contract and often work in pairs. Thus, muscles are also categorized according to their functions when a movement is performed. Two examples of muscle categories are:

  1. Prime movers / agonist – which are the main muscle that carry out a function
  2. Antagonist – the opposite muscle that relaxes when prime movers contracts

 

Here is a video that explains the categories of muscles in details (LondonLeisureCollege)

 

Muscle attaches to bones through an extension of connective tissue called tendon. This structure is characteristically and functionally different from ligament. You are expected to understand how these two differs and examples where they can be located.

 

The attachment of muscle to bone/bones happens at two different sites called:

  1. Origin – part of the muscle which is stationary when movement is made, and
  2. Insertion – part of the muscle which is mobile when movement is made

Because of that, muscle can also be categorized according to the nature of their attachment. There are two categories of muscles with these features:

  1. Intrinsic
  2. Extrinsic

You are advised to understand what do these terms mean and the appropriate examples for each category.

 

Here is a good example of video explaining the origin and insertion as well as movement caused by the muscle pectoralis major (Latin: pectus, breast) (Interactive Biology)

 

Biomechanic of movement

 

The muscle exert force on the bone to cause movement. This resulted in a movement that functions just like the lever system. There are 3 classes of lever systems, each produces different level of mechanical advantage useful in the physiology of anatomy:

  1. First class lever
  2. Second class lever, and
  3. Third class lever

 

The video below give good explanation of the lever systems in anatomy (Dr. Akram Jaffar; Human Anatomy Education)

 

 

Microscopic anatomy of muscle

The muscle tissue is formed by an aggregation of muscle cells. Unlike most cells, muscle cells are long cells than extend to the length of protein fibres, called myofibrils (myo – muscle; fibril – fibre) inside the cells. These myofibrils also produces striation, unique to muscle tissue.

Myofibrils are made of three different filaments, the myofilaments, called thick, thin, and elastic filaments. Microscopically, the myofibres contain:

  • sarcomere – which is a functional unit of muscle
  • Z-disc
  • I band
  • A band
  • M line, and
  • H-zone

 

 

 

 

 

 

 

 

 

The contraction of muscle is made possible by the sliding of myofilaments in the myofibrils through mechanism called ‘the sliding filament’ theory. By now, you should be able to explain how the ‘sliding filament ‘ theory works and how the release of neurotransmitter, acetylcholine (ACH) causes muscle contraction.

 

The following is further explanation how muscle contracts (Biology/Medicine Animations)

 

 

Types of muscle tissues

There are only 3 types of muscle tissues in our body. Although they share similar microscopic anatomy (i.e. made of myofilaments, myofibres, etc), each is characteristically unique to fit their intended role. The 3 types of muscle tissues are:

  1. Skeletal muscle
  2. Cardiac muscle, and
  3. Smooth muscle

You should be able to identify how these muscle tissues are different from each other.

 

Special for the skeletal muscle, the fibres can be categorized into 2 types, which differ by their metabolic activity and their performance. The 2 categories of skeletal muscle fibres are:

  1. Slow oxidative / slow-twitch / type-I fibre – which is commonly known as dark meat
  2. Fast glycolytic / fast-twitch / type-II fibre – which is commonly known as white meat

By understanding the characters of each of the muscle fibre, you should be able to understand where they are located and how they function.

 

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