Muscles

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Types of Muscle

There are three main types of muscle

  • Cardiac muscle
  • Involuntary muscle
  • Voluntary muscle

All muscle types have different structures and features

  • Cardiac muscle has three different sub-types: atrial muscle; ventricular muscle; excitatory and conductive fibres. Some of the fibres are myogenic - they can initiate their own contraction. Cardiac muscle has a striated appearance. It contracts powerfully and does not fatigue.
  • Involuntary muscle is also known as smooth muscle. It is not under conscious control. Examples include sphincter muscles in digestive system and muscles in the wall of the trachea. Involuntary muscle cells are spindle shaped. They contract slowly but do not tire easily.
  • Voluntary muscle is also known as skeletal muscle. It is made of long fibres not cells. Voluntary muscle has a striated appearance. It contracts quickly but can tire.
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Voluntary Muscle Stucture

Voluntary muscles are made up of long muscle fibres. There are no distinct cells. A single fibre is called a myofibril and is made up of shorter sections called sarcomeres. Many sarcomeres are joined end to end to form the myofibril. This is surrounded by a special form of cytoplasm containing cell organelles including sacroplasmic reticulum and encased in a membrane called the sarcolemma.

Each sarcomere is made up of several thin filaments (actin) and several thick filaments (myosin)

                        (http://www.lab.anhb.uwa.edu.au/mb140/corepages/muscle/Images/Mus1ani.gif)

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Muscle Contraction

The mechanism for muscle contraction is called the sliding filament model. It relies on the overlap between the thick and thin filaments

The thin filaments contain actin with two other proteins, troponin and tropomyosin. The troponin binds the tropomyosin to the actin.

The thick filaments contains many myosin molecules, which consist of a tail and two heads.

During contraction

  • The myosin heads attach to any actin overlap surrounding them, forming a cross bridge
  • The head group bends, pulling the actin along, increasing the overlap
  • ADP and Pi are released
  • The cross bridge breaks as ATP attaches to the myosin head
  • The head group returns to its original position, hydrolysing ATP in the process

This only happens in the presence of calcium ions. These are required to bind to the troponin, which then moves the tropomyosin so that myosin binding sites are uncovered.

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