Muscle Structure

An overview of the structure of muscle cells.

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Types of Muscle

Skeletal Muscle (striated, voluntary)- This is always attached to the skeleton (think biceps, triceps etc.) and is under voluntary control via the motor neurones of the somatic nervous system. It is the most abundant and best understood type of muscle. It can be divided into fast twitch (white) and slow twitch (red).

Cardiac Muscle (Involuntary)- This is a special type of slow twitch (red) skeletal muscle, but it is not attached to the skeleton and not under voluntary control (think back to the control of Heart Rate)

Smooth Muscle (Involuntary)- This is found in the internal body organs such as the wall of the gut, the uterus, blood arteries and arterioles, the iris, ciliary body and glandular ducts. It is under involuntary control via the autonomic nervous system or hormones. Smooth muscle usually forms a ring which tightens when it contracts, so therefore doesn't need a skeleton to pull up against (Peristalsis of the Oesophagus, for example)

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Skeletal Muscle- A breakdown

1 Muscle--------> Many Muscle Fibres--------->Many Myofibrils

Skeletal muscle is made up of overlapping striped muscle fibres. They are held together by connective tissue, with a tendon at each end connecting them to the skeleton. When the fibres contract, the muscle shortens, pulls the tendons and moves the bones. The stripes on skeletal muscle fibres can be seen through a light microscope. Like other cells, a striped muscle fibre has Cytoplasm, called Sarcoplasm and is surrounded by a plasma membrane, called a Sarcolemma. There is, however an important difference: striped muscle fibres have many nuclei in the Sarcoplasm, which lie near the surface of the fibre. The central part of the striped muscle fibre consists mainly of much thinner fibre-like structures called myofibrils, which are composed mainly of protein molecules. It is these myofibrils that transfer chemical energy from food into kinetic movement.

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Skeletal Muscle- Ultrastructure

Although it is possible to see the stripes in skeletal muscle using a light microscope, it is only when we look at muscle fibres using and electron microscope that it becomes clear what the stripes actually are. Each myofibril is made up of protein filaments of two types-Thick filaments composed of Myosin and thin filaments composed as Actin. There are a number of things to remember about how these filaments are arranged in accordance to a microscopic view:

  • Where the two sets pf filaments overlap the myofibril appears dark
  • the region where there are only myosin filaments is slightly lighter
  • The region where ther are only actin filaments appears lighter still
  • in the centre of each light band is the Z disc, which holds actin filaments in position. Similarly, M discs hold myosin filaments in position
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