Muscle Structure

Muscle fascicle
Muscle fascicle (Photo credit: Wikipedia)

Gross Structure

Skeletal muscle cells (fibres) are surrounded by endomysium and grouped into fascicles.  These fascicles are surrounded by perimysium and together make up the body of the muscle organ.

Cellular structure

The muscle fibre

This is a long, multinucleated ‘cell’ (the reason why cell is in quote is because a single muscle fibre is embryologically derived from multiple myoblast cells) that usually extends the whole length of the muscle.  They contain a large number of mitochondria to cope with the high energy demand.

The myofibril

These are specialised, cylindrical, contractile elements that extend the entire length of the muscle fibre and make up around 80% of the fibre’s contents.  The myofibril is made up of a regular arrangement of thick and thin filaments.

Thick filaments are 12-18nm in diameter and around 1.6μm in length and are composed of myosin protein molecules.  Myosin molecules are shaped almost like a golf club, with two binding sites at its head- one for actin and one for ATP- and a tail.  In contrast, thin filaments are 5-8nm in diameter and around 1μm long and are composed mainly ofactin protein molecules(NB also troponin and tropomyosin).  The structure of thin filaments is a dimeric helix of actin molecules held by a ‘backbone’ of tropomyosin (troponin is distributed along the backbone).

A top-down view of skeletal muscle
A top-down view of skeletal muscle (Photo credit: Wikipedia)

Thick and thin filaments surround each other within the myofibril.  They connect to each other via a’cross-bridge’that forms between the myosin head and actin.

The sarcomere, A and I bands, Z and M lines, and the H zone

Functionally, sections of the myofibril can be split into sections.  The sarcomere is the functional unit of the muscle organ (i.e. the smallest structure able to perform the organ’s function), and is the region between to Z lines.

NB The following are not so much structures but features that can be seen on electron microscopy.

  • Z lines are essentially a disc of connective tissue that connects thin filaments of adjacent sarcomeres
  • A bands are the region of stacked thick filaments (dark region)
  • I bands are the remaining region of thin filaments that are not overlapping the thick filaments (light region)
  • The M lines are another protein disc that holds the thick filaments together at their centre
  • The H zone is the small area at the centre of the thick filament stack (containing the M line) that is not overlapped by the thin filaments

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