LPT 321: Meat Science (1+1)

MYOFILAMENTS

• Areas of different density are visible within the light and dark bands of the myofibrils.  
• The light band is singly refractive when viewed with polarized light, owing to the fact it exclusively contains actin filaments only, hence it is described as being isotropic and is called the I band.
• The broad dark band is doubly refractive when viewed with polarized light, as it contains both actin and myosin filaments, hence it is described as being anisotropic, and thus referred to as the A band.
• The ‘A’band is much denser than the ‘I’ band. Both the ‘A’ and ‘I’ bands are bisected by relatively thin lines.  A dark thin band called the Z line bisects the ‘I’ band. A narrow dense band, known as the M line, bisects the centre of the ‘A’ band.
• The thick and the thin filaments differ in their dimensions, chemical composition, properties and position within the sarcomere. The thick filaments are approximately 14-16 nm (nanometres) in diameter (1 nm = one-billionth of a metre) and 1.5 µm long.
• The thick filaments constitute the ‘A’ band of the sarcomere. Since they consist almost entirely of protein myosin they are referred to as myosin filaments. They are held in transverse and longitudinal registers by thin cross bands located periodically along the length and by cross connections in the centre of the ‘A’ band.
• The alignment of these cross connections in the centre of the ‘A’ band corresponds to the transverse density characteristics of the M-line
• The thin filaments are about 6-8 nm in diameter and they extend approximately 1.0 µm on either side of the Z line. These filaments constitute I band of the sarcomere. They consist primarily of the protein actin and are referred to as the actin filaments.