7.8.1 Ultra-Structure of Skeletal Muscles

The ultra-structure of skeletal muscles refers to the detailed arrangement of myofilaments within the myofibrils, which gives skeletal muscles their characteristic striated appearance. Under a light microscope, only the striated nature of the myofibrils can be observed. Electron microscope studies reveal the precise arrangement of thin and thick myofilaments.

Definition

Ultra-structure of Skeletal Muscles: The ultra-structure of skeletal muscles refers to the detailed arrangement of myofilaments within the myofibrils, including the bands, lines, and zones that give skeletal muscles their striated appearance.

Main Idea

• Under a light microscope, only the striated nature of the myofibrils can be observed.
• This is seen as a regular alternation of light and dark bands called the I bands and A bands respectively.
• These are transversed by thin, dark lines.
• Electron microscope studies clearly indicate that the bands are due to regular arrangement of thin and thick myofilaments.
• Transversing the middle of each I band is a dark line called the Z line.
• The section of myofibril between two Z lines is called a sarcomere, which is a contractile unit.
• From the Z line, thin myofilaments extend in both directions, while in the centre of the sarcomere are found thick myofilaments.
• In certain regions of the sarcomere, thin and thick myofilaments overlap.
• Transverse sections in these regions indicate that six thin myofilaments surround each thick myofilament.
• The entire length of thick myofilaments constitute the A band because they are anisotropic that can polarize visible light.
• Thin myofilaments alone constitute I band, which is isotropic or nonpolarizing.
• The centre of the A band is lighter than the outer regions in a relaxed sarcomere as there is no overlap between the thin and thick myofilament in this region. It is called the H zone (H stands for 'hele' means bright).
• The H zone itself may be bisected by a dark line, the M line.
• The M line joins adjacent myosin filaments together at a point halfway along their length.

Core Concept

• I band = light band; isotropic; thin myofilaments only
• A band = dark band; anisotropic; thick myofilaments
• Z line = dark line in the middle of I band
• Sarcomere = section between two Z lines; contractile unit
• H zone = lighter centre of A band; no overlap
• M line = dark line bisecting H zone; joins myosin filaments
• Thin myofilaments = extend from Z line
• Thick myofilaments = in the centre of sarcomere
• Six thin myofilaments = surround each thick myofilament

Detailed Explanation

Ultra-structure of Skeletal Muscles

• Under a light microscope only the striated nature of the myofibrils can be observed.
• This is seen as a regular alternation of light and dark bands called the I bands and A bands respectively, transversed by thin, dark lines.
• Electron microscope studies clearly indicate that the bands are due to regular arrangement of thin and thick myofilaments.
• Transversing the middle of each I band is a dark line called the Z line.
• The section of myofibril between two Z lines is called a sarcomere, which is a contractile unit.
• From the Z line thin myofilaments extend in both directions, while in the centre of the sarcomere are found thick myofilaments.
• In certain regions of the sarcomere, thin and thick myofilaments overlap.
• Transverse sections in these regions indicate that six thin myofilaments surround each thick myofilament.
• This arrangement of thin and thick myofilaments results in a number of other bands being recognizable in the sarcomere.
• The entire length of thick myofilaments constitute the A band because they are anisotropic that can polarize visible light.
• Thin myofilaments alone constitute I band, which is isotropic or nonpolarizing.
• The centre of the A band is lighter than the outer regions in a relaxed sarcomere as there are no overlap between the thin and thick myofilament in this region. It is called the H zone (H stands for 'hele' means bright).
• The H zone itself may be bisected by a dark line, the M line.
• The M line joins adjacent myosin filaments together at a point halfway along their length.

Summary Table of Ultra-structure of Skeletal Muscles

Intermediate Level

Ultra-structure of Skeletal Muscles

Detailed Description of Each Structure

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Ultra-structure refers to detailed arrangement of myofilaments.
• 2. Light microscope shows striated nature.
• 3. Striations are due to regular arrangement of thin and thick myofilaments.
• 4. I band is light band.
• 5. I band is isotropic.
• 6. I band contains thin myofilaments only.
• 7. A band is dark band.
• 8. A band is anisotropic.
• 9. A band contains thick myofilaments.
• 10. Z line is in the middle of I band.
• 11. Sarcomere is between two Z lines.
• 12. Sarcomere is the contractile unit.
• 13. Thin myofilaments extend from Z line.
• 14. Thick myofilaments are in the centre of sarcomere.
• 15. Six thin myofilaments surround each thick myofilament.
• 16. H zone is the lighter centre of A band.
• 17. H zone has no overlap.
• 18. M line bisects H zone.
• 19. M line joins myosin filaments.
• 20. A band remains constant during contraction.

Remember

• - I band = light band; isotropic; thin myofilaments only
• - A band = dark band; anisotropic; thick myofilaments
• - Z line = dark line in the middle of I band
• - Sarcomere = section between two Z lines; contractile unit
• - H zone = lighter centre of A band; no overlap
• - M line = dark line bisecting H zone; joins myosin filaments
• - Thin myofilaments = extend from Z line
• - Thick myofilaments = in the centre of sarcomere
• - Six thin myofilaments = surround each thick myofilament

Common Mistakes

• 1. Confusing I band and A band — I band = light; A band = dark.
• 2. Mixing up the composition — I band = thin myofilaments; A band = thick myofilaments.
• 3. Forgetting the role of Z line — In the middle of I band.
• 4. Confusing sarcomere and Z line — Sarcomere is between two Z lines.
• 5. Mixing up H zone and M line — H zone = lighter centre; M line = dark line bisecting H zone.
• 6. Forgetting the number of thin myofilaments — Six surround each thick myofilament.
• 7. Believing A band shortens during contraction — A band remains constant.
• 8. Mixing up isotropic and anisotropic — I band = isotropic; A band = anisotropic.
• 9. Forgetting the role of the M line — Joins myosin filaments.
• 10. Thinking the H zone is always present — H zone disappears during contraction.

Quick Revision

Under a light microscope only the striated nature of the myofibrils can be observed. This is seen as a regular alternation of light and dark bands called the I bands and A bands respectively, transversed by thin, dark lines. Electron microscope studies clearly indicate that the bands are due to regular arrangement of thin and thick myofilaments. Transversing the middle of each I band is a dark line called the Z line. The section of myofibril between two Z lines is called a sarcomere, which is a contractile unit. From the Z line thin myofilaments extend in both directions, while in the centre of the sarcomere are found thick myofilaments. In certain regions of the sarcomere, thin and thick myofilaments overlap. Transverse sections in these regions indicate that six thin myofilaments surround each thick myofilament. This arrangement of thin and thick myofilaments results in a number of other bands being recognizable in the sarcomere. The entire length of thick myofilaments constitute the A band because they are anisotropic that can polarize visible light. Thin myofilaments alone constitute I band, which is isotropic or nonpolarizing. The centre of the A band is lighter than the outer regions in a relaxed sarcomere as there are no overlap between the thin and thick myofilament in this region. It is called the H zone. The H zone itself may be bisected by a dark line, the M line. The M line joins adjacent myosin filaments together at a point halfway along their length.

60-Second Revision

• I band = light band; isotropic; thin myofilaments only
• A band = dark band; anisotropic; thick myofilaments
• Z line = dark line in the middle of I band
• Sarcomere = section between two Z lines; contractile unit
• H zone = lighter centre of A band; no overlap
• M line = dark line bisecting H zone; joins myosin filaments
• Thin myofilaments = extend from Z line
• Thick myofilaments = in the centre of sarcomere
• Six thin myofilaments = surround each thick myofilament
• Warning: A band remains constant during contraction; I band and H zone shorten

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• 7. Support and Movement

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• 7.9 MUSCLE CONTRACTION - SLIDING FILAMENT MODEL

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