Movement and the Structure of Muscle: Quick Revision

A brief note on Movement and the Structure of Muscle. Read it before a test. Try each question at the end before you open its answer.

Movement

• Movement: any change in the position of the body or a part of it. Locomotion: a voluntary movement that changes the place of the whole body.
• Kinds: amoeboid (pseudopodia), ciliary (cilia in the trachea and the female reproductive tract), flagellar (a sperm), muscular.
• Cilia are short and many. Flagella are long and few.
• About 40 to 50 per cent of the body weight of an adult is muscle.

Muscle

• Skeletal: striated, voluntary, next to the bones. Visceral: smooth, involuntary, in hollow organs. Cardiac: striated, branched, involuntary, in the heart.
• Order of size: muscle, bundle (fascicle), fibre, myofibril, filament.
• Fascia holds the bundles. The sarcolemma lines the fibre. The sarcoplasm has many nuclei, so the fibre is a syncytium. The sarcoplasmic reticulum stores calcium ions.

The sarcomere

• Sarcomere: from one Z line to the next. It is the functional unit of contraction.
• I band: light, thin filaments (actin). A band: dark, thick filaments (myosin). Z line: middle of the I band. M line: middle of the A band. H zone: centre of the A band, with no thin filaments.
• Thin filament: F actin (a polymer of G actin), tropomyosin, troponin. Thick filament: meromyosins. The head and short arm are the heavy meromyosin (HMM), an ATPase. The tail is the light meromyosin (LMM).

A worked example

A relaxed model sarcomere is 22 u long. The thick filaments are 16 u and each thin filament is 10 u.

1. A band: 16 u.
2. I band: $22 - 16 = 6$ u.
3. H zone: $22 - 2 \times 10 = 2$ u.

Practice with new numbers

A program to try

The program works out the bands of a relaxed model sarcomere.

Mistakes to avoid

• Calling cardiac muscle voluntary because it is striated. It is striated and involuntary.
• Putting the Z line in the A band. It lies in the middle of the I band.
• Saying that troponin binds myosin. It masks the sites on the actin.
• Calling the myosin head the light meromyosin. The head is part of the heavy meromyosin.

Check yourself

1. Which band is dark, and which protein does it hold?

2. What is stored in the sarcoplasmic reticulum?

3. Which part of the muscle fibre is the plasma membrane?

4. Why is the skeletal muscle fibre a syncytium?

Sources

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 10.
• NCERT (2023). Biology, Textbook for Class XI, chapter 17.