Question

What are myofibrils and myofilaments?

Answer 1

Hint: Muscle is a tissue of mesodermal origin where the cells called myocytes are specialised to contract. They are bound together by connective tissue. All the muscles of the body which bring about various types of movements in internal and external parts of the body constitute the muscular system. There are about 639 muscles which constitutes the bulk of body, 40% of total weight and 50% of soft parts.

Complete answer:
A muscle is made of a large number of cells called muscle fibres.
1. Myofibrils: Contractile apparatus of a striated muscle cell is made of ribbons of unbranched cross-striated longitudinal strands called myofibrils. They are partitioned into contractile units called sarcomeres. Each sarcomere is about 2-5 um in length in a resting muscle fibre. Sarcomeres are separated by densely stained transverse lines called Z-lines. Myofibrils bear alternate light and dark bands. Dark bands are doubly refractive under polarised light They are called anisotropic or A-bands Light bands are mono refractive in polarised light. They are called isotropic or I-bands. Densely stained Z-band bisect the I-bands. The central part of each dark band has a variable light zone called H-zone or Hensen's line.
2. Myofilaments: Myofibrils are made of two types of protein filaments or myofilaments, thicker myosin and thinner actin. Myosin or primary myofilaments are 1-6 um long and about 15 nm in diameter. They are swollen in the middle 150 nm long region called M-line. M-line is bare. Elsewhere the myosin filaments possess projections for forming cross bridges with active sites of actin filaments. Actin or secondary myofilaments are 1-0 um long and 9 nm in diameter. They also possess troponin and tropomyosin proteins. Actin is a globular protein, G-actin, which has a molecular weight of 42000. It is polymerised to produce helically coiled apparently double filament or F-actin. F-actin bears active sites of ADP at intervals.

Note:
Muscles show three unique properties-contractibility, excitability and conductivity. Muscles can shorten by 1/3 to ½ of their length due to contraction. Afterwards they return to their original size. This property of shortening and then returning to a relaxed state is called contractility. For contraction of a muscle, there must be excitation by nerve impulse, hormone, mechanical, thermal or chemical. Stimulus received at one point of a muscle cell spreads almost instantaneously to other parts as well as to neighbouring myocytes.