<< Chapter < Page Chapter >> Page >

The striated appearance of skeletal muscle tissue is a result of repeating bands of the proteins actin and myosin that are present along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded.

Each I band has a dense line running vertically through the middle called a Z disc or Z line. The Z discs mark the border of units called sarcomeres , which are the functional units of skeletal muscle. One sarcomere is the space between two consecutive Z discs and contains one entire A band and two halves of an I band, one on either side of the A band. A myofibril is composed of many sarcomeres running along its length, and as the sarcomeres individually contract, the myofibrils and muscle cells shorten ( [link] ).

Illustration shows part of a tubular myofibril, which consists of many sarcomeres. Zigzagging lines, called Z lines, run perpendicular to the fiber. Each sarcomere starts at one Z line and ends at the next. A straight perpendicular line, called an M line, exists halfway between each Z line. Thick filaments extend out from the M lines, parallel to the length of the myofibril. Thin filaments extend from the Z lines, and extend into the space between the thick filaments.
A sarcomere is the region from one Z line to the next Z line. Many sarcomeres are present in a myofibril, resulting in the striation pattern characteristic of skeletal muscle.

Myofibrils are composed of smaller structures called myofilaments . There are two main types of filaments: thick filaments and thin filaments; each has different compositions and locations. Thick filaments occur only in the A band of a myofibril. Thin filaments attach to a protein in the Z disc called alpha-actinin and occur across the entire length of the I band and partway into the A band. The region at which thick and thin filaments overlap has a dense appearance, as there is little space between the filaments. Thin filaments do not extend all the way into the A bands, leaving a central region of the A band that only contains thick filaments. This central region of the A band looks slightly lighter than the rest of the A band and is called the H zone. The middle of the H zone has a vertical line called the M line, at which accessory proteins hold together thick filaments. Both the Z disc and the M line hold myofilaments in place to maintain the structural arrangement and layering of the myofibril. Myofibrils are connected to each other by intermediate, or desmin, filaments that attach to the Z disc.

Thick and thin filaments are themselves composed of proteins. Thick filaments are composed of the protein myosin. The tail of a myosin molecule connects with other myosin molecules to form the central region of a thick filament near the M line, whereas the heads align on either side of the thick filament where the thin filaments overlap. The primary component of thin filaments is the actin protein. Two other components of the thin filament are tropomyosin and troponin. Actin has binding sites for myosin attachment. Strands of tropomyosin block the binding sites and prevent actin–myosin interactions when the muscles are at rest. Troponin consists of three globular subunits. One subunit binds to tropomyosin, one subunit binds to actin, and one subunit binds Ca 2+ ions.

View this animation showing the organization of muscle fibers.

Sliding filament model of contraction

For a muscle cell to contract, the sarcomere must shorten. However, thick and thin filaments—the components of sarcomeres—do not shorten. Instead, they slide by one another, causing the sarcomere to shorten while the filaments remain the same length. The sliding filament theory of muscle contraction was developed to fit the differences observed in the named bands on the sarcomere at different degrees of muscle contraction and relaxation. The mechanism of contraction is the binding of myosin to actin, forming cross-bridges that generate filament movement ( [link] ).

Questions & Answers

what is abiotic and biotic factors?
Hira Reply
which of the following shows the correct sequence of the cell cycle
Kameishia Reply
who is name virus
Shivam Reply
centromere consist of
meeting point of two chromatids
Explain the function of nematocysts in cnidarians?
Israel Reply
The nemotocyst is used by Cnidarians (hydra, jellyfish, sea anemones) to sting their prey and any threatening enemy.
photosynthesis in plants is an example of what ? (a) excretion (b) irritability (c) nutrition (d) reproduction
Lee Reply
If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?
Israel Reply
Which of the following organisms is most likely to be a diploblast?
what are reactions of photosynthesis?
Maria Reply
what are the probabilities of blood genotypes for the offspring from a cross between a mother lAlA blood and a father with lBi blood?
dayana Reply
what is matter
Emmanuel Reply
matter is anything that has mass and can occupied space
example of matter
You serves as an example of matter Because matter is anything that has mass and occupy space e.g man and every other things that exist on earth.. So think of every other things around you ...
and you too
We generally
What is ecological management
how the kidney functions as osmoregulatory organ
Sam Reply
That true
what is the major connection for sugars in glycolysis?
Ibrahim Reply
Simple term of science
Palesa Reply
what does it mean
it's means what do u know about biology?
what is immunisation
the action of making a person immune to infections ,for immunisation
what is the biology? what do you know about biology
Phathu Reply
biology is the study of living organisms, divided into many specialized fields that cover their morphology, physiology, anatomy, behavior, origin, and distribution.
The study of all aspects of life. The study of all living organisms (such as animal cells and plant cells) in greater detail (their structure and how they function). It's a very broad science.
what is prokaryotic
Bhaskar Reply
what is pathogens
pathogens are a bacterium, virus, or other microorganism that can cause disease.
transistion metals....
Wasik Reply

Get the best Biology course in your pocket!

Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?