In this lesson, you will explore a micro-level view of muscle contractions, looking specifically at sarcomeres and how they act to produce a muscle contraction. Specifically, this lesson will cover:
Muscle contractions are the shortening of fibers within muscles, which act to generate force. The sarcomere is the basic unit of contraction. Sarcomeres are the functional and contractile units of skeletal and cardiac muscles, and they are created by a specific arrangement of myofilaments called actin and myosin.
As you reviewed in a previous lesson, muscle contractions rely on calcium. Calcium is a mineral necessary for the proper development and mineralization as well as proper nerve and muscle function, and it is stored by the sarcoplasmic reticulum and released during a muscle contraction. You will learn more about sarcomeres and contractions throughout this lesson.
terms to know
Contraction
A muscle fiber generates tension, causing the muscle to shorten.
Sarcomere
The functional and contractile units of skeletal and cardiac muscles; created by a specific arrangement of myofilaments called actin and myosin.
Calcium
A mineral necessary for the proper development and mineralization as well as proper nerve and muscle function.
2. The Sarcomere
Skeletal muscle cells appear striated (striped) from the outside. This striation is due to the arrangement of the myofilaments that are composed of alternating thin and thick filaments. When a light is placed behind the cell, as in histology, thin filaments allow more light through, whereas thick filaments block more light. This creates alternating bands of light and dark along the skeletal muscle fiber.
Skeletal Muscle Fiber—Skeletal muscle fibers, shown in histology (left) and diagram (right), are striated on their surface due to the presence of a unique contractile organelle called a myofibril.
A thick filament is a myofilament composed of a protein called myosin. Myosin looks like two golf clubs that were twisted together and contains three regions, the head, neck, and tail. The long, twisted portion (the shaft of the golf club) is called the myosin tails. The two bulbous ends are called the myosin heads, and each contain an actin-binding site and an ATP-binding site. The flexible region that is bent and connects the head to the tails is called the myosin neck or hinge. A thick filament contains many myosin proteins bunched together with their heads sticking out at all angles. At the end of this large group, the thick filament is connected to what you will later learn is called the Z-line by an elastic protein called titin.
A thin filament is a myofilament composed of a complex of three proteins. The primary protein is actin. Individual actin subunits are roughly spherical and bond together like beads on a string to form two filamentous forms of actin that are twisted around one another. Each actin subunit contains a myosin-binding site. However, when a muscle is at rest, this myosin-binding site is blocked by a long, thin protein called tropomyosin (tropo, to change). Tropomyosin is held in place by a third protein called troponin, which contains a calcium-binding site.
Muscle Fiber—A skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. A muscle fiber is composed of many fibrils, which give the cell its striated appearance.
The thick and thin filaments are combined in a highly organized alternating pattern to form a myofibril. This organization has various structures and regions.
The Z-line (also known as the Z-disc and Z-band) is the zigzag point of attachment for thin filaments.
The M-line is the point of attachment for thick filaments.
The I-band is the region of a sarcomere that contains thin filaments only.
The H-zone is the region of a sarcomere that contains thick filaments only.
The A-band is the region of a sarcomere where thick filaments exist. Keep in mind that thin filaments may or may not also be in the A-band depending on the contracted state of the muscle.
The zone of overlap is the region where thin and thick filaments overlap.
A sarcomere is the repeating unit of a muscle fiber and runs from Z-line to Z-line.
hint
There are a couple of tricks that may help you remember what all the structures and regions of a sarcomere are.
The Z-line is at either END of the sarcomere, just like the letter “Z” is at the end of the alphabet.
The M-line is in the M-iddle or M-idline of the sarcomere.
The alternating I- and A-bands create the striations—alternating bands of light and dark—on a muscle fiber.
The I-band is the l-I-ght band of the striation. The letter “I” is also THIN-ner than the letter “A” and is made of thin filaments.
The A-band is the d-A-rk band of the striation. The letter “A” is also THICK-er than the letter “I” and is made of thick filaments.
The H-zone and zone of overlap are located right next to one another.
The Sarcomere—The sarcomere, the region from one Z-line to the next Z-line, is the functional unit of a skeletal muscle fiber.
terms to know
Thick Filament
A myofilament composed of myosin.
Myosin
The protein that forms the thick filament.
Myosin Tail
The long, twisted portion of a myosin molecule.
Myosin Head
The bulbous region of a myosin molecule.
Myosin Neck
The flexible hinge that connects the myosin head and tail together.
Titin
An elastic protein connecting the thick filament to the Z-line.
Thin Filament
A myofilament composed of actin, tropomyosin, and troponin.
Actin
The primary protein that forms the thin filament.
Tropomyosin
A long, thin regulatory protein that covers the myosin-binding site of actin when a muscle is at rest.
Troponin
A regulatory protein that binds tropomyosin to actin.
Z-Line
The point of attachment for thin filaments.
M-Line
The point of attachment for thick filaments.
I-Band
The region of a sarcomere that contains thin filaments only.
H-Zone
The region of a sarcomere that contains thick filaments only.
A-Band
The region of a sarcomere where thick filaments exist.
Zone of Overlap
The region where thin and thick filaments overlap.
3. The Sliding Filament Model
When excited (signaled) by a motor neuron, a skeletal muscle fiber contracts. As a muscle shortens, thin filaments are pulled and then slide past the thick filaments within the fiber’s sarcomeres. This process is known as the sliding filament model (also referred to as the sliding filament theory).
The Sliding Filament Model of Muscle Contraction—When a sarcomere contracts, the Z-lines move closer together, and the I-band becomes smaller. The A-band stays the same width. At full contraction, the thin and thick filaments overlap completely.
try it
As the sarcomere shortens or lengthens, the muscle shortens or lengthens. Recall that the sarcomere contains various regions and structures defined by their position relative to one another.
Directions: As a muscle shortens or lengthens, try and predict how each region changes.
A molecular model of a muscle contraction which explains how thin and thick filaments slide relative to one another to cause a muscle to shorten or lengthen.
4. Types of Contractions
There are two types of contractions that can occur:
Twitch: a single contraction that happens in response to the firing of a motor neuron. Sarcomeres quickly pull inward and create a quick, jerky twitch.
Tetanus: a sustained contraction that's caused by repeated muscle twitches. This term is often used to describe a person who has been exposed to the bacteria Clostridium tetani toxin.
In other words, if a muscle twitches over and over again, it can sustain that contraction. Contractions are caused by impulses from the nervous system. An action potential is the technical term for these impulses, and a motor unit describes one motor neuron and all of the muscle fibers it innervates at once. You will learn more about these contractions in a future lesson.
term to know
Twitch
The way a skeletal muscle contracts; sarcomeres quickly pull inward and create a quick, jerky twitch.
Tetanus
A sustained muscle contraction caused by repeated muscle twitches.
summary
In this lesson, you learned about how muscles contract. First, you reviewed a muscle contraction overview that described essential characteristics of muscle contractions. Then, you learned that the sarcomere is the basic unit of contraction within muscle fibers and is made up of myofibrils. You then explored the sliding filament model, which refers to the process by which a muscle contracts. Finally, you learned that there are two types of contractions: twitch, a single contraction, and tetanus, a prolonged contraction made up of multiple twitches.
The region of a sarcomere where thick filaments exist.
Actin
The primary protein that forms the thin filament.
Calcium
A mineral necessary for the proper development and mineralization as well as proper nerve and muscle function.
Contraction
A muscle fiber generates tension, causing the muscle to shorten.
H-Zone
The region of a sarcomere that contains thick filaments only.
I-Band
The region of a sarcomere that contains thin filaments only.
M-Line
The point of attachment for thick filaments.
Myosin
The protein that forms the thick filament.
Myosin Head
The bulbous region of a myosin molecule.
Myosin Neck
The flexible hinge that connects the myosin head and tail together.
Myosin Tail
The long, twisted portion of a myosin molecule.
Sarcomere
The functional and contractile units of skeletal and cardiac muscles; created by a specific arrangement of myofilaments called actin and myosin.
Sliding Filament Model
A molecular model of a muscle contraction which explains how thin and thick filaments slide relative to one another to cause a muscle to shorten or lengthen.
Tetanus
A sustained muscle contraction caused by repeated muscle twitches.
Thick Filament
A myofilament composed of myosin.
Thin Filament
A myofilament composed of actin, tropomyosin, and troponin.
Titin
An elastic protein connecting the thick filament to the Z-line.
Tropomyosin
A long, thin regulatory protein that covers the myosin-binding site of actin when a muscle is at rest.
Troponin
A regulatory protein that binds tropomyosin to actin.
Twitch
The way a skeletal muscle contracts; sarcomeres quickly pull inward and create a quick, jerky twitch.
Z-Line
The point of attachment for thin filaments.
Zone of Overlap
The region where thin and thick filaments overlap.
