The Muscular System

Keep In Mind

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The connective tissue supports and protects muscle fibers. This tissue also provides a way for nerve and blood vessels to innervate the skeletal muscle.

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The cell membrane that surrounds a skeletal muscle fiber is called a sarcolemma. The cytoplasm of the skeletal muscle fiber is the sarcoplasm. One muscle fiber is filled with several long, cylindrical proteins called myofibrils, which are the contractile units of the fiber. The smallest contractile unit in a myofibril is a sarcomere. Several protein myofilaments make up a myofibril. There are two types of myofilaments: thick bands and thin bands. Thick bands, or myofilaments, are made of several protein molecules called myosin. Several protein molecules, called actin, link together to form the thin bands. These thin actin bands are attached to a Z-disk (or Z-line).

Muscle Contraction

Keep in mind that the dark, striped Z-disc marks where one sarcomere ends and another begins. As shown in the image below, there are light-colored bands called I-bands and dark-colored bands called A-bands. The Z-line is found in the middle of the I-bands, while the H zone is found in the middle of the A-bands. In the middle of the H-zone is the M line, which is the center of the sarcomere.

Test Tip

The following guide can be used to remember the components of the various lines in a skeletal muscle:

A-band	Thick and thin filaments
I-band	Thin filaments only
Z-line	Actin filament attachment site
H-band	Thick filaments only

Slide filament theory explains muscle contraction. According to this theory, actin filaments slide past myosin filaments, pulling the actin filaments closer to the center of the sarcomere, or M line. As shown in the image below, this sliding action happens because of interactions between the heads of actin and myosin. The heads of myosin form attachments with the actin myofilaments. These attachments are known as crossbridges.

Keep In Mind

The head of actin is a round protein shaped like a ball. Several of these round proteins link together to form a long chain, or thin myofilament. Myosin is a thick protein with a head that resembles a golf club. When several myosin proteins join together, they create a myosin filament, where the heads point outward.

With the help of energy in the form of ATP, the myosin heads are energized to attach to binding sites in actin and form a crossbridge. After energy in the myosin head is released, the myosin pulls actin myofilaments closer to the M line. This head can only form another crossbridge when another molecule of ATP attaches to the head, reenergizing it. Calcium also plays an important role in determining when contraction happens. This ion is found in the sarcoplasmic reticulum, which surrounds myofibrils.

Coordinating Movement

Ligaments attach bones to bones. Where ligaments connect bones, they form a joint. Thus, joints are the site where individual bones meet. There are three types of joints:

1. Immovable: Also known as fibrous joints, these consist of bones held together by connective tissues. The bones are in very close contact. An example of an immovable joint is the intersection of cranial bones in the skull.
2. Partly movable: Also known as cartilaginous joints, these consist of bones held together by cartilage. These joints allow some degree of movement. Partly movable joints include the vertebral discs in the spine.
3. Synovial: These allow the largest freedom of movement because the bones are separated by a joint cavity. Examples of synovial joints are the hip and shoulder.

Keep In Mind

While there are three different types of joints, there are six different classes of synovial joints.

Each type is characterized and named for the way that the joint moves.

Use the diagram to the right to identify each type of synovial joint.

The muscular system works with the skeletal system to move the body. Thus, the muscles must be attached to bone. Tendons attach muscle to bone. Tendons consist of tough connective tissue that is found on either side of the joint where two bones are connected. Tendons work with skeletal muscles to move bones. When muscles contract, they shorten. This pulls on the bones, with the help of the tendon, to allow the body to move.

Muscles must work in pairs to move bones at the joint. The muscle that causes a joint to bend is called a flexor muscle. The muscle that contracts and causes a joint to straighten is called an extension muscle. If one muscle in the pair contracts, the other remains elongated.

For Example

Biceps and triceps muscles in the arm work together to bend and lengthen the elbow. As a biceps muscle contracts, the triceps muscle remains elongated, or relaxed. Thus, the biceps is the flexor and the triceps is the extensor of the elbow joint.

Let’s Review

• A muscle is a fibrous tissue that aids in body movement, provides support, and generates heat energy for the body.
• Cardiac, smooth, and skeletal muscles are the three muscle types found in the body.
• Cardiac and smooth muscle are under involuntary control, while skeletal muscle is under voluntary control.
• Cardiac and skeletal muscles are striated, while smooth muscle is non-striated.
• Three types of muscle tissues comprise a skeletal muscle: epimysium, endomysium, and perimysium.
• A single skeletal muscle fiber consists of several contractile units called myofibrils, which consist of actin and myosin myofilaments.
• According to the slide filament theory, actin and myosin myofilaments form crossbridges to shorten a sarcomere, which shortens a skeletal muscle.
• Tendons attach muscle to bone and help bones move.
• Joints are the regions between bones that influence the degree of flexibility with body movement.
• Skeletal muscles move bones by working in muscle pairs to contract and elongate.

Muscular System Flashcards