• Skeletal muscles are made up of many muscle fibers.
• Each muscle fiber has a diameter of about 10–80 micrometers (µm).
• Every muscle fiber is made of smaller subunits arranged inside it
• These subunits become progressively smaller from the whole muscle fiber to its internal structures.
• In most skeletal muscles, each muscle fiber extends from one end of the muscle to the other.
• This means that one muscle fiber usually runs the entire length of the muscle.
• About 98% of muscle fibers receive only one nerve ending.
• This single nerve ending is usually located near the middle of the muscle fiber.
• The nerve ending carries signals that stimulate the entire muscle fiber to contract.

KEY CONCEPT

• Skeletal muscle is composed of numerous muscle fibers measuring 10–80 µm in diameter.
• Each muscle fiber contains progressively smaller subunits.
• Most muscle fibers extend the entire length of the muscle.
• About 98% of muscle fibers are supplied by a single nerve ending.
• The nerve ending is usually located near the middle of the muscle fiber, allowing the entire fiber to contract.

The Sarcolemma Is a Thin Membrane Enclosing a Skeletal Muscle Fiber

• The sarcolemma is a thin membrane that surrounds each skeletal muscle fiber.
• The sarcolemma has two main parts:
  • A true cell membrane, called the plasma membrane.
  • An outer coat made of a thin layer of polysaccharide material containing many thin collagen fibrils.
• At both ends of the muscle fiber, the outer layer of the sarcolemma fuses with a tendon fiber.
• The tendon fibers then join together in bundles.
• These bundles form the muscle tendons.
• The muscle tendons connect the muscles to the bones.

KEY CONCEPT

• The sarcolemma is the thin membrane that encloses a skeletal muscle fiber.
• It consists of the plasma membrane and an outer polysaccharide layer containing collagen fibrils.
• The outer layer of the sarcolemma joins with tendon fibers at both ends of the muscle fiber.
• Tendon fibers bundle together to form muscle tendons, which attach muscles to bones.

Myofibrils Are Composed of Actin and Myosin Filaments

• Each muscle fiber contains several hundred to several thousand myofibrils (Fig. 6.1C).
• Each myofibril is made of about:
  • 1500 myosin filaments.
  • 3000 actin filaments (Fig. 6.1D–E).
• Actin and myosin are large protein molecules responsible for muscle contraction.
• In Fig. 6.1E–L:
  • The thick filaments are myosin.
  • The thin filaments are actin.
• The actin and myosin filaments partially overlap (interdigitate) (Fig. 6.1E).
• This overlapping produces alternating light and dark bands in the myofibril.
• The light bands contain only actin filaments.
• These are called I bands because they are isotropic to polarized light.
• The dark bands contain:
  • Myosin filaments.
  • The overlapping ends of the actin filaments.
• These are called A bands because they are anisotropic to polarized light.
• Small projections are present on the sides of the myosin filaments (Fig. 6.1E–L).
• These projections are called cross-bridges.
• Cross-bridges interact with actin filaments to produce muscle contraction.
• The ends of the actin filaments are attached to the Z disk (Fig. 6.1E).
• From the Z disk, the actin filaments extend in both directions and overlap with the myosin filaments.
• The Z disk is made of filamentous proteins different from actin and myosin.
• The Z disk extends across each myofibril.
• It also connects neighboring myofibrils across the entire muscle fiber.
• Therefore, the entire muscle fiber shows alternating light and dark bands.
• These bands give skeletal muscle and cardiac muscle their striated appearance.
• The portion of a myofibril between two successive Z disks is called a sarcomere (Fig. 6.1E).

• During muscle contraction (bottom of Fig. 6.4), the sarcomere length is about 2 µm.
• At this length:
  • The actin filaments completely overlap the myosin filaments.
  • The tips of the actin filaments just begin to overlap each other.
• At this sarcomere length, the muscle produces its greatest force of contraction.

KEY CONCEPT

• Each muscle fiber contains many myofibrils.
• Each myofibril is composed of thick myosin filaments and thin actin filaments.
• Partial overlap of actin and myosin forms the light I bands and dark A bands.
• Cross-bridges on myosin interact with actin to produce muscle contraction.
• Actin filaments are attached to the Z disk, and the region between two Z disks is called the sarcomere (Fig. 6.1E).
• At a sarcomere length of about 2 µm (Fig. 6.4), the muscle generates its maximum force of contraction.

Titin Filamentous Molecules Hold the Myosin and Actin Filaments in Place

• The side-by-side arrangement of myosin and actin filaments is maintained by titin (also called connectin) (Fig. 6.2).
• Titin is a long filamentous protein.
• Each titin molecule has a molecular weight of about 3.9 million.
• This makes titin one of the largest proteins in the human body.
• Because titin is filamentous, it is highly elastic (springy).
• The springy titin molecules form a framework that:
  • Holds the myosin and actin filaments in their correct positions.
  • Helps the contractile machinery of the sarcomere work effectively.
• One end of the titin molecule is attached to the Z disk.
• This end is elastic and acts like a spring.
• It changes its length as the sarcomere contracts and relaxes.
• The other end of the titin molecule is attached to the thick myosin filament.
• This attachment anchors (tethers) the myosin filament in place.
• Titin may also serve as a template during the initial formation of the contractile filaments of the sarcomere.
• This role is especially important for the formation of myosin filaments.

KEY CONCEPT

• Titin (connectin) maintains the alignment of actin and myosin filaments (Fig. 6.2).
• Titin is one of the largest proteins in the body and is highly elastic.
• It forms a spring-like framework that keeps the sarcomere organized during contraction and relaxation.
• One end of titin is attached to the Z disk, and the other is attached to the myosin filament.
• Titin may also help in the initial formation of the sarcomere, especially the myosin filaments.

Sarcoplasm Is the Intracellular Fluid Between Myofibrils

• Many myofibrils are arranged side by side inside each muscle fiber (Fig. 6.1 and Fig. 6.3).
• The spaces between the myofibrils are filled with an intracellular fluid called sarcoplasm.
• The sarcoplasm contains large amounts of:
  • Potassium (K⁺).
  • Magnesium (Mg²⁺).
  • Phosphate.
  • Many protein enzymes.
• The sarcoplasm also contains large numbers of mitochondria.
• The mitochondria lie parallel to the myofibrils.
• The mitochondria produce energy in the form of adenosine triphosphate (ATP).
• This ATP supplies the energy needed for contraction of the myofibrils.

KEY CONCEPT

• Sarcoplasm is the intracellular fluid present between the myofibrils (Fig. 6.1 and Fig. 6.3).
• It contains large amounts of potassium, magnesium, phosphate, and many protein enzymes.
• Numerous mitochondria are present parallel to the myofibrils.
• Mitochondria produce ATP, which provides the energy required for muscle contraction.

Transverse Tubule–Sarcoplasmic Reticulum System of Skeletal Muscle

• In the sarcoplasm surrounding each myofibril is an extensive membranous network called the sarcoplasmic reticulum (SR) (Fig. 6.3).
• The enlarged ends of the sarcoplasmic reticulum are called the terminal cisternae.
• The terminal cisternae are closely associated with the transverse tubules (T tubules).
• The T tubules are extensions of the cell membrane.
• The T tubules penetrate throughout the muscle fiber.
• The T tubules surround each myofibril.
• This arrangement allows action potentials to spread rapidly through the T tubules to the deep interior of the muscle fiber.
• The T tubule–sarcoplasmic reticulum system is very important for regulating:
  • Storage of Ca²⁺.
  • Release of Ca²⁺.
  • Reuptake of Ca²⁺.
• Therefore, this system plays an essential role in muscle contraction.
• Rapidly contracting muscle fibers have an especially well-developed T tubule–sarcoplasmic reticulum system.

KEY CONCEPT

• The sarcoplasmic reticulum (SR) is a membranous network surrounding the myofibrils (Fig. 6.3).
• Its enlarged ends, called terminal cisternae, are closely associated with the T tubules.
• T tubules are extensions of the cell membrane that carry action potentials deep into the muscle fiber.
• The T tubule–SR system regulates Ca²⁺ storage, release, and reuptake, making it essential for muscle contraction.
• Rapidly contracting muscle fibers have a highly developed T tubule–SR system.

PREPARE AND MADE BY SELF LEARNING DR SHEEN