Video: Skeletal muscle structure

When you look at skeletal muscle, it appears like one single structure. But look closely, and you'll see it's got many layers and levels all arranged with intention for its smooth functioning. And in this tutorial, we'll be learning about the structure of a skeletal muscle. Skeletal muscles are discrete organs made up of different kinds of tissues besides skeletal muscle tissue. They have connective tissue, a rich supply of blood vessels, along with sensory and motor nerves.

Connective tissue organizes the skeletal muscle into hierarchical levels. The epimysium surrounds the whole muscle. The perimysium bundles muscle fibers into muscle fascicles. The endomysium separates and surrounds individual muscle fibers, which is another name for muscle cells. Like typical cells, skeletal muscle fibers have a plasma membrane and cytoplasm, which are known as the sarcolemma and sarcoplasm.

But unlike typical cells, skeletal muscle fibers are long, cylindrical, multinucleated, and striated cells. The sarcoplasm has several structures important for muscle contraction, including bundles of myofibrils, the main contractile organelles of the cell, lots of mitochondria, required for ATP production glycogen granules, which store energy and myoglobin, a protein that binds and stores oxygen. Other structures ensure that skeletal muscle fibers contract as directed by the central nervous system. One of these structure is a specialized form of smooth endoplasmic reticulum known as the sarcoplasmic reticulum. It forms a network around the myofibrils and functions as a reservoir for calcium ions.

These ions are essential for the initiation of muscle contractions. Transverse tubules, or T-tubules, are regions of the sarcolemma that dip into the sarcoplasm. Each T-tubule is flanked by expanded ends of the sarcoplasmic reticulum known as the terminal cisternae. T-tubules allow electrical impulses to reach the interior of the cell. One T-tubule and two flanking terminal cisternae together form a triad.

Now let's focus on our contractile organelles, the myofibrils. Myofibrils are made up of proteins with different functions. Contractile proteins like actin and myosin generate muscle tension. Regulatory proteins such as troponin and tropomyosin control the start and end of the contraction. Structural proteins such as titin and nebulin keep the myofilaments aligned and ensure stability.

These proteins are organized into myofilaments, which include thick, thin, and elastic filaments. Each thick filament consists of hundreds of myosin molecules bundled together with their tails at the center and globular heads projecting outward towards the thin filaments. Each myosin molecule has two globular heads and intertwined polypeptide chains that form the tail. Between the head and the tail is a flexible hinge region, which allows movement. Why would it need to move?

The head contains binding sites for actin, and the attachment of myosin to actin is a key step in muscle contraction. Myosin heads also have a binding site for ATP, where an ATPase enzyme breaks down ATP and stores the energy. Thin filaments are primarily composed of actin subunits, which form intertwined strands. The actin subunits contain binding sites for myosin. Along with actin, thin filaments also have the regulatory proteins troponin and tropomyosin.

Troponin consists of three polypeptides forming a complex. It binds to actin and has binding sites for calcium. Troponin plays an important role in regulating muscle contraction because it determines the position of tropomyosin. Tropomyosin forms strands that cover the myosin binding sites on actin when the muscle is at rest, ensuring actin and myosin cannot bind. When the muscle needs to contract, tropomyosin shifts position to expose the binding sites.

Elastic filaments contain a large structural protein known as titin. It anchors the thick filaments and maintains elasticity such that when the muscle is stretched, titin helps bring it back to the resting length. The arrangement of myofilaments is responsible for the striations on a muscle fiber. They form alternating dark and light bands. The A band is a dark band that spans the entire length of thick filaments and includes the zone of overlap between thick and thin filaments.

Thus, it includes both thick and thin filaments. The H zone is a slightly lighter region within the A band with only thick filaments. It excludes the region of overlap with thin filaments. The M line is a dark line in the center of the H zone, which holds adjacent thick filaments together. The I band is a light colored band that contains only thin filaments.

The I band is bisected by a zigzag Z disc, which contains structural proteins that anchor thin filaments and form an attachment point for elastic filaments. The region between two consecutive Z discs is known as the sarcomere, the fundamental functional unit of muscle contraction. During muscle contraction, the Z disks come closer together as the sarcomere shortens. This process is explained by the sliding filament model. According to this model, muscles contract because the myosin heads attach to actin and pull the thin filaments toward the M line.

This increases the amount of overlap, bringing the Z disks closer together, shortening the sarcomere and generating muscle tension. However, the myofilaments do not shorten during muscle contraction. When the thin filaments slide, the I band and the H zone narrow. But as the thick filaments stay in place, the width of the a band and the position of the m line remain the same. Thus, the structure of skeletal muscle is carefully designed for its function, contraction, and movement.

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