In this lesson, you will learn about the general functions of skeletal muscle and its underlying anatomical organization. Specifically, this lesson will cover:
Recall that skeletal muscle is a voluntary muscle tissue that is attached to bones or skin. This tissue combines with epithelial, connective, and nervous tissues to form the muscles of the muscular system, performing a range of functions.
Contract and cause movement: When activated, muscle tissue shortens, causing the bones (or skin) that it is attached to move.
Contract and resist movement: The activation of certain muscles can stop movement instead of causing it. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position, resisting gravity. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. Joints can become misaligned or dislocated entirely by pulling on the associated bones; muscles work to keep joints stable.
Regulate body openings: Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions (such as swallowing, urination, and defecation) to be under voluntary control.
Protect internal organs: Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs.
Generate heat: Skeletal muscles perform contractions which require energy. When ATP is broken down, heat is produced. This heat contributes to the maintenance of homeostasis. During exercise (sustained muscle movement), it can also be very noticeable, causing body temperature to rise. In cases of extreme cold, shivering produces random skeletal muscle contractions to generate heat.
Each skeletal muscle is an organ that consists of various integrated tissues and structures. These tissues include blood vessels, nerve fibers, connective tissue, and skeletal muscle fibers, also known as skeletal muscle cells. Each skeletal muscle has three layers of connective tissue (called “mysia”) that enclose it, compartmentalize its muscle fibers, and provide structure to the muscle as a whole. Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and move powerfully while maintaining its structural integrity. The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently.
The Three Connective Tissue Layers - Bundles of muscle fibers, called fascicles, are covered by the perimysium. Muscle fibers are covered by the endomysium.
Inside each skeletal muscle, several muscle fibers are organized into individual bundles, each called a muscle fascicle, by a middle layer of connective tissue called the perimysium. This fascicular organization is common in muscles of the limbs; it allows the nervous system to be selective in how much of a muscle it activates at one time. Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium. The endomysium contains extracellular fluid and nutrients to support the muscle fiber. These nutrients are supplied via blood to the muscle tissue.
The connective tissue wrappings of a skeletal muscle continue at each end of the muscle to form a tendon. Recall that a tendon is a cord-like dense regular connective tissue structure that attaches a skeletal muscle to a bone. The collagen in the three tissue layers (the mysia) intertwines with the collagen of a tendon. At the other end of the tendon, it fuses with the periosteum coating the bone. The tension created by contraction of the muscle fibers is then transferred through the mysia, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton. In other places, the mysia may fuse with a broad, tendon-like sheet called an aponeurosis. Muscles that attach to small single points of a bone do so using tendons (i.e., biceps brachii). Muscles that attach to broad or wide regions of a bone use an aponeurosis (i.e., frontalis). Some muscles use a tendon on one end and an aponeurosis at the other (i.e., latissimus dorsi or “lats”).
Every skeletal muscle is also richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal. In addition, every muscle fiber in a skeletal muscle is supplied by a neuron, which signals the fiber to contract. Unlike cardiac and smooth muscle, the only way to functionally contract a skeletal muscle is through signaling from the nervous system.
watch
View the following video for more information on this topic.
terms to know
Skeletal Muscle Fibers
Skeletal muscle cells.
Epimysium
A sheet of connective tissue which is wrapped around a muscle.
Muscle Fascicle
A bundle of muscle fibers.
Perimysium
A sheet of connective tissue which is wrapped around a muscle fascicle.
Endomysium
A sheet of connective tissue which is wrapped around a muscle fiber.
Aponeurosis
A broad tendon-like sheet of connective tissue which attaches a skeletal muscle to a bone or other structure.
2. Skeletal Muscle Fibers
key concept
Much of the terminology associated with muscle fibers that you will learn throughout this and future lessons is rooted in two Greek root words. The root sarco means flesh and the root myo means muscle. Keep these in mind as you compile your anatomical language. Anything that starts with "sarco-" or "myo-" is associated with muscle tissue.
Skeletal muscle cells are long and cylindrical and can be quite large for human cells, with diameters up to 100 μm (micrometre or micrometer, also commonly known as a micron and in this case 0.004 in) and lengths up to 30 cm (11.8 in). During early development, embryonic stem cells called myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form a single multinucleated skeletal muscle fiber. Multiple nuclei mean multiple copies of genes, permitting the production of large amounts of proteins and enzymes needed for muscle contraction.
Recall that most cells, including muscle fibers, contain multiple organelles and structures. The muscle fiber, however, has specific names for a few of the common structures. The plasma membrane of muscle fibers is called the sarcolemma, the cytoplasm is referred to as sarcoplasm, and the specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca⁺⁺) is called the sarcoplasmic reticulum (SR). Each muscle fiber also contains hundreds to thousands of unique cylindrical organelles called myofibrils composed of myofilaments, or protein filaments.
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.
terms to know
Myoblast
An embryonic stem cell which fuses to form skeletal muscle fibers.
Sarcolemma
The plasma membrane of a skeletal muscle fiber.
Sarcoplasm
The cytoplasm of a skeletal muscle fiber.
Sarcoplasmic Reticulum
The specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions.
Myofibril
A cylindrical organelle of muscle fibers composed of myofilaments.
Myofilaments
Protein filaments.
summary
In this lesson, you learned about the function of skeletal muscles and the anatomical arrangement of their components. You also learned about the formation and organization of the individual skeletal muscle fibers that make up skeletal muscles.
