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. 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.
Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced.
This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractions to generate heat. Each skeletal muscle is an organ that consists of various integrated tissues.
These tissues include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. 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. Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a 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 trigger a specific movement of a muscle by activating a subset of muscle fibers within a bundle, or fascicle of the muscle.
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 the extracellular fluid and nutrients to support the muscle fiber. These nutrients are supplied via blood to the muscle tissue. In skeletal muscles that work with tendons to pull on bones, 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 though 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 , or to fascia, the connective tissue between skin and bones.
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 the axon branch of a somatic motor 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. Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers.
During early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers. Multiple nuclei mean multiple copies of genes, permitting the production of the large amounts of proteins and enzymes needed for muscle contraction.
As will soon be described, the functional unit of a skeletal muscle fiber is the sarcomere, a highly organized arrangement of the contractile myofilaments actin thin filament and myosin thick filament , along with other support proteins.
During flexing of the forearm, the brachioradialis and brachialis act as synergist muscles, aiding the biceps brachii in pulling the forearm up towards the shoulder. The muscles of the rotator cuff are also synergists in that they fix the shoulder joint allowing the bicepps brachii to exert a greater force. Flexing of the forearm by the biceps brachii : The biceps brachii is the agonist, or primer mover, responsible for flexing the forearm.
The triceps brachii not shown acts as the antagonist. The brachioradialis and brachialis are synergist muscles, and the rotator cuff not shown fixes the shoulder joint allowing the biceps brachii to exert greater force. Learning Objectives Differentiate between agonist and antagonist muscles. Key Points Agonist muscles shorten with contraction to produce a movement.
Following contraction, the antagonist muscle paired to the agonist muscle returns the limb to the previous position. Synergist muscles act around a movable joint to produce motion similar to or in concert with agonist muscles, allowing for a range of possible movements. Key Terms antagonist : This type of muscle acts as opposing muscle to agonists, usually contracting as a means of returning the limb to its original resting position.
Enthesitis is a condition in which the sites where tendons and ligaments attach to the bones become inflamed. Learn more about its causes and…. This article looks at the different types of myopathy, their symptoms and treatments, and the outlook for people with these muscle disorders.
Elbow pain can occur when lifting an object for several reasons, including tennis elbow and trapped nerves. Learn more about some causes and…. Many factors, such as muscle strain and bruising, and more rarely, a heart attack, can cause pain in the middle of the bicep.
Learn more here. How do muscles work? Written by Yella Hewings-Martin, Ph. Share on Pinterest Muscles use energy from our food to produce movement. Contraction gets muscles moving. Calcium stimulates contraction. Muscle repair. Latest news Could 'cupping' technique boost vaccine delivery?
Scientists identify new cause of vascular injury in type 2 diabetes. Adolescent depression: Could school screening help? Related Coverage.
0コメント