Continued from Above… Continued from Above… Visceral Muscle Visceral muscles are found in organs such as the stomach, intestines and blood vessels. Visceral muscle, the weakest muscle tissue, causes organs to contract to move substances within an organ. Visceral muscle, which is controlled by the unconscious brain part, is called involuntary muscle. It cannot be controlled directly by the conscious mind. Visceral muscle is commonly called “smooth” because of its uniform, smooth appearance under a microscope. This contrasts sharply with the skeletal and cardiac muscles’ striated appearances. Cardiac Muscle The heart is the only place where cardiac muscle can be found. It pumps blood throughout the body. Because cardiac muscle tissue can’t be controlled by the brain, it is involuntary. The rate at which cardiac muscle contracts is controlled by hormones and brain signals. However, the heart muscle itself contract. Cardiac muscle tissue is the natural pacemaker for the heart. It stimulates other cardiac muscles cells to contract. The self-stimulation of cardiac muscle makes it intrinsically controlled or autorhythmic. When viewed under a microscope, the cells of cardiac muscle tissue appear to be striated. This means that they have light and dark stripes. These dark and light bands are caused by the arrangement of protein fibers within the cells. Striations are a sign that a muscle cell has strong bones, which is not the case with visceral muscles. Intercalated disks are special junctions that connect cells in cardiac muscle. They can be branched into X- or Y-shaped cells. Intercalated disks consist of projections that look like fingers from neighboring cells and create a strong bond between cells. Intercalated disks and a branched structure allow muscle cells to withstand high blood pressures and the strains of pumping blood for a lifetime. These features allow the cells to quickly transmit electrochemical signals from one cell to another, allowing for the heart to beat as a whole. Skeletal Muscle Skeletal muscles are the only tissue of voluntary muscle in the body. It can be controlled consciously. Each physical action a person performs (e.g., walking, writing) is controlled consciously. Skeletal muscle is required for speaking, walking, and writing. Skeletal muscle contracts to move body parts closer to the bone it is attached to. Skeletal muscles attach to two bones at a time, so they move different parts of the bones closer together. Skeletal muscle cells are formed when many smaller progenitor cell cells combine to form long, straight, multinucleated fibers. These skeletal muscle fibers, which are similar to cardiac muscle, are extremely strong. Skeletal muscle gets its name because these muscles connect to the skeleton at least once. Gross Anatomy of Skeletal Muscle The majority of skeletal muscles are connected to two bones by tendons. Tendons are strong, dense bands of regular connective tissue that hold the bones together. Their collagen fibers attach them to their muscles. Tendons can be pulled on by muscles, so they are extremely strong and are woven into both the muscles and bone coverings. Muscles work by shortening the length of their bones, pulling on tendons and moving closer together. The other bone is pulled towards one of the bones, while the other bone remains stationary. The origin is the location on the stationary bone that connects via tendons to a muscle. The insertion is the location on the moving bone that connects to the muscle via the tendons. The actual contraction takes place in the belly of the muscle, which is the fleshy portion of the muscle between the tendons. Skeletal Muscles Names Skeletal Muscles are given names based on many factors such as their location, origin, insertion, number, shape, size and function. * Location. * Location. Many muscles get their names from the anatomical area. For example, the abdominal region is home to the rectus abdominis (or transverse abdominis) and transverse abdominis (or both). The anterior portion of the bone that the muscles attach to, such as the tibialis, is the name of some of them, including the tibialis. Some muscles combine both of these names, such as the brachioradialis. It is named after a specific region (brachial and bone) respectively. * Insertion and Origin. Some muscles are named according to their connection with a stationary bone (origin), and a moveable bone (insertion). Once you know the names and locations of the bones they are attached to, these muscles can be easily identified. This type of muscle includes the sternocleidomastoid, which connects the sternum and skull to the mastoid process. The occipitofrontalis connects the occipital and frontal bones. * The number of origins. Some muscles can connect to more than 1 bone or more than one location on a bone. These muscles have multiple origins. A biceps muscle is one with two origins. A triceps muscle is one with three origins. A quadriceps muscle is a muscle that has four origins. * Size, shape, and direction. Also, we classify muscles based on their shapes. The deltoids, for example, have a triangular or delta shape. The serratus muscles have a saw-like or serrated shape. The rhomboid minor is a rhombus, or diamond shape. It is possible to tell the difference between two muscles located in the same area by the size of the muscle. There are three types of muscles in the gluteal area: the gluteus maximus, gluteus medius and gluteus minimalus. The direction that the muscle fibers run can also be used to identify a particular muscle. There are many sets of flat, wide muscles in the abdominal region. The rectus abdominis are muscles whose fibers run straight upwards and downwards. The transverse abdominis are those whose fibers run transversely (left-to-right) and the obliques are those whose fibers run at an angle. * Function. Sometimes, muscles are classified according to the function they perform. Because they are all located in the same area and have similar shapes, most of the muscles found in the forearms can be named after their function. The flexor muscle of the forearm, for example, flexes the wrist and fingers. The supinator muscle supinates the wrist, by rolling it up to face palm-up. Adductors are the muscles that pull together the legs. Skeletal Muscles in Groups: Skeletal muscles rarely perform by themselves to produce movements in the body. They work together more often to achieve precise movements. An agonist, or prime mover, is a muscle that causes a particular movement in the body. An antagonist muscle produces the opposite effect and pairs with the agonist. The biceps brachii muscles flexes the arm at elbow. The triceps brachii muscles extends the arm at elbow as the antagonist to this motion. The antagonist would be the triceps brachii muscle when the arm is extended by the triceps. Other muscles support the movements made by the antagonist, in addition to the agonist/antagonist pair. Synergists are muscle groups that stabilize and decrease extraneous movement. They are often found near the agonist and connect to the same bone. Fixator muscles help in movement by stabilizing the origin. Skeletal muscles move the insertion closer towards the immobile origin. Fixators in your trunk area help you to maintain balance when lifting heavy objects with your arms. Histology of Skeletal Muscle Skeletal muscle fibers are very different from other tissues due to their highly specialized functions. Many organelles that make muscle fibers are unique to this cell type. The cell membrane of muscle fibres is the sarcolemma. The sarcolemma is a conductor of electrochemical signals that can stimulate muscle cells. Transverse tubules (T–tubules), which help transport these electrochemical signals into muscle fibers, are connected to the Sarcolemma. The sarcoplasmic retina is a storage area for calcium ions (Ca2+), which are essential to muscle contraction. The “powerhouses” of the cell, mitochondria, are found in muscle cells. They break down sugars and give energy as ATP to activate muscles. Myofibrils are the contractile structure of the cell and make up most of muscle fiber structure. Myofibrils consist of many proteins fibers that are arranged in repeating subunits known as sarcomeres. The functional unit for muscle fibers is the sarcomere. For more information on the roles of sugars, proteins and other nutrients, see Macronutrients. Sarcomere Structure Sarcomeres consist of two types protein fibers: thick and thin filaments. * Thick filaments. Thick filaments are composed of many bonded units from the protein myosin. Myosin is the protein responsible for muscles contracting. * Thin filaments. Three proteins make up thin filaments: 1. Actin. Actin is a helical structure which makes up the bulk the thin filament mass. Myosin-binding points in actin allow myosin connect to and move the actin during muscle contraction. 2. Tropomyosin. Tropomyosin, a long-chain protein fiber that wraps around and covers myosin binding points on actin, is also known as Tropomyosin. 3. Troponin. Troponin is tightly bound to tropomyosin. This allows troponin to move tropomyosin from myosin-binding sites during muscle contraction. Muscular System Physiology Function Of Muscle Tissue Movement is the main function of a muscular system. Muscles are the only tissues in the body capable of contracting and moving other parts. The second function of the muscular system, which is related to movement, is maintaining posture and body position. The muscles contract more to keep the body still and in a certain position than to cause movement. The muscles that control the body’s posture are the most resilient of all the muscles. They can hold the body up throughout the day and not become tired. The movement of substances within the body is another function that is related to movement. Transporting substances such as blood and food from one area of the body is primarily the responsibility of the cardiac and visceral muscle. The generation of heat is the final function of muscle tissue. Our muscular system generates a lot of heat waste because of its high metabolism when we contract muscle. Our body’s natural heat is produced by small muscle contractions. Extra muscle contractions can cause a rise in body heat and eventually sweating if we exert ourselves beyond our normal levels. The Skeletal Muscles are Levers. Skeletal muscles, bones, and joints work together to create lever systems. The muscle acts like an effort force, the joint as the fulcrum, and the bone it moves as the lever. The object being moved acts the load. Although there are three types of levers available, the majority of the levers found in the body belong to the third-class category. A third-class lever is one in which the fulcrum lies at the end of a lever, and the effort is divided between the fulcrum or the load at the opposite end. Third class levers are used to increase the distance the load moves relative to how far the muscle contracts. This distance increase is due to the fact that the force required for the load to be moved must be greater than its mass. The biceps brachia pulls on the radius forearm and causes flexion at the elbow joint using a third-class lever system. Although a slight variation in the length of biceps can cause a greater movement of the hand and forearm, the force exerted by the biceps must exceed the weight of the muscle. Motor Units The skeletal muscles are controlled by motor neurons, which are nerve cells. Motor units are a group of muscle cells controlled by a single motor neuron. A motor neuron stimulates all the motor units in its motor unit when it receives a signal. Depending on the function of each muscle, the size of the motor units can vary throughout the body. Motor units that are used to perform fine movements, such as those of the eyes or fingers, have fewer muscle fibers per motor unit. This improves the brain’s control of these structures. Many motor units contain many muscle cells. Muscles that require a lot of strength, such as leg or arm muscles, have many motor units. The body can control how many motor units are activated for each function by controlling the strength of each muscle. This is why the same muscles used to pick a pencil can also be used to pick a bowling ball up. When motor neurons send signals to their motor neurons, the contraction cycle occurs. The Neuromuscular Junction (NMJ) is where motor neurons touch muscle cells. At the NMJ, motor neurons release neurotransmitter chemicals that attach to a part of the Sarcolemma called the motor end plate. Many ion channels in the motor end plate are open to allow positive ions into the muscle fiber. Positive ions create an electrochemical gradient inside the cell. This gradient spreads through the sarcolemma, T-tubules and further ion channels. Ca2+ ions can flow into myofibrils when the positive ions reach sarcoplasmic retina. The Ca2+ ions attach to troponin. This causes the troponin molecule’s shape to change and moves nearby tropomyosin molecules. Myosin binding sites on tropomyosin are removed from tropomyosin, which allows actin and myosin bind together. Myosin proteins in thick filaments are powered by ATP molecules, which allow them to pull on actin in thin filaments. Myosin proteins pull the thin filaments closer together to the center of the sarcomere. The sarcomere contracts and shortens as the thin filaments are pulled together. Muscle fibers’ myofibrils are composed of multiple sarcomeres. When all the sarcomeres contract the muscle cells shrinks proportionally to its size. As long as a neurotransmitter is released, muscle contractions will continue. The contraction process reverses when a motor neuron stops releasing the neurotransmitter. Calcium is returned to the sarcoplasmic retina; troponin, tropomyosin and troponin return to their normal positions; actin and myosin cannot be bound. Once the actin pulling force has stopped, the force of myosin pulling at actin stops, sarcomeres will return to their extended resting state. Myoclonus and other conditions can cause muscle contractions to be affected. Learn more about musculoskeletal conditions and diseases in our section dedicated to diseases and conditions. Learn more about DNA health testing, which helps us to understand the genetic risk of developing primary dystonia. Two factors can control the strength of muscle contraction: the number and stimulus received from the nervous system. One motor neuron nerve impulse will cause a motor to contract briefly, then relax. This is called a “twitch contraction”. The muscle contraction strength and duration will increase if the motor neuron sends several signals in a short time. Temporal summation is a term that describes this phenomenon. The muscle can enter the state known as tetanus if it receives many nerve impulses at once. A muscle can remain in tetanus for as long as the nerve signal rate slows down or the muscle is too tired to sustain the tetanus. All muscle contractions do not produce movement. Isometric contractions, which increase muscle tension without causing movement in any body part, are called light contractions. An isometric contraction occurs when people feel stressed and tighten their muscles. Isometric contractions can also result in maintaining posture and holding an object still. An isotonic contraction is one that produces movement. To build muscle mass by weight lifting, you will need isotonic contractions. Muscle tone refers to a natural state in which skeletal muscles remain partially contracted at all time. Muscle tone is a natural condition that places a slight tension on the muscles to prevent injury to the joints and muscle from sudden movements. It also helps maintain the body’s posture. Unless the nerves have been damaged, all muscles retain some muscle tone. Functional Types of Skeletal Muscle Fibres Skeletal muscle fibers are divided into Type I and II based on how they generate and use energy. 1. Type I fibers are slow and deliberate in the way they contract. Because they use aerobic respiration to make energy from sugar, they are extremely resistant to fatigue. Type I fibers are found in all muscles for their ability to maintain stamina, and good posture. Type I fibers are found in high amounts near the spine and neck regions. They keep the body upright throughout the day. 2. Type II fibers can be broken down into Type II A or Type II B. * Type IIA fibers are stronger and faster than Type I fibers but have less endurance. Type IIA fibers can be found all over the body, but they are most prominent in the legs. They help support your body during long days of standing and walking. Type IIB fibers are stronger and faster than Type IIA, but they have less endurance. Due to the lack of oxygen-storing pigment myoglobin (a type of myoglobin), Type II B fibers are lighter than Type I and II A. Type IIB fibers are found throughout the body, especially in the upper body. They provide speed and strength to the chest and arms at the expense of stamina. Depending on the environment in which the muscle is operating, the energy that they receive comes from different sources. When we ask muscles to produce low to moderate force, they use aerobic respiration. Aerobic respiration needs oxygen to make about ATP molecules out of a glucose molecule. Aerobic respiration can be very efficient and can last as long as the muscle has enough glucose and oxygen to continue contracting. Muscles that are used to produce high levels of force can become so tightened that oxygen-carrying blood cannot enter the muscles. This causes the muscle to produce energy by using lactic acid fermentation (anaerobic respiration). Anaerobic respiration produces 2 ATP for every molecule of glucose, which is less efficient than aerobic respiration. Anaerobic respiration causes muscle fatigue quickly as it exhausts their energy reserves. Muscle fibers are rich in important energy molecules that keep them working longer. Myoglobin is a red pigment that can be found in muscles and it stores oxygen in a similar way to hemoglobin in blood. Myoglobin’s oxygen allows muscles to carry on aerobic respiration even when there is no oxygen. Creatine phosphate is another chemical that keeps muscles functioning. Muscles are able to use energy as ATP. To release this energy, they convert ATP to ADP. Creatine phosphate gives its phosphate group back to ADP in order to give extra energy to the muscles. Finally, muscle fibers are made up of energy-storing glycogen. This is a macromolecule that contains many linked glucoses. To provide internal fuel, active muscles extract glucose from glycogen molecules. The muscle tires quickly and stops being able to contract when it runs out of energy, whether through aerobic or anaerobic respiratory. This is called muscle fatigue. A tired muscle has very low or no oxygen, glucose, or ATP and instead contains many waste products from respiration like lactic acid or ADP. After exertion, the body must absorb additional oxygen to replace the oxygen in muscle fibers and to power aerobic respiration which will replenish the cells’ energy supply. The body must absorb additional oxygen to bring the muscle cells back to rest. This is called oxygen debt or recovery oxygen uptake. This is why you may feel short of breath after strenuous exercise. Your body is trying its best to return to normal.
