 In this video, I will list and describe the functions of the muscular system and compare the structural and functional properties of skeletal muscle, cardiac muscle, and smooth muscle. Movement is a major function of the muscular system. For example, contraction of a skeletal muscle pulls on a bone at an insertion, which is where the tendon attaches to a bone from a muscle. Another major function of the muscular system is posture, muscles stabilize joints by producing muscle tone with continuous contractions in order to prevent movement. Protection is another major function of the muscular system. For example, the skeletal muscles of the abdominal wall, such as the rectus abdominis and external abdominal oblique and internal abdominal oblique help to protect visceral organs located deep within the abdominal cavity. Another major function of the muscular system is thermogenesis or heat production. Metabolism of cells will generate heat and the majority of the mass of the body is muscle mass as muscles contract their metabolic rate increases and shivering is a mechanism that increases the metabolic rate of skeletal muscles, primarily for the function of increasing the temperature of the body. Excitability and contractility are functional properties of muscle tissue. Excitability refers to the fact that muscles respond to stimulation. In the case of skeletal muscles as shown in this illustration, the stimulus comes from a motor neuron releasing neurotransmitters and these neurotransmitters bind to receptors on the surface of the skeletal muscle cells. This excitation then stimulates contraction. Contractility is the functional property referring to how muscles shorten when they are stimulated. There are three types of muscle tissue. The image in the top left shows skeletal muscle. In the middle we see smooth muscle and the bottom left is cardiac muscle. Cardiac muscle is located in the walls of the heart. Cardiac muscle tissue consists of branching striated cells called cardiomyocytes. Each has a central round nucleus and cardiomyocytes form cell junctions known as intercalated discs. Intercalated discs contain desmosomes and gap junctions. The desmosomes hold two intercalated discs tightly together so that as a cardiomyocyte contracts and pulls on the adjacent cell, the cells are not pulled apart. Gap junctions allow ions to flow from one cardiomyocyte to the next and these ions spreading through gap junctions stimulate the excitation of cardiomyocytes to contract. Cardiac muscle is under involuntary control. There are local pacemaker cells found within the heart that generate the excitation that spreads through the heart by traveling through gap junctions from one cell to the next to stimulate contraction. Although the contraction mechanism is involuntary and stimulated by that local internal pacemaker, the activity of the heart can still be regulated by the nervous system. The autonomic nervous system provides this involuntary regulation of the heart. Smooth muscle tissue consists of muscle fibers that have a spindle shape with tapered ends and a centrally located nucleus. There are no striations in smooth muscle and this is why we call it smooth muscle. Smooth muscles found in the walls of hollow organs, for example, in the digestive tract. The stomach and intestines contain smooth muscles that can contract in order to help with digestion and to propel the contents through the digestive tract. There's also smooth muscles in the eyes. The iris is the smooth muscle that controls the diameter of the pupil to regulate the amount of light entering the eye and the ciliary muscles control the shape of the lens in order to focus light onto the photoreceptors in the retina of the eye. Smooth muscle is involuntary. It is regulated by the autonomic nervous system. There are different types of smooth muscle known as single unit and multi unit smooth muscle. The single unit smooth muscle is stimulated by local pacemaker cells similar to the way that cardiac muscle has local pacemaker cells. Whereas multi unit smooth muscles are individually stimulated by a autonomic motor neuron. Skeletal muscle tissue consists of long cylindrical cells known as skeletal muscle fibers that each contain multiple nuclei and have striations, alternating light and dark bands resulting from the overlapping thick and thin myofilaments organized into sarcomeres which are the structural and functional units of contraction. Multiple skeletal muscle fibers are bundled together in fascicles wrapped with connective tissue and multiple fascicles are bundled together to form a skeletal muscle organ. The ends of the skeletal muscle organ are tendons that connect the skeletal muscle to bones at the origin and insertion. The origin is the bony attachment where the skeletal muscle remains stationary during the action of that muscle and the insertion is the location where the muscle's tendon attaches to a bone that will move during the action performed by that muscle. For example, the biceps brachii performs the action of flexion at the humeral ulnar joint commonly known as the elbow joint. The origin of the biceps brachii is on the scapula at the supraglenoid tubercle and the corcoid process. The scapula remains stationary whereas the insertion of the biceps brachii on the radial tuberosity is what moves during the action of flexion of the glenohumeral joint. A broad tendon is known as an aponeurosis. The epicranial aponeurosis is an example. This is a broad tendon located on the surface of the cranium on the superior surface of the cranium and it connects to the frontalis muscle found on the anterior of the cranium overlying the frontal bone as well as the occipitalis muscle located on the posterior of the cranium overlying the occipital bone. Skeletal muscle contraction is voluntary. We will see the mechanism of excitation contraction coupling in skeletal muscle where a somatic motor neuron will send a command by releasing a neurotransmitter to stimulate the contraction of skeletal muscle. And this excitation mechanism is under voluntary control. You can decide when you want to contract any of your skeletal muscles voluntarily. Surrounding a skeletal muscle organ is a layer of connective tissue known as epimysium. The epimysium is continuous with the tendons of skeletal muscles. The bundles of muscle fibers known as muscle fascicles found within a skeletal muscle organ are each wrapped with a layer of connective tissue known as perimysium. Inside of a muscle fascicle there's loose areolar connective tissue surrounding the muscle fibers and this loose connective tissue is known as endomyceum. The plasma membrane on the surface of a muscle fiber is known as the sarcolemma and then within a muscle fiber are non-membranous organelles known as myofibrils that are formed from the overlapping thick and thin myofilaments. The sarcoplasmic reticulum is a version of the endoplasmic reticulum found in muscle fibers. The sarcoplasmic reticulum is found wrapped around myofibrils and its primary function for muscles is the storage and release of calcium during excitation to stimulate contraction. Transverse tubules or T-tubules are deep indentations on the sarcolemma, the surface of the plasma membrane which form a network of narrow fluid filled spaces or channels. Transverse tubules contain extracellular fluid and enable that extracellular fluid to come in close contact with the sarcoplasmic reticulum surrounding each myofibril. The terminal cisternae are expanded chambers of the sarcoplasmic reticulum located adjacent to the T-tubules. When excitation from a motor neuron travels through a skeletal muscle it will travel along the sarcolemma and down the T-tubules then stimulate the release of calcium from the terminal cisternae of the sarcoplasmic reticulum and then that calcium will activate the myofilaments within the myofibrils to cause contraction. The sarcomere is the structural and functional unit of skeletal muscle and cardiac muscle that is the reason that these tissues have striations. So the striations are alternating light and dark bands, the dark bands are known as the A bands and the light bands are known as the I bands. The A bands contain the thick myofilaments and the motor protein myosin ATPase is found in the A band as the major protein of the thick myofilaments. The I bands contain the thin myofilaments and the major proteins found in the thin myofilaments are actin, tropomyosin and troponin. Actin is the same protein that forms the myofilaments of the cytoskeleton in all cells and actin has binding sites for myosin. So the myosin motor protein will pull on actin to produce contraction and there are two regulatory proteins in the thin filaments known as troponin and tropomyosin that regulate the binding of myosin to actin. The ends of the sarcomeres are known as the Z lines and these contain proteins that anchor the thin filaments in place. The H band is a central region within the sarcomere that contains thick filaments with no overlapping thin filaments. The M line is the center of the H band within the A band. And so while the A band is the entire length of the thick myofilaments, there are some overlapping thin myofilaments within the A band. And the I band is the Z discs as well as the regions of the thin myofilaments that have no overlapping thick myofilaments. Contraction of skeletal muscle is voluntary. The excitation mechanism involves a somatic motor neuron releasing the neurotransmitter acetylcholine to stimulate excitation of the skeletal muscle. This excitation generates an action potential which is an electrical impulse that spreads along the sarcolemma down the T-tubules and activates the release of calcium from the terminal cisternae of the sarcoplasmic reticulum. Then calcium will bind to troponin within the thin myofilaments causing tropomyosin to move off of the binding sites of actin, enabling myosin to form a cross bridge to bind to actin. Myosin then perform the power stroke cycle involving a pivoting of the myosin head, pulling on actin, producing the movement of contraction.