Muscular System. Human Physiology

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1 Human Physiology 2 nd Stage Assist lecturer: Karrar Abbas Lec: 3 Muscular System The muscular system is an organ system consisting of skeletal, smooth and cardiac muscles. It permits movement of the body, maintains posture, producing body heat and circulates blood throughout the body. The muscular system in vertebrates is controlled through the nervous system, although some muscles (such as the cardiac muscle) can be completely autonomous. Together with the skeletal system it forms the musculoskeletal system, which is responsible for movement of the human body. Muscles There are three distinct types of muscles: skeletal muscles, cardiac muscles and smooth (non-striated) muscles. Muscles provide strength, balance, posture, movement, and heat of the body. Skeletal muscle There are approximately 639 skeletal muscles in the human body. Skeletal muscle is a form of striated muscle tissue which is under the control of the somatic nervous system. Most skeletal muscles are attached to bones by bundles of collagen fibers known as tendons. Skeletal muscle is made up of individual muscle cells or myocytes, known as muscle fibers. Muscle fibers are cylindrical, and multinucleated. Muscle fibers are in turn composed of myofibrils. The myofibrils are composed of two kinds of protein filaments: actin and myosin myofilaments, repeated in units called sarcomeres, the basic structural and functional units of the muscle fiber, because it is the smallest portion of skeletal muscle capable of contracting. Each sarcomere extends from one Z line to another Z line. The arrangement of the actin and myosin filaments is responsible for the striated appearance of skeletal muscle, and forms the basic machinery necessary for muscle contraction. 1

2 The light bands contain only actin filaments and are called I bands. The dark bands contain myosin filaments, as well as the ends of the actin filaments where they overlap the myosin, and are called A bands. When the muscle fiber is contracted, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. Smooth muscles Smooth muscle is an involuntary non-striated muscle, controlled directly by the autonomic nervous system. Smooth muscle is found within the walls of blood vessels such as in the aorta and small arteries, arterioles and veins. Smooth muscle is also found in lymphatic vessels, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract and iris of the eye. The structure and function is basically the same in smooth muscle cells in different organs, but the inducing stimuli differ substantially, in order to perform individual effects in the body at individual times. 2

3 Smooth muscle is different from skeletal muscle and cardiac muscle in terms of structure, function and regulation of contraction. Smooth muscle cells known as myocytes, have a fusiform shape with single nuclei is seen and, like striated muscle, can tense and relax. However, smooth muscle tissue tends to demonstrate greater elasticity and function within a larger length tension curve than striated muscle. This ability to stretch and still maintain contractility is important in organs like the intestines and urinary bladder. Cardiac muscle Cardiac muscle (heart muscle) is involuntary striated muscle that is found in the walls and histological foundation of the heart, specifically the myocardium, is the muscle tissue of the heart. Cardiac muscles are distinct from skeletal muscles because the muscle fibers are laterally connected to each other. Furthermore, as with smooth muscles, they are not controlling themselves. Heart muscles are controlled by the sinus node influenced by the autonomic nervous system. Like skeletal muscle, the primary structural proteins of cardiac muscle are myosin and actin. The actin filaments are thin, causing the lighter appearance of I bands in striated muscle, whereas the myosin filament is thicker, lending a darker appearance to A bands. The cells that constitute cardiac muscle, called cardiomyocytes, contain only three nuclei. In contrast to skeletal muscle, cardiac muscle cells are typically branch-like instead of cylindrical. Coordinated contractions of cardiac muscle cells in the heart propel blood through circulatory systems. Neuromuscular junction Motor neurons are nerve cells that carry action potentials to skeletal muscle fibers. Its branch forms a junction with a muscle fiber, called a neuromuscular junction or synapse, is a junction between nerve and muscle by which that a motor neuron is able to transmit a signal to the muscle fiber, causing muscle contraction. Motor neurons release acetylcholine (ACh), a small molecule neurotransmitter, when an action potential reaches the presynaptic terminal of a motor neuron. Acetylcholine 3

4 diffuses across the synaptic and binds to its receptors on the cell membrane of the muscle fiber, also known as the sarcolemma. Muscle contraction Muscle contraction is the activation of tension generating sites within muscle fibers. In physiology, muscle contraction does not mean muscle shortening because muscle tension can be produced without changes in muscle length such as holding a heavy book. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state. General Mechanism of Muscle Contraction 1. An action potential travels along a motor nerve to its endings on muscle fibers. 2. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine. 3. The acetylcholine acts on a local area of the muscle fiber membrane to open multiple acetylcholine-gated channels through protein molecules floating in the membrane. 4. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This initiates an action potential at the membrane. 5. The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes. 6. The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum. 7. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process. 8. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca ++ membrane pump, and they remain stored in the reticulum until a new muscle action potential comes along; this removal of calcium ions from the myofibrils causes the muscle contraction to cease. 4

5 1. Action potential travels along a motor nerve 11. Removal of muscle contraction 2. Secretes acetylcholine 10. Removal of calcium ions from the myofibrils 3. Open multiple acetylcholinegated channels 9. Calcium ions are pumped back into sarcoplasmic reticulum 4. Sodium ions diffuse to the interior of the muscle fiber membrane 8. Initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other 5. Initiates action potential travels along the muscle fiber membrane 7. Sarcoplasmic reticulum release large quantities of calcium ions 6. Depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber Muscle Fatigue Muscle fatigue, or physical fatigue, is the decline in ability of a muscle to generate force. It can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction. There are two main causes of muscle fatigue: 1. The limitations of a nerve s ability to generate a suitable signal (neural fatigue). After a period of maximum contraction, the nerve s signal reduces in frequency and the force generated by the contraction diminishes. 2. The reduced ability of the muscle fiber to contract (metabolic fatigue). Due to the direct or indirect effects of two main factors: 5

6 i. Shortage of fuel (substrates) within the muscle fiber. ii. Accumulation of substances (metabolites) within the muscle fiber, which interfere either with the release of calcium (Ca 2+ ) or with the ability of calcium to stimulate muscle contraction. Myalgia Myalgia, or muscle pain, is a symptom of many diseases and disorders. The most common causes are the overuse injury or strain. Longer-term myalgias may be indicative of a metabolic myopathy, some nutritional deficiencies or chronic fatigue syndrome. 6