Learning Objectives List the four traits that all muscle types have in common. CHAPTER 6 The Muscular System Demonstrate and explain the use of antagonistic muscle pairs. Describe the attachment of muscle to bone and how it leads to movement. Define the terms origin and insertion as they apply to muscle anatomy. Compare the terms muscle fiber and fascicles. Discuss the banded appearance of striated muscles using the terms myofibril, myofilaments, actin, and myosin. Explain muscle contraction at the molecular level by describing the sliding filament model. Explain why ATP is essential to muscle contraction and relaxation. Describe the role of the tropomyosin-troponin complex in muscle contraction. Draw a neuromuscular junction, and explain the role of acetylcholine in muscle contraction and the release of calcium ions. Describe the ways to vary the contraction of whole muscles. Explain the process of motor unit recruitment. Differentiate between a muscle twitch, a stronger contraction resulting from wave summation, tetanus, and fatigue. List the sources of ATP for muscle contraction, and describe in detail where and how the ATP is generated. Compare and contrast slow-twitch and fast-twitch muscles, including where they are located in the body and when they are utilized in different physical activities. Describe the best way to build muscle endurance and the requirements for building larger muscle mass. Function and Characteristics of Muscles All muscles are Excitable (they respond to stimuli) Contractile (they can shorten) Extensible (they can stretch) Elastic (they can return to their original length after being shortened or stretched) Three types of muscle Skeletal Cardiac Smooth Function and Characteristics of Muscles Skeletal muscles are voluntary muscles responsible for Moving our body Maintaining posture Supporting internal organs Pushing against veins and lymphatic vessels to move blood and lymph along Generating heat Skeletal Muscles Working in Pairs Figure 6.1 Skeletal muscle. The body has more than 600 skeletal muscles Synergistic muscles Muscles that must contract at the same time to cause movement Antagonistic muscles Movement is produced when one muscle of the pair contracts and the other relaxes Example: the biceps muscle and triceps muscle of the upper arm 1
Figure 6.2 Some major muscles of the body. Damage to Skeletal Muscles or Tendons Most of the major muscles we use for locomotion, manipulation, and other voluntary movements are attached to bones Tendon Band of connective tissue that attaches a muscle to a bone Origin of a muscle The end attached to the bone that remains relatively stationary during movement Insertion of a muscle The end attached to the bone that moves Damage to Skeletal Muscles or Tendons Tendinitis Condition of having an inflamed tendon Caused by overuse, misuse, or age Healing is slow because tendons have a poor blood supply Most effective treatment is rest Damage to Skeletal Muscles or Tendons Muscle pull Also called a muscle strain or tear Caused by overstretching that damages the muscle or tendon Treatment includes ice to reduce swelling and keeping the muscle stretched An entire, intact muscle is formed from individual muscle fibers grouped in increasingly larger bundles, each wrapped in a connective tissue sheath Fascicle: a bundle of muscle cells A skeletal muscle has many fascicles Each fascicle is surrounded by its own connective tissue sheath The connective tissue sheaths of fascicles merge at the ends of muscles to form tendons that attach the muscle to bone A muscle cell a muscle fiber When skeletal muscle cells are viewed under a microscope, they have distinct bands called striations formed by the arrangement of myofibrils within the cell Myofibrils are specialized bundles of proteins Each myofibril contains two types of myofilaments Myosin (thick) filaments Actin (thin) filaments are more numerous 2
Figure 6.3 The structure of a skeletal muscle. Each myofibril has tens of thousands of contractile units, called sarcomeres. The ends of each sarcomere are marked by dark protein bands called Z lines. Within each sarcomere the actin and myosin filaments are specifically arranged One end of each actin filament is attached to a Z line Myosin filaments lie in the middle of the sarcomere, and their ends partially overlap with surrounding actin filaments. The degree of overlap increases when the muscle contracts Muscle contraction occurs at the molecular level According to the sliding filament model, a muscle contracts when actin filaments slide past myosin filaments, shortening the sarcomere Myosin molecules are shaped like two-headed golf clubs. The club-shaped myosin heads are key to moving actin filaments The myosin head, also known as a cross-bridge, attaches to a nearby actin filament Then the head bends and swivels, pulling the actin filament toward the midline of the sarcomere The myosin head disengages from the actin filament The movements of myosin require ATP The cycle begins again Figure 6.4 When a muscle contracts, actin filaments slide past myosin filaments. Muscle contraction is controlled by the availability of calcium ions Muscle cells contain the proteins troponin and tropomyosin The troponin-tropomyosin complex and calcium ions regulate muscle contraction at the actin-myosin binding sites 3
Figure 6.5 Calcium ions initiate muscle contraction. When a muscle is relaxed, the troponintropomyosin complex covers the actin-myosin binding sites Muscle contraction occurs when calcium ions bind to troponin, causing it to change shape This change in shape moves tropomyosin, exposing the actin-myosin binding sites Calcium Ions and Regulatory Proteins Calcium Ions and Regulatory Proteins Calcium ions are stored in the sarcoplasmic reticulum, a form of smooth endoplasmic reticulum found in muscle cells Transverse tubules (T tubules) Pockets in the plasma membrane of a muscle cell Carry signals from motor neurons deep into the muscle cell to every sarcomere Rigor mortis Muscle contraction will occur as long as ATP is present Without ATP, cross-bridges cannot be broken Within 3 to 4 hours after death, the muscles become stiff rigor mortis Actin and myosin gradually break down and muscles relax again after 2 to 3 days Role of Nerves Role of Nerves Neuromuscular junction Junction between the tip of a motor neuron and a skeletal muscle cell A nerve impulse travels down a motor neuron to the neuromuscular junction, where it causes the release of acetylcholine (a neurotransmitter) from the motor neuron Acetylcholine diffuses across a small gap and binds to receptors on the plasma membrane of the muscle cell The acetylcholine causes changes in the permeability of the muscle cell, resulting in an electrochemical message similar to a nerve impulse The message travels along the plasma membrane into the T tubules and then to the sarcoplasmic reticulum, releasing calcium ions for muscle contraction 4
Role of Nerves Figure 6.6 Neuromuscular junction. Web Activity: Muscle Structure and Function Muscular Dystrophy (MD) A group of inherited conditions in which muscles weaken: if too many calcium ions enter a muscle cell, proteins may be destroyed, eventually causing the cell to die; on a large scale, muscles weaken Duchenne muscular dystrophy One of the most common forms The gene for production of the protein dystrophin is defective Lack of dystrophin allows excess calcium ions to enter muscle cells, eventually killing the cells Voluntary Movement Motor unit: a motor neuron and all the muscle cells it stimulates All the muscle cells in a given motor unit contract together The number of muscle cells in a motor unit is highly variable Muscles responsible for precise movements have fewer muscle cells in each motor unit than do muscles responsible for less precise movements On average, there are 150 muscle cells in a motor unit Figure 6.7 A motor unit. Motor Units and Recruitment Increasing the number of motor units that are stimulated increases the strength of muscle contraction. This process, performed by the nervous system, is called recruitment. Muscle twitch Contraction of a muscle in response to a single stimulus Twitches are very brief and typically not part of normal movements If a second stimulus is received before the muscle is fully relaxed, the second twitch will be stronger than the first, due to summation 5
Muscle Twitches, Summation, and Tetanus A sustained, powerful contraction caused by very frequent stimuli Fatigue sets in when a muscle is unable to contract even when stimulated Changing the frequency of stimulation is another way to vary the contraction of muscles Figure 6.8 Muscle contraction shown graphically. Energy for Muscle Contraction Figure 6.9 Energy sources for muscle contraction. Muscle contraction requires an enormous amount of energy ATP for muscle contraction comes from many sources, typically used in sequence ATP stored in muscle cells Creatine phosphate stored in muscle cells Anaerobic metabolic pathways Aerobic respiration Slow-Twitch and Fast-Twitch Muscle Cells Slow-twitch muscle cells Contract slowly, with great endurance Abundant mitochondria Packed with myoglobin (oxygen-binding pigment) Dark, reddish appearance Myoglobin Rich blood supply Slow-Twitch and Fast-Twitch Muscle Cells Fast-twitch muscle cells Contract rapidly and powerfully but with much less endurance Can make and break cross-bridge attachments more rapidly Have more actin and myosin Rely on anaerobic metabolic pathways to generate ATP and therefore tire quickly 6
Figure 6.10 Slow- and fast-twitch muscle cells. Building Muscle Aerobic exercise Enough oxygen is delivered to the muscles to keep them going for long periods Increases endurance and coordination Promotes development of new blood vessels Increases the number of mitochondria Typically does not increase size of muscles Examples: walking, jogging, swimming Building Muscle Resistance exercise Builds strength Muscles increase in size when they are repeatedly made to exert more than 75% of their maximum force Increases in muscle size reflect increases in the diameter of existing muscle cells Example: weight lifting 7