Copyright © 2012 Pearson Education, Inc. Chapter 6 The Muscular System Betty McGuire Cornell University Lecture Presentation

Copyright © 2012 Pearson Education, Inc. The Muscular System  Function and characteristics of muscles  Skeletal muscles working in pairs  Contraction of muscles  Voluntary movement  Energy for muscle contraction  Slow-twitch and fast-twitch muscle cells  Building muscle

Copyright © 2012 Pearson Education, Inc. Function and Characteristics of Muscles  Three types of muscle  Skeletal  Cardiac  Smooth

Copyright © 2012 Pearson Education, Inc. 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)

Copyright © 2012 Pearson Education, Inc. 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

Copyright © 2012 Pearson Education, Inc. Skeletal Muscles Working in Pairs  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

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Skeletal Muscles Working in Pairs  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

Copyright © 2012 Pearson Education, Inc. Skeletal Muscles Working in Pairs  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

Copyright © 2012 Pearson Education, Inc. Skeletal Muscles Working in Pairs  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

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  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

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  A muscle cell = a muscle fiber  When skeletal muscle cells are viewed under a microscope, they have distinct bands called striations  The striations are formed by the arrangement of myofibrils within the cell  Myofibrils are specialized bundles of proteins

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  Each myofibril contains two types of myofilaments  Myosin (thick) filaments  Actin (thin) filaments  Actin filaments are more numerous

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  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

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  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

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Contraction of Muscles  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

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  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

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Contraction of Muscles  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

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  When a muscle is relaxed, the troponin- tropomyosin 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

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Contraction of Muscles  Sarcoplasmic reticulum  Form of smooth endoplasmic reticulum found in muscle cells  Stores calcium ions  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

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  Rigor mortis  Muscle contraction will occur as long 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

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  Role of nerves in muscle contraction  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

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  Role of nerves in muscle contraction (cont.)  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

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Contraction of Muscles Web Activity: Muscle Structure and Function

Copyright © 2012 Pearson Education, Inc. Contraction of Muscles  Muscular dystrophy (MD)  If too many calcium ions enter a muscle cell, then proteins may be destroyed, eventually causing the cell to die; on a large scale, muscles weaken  MD = a group of inherited conditions in which 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

Copyright © 2012 Pearson Education, Inc. 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

Copyright © 2012 Pearson Education, Inc. Voluntary Movement  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

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Voluntary Movement  Motor units and recruitment  The strength of muscle contraction can be increased by increasing the number of motor units that are stimulated  This process, performed by the nervous system, is called recruitment

Copyright © 2012 Pearson Education, Inc. Voluntary Movement  Muscle twitch  Contraction of a muscle in response to a single stimulus  Twitches are very brief and typically not part of normal movements

Copyright © 2012 Pearson Education, Inc. Voluntary Movement  If a second stimulus is received before the muscle is fully relaxed, the second twitch will be stronger than the first, due to summation

Copyright © 2012 Pearson Education, Inc. Voluntary Movement  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

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Energy 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

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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

Copyright © 2012 Pearson Education, Inc. 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

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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

Copyright © 2012 Pearson Education, Inc. 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

Copyright © 2012 Pearson Education, Inc. Building Muscle PLAY | Mitchell Report

Copyright © 2012 Pearson Education, Inc. Building Muscle PLAY | Steroids