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© 2014 Pearson Education, Inc. Human Biology Concepts and Current Issues Seventh Edition Michael D. Johnson Lecture Presentations by Robert J. Sullivan Marist College 6 The Muscular System
© 2014 Pearson Education, Inc. Introduction to Muscles Muscle tissue is found in every organ Muscles participate in every activity that requires movement Large proportion of body weight is muscle –40% of body weight in males –32% of body weight in females Skeletal muscle: attaches to skeleton and provides strength and mobility Cardiac muscle: exclusively in the heart Smooth muscle: walls of digestive tract, blood vessels, uterus, ureters
© 2014 Pearson Education, Inc. Muscles Produce Movement or Generate Tension Muscles may produce movement –Voluntary: conscious control over movement (picking up a pen) –Involuntary: unconscious control over movement (beating of heart) Many muscles resist movement –Maintenance of posture –Maintenance of blood pressure Muscles generate heat
© 2014 Pearson Education, Inc. The Fundamental Activity of Muscle Is Contraction Excitable: contract in response to electrical or chemical stimuli All muscle cells have one mechanism of action: –They contract (shorten), then relax (lengthen)
© 2014 Pearson Education, Inc. Figure 6.1 Pectoralis major Draws arm forward and toward the body Serratus anterior Helps raise arm Contributes to pushes Draws shoulder blade forward Biceps brachii Bends forearm at elbow Rectus abdominus Compresses abdomen Bends backbone Compresses chest cavity External oblique Lateral rotation of trunk Compresses abdomen Adductor longus Flexes thigh Rotates thigh laterally Draws thigh toward body Sartorius Bends thigh at hip Bends lower leg at knee Rotates thigh outward Quadriceps group Flexes thigh at hips Extends leg at knee Tibialis anterior Flexes foot toward knee Trapezius Lifts shoulder blade Braces shoulder Draws head back Deltoid Raises arm Triceps brachi Straightens forearm at elbow Latissimus dorsi Rotates and draws arm backward and toward body Gluteus maximus Extends thigh Rotates thigh laterally Hamstring group Draws thigh backward Bends knee Gastrocnemius Bends lower leg at knee Bends foot away from knee Achilles tendon Connects gastrocnemius muscle to heel
© 2014 Pearson Education, Inc. Skeletal Muscles Cause Bones to Move 600 skeletal muscles Synergistic muscles: work together to created the same movement Antagonistic muscles: muscles that oppose each other Many muscles attach to bones via tendons Origin: end of muscle that attaches to relatively stationary bone Insertion: end of muscle attached to another bone across a joint, action pulls insertion toward origin
© 2014 Pearson Education, Inc. Figure 6.2 Scapula Shoulder joint Origins from scapula and humerus Triceps muscle Tendon Elbow joint Ulna Radius Tendon Biceps muscle Humerus Tendons Triceps relaxes Triceps contracts, pulling forearm down Biceps relaxes Biceps contracts, pulling forearm up Origins from scapula Insertion on ulna Insertion on radius Origin and insertion. The point of attachment of a muscle to the stationary bone is its origin; the point of attachment to the movable bone is its insertion. Movement. Antagonistic muscles produce opposite movements. The forearm bends when the biceps contracts and the triceps relaxes. The forearm straightens when the biceps relaxes and the triceps contracts.
© 2014 Pearson Education, Inc. A Muscle Is Composed of Many Muscle Cells Muscles –Group of muscle cells with same origin, insertion, and function Fasicles –Bundles of muscle fibers (cells) wrapped with connective tissue (fascia) Muscle fibers (muscle cells) –Long, tube shaped –Vary in length from few mm to 30 cm –Multinucleate –Packed with myofibrils, which are long cylindrical structures which contain proteins actin and myosin
© 2014 Pearson Education, Inc. Figure 6.3 Muscle bundle (fascicle) surrounded by connective tissue (fascia) Whole muscle Single muscle cell (fiber) Tendon Bone
© 2014 Pearson Education, Inc. Figure 6.4 Muscle cell Myofibril Nuclei Muscle cell A single muscle cell contains many individual myofibrils and has more than one nucleus. A photograph of portions of several skeletal muscle cells.
© 2014 Pearson Education, Inc. Animation: Muscle Structure and Function Right-click and select Play
© 2014 Pearson Education, Inc. The Muscle Contractile Unit Is the Sarcomere Sarcomere: contractile unit –Myosin: forms thick filaments –Actin: forms thin filaments Z Lines: attachment points for sarcomeres A sarcomere is a segment of myofibril extending from one Z line to the next Arrangement of filaments gives rise to striated appearance of skeletal muscle
© 2014 Pearson Education, Inc. Figure 6.5 Myofibril Z-line Sarcomere Myosin Actin Thin filament (actin) Thick filament (myosin) A closer view of a section of a myofibril showing that it is composed of sarcomeres joined end to end at the Z-line. An electron micrograph cross section of a sarcomere in a region that contains both actin and myosin. Sarcomeres contain thin filaments of actin that attach to the Z-lines and thicker filaments of myosin that span the gap between actin molecules. A transmission electron micrograph ( 11,300) of a longitudinal section of a sarcomere. The rounded red objects are mitochondria.
© 2014 Pearson Education, Inc. Individual Muscle Cells Contract and Relax Muscle contraction: each sarcomere shortens a little Basic process of contraction: 1.Skeletal muscle must be activated by a nerve 2.Nerve activation increases the concentration of calcium ions in the vicinity of the contractile proteins 3.Presence of calcium permits contractions 4.When nerve stimulation stops, contraction stops
© 2014 Pearson Education, Inc. Nerves Activate Skeletal Muscles Acetylcholine is released from motor neuron at neuromuscular junction Electrical impulse transmitted along T tubules Calcium (Ca ) is released from sarcoplasmic reticulum (modified smooth endoplasmic reticulum) Ca initiates chain of events that cause contraction when it contacts the myofibrils
© 2014 Pearson Education, Inc. Figure 6.6 Motor neuron Electrical impulse T tubule Sarcoplasmic reticulum Muscle cell plasma membrane Z-line Myofibrils Ca 2 Acetylcholine The release of acetylcholine at the neuromuscular junction causes an electrical impulse to be generated in the muscle cell plasma membrane The electrical impulse triggers the release of Ca 2 from the sarcoplasmic reticulum The electrical impulse ( ) is carried to the cell’s interior by the T tubules 1 2 3
© 2014 Pearson Education, Inc. Calcium Initiates the Sliding Filament Mechanism Thick filaments: myosin Thin filaments: actin Contraction: formation of cross-bridges between thin and thick filaments Ca must be present for cross-bridges to form –In absence of Ca , protein complex of troponin- tropomyosin covers myosin binding sites on actin molecules –Presence of Ca , binding sites available
© 2014 Pearson Education, Inc. Figure 6.7 Myofibril Thick filament Thin filament Myosin molecule head Myosin molecule Actin molecule Relaxed state. The myosin heads do not make contact with actin. Contraction. The myosin heads form cross-bridges with actin and then bend, pulling the actin filaments toward the center of the sarcomere.
© 2014 Pearson Education, Inc. When Nerve Activation Ends, Contraction Ends 1.Calcium is released from sarcoplasmic reticulum 2.Calcium binds to troponin 3.Troponin–tropomyosin complex shifts position 4.Myosin binding site is exposed 5.Myosin heads form cross-bridges with actin 6.Actin filaments are pulled toward center of sarcomere 7.Sarcomere shortens
© 2014 Pearson Education, Inc. Figure 6.8 Myofibril Sarcoplasmic reticulum Thick and thin filaments Tropomyosin Actin filament Troponin Myosin head Myosin filament Sarcoplasmic reticulum Electrical impulse Calcium release Myosin binding sites Cross- bridge Ca 2 Resting sarcomere. In the absence of calcium the muscle is relaxed because the myosin heads cannot form cross-bridges with actin. Cross-bridge attachment. The binding of calcium to troponin causes a shift in the troponin-tropomyosin complex, allowing cross-bridges to form.
© 2014 Pearson Education, Inc. Muscles Require Energy to Contract and to Relax Nerve activation ends, contraction ends Ca pumped back into sarcoplasmic reticulum (requires ATP) Ca no longer bound to troponin Myosin binding site covered ATP must bind to myosin before myosin heads can detach from actin No calcium no cross-bridges Muscle relaxes
© 2014 Pearson Education, Inc. Muscles Require Energy to Contract and to Relax Principle source of energy: ATP ATP required for contraction ATP required for relaxation ATP is replenished by a variety of means –Creatine phosphate –Stored glycogen –Aerobic metabolism of glucose, fatty acids, and other high-energy molecules
© 2014 Pearson Education, Inc. Table 6.1
© 2014 Pearson Education, Inc. The Activity of Muscles Can Vary Isotonic contractions: muscle shortens, while maintaining a constant force, movement occurs Isometric contractions: force generated, muscle doesn’t shorten, no movement Degree of nerve activation influences force Terms to know: –Motor unit –Muscle tension –All-or-none principle –Muscle tone –Recruitment
© 2014 Pearson Education, Inc. The Degree of Nerve Activation Influences Force Motor unit –Motor neuron and all the muscle cells it controls –Smallest functional unit of muscle contraction Muscle tension –Mechanical force that muscles generate when they contract –Determined by –Motor unit size –Number of active motor units –Frequency of stimulation of motor units
© 2014 Pearson Education, Inc. The Degree of Nerve Activation Influences Force All-or-none principle –Individual muscle cells are completely contracting or are relaxed Muscle tone –Whole muscles—maintain intermediate level of force known as muscle tone Recruitment –Activation of additional motor units increases muscle tone
© 2014 Pearson Education, Inc. Figure 6.9 Muscle Muscle cells Neuromuscular junctions Two motor neurons A motor unit consists of a motor neuron and all of the muscle cells it controls. Any one muscle cell is controlled by only one motor neuron, but a motor neuron controls more than one muscle cell. Photograph of the muscle cells in a motor unit, showing branches of the motor neuron and neuromuscular junctions.
© 2014 Pearson Education, Inc. The Degree of Nerve Activation Influences Force Complete cycle of contraction-relaxation in response to stimulus Can be observed using a myogram (laboratory recording of muscle activity) –Latent period –Contraction –Relaxation –Summation –Tetanic contraction
© 2014 Pearson Education, Inc. Figure 6.10 Tetanus Summation Latent period Contraction Relaxation 500 Stimulus 0 Time (msec) Muscle force
© 2014 Pearson Education, Inc. Slow Twitch versus Fast Twitch Fibers Slow Twitch Contract slowly Make ATP as needed by aerobic metabolism Many mitochondria Well-supplied with blood vessels Store very little glycogen “Red” muscle Used for endurance activities Fast Twitch Contract quickly Rapidly break down ATP Fewer mitochondria Little or no mitochondria Store a lot of glycogen “White” muscle Capable of anaerobic metabolism Used for brief high-intensity activities
© 2014 Pearson Education, Inc. Exercise Training Improves Muscle Mass, Strength, and Endurance Strength training –Resistance training –Short, intense –Builds more myofibrils, particularly in fast-twitch fibers Aerobic training –Builds endurance –Increases blood supply to muscle cells –Increase in mitochondria and myoglobin –Reach target heart rate for at least 20 minutes, three times a week
© 2014 Pearson Education, Inc. Cardiac and Smooth Muscles Have Special Features Involuntary Able to contract entirely on their own in absence of nerve stimulation Cardiac muscle cells are joined by intercalated disks –Have gap junctions allowing cells to electrically stimulate the next one Smooth muscle cells joined by gap junctions allowing cells to activate each other Cardiac and smooth muscle cells respond to stimulation from autonomic nervous system, which can modify the degree of contraction
© 2014 Pearson Education, Inc. Figure 6.12 Intercalated disc Cardiac muscle cell Adhesion junction Protein channel Gap junction Cell membranes of adjacent cells A closer view showing that intercalated discs are bridged by gap junctions that permit direct electrical connections between cells. A view of several adjacent cardiac muscle cells showing their blunt shape and the intercalated discs that join them together.
© 2014 Pearson Education, Inc. Speed and Sustainability of Contraction Skeletal muscle: fastest Cardiac muscle: moderate Smooth muscle: –Very slow –Partially contracted all of the time –Almost never fatigues
© 2014 Pearson Education, Inc. Arrangement of Myosin and Actin Filaments Cardiac muscle –Sarcomere arrangement of thick and thin filaments –Striated appearance Smooth muscle –Filaments arranged in criss-crossed bundles, not sarcomeres –No striations
© 2014 Pearson Education, Inc. Figure 6.13 Filament bundles Thin filament Thick filament Cell membrane protein Relaxed state. Contracted state. The crisscross arrangement of bundles of contractile filaments causes the cell to become shorter and fatter during contraction. A closer view showing how actin filaments are attached to cell membrane proteins.
© 2014 Pearson Education, Inc. Diseases and Disorders of the Muscular System Muscular Dystrophy –Genetic disease: Duchenne Muscular Dystrophy –Modified dystrophin protein enables leakage of Ca into cells –Extra Ca activates enzymes that destroy muscle proteins –Muscle weakening and wasting –Muscle mass is replaced with fibrous connective tissue –Life expectance: approx. 30 years
© 2014 Pearson Education, Inc. Diseases and Disorders of the Muscular System Tetanus –Infection of deep wound by bacteria, Clostridium tetani –Bacteria produce tetanus toxin which causes muscles to contract forcefully –Death due to respiratory failure –Preventable by tetanus vaccine
© 2014 Pearson Education, Inc. Diseases and Disorders of the Muscular System Muscle cramps: often caused by dehydration and ion imbalances Pulled muscles: result from overstretching of a muscle, fibers tear apart Fasciitis: inflammation of fascia –Plantar fasciitis: sole of foot
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