Essentials of Anatomy & Physiology, 4th Edition Martini / Bartholomew PowerPoint ® Lecture Outlines prepared by Alan Magid, Duke University The Muscular System The Muscular System 7 7 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings C

Muscle Mechanics Some Basic Muscle Definitions Muscle tension—The pulling force on the tendons that muscle cells generate when contracting Muscle twitch—A brief contraction-relaxation response to a single action potential Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Mechanics The Twitch and Development of Tension Figure 7-6

Muscle Mechanics The Frequency of Muscle Fiber Stimulation Summation—Addition of twitch tension when a stimulus is applied before tension has completely relaxed Incomplete tetanus—Tension peaks and falls repeatedly and builds up beyond twitch tension Complete tetanus—Tension is steady (no relaxation phase) and largest if stimuli arrive at very high rates Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Mechanics The Effects of Repeated Stimulations Figure 7-7

Muscle Mechanics Motor Units Motor Unit —A motor neuron and all the muscle cells it controls Recruitment—To increase muscle tension by activating more motor units Small motor units provide finer control Motor units are intermixed in the muscle to pull evenly on the tendon Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Mechanics Motor Units Figure 7-8

Muscle Mechanics Key Note All voluntary (intentional) movements involve the sustained, sub-tetanic contractions of skeletal muscle fibers organized into distinct motor units. The force generated can be increased by increasing the frequency of action potentials (=impulses) or by recruiting additional motor units. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Mechanics Muscle tone—Tension in a “resting” muscle produced by a low level of spontaneous motor neuron activity. Distinct from resting tension produced by passive stretching. Function of muscle tone Stabilizes bones, joints Prevents atrophy (muscle wasting ) Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Mechanics Types of Contractions Isotonic contraction The tension (load) on a muscle stays constant (iso = same, tonic = tension) during a movement. (Example: lifting a baby) Isometric contraction The length of a muscle stays constant (iso = same, metric = length) during a “contraction” (Example: holding a baby at arms length) Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Mechanics Muscle Elongation Muscle contracts actively Muscles can only pull Muscles never push Muscle elongates passively Elastic forces Contraction of opposing muscles Effects of gravity Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Energetics of Muscle Contraction ATP and Creatine Phosphate Reserves Muscle contraction consumes much ATP ATP transfers energy directly to cycling cross-bridges and calcium pumping CP stores energy and regenerates ATP CP transfers its energy to ADP Creatine phosphokinase (CPK) catalyzes ADP (2 “P”s) becomes ATP(3 “P”s) CP levels greatly exceed ATP levels Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Energetics of Muscle Contraction ATP Generation Light activity Aerobic metabolism of fatty acids Storage of glucose as glycogen Moderate activity Breakdown of glycogen to glucose Glycolysis of glucose Peak activity Anerobic breakdown of glucose Production of lactic acid Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Energetics of Muscle Contraction Muscle Metabolism Figure 7-9(a)

Energetics of Muscle Contraction Muscle Metabolism Figure 7-9(b)

Energetics of Muscle Contraction Muscle Metabolism Figure 7-9(c)

Energetics of Muscle Contraction Muscle Fatigue—When a muscle loses ability to contract due to a low pH (lactic acid buildup), low ATP levels, or other problems Recovery Period—Time after muscle activity that it takes to restore pre- exertion conditions Oxygen Debt—Amount of excess oxygen used during the recovery period Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Energetics of Muscle Contraction Key Note Skeletal muscles at rest metabolize fatty acids and store glycogen. During light activity, muscles can generate ATP through the aerobic breakdown of carbohydrates, lipids, or amino acids. At peak levels of activity, most of the energy is provided by anaerobic reactions that generate lactic acid. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Performance Two Types of Skeletal Muscle Fibers Fast fibers Large diameter, abundant myofibrils, ample glycogen, scant mitochondria. Produce powerful, brief contractions Slow fibers Smaller diameter, rich capillary supply, many mitochondria, much myoglobin. Produce slow, steady contractions Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Performance Physical Conditioning Anaerobic endurance Time over which a muscle can contract effectively under anerobic conditions. Hypertrophy Increase in muscle bulk. Can result from anerobic training. Aerobic endurance Time over which a muscle can contract supported by mitochondria. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Performance Key Note What you don’t use, you lose. When motor units are inactive for days or weeks, muscle fibers break down their contractile proteins and grow smaller and weaker. If inactive for long periods, muscle fibers may be replaced by fibrous tissue. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings