Muscular System- contains over 700 muscles

Some functions of Skeletal Muscle Movement of body parts Maintains posture Muscle contraction generate heat which helps regulate body temperature Allow us to change our facial expressions

Characteristics of Muscles All muscles are excitable, contractile, extensible, and elastic Excitable: respond to stimuli Contractile: shorten Extensible: stretch Elastic: return to their original length after being shortened or stretched

Skeletal Muscles Work in Opposing Pairs Each muscle is attached to a bone by a tendon The origin of the muscle is attached to the bone that remains stationary during movement The insertion is attached to the bone that moves Bones act as levers in working with skeletal muscles to produce movement

Skeletal Muscles Work in Pairs Muscles work in opposing pairs called antagonistic pairs Agonist- prime mover; It contracts during a movement Antagonist-muscle that relaxes during a movement Synergist- muscle that assist in a movement

Ways Skeletal Muscles are Named Direction of muscle fibers Size of muscle Shape of the muscle Location of the muscle Number of heads of origins Origin & Insertion Action of the muscle

Naming Some Superficial Skeletal Muscles Triceps brachii • Straightens arm at elbow Pectoralis major • Flexes and rotates arm Serratus anterior • Boxer’s muscle; important in Reaching and pushing External oblique • lateral rotation of trunk (twisting from side to side) compresses abdomen Rectus abdominis • Compresses abdomen and Flex the trunk Adductor longus • adducts and flexes thigh At the hip joint Sartorius • Bends lower leg at knee, bends and rotates thigh at hip Quadriceps femoris • extends leg at knee Tibialis anterior • Flexes foot toward knee (a) Front view Figure 6.2a

Naming Some Superficial Skeletal Muscles Biceps brachii • Bends forearm at elbow, Deltoid • Raises arm Trapezius • Raises, turns, and lowers the shoulder; turns head Latissimus dorsi • “Swimmer’s muscle” Rotates and draws arm backwards (toward the body) Gluteus maximus • Extends and rotates thigh when walking Hamstring group • bend the leg at the knee Gastrocnemius • Raises the foof off the floor (b) Back view Figure 6.2b

Parts of a skeletal muscle

Parts of a skeletal muscle A muscle is made up of bundles of muscle cells (fibers) Each bundle is called a fascicle Epimysium- connective tissue covering of a muscle Perimysium- connective tissue covering of fascicle Endomysium- connective tissue covering of each muscle cell (fiber)

Myofibrils Myofibril- is a rod-like structure within a muscle cell (fiber) Figure 6.3

Myofibrils Myofibril- contain striations produced by alternating thick (myosin) and thin (actin) filaments Figure 6.3

Sacromeres (c) A diagram and Z line One sarcomere (c) A diagram and electron micrograph of a myofibril Z line Z line One sarcomere Actin Myosin (d) A sarcomere, the contractile unit of a skeletal muscle, contains actin and myosin myofilaments. Figure 6.3c–d

Sacromeres A Sarcomere is a unit on a myofibril The ends of each sarcomere are marked by dark protein bands called Z lines actin and myosin proteins alternate within the sarcomere

Sacromeres Actin is attached to the z-line. Myosin filaments lie in the middle of the sarcomere, they are not attached directly to the z-line. Video Myosin attaches to actin and pulls it toward the center of the sarcomere to produce muscle contraction

Neuromuscular Junction Neuromuscular Junction is a junction between a motor neuron and a muscle membrane Figure 6.7 (1 of 2)

Skeletal Muscle Contraction Contraction is triggered when a nerve impulse travels down a motor neuron until it reaches the neuromuscular junction The motor neuron releases acetylcholine into the synaptic cleft. Acetylcholine binds to receptors on the muscle membrane

Skeletal Muscle Contraction The binding of acetylcholine to the muscle membrane causes Ions to flow across the membrane generating an impulse Figure 6.7 (2 of 2)

Skeletal Muscle Contraction Structures called T- tubules carry the impulse inside the muscle cell to the sarcoplasmic reticulum which release Ca++ When Ca ++ is free inside a muscle cell, myosin attaches to actin and pulls it producing muscle contraction Figure 6.7 (2 of 2)

Muscle Contraction Spinal cord Motor neuron Neuromuscular junction Skeletal muscle cells Motor unit Figure 6.8

Skeletal Muscle Contraction Contraction is triggered when a nerve impulse travels down a motor neuron until it reaches the neuromuscular junction The motor neuron releases acetylcholine into the synaptic cleft. Acetylcholine binds to receptors on the muscle membrane The binding of acetylcholine to the muscle membrane causes Ions to flow across the membrane generating an impulse Structures called T- tubules carry the impulse inside the muscle cell to the sarcoplasmic reticulum which release Ca++ When Ca ++ is free inside a muscle cell, myosin attaches to actin and pulls it producing muscle contraction

Myogram- a recording of a muscle contraction Figure 6.9

Myograms Muscle twitch- single contraction Wave summation- three to four contractions Taken to the extreme, a sustained powerful contraction is called tetanus

ATP Comes from Many Sources Figure 6.10

ATP for Muscle Contraction Comes from Many Sources Sources of ATP: The initial source is the ATP stored in the muscle cells then the ATP formed from the creatine phosphate reserves When those sources are depleted, the muscles depend upon stored glycogen Glycogen is converted to glucose and then to ATP through aerobic respiration or lactic acid fermentation

Skeletal muscles contain both Slow and Fast twitch cells Slow-twitch cells Loaded with mitochondria, and therefore deliver prolonged, strong contractions Fast-twitch cells Contract rapidly and powerfully but with much less endurance They rely on lactic acid fermentation as their source of energy, and therefore tire quickly

Slow twitch Fibers Figure 6.11 (1 of 2)

Fast-Twitch Fibers Figure 6.11 (2 of 2)

Aerobic Exercise; Resistance Exercise Increases endurance and coordination Resistance exercise Builds strength