Chapter 9 Muscular System

A. All movements require muscle. CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Introduction: A. All movements require muscle. B. The three types of muscle in the body are skeletal, smooth, and cardiac muscle. C. This chapter focuses on skeletal muscle.

Muscles CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. A. Each muscle is an organ, composed of specialized cells that use chemical energy to contract and relax. *Muscles are composed of muscle tissue, connective tissues, nervous tissue, and blood. Muscle functions include: 1)Body movement 2)Muscle tone 3) Propel food and fluids through the body 4)Generate the heartbeat 5)Distribute heat

B. Skeletal Muscle Fibers CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. B. Skeletal Muscle Fibers Muscle fiber = single, long, cylindrical muscle cell, which is multinucleated and has many mitochondria. It is filled with many threadlike myofibrils that lie parallel to one another.

CopyrightThe McGraw-Hill Companies, Inc CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Study Analogy Pretend you are going to play a joke on someone and give them 100 pencils. The pencils will represent muscle fibers. First you wrap each individual pencil in tissue paper (dense tissue paper of course!). This would be endomysium. Then you take about 10 pencils in a bundle (a fascicle) and wrap them in paper (perimysium). After that you take all the bundles and wrap them in gift wrap (epimysium). But you are going to mail this joke, so you also have to wrap it in brown paper representing the fascia.

Pages 172-173 (old text): Muscle Structure Fascia – layers of fibrous C.T. (connective tissue) wrapped around a muscle. -Tendon – cord of fascia at the ends of muscle that connects the muscle to bone -Aponeurosis – sheet of fascia that attaches muscle to muscle Epimysium – thin sheet of C.T. that surrounds a muscle Perimysium – thin sheet of C.T. that extends inward and surrounds a group of muscle fibers, forming a fascicle Fascicle – bundle of muscle fibers surrounded by perimysium Endomysium – thin sheet of C.T. that surrounds a muscle fiber Muscle fiber = muscle cell Sarcolemma – cell membrane of a muscle cell Sarcoplasm – cytoplasm of a muscle cell Myofibrils – threadlike fibers responsible for muscle contraction. Made of two types of filaments: Myosin – thick filaments Actin – thin filaments

Sarcomere The structural and functional unit of a myofibril. Extends from Z line to Z line Actin filaments extend inward from Z line. Myosin filaments remain stationary in center portion of sarcomere Myofibril = long chain of sarcomeres attached end to end.

Sliding Filament Model Sarcomere shortens when actin filaments slide inward. Actin and myosin filaments stay the same length. Cross-bridges on myosin (thick filament) attach to actin (thin filament) and pull them toward the center. The thin filaments slide past the thick filaments, shortening the sarcomere. Myofibril is millions of sarcomeres attached end to end. When sarcomeres contract, the myofibril shortens, contracting the muscle.

Contraction When a nervous impulse stimulates a muscle cell: 1) S.R. (sarcoplasmic reticulum) is stimulated. 2) Calcium channels in S.R. open. 3) Calcium ions diffuse from the S.R. into the sarcoplasm and bind to Troponin molecules. 4) Tropomyosin molecules move and expose binding sites on Actin. 5) “Cocked” Myosin crossbridges bind Actin and pull Actin towards the center of the sarcomere. 6) Muscle fiber contracts.

Muscle Relaxation 1) Acetylcholinesterase decomposes acetylcholine, so that muscle fiber is no longer stimulated 2) Calcium ions are pumped back into the sarcoplasmic reticulum. (Requires ATP) 3) ATP binds with Myosin crossbridge causes the bond between actin and myosin to break. (ATP is not hydrolyzed yet.) 4) ATP hydrolyzed by ATPase in Myosin crossbridge – this “cocks” the crossbridge. 5)Troponin and Tropomyosin inhibit interaction between Actin and the Myosin crossbridges. 6) Muscle fiber relaxed until the next stimulus.

Neuromuscular Junction Neuromuscular junction – Where axon of motor neuron synapses with the muscle fiber. Synapse – functional connection between synaptic knob of neuron and cell membrane of muscle fiber Synaptic knob – Enlargement at end of axon that secretes neurotransmitter (Acetylcholine) Motor end plate – specialized part of sarcolemma that forms the neuromuscular junction – highly folded, many mitochondria Synaptic cleft – narrow space between the synaptic knob and motor end plate. Neurotransmitter – chemical secreted at the synaptic knob that stimulates a muscle fiber or neuron. Acetylcholine (Ach) – neurotransmitter secreted by motor neurons to stimulate muscle fibers

Stimulus for Muscle Contraction Nerve impulse (Action potential) stimulates release of Ach (Acetylcholine) from axon  synaptic cleft Ach diffuses across synaptic cleft  binds Ach receptors on motor end plate Increased permeability of muscle cell to Na+ ions Na+ rushes in  stimulates muscle impulse (Action potential) Muscle impulse travels throughout the sarcolemma  T- tubules  to the sarcoplasmic reticulum Sarcoplasmic reticulum releases Ca++ ions from S.R. Ca++ binds Troponin  Tropomyosin moves to expose binding sites on the Actin.

Neuromuscular Junction