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Lecture 21

01:119:116 Lecture 21: Lecture 21 - Musculoskeletal System

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Rutgers University
Biological Sciences
Dr.Christy Beal

Lecture 21 - Musculoskeletal System I. Vertebrate Skeletal Muscle A. Introduction to Muscle ➢ Muscle tissue generates mechanical force in response to nervous system (What?) ○ What are some of the functions of muscles? ➢ 3 types of muscle (in verts) ○ Skeletal ○ Smooth ○ Cardiac ➢ Movement ○ Attached to bones by tendons ○ Most abundant tissue ➢ Muscle fiber = muscle cell ○ Multinucleate ○ T (transverse) tubules connect to impulses to.. ○ Sarcoplasmic reticulum (SR) ○ Myofibrils inside cell ■ Fig 50.30 B. Muscle Fibers ➢ Long fibers run lengthwise ➢ Thin filaments have 3 types of proteins ○ 2 strands of actin (contractile) ○ Tropomyosin - regulatory ○ Troponin complex - regulatory ➢ Thick filaments = 350 myosin molecules ➢ Actin and myosin proteins ○ Muscle appears striped (Striated) C. Sarcomere ➢ Sarcomere - basic unit of contraction within myofibril ➢ Overlapping thick + thin ➢ 100’s connect to form myofibril ➢ Z lines - join sarcomeres at ends ➢ M lines - “middle” ➢ Thin filaments attach to Z ➢ Thick filaments anchor at M (center in sarcomere) During Contraction Fig 50.26 ➢ Thick and thin filaments slide & break old connections ➢ Form new ones - sarcomere gets shorter ➢ Distance between Z lines change ○ Overlap changes - NOT length of filament D. Sliding Filament Model 1. Muscle fiber at rest ○ Tropomyosin covers myosin binding sites of actin ■ Fig 50.29 2. Motor neuron transmit AP ○ Motor unit - connects with ~ 150 muscle fibers ■ Each one is a neuromuscular junction ○ Neuron secretes ACh (acetylcholine) ■ Fig 48.16 ➢ Muscle cells are large ○ T-tubules facilitate AP to affect myofibrils ➢ AP travels ○ Plasma membrane → down T-tubules → SR ○ SR has high Ca 2+ ○ Ca 2+channels open enters cytosol ○ Later Ca2+pump returns Ca to SR Fig 50.28 1. Contraction a. Ca binds to troponin complex on thick filaments i. Now it is ready to contract b. Myosin is an ATPase - splits ATP i. ATP is hydrolyzed - released E → shape change 2. Myosin binds to actin binding site a. Cross-bridge is formed linking thick and thin 3. ADP + P rileased a. Myosin returns to low energy state & pulls thin filament to center of sarcomere b. Power Stroke! 4. ATP molecule binds to low-energy myosin head cross bridge is broken
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