Biol 2401 Biol 2401 Fundamentals of Anatomy and Physiology Mrs. Willie Grant wgrant4@alamo.edu (210) 486-2870

© 2015 Pearson Education, Inc.

10-1 An Introduction to Muscle Tissue Learning Outcomes 10-1 Specify the functions of skeletal muscle tissue. 10-2 Describe the organization of muscle at the tissue level. 10-3 Explain the characteristics of skeletal muscle fibers, and identify the structural components of a sarcomere. 10-4 Identify the components of the neuromuscular junction, and summarize the events involved in the neural control of skeletal muscle contraction and relaxation. 10-5 Describe the mechanism responsible for tension production in a muscle fiber, and compare the different types of muscle contraction. 10-6 Describe the mechanisms by which muscle fibers obtain the energy to power contractions. 10-7 Relate the types of muscle fibers to muscle performance, and distinguish between aerobic and anaerobic endurance. 10-8 Identify the structural and functional differences between skeletal muscle fibers and cardiac muscle cells. 10-9 Identify the structural and functional differences between skeletal muscle fibers and smooth muscle cells, and discuss the roles of smooth muscle tissue in systems throughout the body.

An Introduction to Muscle Tissue A primary tissue type, divided into: Skeletal muscle tissue Cardiac muscle tissue Smooth muscle tissue Skeletal Muscles Are attached to the skeletal system Allow us to move

10-1 Functions of Skeletal Muscle Tissue Six Functions of Skeletal Muscle Tissue Produce skeletal movement Maintain posture and body position Support soft tissues Guard entrances and exits Maintain body temperature Store nutrient reserves Skeletal Muscle Organization Muscle tissue (muscle cells or fibers) Connective tissues Nerves Blood vessels

10-2 Organization of Muscle Organization of Connective Tissues Muscles have three layers of connective tissues Epimysium Exterior collagen layer Connected to deep fascia Separates muscle from surrounding tiss Perimysium Separates muscle from surrounding tissue Endomysium Surrounds individual muscle cells (muscle fibers) Contains capillaries and nerve fibers contacting muscle cells Contains myosatellite cells (stem cells) that repair damage

Figure 10-2 The Formation of a Multinucleate Skeletal Muscle Fiber Muscle fibers develop through the fusion of mesodermal cells called myoblasts. Cells are long Cells develop from myoblasts Cells become very large Cells contain hundreds of nuclei Myoblasts A muscle fiber forms by the fusion of myoblasts. Muscle fiber LM  612 Nuclei Sarcolemma Myofibrils Myosatellite cell Nuclei Mitochondria Immature muscle fiber 1 Which structure shown releases calcium ions to trigger muscle contraction? A diagrammatic view and a micrograph of one muscle fiber. Myosatellite cell Up to 30 cm in length Mature muscle fiber 8

10-2 Organization of Muscle Organization of Connective Tissues Muscle Attachments Endomysium, perimysium, and epimysium come together: At ends of muscles To form connective tissue attachment to bone matrix tendon (bundle) or aponeurosis (sheet) Blood Vessels and Nerves Muscles have extensive vascular systems that: Supply large amounts of oxygen Supply nutrients Carry away wastes Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord)

10-3 Characteristics of Skeletal Muscle Fibers The Sarcolemma and Transverse Tubules The sarcolemma—cell membrane of a muscle fiber Surrounds the sarcoplasm (cytoplasm of muscle fiber) A change in transmembrane potential begins contractions Transverse tubules (T tubules) Transmit action potential through cell Allow entire muscle fiber to contract simultaneously Have same properties as sarcolemma

10-3 Characteristics of Skeletal Muscle Fibers Myofibrils Lengthwise subdivisions within muscle fiber Made up of bundles of protein filaments (myofilaments) Myofilaments are responsible for muscle contraction Types of myofilaments: Thin filaments Made of the protein actin Thick filaments Made of the protein myosin

10-3 Characteristics of Skeletal Muscle Fibers The Sarcoplasmic Reticulum (SR) A membranous structure surrounding each myofibril Helps transmit action potential to myofibril Similar in structure to smooth endoplasmic reticulum Forms chambers (terminal cisternae) attached to T tubules Triad Is formed by one T tubule and two terminal cisternae Cisternae Concentrate Ca2+ (via ion pumps) Release Ca2+ into sarcomeres to begin muscle contraction

Figure 10-3 The Structure of a Skeletal Muscle Fiber Myofibril Sarcolemma Nuclei Sarcoplasm MUSCLE FIBER Mitochondria Terminal cisterna Sarcolemma Sarcolemma Sarcoplasm Myofibril Myofibrils Thin filament Thick filament Triad Sarcoplasmic reticulum T tubules 13

10-3 Structural Components of a Sarcomere Sarcomeres The contractile units of muscle Structural units of myofibrils Form visible patterns within myofibrils A striped or striated pattern within myofibrils Alternating dark, thick filaments (A bands) and light, thin filaments (I bands)

10-3 Structural Components of a Sarcomere Sarcomeres The A Band M line The center of the A band At midline of sarcomere The H Band The area around the M line Has thick filaments but no thin filaments Zone of overlap The densest, darkest area on a light micrograph Where thick and thin filaments overlap The I Band Z lines The centers of the I bands At two ends of sarcomere Titin Are strands of protein Reach from tips of thick filaments to the Z line Stabilize the filaments 2 Which proteins are present in the A band? In the I band?

A superficial view of a sarcomere Figure 10-5 Sarcomere Structure, Superficial and Cross-Sectional Views. Sarcomere Myofibril a A superficial view of a sarcomere Thin filament Thick filament Actinin filaments Thin filaments Titin filament Thick filaments Attachment of titin Z line I band M line H band Zone of overlap b Cross-sectional views of different regions of a sarcomere

Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Myofibril Surrounded by: Epimysium Surrounded by: Sarcoplasmic reticulum Epimysium Contains: Muscle fascicles Consists of: Sarcomeres (Z line to Z line) Sarcomere I band A band Muscle Fascicle Contains: Thick filaments Surrounded by: Perimysium Thin filaments Perimysium Contains: Muscle fibers Z line M line Titin Z line H band Muscle Fiber 3 Which of the following is the smallest: muscle fiber, thick filament, myofibril? Which is the largest? Surrounded by: Endomysium Endomysium Contains: Myofibrils 18

10-3 Structural Components of a Sarcomere Thin Filaments F-actin (filamentous actin) Is two twisted rows of globular G-actin The active sites on G-actin strands bind to myosin Nebulin Holds F-actin strands together Tropomyosin Is a double strand that prevents actin–myosin interaction Troponin A globular protein that binds tropomyosin to G-actin Controlled by Ca2+

10-3 Structural Components of a Sarcomere Thick Filaments Contain about 300 twisted myosin subunits Contain titin strands that recoil after stretching The mysosin molecule: Tail (binds to other myosin molecules) Head (reaches the nearest thin filament) Myosin Action During contraction, myosin heads: Interact with actin filaments, forming cross-bridges Pivot, producing motion

10-3 Structural Components of a Sarcomere Sliding Filaments and Muscle Contraction Sliding Filament Theory Thin filaments of sarcomere slide toward M line, alongside thick filaments The width of A zone stays the same Z lines move closer together

4 What happens to the I band and the H zone as muscles contract 4 What happens to the I band and the H zone as muscles contract? Do the lengths of the thick and thin filaments change?

10-4 Components of the Neuromuscular Junction The Control of Skeletal Muscle Activity The neuromuscular junction (NMJ) Special intercellular connection between the nervous system and skeletal muscle fiber Controls calcium ion release into the sarcoplasm A single axon may branch to control more than one skeletal muscle fiber, but each muscle fiber has only one neuromuscular junction (NMJ). At the NMJ, the synaptic terminal of the neuronlies near the motor end plate of the muscle fiber. 5 What part of the sarcolemma contains acetylcholine receptors?

Figure 10-12 The Contraction Cycle Resting Sarcomere Zone of overlap (shown in sequence above) 28

Figure 10-12 The Contraction Cycle Contracted Sarcomere 29

10-4 Skeletal Muscle Relaxation Contraction Duration depends on: Duration of neural stimulus Number of free calcium ions in sarcoplasm Availability of ATP Ca2+ concentrations fall Ca2+ detaches from troponin Rigor Mortis A fixed muscular contraction after death Caused when: Ion pumps cease to function; ran out of ATP Calcium builds up in the sarcoplasm Summary Skeletal muscle fibers shorten as thin filaments slide between thick filaments Free Ca2+ in the sarcoplasm triggers contraction SR releases Ca2+ when a motor neuron stimulates the muscle fiber Contraction is an active process Relaxation and return to resting length are passive

Table 10-1 Steps Involved in Skeletal Muscle Contraction and Relaxation Steps in Initiating Muscle Contraction Steps in Muscle Relaxation Synaptic terminal Motor end plate T tubule Sarcolemma Action potential reaches T tubule ACh released, binding to receptors ACh broken down by AChE Sarcoplasmic reticulum releases Ca2 Sarcoplasmic reticulum recaptures Ca2 Ca2 Active site exposure, cross-bridge formation Actin Active sites covered, no cross-bridge interaction Myosin Contraction ends Contraction begins Relaxation occurs, passive return to resting length 31

10-5 Tension Production and Contraction Types Tension Production by Muscles Fibers As a whole, a muscle fiber is either contracted or relaxed Depends on: The number of pivoting cross-bridges The fiber’s resting length at the time of stimulation The frequency of stimulation Length–Tension Relationships Number of pivoting cross-bridges depends on: Amount of overlap between thick and thin fibers Optimum overlap produces greatest amount of tension Too much or too little reduces efficiency Normal resting sarcomere length Is 75% to 130% of optimal length

Figure 10-14 The Effect of Sarcomere Length on Active Tension A muscle fiber is either “on” (producing tension) or “off” (relaxed). In a skeletal muscle fiber, the amount of tension generated during a contraction depends on the number of pivoting cross-bridges in each of the myofibrils. The number of cross-bridges that can form depends on the degree of overlap between thick and thin filaments with these sarcomeres. Skeletal muscle fibers contract most forcefully when stimulated over a narrow range of lengths. Tension (percent of maximum) Normal range Decreased length Increased sarcomere length Optimal resting length: The normal range of sarcomere lengths in the body is 75 to 130 percent of the optimal length. 34

10-5 Tension Production and Contraction Types Tension Production by Muscles Fibers The Frequency of Stimulation A single neural stimulation produces: A single contraction or twitch Which lasts about 7–100 msec. Sustained muscular contractions Require many repeated stimuli

10-5 Tension Production and Contraction Types Tension Production by Muscles Fibers Twitches Latent period The action potential moves through sarcolemma Causing Ca2+ release Contraction phase Calcium ions bind Tension builds to peak Relaxation phase Ca2+ levels fall Active sites are covered and tension falls to resting levels

Figure 10-15b The Development of Tension in a Twitch Maximum tension development Tension Stimulus Resting phase Latent period Contraction phase Relaxation phase The details of tension over time for a single twitch in the gastrocnemius muscle. Notice the presence of a latent period, which corresponds to the time needed for the conduction of an action potential and the subsequent release of calcium ions by the sarcoplasmic reticulum. 37

Figure 10-15a The Development of Tension in a Twitch Eye muscle Gastrocnemius Soleus Tension Time (msec) Stimulus A myogram showing differences in tension over time for a twitch in different skeletal muscles. 38

10-5 Tension Production and Contraction Types Tension Production by Muscles Fibers Treppe A stair-step increase in twitch tension Repeated stimulations immediately after relaxation phase Stimulus frequency <50/second Causes a series of contractions with increasing tension

10-5 Tension Production and Contraction Types Tension Production by Muscles Fibers Wave summation Increasing tension or summation of twitches Repeated stimulations before the end of relaxation phase Stimulus frequency >50/second Causes increasing tension or summation of twitches

10-5 Tension Production and Contraction Types Tension Production by Muscles Fibers Incomplete tetanus Twitches reach maximum tension If rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension

Tension Production and Contraction Types Complete tetanus If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction

10-5 Tension Production and Contraction Types Tension Production by Skeletal Muscles Depends on: Internal tension produced by muscle fibers External tension exerted by muscle fibers on elastic extracellular fibers Total number of muscle fibers stimulated Motor units (all muscle fibers controlled by a single motor neuron) in a skeletal muscle: Contain hundreds of muscle fibers That contract at the same time Controlled by a single motor neuron

10-5 Tension Production and Contraction Types Motor Units and Tension Production Recruitment (multiple motor unit summation) In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated Maximum tension Achieved when all motor units reach tetanus Can be sustained only a very short time

10-5 Tension Production and Contraction Types Motor Units and Tension Production Sustained tension Less than maximum tension Allows motor units rest in rotation Muscle tone The normal tension and firmness of a muscle at rest Muscle units actively maintain body position, without motion Increasing muscle tone increases metabolic energy used, even at rest

Figure 10-9 An Overview of Skeletal Muscle Contraction Neural control Contraction occurs when skeletal muscle Fibers are activated by neurons. Excitation–contraction coupling Calcium ions are released from SR. Excitation Calcium release Calcium ions trigger interaction between thick/thin filaments. triggers ATP Thick-thin filament interaction Muscle fiber contraction Mulscle fiber contracts. leads to Tension is produced. Tension production 47