10 Muscle Tissue

An Introduction to Muscle Tissue What are some functions of skeletal muscle tissue? Think, pair, share! Movement Maintenance of posture Joint stabilization Heat generation

An Introduction to Muscle Tissue Define striated and voluntary. Muscle Tissue A primary tissue type, divided into: Skeletal muscle tissue (striated, voluntary) Cardiac muscle tissue (striated, involuntary) Smooth muscle tissue (non-striated, involuntary) Divided based on: how the contractile machinery is organized how the nervous system figures in its control.

Skeletal, Cardiac, or Smooth? Smooth –hallow walls of organs , cardiac—only in the heart , skeletal—attached to bones Where are these tissues normally found?

Skeletal Muscle voluntary, striated muscle attached to one or more bones, long, threadlike cells – muscle fibers voluntary – conscious control over skeletal muscles striations - alternating light and dark transverse bands , results from an overlapping of internal contractile proteins contains multiple nuclei adjacent to plasma membrane

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

10-2 Organization of Muscle Skeletal Muscle Connective tissues Nerves Blood vessels Muscle tissue (muscle cells or fibers)

10-2 Organization of Muscle Organization of Connective Tissues Muscles have three layers of connective tissues Epimysium Perimysium Endomysium

Connective Tissue of Muscle Epimysium: surrounds whole muscle Endomysium is around each muscle fiber Perimysium is around fascicle (muscle fiber bundles)

Exterior collagen layer Connected to deep fascia Epimysium Exterior collagen layer Connected to deep fascia Separates muscle from surrounding tissues 10

Perimysium Surrounds muscle fiber bundles (fascicles) Contains blood vessel and nerve supply to fascicles

Endomysium Surrounds individual muscle cells (muscle fibers) Contains capillaries and nerve fibers contacting muscle cells Contains cells that repair damage

Figure 10-1 The Organization of Skeletal Muscles Skeletal Muscle (organ) Epimysium Perimysium Endomysium Nerve Muscle fascicle Muscle fibers Blood vessels Epimysium Blood vessels and nerves Tendon Endomysium Perimysium 13

Organization of Connective Tissues Muscle Attachments Endomysium, perimysium, and epimysium come together: At ends of muscles To form connective tissue attachment to bone matrix I.e., tendon (bundle) or aponeurosis (sheet)

10-2 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)

Figure 10-2b The Formation of a Multinucleate Skeletal Muscle Fiber 10-3 Skeletal Muscle cells and fibers Muscle fiber LM  612 Nuclei Sarcolemma Myofibrils Mitochondria A diagrammatic view and a micrograph of one muscle fiber. 16

Muscle Fibers sarcolemma – plasma membrane of a muscle fiber sarcoplasm – cytoplasm of a muscle fiber mitochondria – packed in spaces between myofibrils sarcoplasmic reticulum (SR) - smooth ER that forms a network around each myofibril – calcium reservoir The repeating unit of a skeletal muscle fiber is the sarcomere

10-3 Characteristics of Skeletal Muscle Fibers Fibers (each is one cell) have striations Myofibrils are organelles of the cell: these are made up of myofilaments Sarcomere Basic unit of contraction Myofibrils are long rows of repeating sarcomeres Boundaries: Z discs (or lines) -an organelle

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

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 20

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

10-3 Structural Components of a Sarcomere Sarcomeres 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

Figure 10-4a Sarcomere Structure, Part I I band A band H band Z line Titin A longitudinal section of a sarcomere, showing bands Zone of overlap M line Thin filament Thick filament Sarcomere 24

Figure 10-4b Sarcomere Structure, Part I I band A band H band Z line A corresponding view of a sarcomere in a myofibril from a muscle fiber in the gastrocnemius muscle of the calf Myofibril TEM  64,000 Z line Zone of overlap M line Sarcomere 25

Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Sarcomere I band A band Contains: Thick filaments Thin filaments Z line M line Titin Z line H band 26

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

10-3 Structural Components of a Sarcomere Thin Filaments Tropomyosin Is a double strand Prevents actin–myosin interaction Troponin A globular protein Binds tropomyosin to G-actin Controlled by Ca2+

Figure 10-7ab Thick and Thin Filaments Sarcomere H band Actinin Z line Titin Myofibril The gross structure of a thin filament, showing the attachment at the Z line Troponin Active site Nebulin Tropomyosin G-actin molecules Z line M line F-actin strand The organization of G-actin subunits in an F-actin strand, and the position of the troponin–tropomyosin complex 29

Figure 10-8b Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber I band A band Z line H band Z line During a contraction, the A band stays the same width, but the Z lines move closer together and the I band gets smaller. When the ends of a myofibril are free to move, the sarcomeres shorten simultaneously and the ends of the myofibril are pulled toward its center. 30

10-3 Structural Components of a Sarcomere Skeletal Muscle Contraction The process of contraction Neural stimulation of sarcolemma Causes excitation–contraction coupling Muscle fiber contraction Interaction of thick and thin filaments Tension production

Figure 10-9 An Overview of Skeletal Muscle Contraction Neural control Excitation–contraction coupling Excitation Calcium release triggers ATP Thick-thin filament interaction Muscle fiber contraction leads to Tension production 32

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&P FLIX Events at the Neuromuscular Junction

Neuromuscular Junction Motor neurons innervate muscle fibers Motor end plate is where they meet Neurotransmitters are released by nerve signal: this initiates calcium ion release and muscle contraction Motor Unit: a motor neuron and all the muscle fibers it innervates (these all contract together) Average is 150, but range is four to several hundred muscle fibers in a motor unit The finer the movement, the fewer muscle fibers /motor unit The fibers are spread throughout the muscle, so stimulation of a single motor unit causes a weak contraction of the entire muscle

Figure 10-11 Skeletal Muscle Innervation Motor neuron Path of electrical impulse (action potential) Axon Neuromuscular junction Synaptic terminal SEE BELOW Sarcoplasmic reticulum Motor end plate Myofibril Motor end plate 35

10-4 Components of the Neuromuscular Junction Excitation–Contraction Coupling Action potential reaches a triad Releasing Ca2+ Triggering contraction Requires myosin heads to be in “cocked” position Loaded by ATP energy A&P FLIX Excitation-Contraction Coupling

Figure 10-10 The Exposure of Active Sites SARCOPLASMIC RETICULUM Calcium channels open Myosin tail (thick filament) Tropomyosin strand Troponin G-actin (thin filament) Active site Nebulin In a resting sarcomere, the tropomyosin strands cover the active sites on the thin filaments, preventing cross-bridge formation. When calcium ions enter the sarcomere, they bind to troponin, which rotates and swings the tropomyosin away from the active sites. Cross-bridge formation then occurs, and the contraction cycle begins. 38

10-4 Skeletal Muscle Contraction The Contraction Cycle Contraction Cycle Begins Active-Site Exposure Cross-Bridge Formation Myosin Head Pivoting Cross-Bridge Detachment Myosin Reactivation A&P FLIX The Cross Bridge Cycle

10-4 Skeletal Muscle Contraction The Contraction Cycle Contraction Cycle Begins Active-Site Exposure Cross-Bridge Formation Myosin Head Pivoting Cross-Bridge Detachment Myosin Reactivation A&P FLIX The Cross Bridge Cycle

Figure 10-12 The Contraction Cycle Contraction Cycle Begins The contraction cycle, which involves a series of interrelated steps, begins with the arrival of calcium ions within the zone of overlap. Myosin head Troponin Tropomyosin Actin 41

Figure 10-12 The Contraction Cycle Active-Site Exposure Calcium ions bind to troponin, weakening the bond between actin and the troponin– tropomyosin complex. The troponin molecule then changes position, rolling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads. Sarcoplasm Active site 42

Figure 10-12 The Contraction Cycle Cross-Bridge Formation Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges. 43

Figure 10-12 The Contraction Cycle Myosin Head Pivoting After cross-bridge formation, the energy that was stored in the resting state is released as the myosin head pivots toward the M line. This action is called the power stroke; when it occurs, the bound ADP and phosphate group are released. 44

Figure 10-12 The Contraction Cycle Cross-Bridge Detachment When another ATP binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken. The active site is now exposed and able to form another cross-bridge. 45

Figure 10-12 The Contraction Cycle Myosin Reactivation Myosin reactivation occurs when the free myosin head splits ATP into ADP and P. The energy released is used to recock the myosin head. 46

In other words… Myosin heads sticking off the thick filaments use ATP to pull the actin filaments toward the middle of the A band, shortening the sarcomeres and thus pulling on an external load.

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

Myofibrils Made of three types of filaments (or myofilaments): Thick (myosin) Thin (actin) Elastic (titin) ______actin _____________myosin titin_____

Sliding Filament Model __relaxed sarcomere__ _partly contracted_ fully contracted Sarcomere shortens because actin pulled towards its middle by myosin cross bridges “A” band constant because it is caused by myosin, which doesn’t change length Titin resists overstretching

Another view

10-4 Skeletal Muscle Contraction and Relaxation 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

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

10-6 Energy to Power Contractions ATP Provides Energy For Muscle Contraction Sustained muscle contraction uses a lot of ATP energy Muscles store enough energy to start contraction Muscle fibers must manufacture more ATP as needed Rigor mortis is an interesting application of the biochemistry of the cross-bridge cycle. Using Figure 10–12, ask the students where the cycle would stop if ATP were absent [Step 3—angled and attached]. Point out that after death, ATP becomes depleted. Many will have an “a-ha” reaction when you ask if they now understand the why behind the “stiffness of death”? The rigor mortis eventually resolves as internal proteases break up the myofibrils at the Z lines. This endogenous proteolysis plays a major role in meat tenderness that improves as meat is “aged.” How does Rigor mortis relate to ATP depletion during death?

10-6 Energy to Power Contractions Energy Use and the Level of Muscular Activity Skeletal muscles at rest metabolize fatty acids and store glycogen During light activity, muscles generate ATP through the breakdown of carbohydrates, lipids, or amino acids At peak activity, energy is provided by certain reactions that generate lactic acid as a byproduct

10-6 Energy to Power Contractions Muscle Fatigue When muscles can no longer perform a required activity, they are fatigued Results of Muscle Fatigue Depletion of metabolic reserves Damage to sarcolemma and sarcoplasmic reticulum Low pH (lactic acid) Muscle exhaustion and pain Why do we get sore muscles after a hard work out?

10-6 Energy to Power Contractions The Recovery Period The time required after exertion for muscles to return to normal Oxygen becomes available Mitochondrial activity resumes

10-6 Energy to Power Contractions Heat Production and Loss Active muscles produce heat Up to 70% of muscle energy can be lost as heat, raising body temperature

10-7 Types of Muscles Fibers and Endurance Muscle Performance Force The maximum amount of tension produced Endurance The amount of time an activity can be sustained Force and endurance depend on: The types of muscle fibers Physical conditioning

10-7 Types of Muscles Fibers and Endurance Muscle Hypertrophy Muscle growth from heavy training Increases diameter of muscle fibers Increases number of myofibrils Increases mitochondria, glycogen reserves Muscle Atrophy Lack of muscle activity Reduces muscle size, tone, and power

10-7 Types of Muscles Fibers and Endurance Importance of Exercise What you don’t use, you lose Muscle tone indicates base activity in motor units of skeletal muscles Muscles become flaccid when inactive for days or weeks Muscle fibers break down proteins, become smaller and weaker With prolonged inactivity, fibrous tissue may replace muscle fibers

10-8 Cardiac Muscle Tissue limited to the heart myocytes or cardiocytes are much shorter, branched, and notched at ends contain one centrally located nucleus surrounded by light staining glycogen intercalated discs join cardiocytes end to end provide electrical and mechanical connection striated, and involuntary (not under conscious control)

Figure 10-22a Cardiac Muscle Tissue Cardiac muscle cell Intercalated discs Nucleus Cardiac muscle tissue LM  575 A light micrograph of cardiac muscle tissue. 63

10-9 Smooth Muscle Tissue Smooth Muscle in Body Systems Forms around other tissues lacks striations and is involuntary visceral muscle – forms layers of digestive, respiratory, and urinary tract: blood vessels, uterus and other viscera propels contents through an organ, regulates diameter of blood vessels

Figure 10-23b Smooth Muscle Tissue Relaxed (sectional view) Dense body Actin Myosin Relaxed (superficial view) Intermediate filaments (desmin) Adjacent smooth muscle cells are bound together at dense bodies, transmitting the contractile forces from cell to cell throughout the tissue. Contracted (superficial view) A single relaxed smooth muscle cell is spindle shaped and has no striations. Note the changes in cell shape as contraction occurs. 65