Muscle Tissue

PowerPoint® Lecture Presentations prepared by Jason LaPres Lone Star College—North Harris

• 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.

• Learning Outcomes • 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.

• Learning Outcomes • 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.

• 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 • The muscular system

• Includes only skeletal muscles

• Six Functions of Skeletal Muscle Tissue 1. Produce skeletal movement 2. Maintain posture and body position 3. Support soft tissues 4. Guard entrances and exits 5. Maintain body temperature 6. Store nutrient reserves

• Skeletal Muscle • Muscle tissue (muscle cells or fibers) • Connective tissues • Nerves • Blood vessels

• Organization of Connective Tissues • Muscles have three layers of connective tissues

1. Epimysium 2. Perimysium 3. Endomysium

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

• 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 myosatellite cells (stem cells) that repair damage

Epimysium Perimysium Endomysium Nerve

Muscle Muscle Blood fascicle fibers vessels

Epimysium

Blood vessels and nerves

Tendon

Endomysium

Perimysium

Muscle Fascicle (bundle of fibers)

Perimysium

Muscle fiber Epimysium

Blood vessels and nerves Endomysium

Tendon

Endomysium

Perimysium

Muscle Fiber (cell) Capillary Myofibril Endomysium Sarcoplasm Epimysium Mitochondrion Blood vessels and nerves Myosatellite cell Sarcolemma Nucleus Tendon Axon of neuron

Endomysium

Perimysium

• 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)

• 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)

• Skeletal Muscle Cells

• Are very long • Develop through fusion of mesodermal cells (myoblasts) • Become very large • Contain hundreds of nuclei

Muscle fibers develop through the fusion of mesodermal cells called myoblasts.

Myoblasts

A muscle fiber forms by the Muscle fiber LM × 612 fusion of myoblasts. Nuclei Sarcolemma Myofibrils Myosatellite cell

Nuclei Mitochondria Immature muscle fiber

A diagrammatic view and a Myosatellite cell micrograph of one muscle fiber.

Up to 30 cm in length

Mature muscle fiber

Muscle fibers develop through the fusion of mesodermal cells called myoblasts.

Myoblasts

A muscle fiber forms by the fusion of myoblasts. Myosatellite cell

Nuclei Immature muscle fiber

Myosatellite cell

Up to 30 cm in length

Mature muscle fiber

Muscle fiber LM × 612

Nuclei Sarcolemma

Myofibrils

Mitochondria

• The Sarcolemma and Transverse Tubules

• The sarcolemma • The cell membrane of a muscle fiber (cell)

• Surrounds the sarcoplasm (cytoplasm of muscle fiber) • A change in transmembrane potential begins contractions

• The Sarcolemma and Transverse Tubules

• Transverse tubules (T tubules) • Transmit action potential through cell • Allow entire muscle fiber to contract simultaneously • Have same properties as sarcolemma

• 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

• 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

• The Sarcoplasmic Reticulum (SR)

• 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

Myofibril

Sarcolemma Nuclei

Sarcoplasm MUSCLE FIBER

Mitochondria

Terminal cisterna Sarcolemma Sarcolemma

Sarcoplasm

Myofibril Myofibrils

Thin filament

Thick filament Triad Sarcoplasmic T tubules reticulum

Myofibril

Sarcolemma Nuclei

Sarcoplasm MUSCLE FIBER

Mitochondria

Terminal cisterna Sarcolemma Sarcolemma

Sarcoplasm

Myofibril Myofibrils

Thin filament

Thick filament Triad Sarcoplasmic T tubules reticulum

Mitochondria

Sarcolemma

Myofibril

Thin filament

Thick filament

Terminal cisterna Sarcolemma

Sarcoplasm

Myofibrils

Triad Sarcoplasmic T tubules reticulum

• 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)

• 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

• 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

I band A band H band Z line Titin

A longitudinal section of a sarcomere, showing bands

Zone of overlap M line Thin Thick filament filament Sarcomere

I band A band H band Z line

A corresponding view of a sarcomere in a myofibril from a muscle fiber in the Myofibril TEM × 64,000 gastrocnemius muscle of the calf Z line Zone of overlap M line Sarcomere

Sarcomere

Myofibril A superficial view of a sarcomere Thin Thick filament filament

Actinin Titin filaments filament

Attachment of titin

Z line I band M line H band Zone of overlap Cross-sectional views of different portions of a sarcomere

Surrounded by: Surrounded by: Epimysium Sarcoplasmic Epimysium reticulum 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

Surrounded by: Endomysium Endomysium Contains: Myofibrils

Skeletal Muscle

Surrounded by: Epimysium Epimysium Contains: Muscle fascicles

Muscle Fascicle

Surrounded by: Perimysium Perimysium Contains: Muscle fibers

Muscle Fiber

Surrounded by: Endomysium Endomysium Contains: Myofibrils

Myofibril

Surrounded by: Sarcoplasmic reticulum

Consists of: Sarcomeres (Z line to Z line)

Sarcomere I band A band

Contains: Thick filaments

Thin filaments

Z line M line Titin Z line H band

• 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

• Thin Filaments

• Tropomyosin • Is a double strand • Prevents actin–myosin interaction

• Troponin • A globular protein • Binds tropomyosin to G-actin • Controlled by Ca2+

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 Z line M line molecules

F-actin strand The organization of G-actin subunits in an F-actin strand, and the position of the troponin–tropomyosin complex

Actinin Z line Titin

The gross structure of a thin filament, showing the attachment at the Z line

Troponin Active site Nebulin Tropomyosin G-actin molecules

F-actin strand The organization of G-actin subunits in an F-actin strand, and the position of the troponin–tropomyosin complex

• Initiating Contraction

• Ca2+ binds to receptor on troponin molecule • Troponin–tropomyosin complex changes

• Exposes active site of F-actin

• 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 • Made of two globular protein subunits • Reaches the nearest thin filament

Titin

The structure of thick filaments, M line Myosin head showing the orientation of the Myosin tail Hinge myosin molecules The structure of a myosin molecule

• Myosin Action • During contraction, myosin heads:

• Interact with actin filaments, forming cross- bridges • Pivot, producing motion

• 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

I band A band

Z line H band Z line A relaxed sarcomere showing location of the A band, Z lines, and I band. © 2012 Pearson Education, Inc. 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. © 2012 Pearson Education, Inc. 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

Neural control

Excitation–contraction coupling

Excitation

Calcium release

triggers ATP

Thick-thin filament interaction

Muscle fiber contraction

leads to

Tension production

Neural control

Excitation

Calcium release

ATP triggers

Thick-thin filament interaction

Muscle fiber contraction

leads to

Tension production

• 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

Motor neuron Path of electrical impulse (action potential) Axon

Neuromuscular junction

Synaptic terminal SEE BELOW

Sarcoplasmic Motor reticulum end plate

Myofibril Motor end plate

The cytoplasm of the synaptic terminal contains vesicles filled with molecules of acetylcholine, or ACh. Acetylcholine is a neurotransmitter, a chemical released by a neuron to change the permeability or other properties of another cell’s plasma membrane. The synaptic cleft and the motor end plate contain molecules of the enzyme acetylcholinesterase (AChE), which breaks down ACh.

Vesicles ACh

The synaptic cleft, a narrow space, separates the synaptic terminal of the neuron from the opposing motor end Junctional AChE plate. fold of motor end plate © 2012 Pearson Education, Inc. Figure 10-11 Skeletal Muscle Innervation

The stimulus for ACh release is the arrival of an electrical impulse, or action potential, at the synaptic terminal. An action potential is a sudden change in the transmembrane potential that travels along the length of the axon.

Arriving action potential

When the action potential reaches the neuron’s synaptic terminal, permeability changes in the membrane trigger the exocytosis of ACh into the synaptic cleft. Exocytosis occurs as vesicles fuse with the neuron’s plasma membrane.

Motor end plate

ACh molecules diffuse across the synatpic cleft and bind to ACh receptors on the surface of the motor end plate. ACh binding alters the membrane’s permeability to sodium ions. Because the extracellular fluid contains a high concentration of sodium ions, and sodium ion concentration inside the cell is very low, sodium ions rush into the sarcoplasm.

ACh receptor site

The sudden inrush of sodium ions results in the generation of an action potential in the sarcolemma. AChE quickly breaks down the ACh on the motor end plate and in the synaptic cleft, thus inactivating the ACh receptor sites.

Action potential

AChE

• 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

SARCOPLASMIC RETICULUM

Calcium channels open

Myosin tail (thick filament) Tropomyosin strand Troponin G-actin (thin filament) Active site Nebulin

In a resting sarcomere, the When calcium ions enter Cross-bridge tropomyosin strands cover the sarcomere, they bind formation then occurs, the active sites on the thin to troponin, which and the contraction filaments, preventing rotates and swings the cycle begins. cross-bridge formation. tropomyosin away from the active sites. © 2012 Pearson Education, Inc. 10-4 Skeletal Muscle Contraction

• The Contraction Cycle 1. Contraction Cycle Begins 2. Active-Site Exposure 3. Cross-Bridge Formation 4. Myosin Head Pivoting 5. Cross-Bridge Detachment 6. Myosin Reactivation

A&P FLIX The Cross Bridge 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

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

Cross-Bridge Formation Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges.

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.

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.

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.

Resting Sarcomere

Zone of overlap (shown in sequence above)

Contracted Sarcomere

• Fiber Shortening • As sarcomeres shorten, muscle pulls together, producing tension • Muscle shortening can occur at both ends of the muscle, or at only one end of the muscle

• This depends on the way the muscle is attached at the ends

When both ends are free to move, the ends of a contracting muscle fiber move toward the center of the muscle fiber.

When one end of a myofibril is fixed in position, and the other end free to move, the free end is pulled toward the fixed end.

• Relaxation • Contraction Duration

• Depends on:

• Duration of neural stimulus • Number of free calcium ions in sarcoplasm • Availability of ATP

• Relaxation • Ca2+ concentrations fall • Ca2+ detaches from troponin • Active sites are re-covered by tropomyosin

• 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

Steps in Initiating Muscle Contraction Steps in Muscle Relaxation

Synaptic Motor terminal end plate T tubule Sarcolemma

Action ACh released, binding potential ACh broken down by AChE to receptors reaches T tubule Sarcoplasmic Sarcoplasmic reticulum reticulum releases Ca2+ recaptures Ca2+ Ca2+

Actin Active site Active sites exposure, covered, no cross-bridge Myosin cross-bridge formation interaction

Contraction ends Contraction begins Relaxation occurs, passive return to resting length

• 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

• Tension Production by Muscles Fibers • 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

Normal range Tension Tension (percent of maximum)

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.

• 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

• Tension Production by Muscles Fibers • Twitches 1. Latent period • The action potential moves through sarcolemma • Causing Ca2+ release 2. Contraction phase • Calcium ions bind • Tension builds to peak 3. Relaxation phase • Ca2+ levels fall • Active sites are covered and tension falls to resting levels © 2012 Pearson Education, Inc. Figure 10-15a The Development of Tension in a Twitch Eye muscle Gastrocnemius

Soleus

Tension

Maximum tension development Tension

Stimulus

Resting Latent Contraction Relaxation phase period phase 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. © 2012 Pearson Education, Inc. 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

• 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

= Stimulus Maximum tension (in tetanus)

Tension

Maximum tension (in treppe)

Time Time Treppe. Treppe is an increase in Wave summation. Wave peak tension with each summation occurs when successive stimulus delivered successive stimuli arrive shortly after the completion of before the relaxation phase the relaxation phase of the has been completed. preceding twitch.

• 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 • Complete tetanus • If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction

Maximum tension (in tetanus)

Tension

Time Time Incomplete tetanus. Complete tetanus. During Incomplete tetanus occurs if the complete tetanus, the stimulus stimulus frequency increases frequency is so high that the further. Tension production rises relaxation phase is eliminated; to a peak, and the periods of tension plateaus at maximal relaxation are very brief. levels.

• 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 and Tension Production

• Motor units in a skeletal muscle: • Contain hundreds of muscle fibers • That contract at the same time • Controlled by a single motor neuron

• 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

Axons of motor neurons

SPINAL CORD Motor nerve

KEY Muscle fibers Motor unit 1

Motor unit 2 Motor unit 3

Muscle fibers of different motor units are intermingled, so the forces applied to the tendon remain roughly balanced regardless of which motor units are stimulated. © 2012 Pearson Education, Inc. Figure 10-17b The Arrangement and Activity of Motor Units in a Skeletal Muscle Tension in tendon

Motor Motor Motor

unit 1 unit 2 unit 3 Tension

Time The tension applied to the tendon remains relatively constant, even though individual motor units cycle between contraction and

• 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

• Motor Units and Tension Production • Contraction are classified based on pattern of tension production

• Isotonic contraction • Isometric contraction

• Isotonic Contraction • Skeletal muscle changes length

• Resulting in motion • If muscle tension > load (resistance):

• Muscle shortens (concentric contraction) • If muscle tension < load (resistance):

• Muscle lengthens (eccentric contraction)

Tendon

Muscle contracts (concentric contraction)

2 kg 2 kg

Amount of Muscle Muscle load relaxes tension Peak tension (kg) production

Contraction begins Resting length

Support removed when contraction Muscle Peak tension begins tension (eccentric contraction) production (kg) Muscle length Support removed, (percent contraction begins of resting 6 kg Resting length length)

6 kg

Time

• Isometric Contraction • Skeletal muscle develops tension, but is prevented from changing length • iso- = same, metric = measure

Amount of load

Muscle Muscle relaxes Muscle tension contracts (kg) Peak tension (isometric production contraction)

Contraction begins Length unchanged

Muscle 6 kg 6 kg length (percent of resting length) Time

• Load and Speed of Contraction • Are inversely related • The heavier the load (resistance) on a muscle

• The longer it takes for shortening to begin • And the less the muscle will shorten

Small load

Intermediate load Distance shortened Large load

Time (msec) Stimulus

• Muscle Relaxation and the Return to Resting Length • Elastic Forces • The pull of elastic elements (tendons and ligaments) • Expands the sarcomeres to resting length • Opposing Muscle Contractions • Reverse the direction of the original motion • Are the work of opposing skeletal muscle pairs

• Muscle Relaxation and the Return to Resting Length • Gravity

• Can take the place of opposing muscle contraction to return a muscle to its resting state

• 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

• ATP and CP Reserves • Adenosine triphosphate (ATP) • The active energy molecule • Creatine phosphate (CP) • The storage molecule for excess ATP energy in resting muscle • Energy recharges ADP to ATP • Using the enzyme creatine kinase (CK) • When CP is used up, other mechanisms generate ATP

• ATP Generation • Cells produce ATP in two ways

1. Aerobic metabolism of fatty acids in the mitochondria

2. Anaerobic glycolysis in the cytoplasm

• Aerobic Metabolism • Is the primary energy source of resting muscles • Breaks down fatty acids • Produces 34 ATP molecules per glucose molecule

• Glycolysis • Is the primary energy source for peak muscular activity • Produces two ATP molecules per molecule of glucose • Breaks down glucose from glycogen stored in skeletal muscles

• 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 anaerobic breakdown of carbohydrates, lipids, or amino acids • At peak activity, energy is provided by anaerobic reactions that generate lactic acid as a byproduct

Fatty acids Blood vessels

Glucose Glycogen

Pyruvate

Mitochondria Creatine To myofibrils to support muscle contraction Resting muscle: Fatty acids are catabolized; the Moderate activity: Glucose and fatty acids are ATP produced is used to build energy reserves of ATP, catabolized; the ATP produced is used to power CP, and glycogen. contraction.

Lactate

Glucose Glycogen

Pyruvate Creatine Lactate To myofibrils to support muscle contraction

Peak activity: Most ATP is produced through glycolysis, with lactate as a by-product. Mitochondrial activity (not shown) now provides only about one-third of the ATP consumed. © 2012 Pearson Education, Inc. Figure 10-20a Muscle Metabolism

Fatty acids Blood vessels

Glucose Glycogen

Mitochondria Creatine

Glucose Glycogen

Pyruvate

To myofibrils to support muscle contraction Moderate activity: Glucose and fatty acids are catabolized; the ATP produced is used to power contraction. © 2012 Pearson Education, Inc. Figure 10-20c Muscle Metabolism Lactate

Glucose Glycogen

Pyruvate Creatine Lactate To myofibrils to support muscle contraction

• 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

• The Recovery Period • The time required after exertion for muscles to return to normal • Oxygen becomes available • Mitochondrial activity resumes

• Lactic Acid Removal and Recycling

• The Cori Cycle • The removal and recycling of lactic acid by the liver • Liver converts lactate to pyruvate • Glucose is released to recharge muscle glycogen reserves

• The Oxygen Debt • After exercise or other exertion:

• The body needs more oxygen than usual to normalize metabolic activities • Resulting in heavy breathing

• Also called excess postexercise oxygen consumption (EPOC)

• Heat Production and Loss • Active muscles produce heat • Up to 70% of muscle energy can be lost as heat, raising body temperature

• Hormones and Muscle Metabolism • Growth hormone • Testosterone • Thyroid hormones • Epinephrine

• 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

• Three Major Types of Skeletal Muscle Fibers

1. Fast fibers 2. Slow fibers 3. Intermediate fibers

• Fast Fibers • Contract very quickly • Have large diameter, large glycogen reserves, few mitochondria • Have strong contractions, fatigue quickly

• Slow Fibers • Are slow to contract, slow to fatigue • Have small diameter, more mitochondria • Have high oxygen supply

• Contain myoglobin (red pigment, binds oxygen)

• Intermediate Fibers • Are mid-sized • Have low myoglobin • Have more capillaries than fast fibers, slower to fatigue

Slow fibers Smaller diameter, darker color due to myoglobin; fatigue resistant

LM × 170

Fast fibers Larger diameter, paler color; easily fatigued

LM × 170 LM × 783

• Muscle Performance and the Distribution of Muscle Fibers • White muscles • Mostly fast fibers • Pale (e.g., chicken breast) • Red muscles • Mostly slow fibers • Dark (e.g., chicken legs) • Most human muscles • Mixed fibers • Pink © 2012 Pearson Education, Inc. 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

• Physical Conditioning • Improves both power and endurance

• Anaerobic activities (e.g., 50-meter dash, weightlifting)

• Use fast fibers • Fatigue quickly with strenuous activity • Improved by:

• Frequent, brief, intensive workouts • Causes hypertrophy

• Physical Conditioning • Improves both power and endurance

• Aerobic activities (prolonged activity) • Supported by mitochondria • Require oxygen and nutrients • Improves:

• Endurance by training fast fibers to be more like intermediate fibers • Cardiovascular performance

• 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

• Cardiac Muscle Tissue • Cardiac muscle cells are striated and found only in the heart • Striations are similar to that of skeletal muscle because the internal arrangement of myofilaments is similar

• Structural Characteristics of Cardiac Muscle Tissue • Unlike skeletal muscle, cardiac muscle cells (cardiocytes): • Are small • Have a single nucleus • Have short, wide T tubules • Have no triads • Have SR with no terminal cisternae • Are aerobic (high in myoglobin, mitochondria) • Have intercalated discs

• Intercalated Discs • Are specialized contact points between cardiocytes • Join cell membranes of adjacent cardiocytes (gap junctions, desmosomes) • Functions of intercalated discs: • Maintain structure • Enhance molecular and electrical connections • Conduct action potentials

• Intercalated Discs • Coordination of cardiocytes

• Because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells

Cardiac muscle cell Intercalated discs Nucleus

Cardiac muscle tissue LM × 575

A light micrograph of cardiac muscle tissue.

Cardiac muscle cell (intact)

Intercalated disc (sectioned)

A diagrammatic view of cardiac muscle. Note the striations and intercalated discs. Mitochondria

Nucleus

Cardiac muscle cell Myofibrils (sectioned)

Entrance to T tubule Sarcolemma Mitochondrion

Contact of sarcoplasmic reticulum with T tubule Sarcoplasmic Myofibrils reticulum Cardiac muscle tissue showing short, broad T-tubules and SR that lacks terminal cisternae. © 2012 Pearson Education, Inc. 10-8 Cardiac Muscle Tissue

• Functional Characteristics of Cardiac Muscle Tissue • Automaticity • Contraction without neural stimulation • Controlled by pacemaker cells • Variable contraction tension • Controlled by nervous system • Extended contraction time • Ten times as long as skeletal muscle • Prevention of wave summation and tetanic contractions by cell membranes • Long refractory period

• Smooth Muscle in Body Systems • Forms around other tissues • In integumentary system • Arrector pili muscles cause “goose bumps” • In blood vessels and airways • Regulates blood pressure and airflow • In reproductive and glandular systems • Produces movements • In digestive and urinary systems • Forms sphincters • Produces contractions

• Structural Characteristics of Smooth Muscle Tissue

• Nonstriated tissue • Different internal organization of actin and myosin • Different functional characteristics

Circular muscle layer

Longitudinal muscle layer

Smooth muscle tissue LM × 100

Many visceral organs contain several layers of smooth muscle tissue oriented in different directions. Here, a single sectional view shows smooth muscle cells in both longitudinal (L) and transverse (T) sections. © 2012 Pearson Education, Inc. Figure 10-23b Smooth Muscle Tissue Relaxed (sectional view) Dense body

Actin Myosin

Relaxed (superficial view)

Intermediate Adjacent smooth muscle cells are filaments (desmin) bound together at dense bodies, transmitting the contractile forces from cell to cell throughout the tissue. Contracted (superficial view)

• Characteristics of Smooth Muscle Cells • Long, slender, and spindle shaped • Have a single, central nucleus • Have no T tubules, myofibrils, or sarcomeres • Have no tendons or aponeuroses • Have scattered myosin fibers • Myosin fibers have more heads per thick filament • Have thin filaments attached to dense bodies • Dense bodies transmit contractions from cell to cell

• Functional Characteristics of Smooth Muscle Tissue 1. Excitation–contraction coupling 2. Length–tension relationships 3. Control of contractions 4. Smooth muscle tone

• Excitation–Contraction Coupling • Free Ca2+ in cytoplasm triggers contraction

• Ca2+ binds with calmodulin • In the sarcoplasm

• Activates myosin light–chain kinase • Enzyme breaks down ATP, initiates contraction

• Length–Tension Relationships • Thick and thin filaments are scattered • Resting length not related to tension development • Functions over a wide range of lengths (plasticity)

• Control of Contractions

• Multiunit smooth muscle cells • Connected to motor neurons

• Visceral smooth muscle cells • Not connected to motor neurons • Rhythmic cycles of activity controlled by pacesetter cells

• Smooth Muscle Tone • Maintains normal levels of activity • Modified by neural, hormonal, or chemical factors