An Introduction to Muscle Tissue A primary tissue type, divided into Skeletal muscle Cardiac muscle Smooth muscle

An Introduction to Muscle Tissue Skeletal Muscles Are attached to the skeletal system Allow us to move The muscular system Includes only skeletal muscles

Functions of Skeletal Muscles Produce skeletal movement Maintain body position Support soft tissues Guard openings Maintain body temperature Store nutrient reserves

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

Skeletal Muscle Structures Organization of Connective Tissues Muscles have three layers of connective tissues 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

Skeletal Muscle Structures Figure 10–1 The Organization of Skeletal Muscles.

Skeletal Muscle Structures 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)

Skeletal Muscle Structures Nerves Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord) Blood Vessels Muscles have extensive vascular systems that Supply large amounts of oxygen Supply nutrients Carry away wastes

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

Skeletal Muscle Fibers Figure 10–2 The Formation of a Multinucleate Skeletal Muscle Fiber.

Skeletal Muscle Fibers Figure 10–2a The Formation of a Multinucleate Skeletal Muscle Fiber.

Skeletal Muscle Fibers Figure 10–2b The Formation of a Multinucleate Skeletal Muscle Fiber.

Skeletal Muscle Fibers Internal Organization of Muscle Fibers The sarcolemma The cell membrane of a muscle fiber (cell) Surrounds the sarcoplasm (cytoplasm of muscle fiber) A change in transmembrane potential begins contractions

Skeletal Muscle Fibers Internal Organization of Muscle Fibers Transverse tubules (T tubules) Transmit action potential through cell Allow entire muscle fiber to contract simultaneously Have same properties as sarcolemma

Skeletal Muscle Fibers Internal Organization of 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

Skeletal Muscle Fibers Internal Organization of Muscle Fibers 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

Skeletal Muscle Fibers Internal Organization of Muscle Fibers 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

Skeletal Muscle Fibers Figure 10–3 The Structure of a Skeletal Muscle Fiber.

Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcomeres The contractile units of muscle Structural units of myofibrils Form visible patterns within myofibrils Muscle striations A striped or striated pattern within myofibrils: alternating dark, thick filaments (A bands) and light, thin filaments (I bands)

Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcomeres M Lines and Z Lines: M line: the center of the A band at midline of sarcomere Z lines: the centers of the I bands at two ends of sarcomere

Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcomeres Zone of overlap: the densest, darkest area on a light micrograph where thick and thin filaments overlap The H Band: the area around the M line has thick filaments but no thin filaments

Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcomeres Titin: are strands of protein reach from tips of thick filaments to the Z line stabilize the filaments

Skeletal Muscle Fibers Figure 10–4a Sarcomere Structure.

Skeletal Muscle Fibers Figure 10–4b Sarcomere Structure.

Skeletal Muscle Fibers Figure 10–5 Sarcomere Structure.

Skeletal Muscle Fibers Figure 10–6 Levels of Functional Organization in a Skeletal Muscle.

Skeletal Muscle Fibers Figure 10–6 Levels of Functional Organization in a Skeletal Muscle.

Skeletal Muscle Fibers Sarcomere Function Transverse tubules encircle the sarcomere near zones of overlap Ca2+ released by SR causes thin and thick filaments to interact

Skeletal Muscle Fibers Muscle Contraction Is caused by interactions of thick and thin filaments Structures of protein molecules determine interactions

Skeletal Muscle Fibers Four Thin Filament Proteins 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 Prevents actin–myosin interaction Troponin A globular protein Binds tropomyosin to G-actin Controlled by Ca2+

Skeletal Muscle Fibers Figure 10–7a, b Thick and Thin Filaments.

Skeletal Muscle Fibers Initiating Contraction Ca2+ binds to receptor on troponin molecule Troponin–tropomyosin complex changes Exposes active site of F-actin

Skeletal Muscle Fibers Thick Filaments Contain 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

Skeletal Muscle Fibers Figure 10–7c, d Thick and Thin Filaments.

Skeletal Muscle Fibers Myosin Action During contraction, myosin heads Interact with actin filaments, forming cross-bridges Pivot, producing motion

Skeletal Muscle Fibers Skeletal 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

Skeletal Muscle Fibers Figure 10–8a Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber.

Skeletal Muscle Fibers Figure 10–8b Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber.

Skeletal Muscle Fibers Skeletal Muscle Contraction The process of contraction Neural stimulation of sarcolemma: causes excitation–contraction coupling Cisternae of SR release Ca2+: which triggers interaction of thick and thin filaments consuming ATP and producing tension

Skeletal Muscle Fibers Figure 10–9 An Overview of Skeletal Muscle Contraction.

The Neuromuscular Junction Is the location of neural stimulation Action potential (electrical signal) Travels along nerve axon Ends at synaptic terminal Synaptic terminal: releases neurotransmitter (acetylcholine or ACh) into the synaptic cleft (gap between synaptic terminal and motor end plate)

The Neuromuscular Junction Figure 10–10a, b Skeletal Muscle Innervation.

The Neuromuscular Junction Figure 10–10c Skeletal Muscle Innervation.

The Neuromuscular Junction Figure 10–10c Skeletal Muscle Innervation.

The Neuromuscular Junction The Neurotransmitter Acetylcholine or ACh Travels across the synaptic cleft Binds to membrane receptors on sarcolemma (motor end plate) Causes sodium–ion rush into sarcoplasm Is quickly broken down by enzyme (acetylcholinesterase or AChE)

The Neuromuscular Junction Figure 10–10c Skeletal Muscle Innervation.

The Neuromuscular Junction Action Potential Generated by increase in sodium ions in sarcolemma Travels along the T tubules Leads to excitation–contraction coupling Excitation–contraction coupling: action potential reaches a triad: releasing Ca2+ triggering contraction requires myosin heads to be in “cocked” position: loaded by ATP energy

The Neuromuscular Junction Figure 10–11 The Exposure of Active Sites.

The Contraction Cycle Five Steps of the Contraction Cycle Exposure of active sites Formation of cross-bridges Pivoting of myosin heads Detachment of cross-bridges Reactivation of myosin

The Contraction Cycle Figure 10–12 The Contraction Cycle.

The Contraction Cycle [INSERT FIG. 10.12, step 1] Figure 10–12 The Contraction Cycle.

The Contraction Cycle Figure 10–12 The Contraction Cycle.

The Contraction Cycle Figure 10–12 The Contraction Cycle.

The Contraction Cycle Figure 10–12 The Contraction Cycle.

The Contraction Cycle Figure 10–12 The Contraction Cycle.

The Contraction Cycle Fiber Shortening Contraction Duration As sarcomeres shorten, muscle pulls together, producing tension Contraction Duration Depends on Duration of neural stimulus Number of free calcium ions in sarcoplasm Availability of ATP

The Contraction Cycle Figure 10–13 Shortening during a Contraction.

The Contraction Cycle Relaxation Rigor Mortis Ca2+ concentrations fall Ca2+ detaches from troponin Active sites are re-covered by tropomyosin Sarcomeres remain contracted 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

The Contraction Cycle 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

The Contraction Cycle

ATP and 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 Muscle Contraction 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 phosphokinase (CPK or CK) When CP is used up, other mechanisms generate ATP

ATP and Muscle Contraction ATP Generation Cells produce ATP in two ways Aerobic metabolism of fatty acids in the mitochondria Anaerobic glycolysis in the cytoplasm

ATP and Muscle Contraction ATP Generation Aerobic metabolism Is the primary energy source of resting muscles Breaks down fatty acids Produces 34 ATP molecules per glucose molecule Anaerobic 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

ATP and Muscle Contraction

ATP and Muscle Contraction Energy Use and Muscle Activity At peak exertion Muscles lack oxygen to support mitochondria Muscles rely on glycolysis for ATP Pyruvic acid builds up, is converted to lactic acid

ATP and Muscle Contraction Figure 10–20 Muscle Metabolism.

ATP and Muscle Contraction Figure 10–20a Muscle Metabolism.

ATP and Muscle Contraction Figure 10–20b Muscle Metabolism.

ATP and Muscle Contraction Figure 10–20c Muscle Metabolism.

ATP and 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

ATP and Muscle Contraction The Recovery Period The time required after exertion for muscles to return to normal Oxygen becomes available Mitochondrial activity resumes

ATP and Muscle Contraction The Cori Cycle The removal and recycling of lactic acid by the liver Liver converts lactic acid to pyruvic acid Glucose is released to recharge muscle glycogen reserves Oxygen Debt After exercise or other exertion The body needs more oxygen than usual to normalize metabolic activities Resulting in heavy breathing

ATP and Muscle Contraction 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

ATP and Muscle Contraction 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

ATP and Muscle Contraction Muscle Performance Power The maximum amount of tension produced Endurance The amount of time an activity can be sustained Power and endurance depend on The types of muscle fibers Physical conditioning

Muscle Fiber Types Three Types of Skeletal Muscle Fibers Fast fibers Slow fibers Intermediate fibers

Muscle Fiber Types Three Types of Skeletal Muscle Fibers Fast fibers Contract very quickly Have large diameter, large glycogen reserves, few mitochondria Have strong contractions, fatigue quickly

Muscle Fiber Types Three Types of Skeletal Muscle Fibers Slow fibers Are slow to contract, slow to fatigue Have small diameter, more mitochondria Have high oxygen supply Contain myoglobin (red pigment, binds oxygen)

Muscle Fiber Types Three Types of Skeletal Muscle Fibers Intermediate fibers Are mid-sized Have low myoglobin Have more capillaries than fast fibers, slower to fatigue

Muscle Fiber Types Figure 10–21 Fast versus Slow Fibers.

Muscle Fiber Types Muscle Hypertrophy Muscle Atrophy 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

Muscle Fiber Types 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 hypertrophy

Muscle Fiber Types Physical Conditioning Improves both power and endurance Aerobic activities (prolonged activity): supported by mitochondria require oxygen and nutrients Improved by: repetitive training (neural responses) cardiovascular training

Muscle Fiber Types 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 Structure of Cardiac Tissue Cardiac muscle is striated, found only in the heart

Cardiac Muscle Tissue Seven Characteristics of Cardiocytes 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

Cardiac Muscle Tissue 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

Cardiac Muscle Tissue Figure 10–22 Cardiac Muscle Tissue.

Cardiac Muscle Tissue Figure 10–22a Cardiac Muscle Tissue.

Cardiac Muscle Tissue Figure 10–22c Cardiac Muscle Tissue.

Cardiac Muscle Tissue 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 Tissue Four Functions of Cardiac 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 Tissue Smooth Muscle in Body Systems Forms around other tissues In blood vessels Regulates blood pressure and flow In reproductive and glandular systems Produces movements In digestive and urinary systems Forms sphincters Produces contractions In integumentary system Arrector pili muscles cause “goose bumps”

Smooth Muscle Tissue Structure of Smooth Muscle Nonstriated tissue Different internal organization of actin and myosin Different functional characteristics

Smooth Muscle Tissue Figure 10–23a Smooth Muscle Tissue.

Smooth Muscle Tissue Figure 10–23b Smooth Muscle Tissue.

Smooth Muscle Tissue Eight 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

Smooth Muscle Tissue Functional Characteristics of Smooth Muscle Excitation–contraction coupling Length–tension relationships Control of contractions Smooth muscle tone

Smooth Muscle Tissue Functional Characteristics of Smooth Muscle 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

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

Smooth Muscle Tissue Functional Characteristics of Smooth Muscle 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 Tissue Functional Characteristics of Smooth Muscle Smooth muscle tone Maintains normal levels of activity Modified by neural, hormonal, or chemical factors