MUSCLE and MUSCLE TISSUE Chapter 11

Muscles are distinguished by their ability to turn ATP (chemical energy) into work (mechanical energy) Muscles do their work in one way – they contract myo – (Greek) “muscle” Sarco – (Greek) “flesh”

Muscle Types Skeletal Attached to and covers bones Striations “voluntary” Powerful but tire easily

Cardiac Isolated; found only in heart Striated “involuntary” Contraction is regular and controlled by neural transmission

Visceral “Smooth” Involuntary No striatons Contraction is slow and sustained

Functions Movement Locomotion and manipulation by skeletal muscle Blood coursing – cardiac muscle Propulsion and/or squeezing of substances – smooth muscle

Posture Maintenance Skeletal muscle Counteracts gravity Heat Generation By-product of muscle metabolism and contractile activity Skeletal muscle (40% of body mass) is responsible

Properties Excitability – ability to receive and respond to a stimulus (usually a neurotransmitter) Contractility – ability to shorten when stimulated Extensibility – ability to be stretched or extended – to a point Elasticity – resumes its resting length

Skeletal Muscle – Gross Anatomy

Epimysium – connective tissue covering entire muscle structure Fascicle – bound and separated by perimysium Muscle fibers – bound and separated by endomysium Myofibrils- composed of myofilaments

Myofilaments - composed of actin and myosin protein fibers Connective tissue sheaths provide strength to the fragile muscle fibers; allow a route of entry and exit for blood and nerve fibers

Metabolic Supply Nerve endings Vascularity

Muscle Attachment Direct attachment: epimysium to periosteum of bone. Indirect attachment: epimysium forms a tendon to epimysium of another muscle or periosteum of bone.

Fascicle Arrangement Pattern influences range of motion and power Long fibers = more range of motion Power related to number of muscle fibers See page 244 of text

Microscopic Anatomy of Skeletal Muscle Typically cylindrical, long, multinucleated

Sarcolemma (cell membrane) – forms elongated tubes deep into the cell interior – “T tubules” Sarcoplasm – with stored glycogen and myoglobin

Sarcoplasmic Reticulum Specialized smooth E.R. Connected to sarcolemma (plasma membrane); provides passage for neurotransmitter, glucose, oxygen, etc. into fiber Regulates intracellular levels of Ca+ (holds and releases Ca+ on demand) Sarcomeres – smallest contractile unit of muscle fiber; made of myofibrils

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Myofibrils – 80% of cell volume; contractile element of cell Alternating bands of dark and light along length; give myofibril “striated” appearance Banding due to 2 distinct protein filaments (“myofilaments”) within sarcomere

Myofilaments

Thick filaments – composed of myosin protein Tail with 2 heads at one end – “cross bridges” Heads contain ATPases Tails bundled together with heads studded outward

Thin filaments – composed of actin protein Strands of protein coil around each other to give appearance of twisted pearl necklace

Sliding Filament Theory Thin filaments slide past thick filaments causing a myofibril overlap During contraction, thin filaments penetrate deeper Causes sliding

Physiology of Muscle Contraction Requires ACh & Ca+ Stimulation by neurotransmitter ACh receptors on sarcolemma cause Na+ to flood muscle cell Na+ causes sarcoplasmic reticulum to release Ca+ Ca+ binds to actin = exposing actin/myosin binding sites

Myosin heads bind to exposed actin heads (exposed in presence of Ca+ which moves molecules covering actin binding sites)

Myosin head binds, forming cross bridges, using energy from previous ATP hydrolysis Myosin head releases ADP (energy), thereby pulling myofilaments = “power stroke”

Ca+ is reabsorbed, myosin cross bridges are broken, muscle relaxes. ???What causes rigor mortis???

Muscle Contraction – Gross Level Muscles respond in an “all-or-none” fashion 150 muscle fibers/motor nerve, one neuron + all the fibers it innervates = one motor unit

Fibers within a motor unit are scattered throughout Muscles requiring fine motor control have fibers of one nerve close together - fingers

Muscle Mechanics Wave summation – recruit more cells Tetanus – sustained contraction Series of twitches Summation Tetanic contraction

Isotonic – muscle length varies Isometric – muscle length stays the same Muscle tone – slight contraction at rest

Muscle Metabolism When ATP synthesis < ATP use then one of several things happens because muscles store very little ATP: ADP + creatine phosphate Aerobic respiration Anaerobic respiration Muscle fatigue Contractures – permanent shortening Oxygen debt – lactic acid buildup

Animation: Energy Sources for Prolonged Exercise Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.

Force, Velocity, & Duration of Contraction Velocity and duration – dependent upon fiber types Slow twitch, fatigue resistant Fast twitch, fatigue vulnerable Fast twitch, fatigue resistant Most muscles are a mixture; some areas of body specialize

Slow Twitch Muscle Fibers Red fibers Most myoglobin Good blood supply Ex. Chicken leg marathon runners Slow-twitch fibers (Type I) Always oxidative Resistant to fatigue

Fast Twitch Fibers Fast-twitch glycolytic fibers (Type IIb) White fibers (less myoglobin) Poorer blood supply Susceptible to fatigue Least endurance Contract rapidly Ex. Chicken breast; sprinters

Disuse Muscle atrophy when not used; this is due either to enforced bedrest, immobilization, paralysis, etc. Muscles atrophy 5% of their strength/day when immobile; thus the reason for “skinny” limbs that have been in a cast.