Skeletal MuscleSkeletal Muscle

~40-45% of BW~40-45% of BW

400+ in body400+ in body

Responsible for all movement and Responsible for all movement and supportsupport

CompositionComposition

75% water75% water

20% protein20% protein

5% other: fats, CHO, high energy 5% other: fats, CHO, high energy phosphates, urea, lactic acid, phosphates, urea, lactic acid, enzymes, Na+, K+, Cl-, and minerals: enzymes, Na+, K+, Cl-, and minerals: Ca2+, Mg, and phosphorousCa2+, Mg, and phosphorous

Striated in ApperanceStriated in Apperance

Structure is fascia and connective tissue Structure is fascia and connective tissue sheaths which separate individual sheaths which separate individual muscles and hold muscles in placemuscles and hold muscles in place

Under fascia: epimysium, CT Under fascia: epimysium, CT surrounding musclesurrounding muscle

Under epimysium: fascicles, bundles of Under epimysium: fascicles, bundles of muscle fibers enclosed by perimysiummuscle fibers enclosed by perimysium– There are ~150 fibers/fasciculusThere are ~150 fibers/fasciculus

FasciulusFasciulusBundle of muscle cells (fibers) Bundle of muscle cells (fibers) surrounded by perimysiumsurrounded by perimysiumEach fiber in fasciulus is surrounded Each fiber in fasciulus is surrounded by endomsysiumby endomsysiumUnder endomysium is sarcolemma, Under endomysium is sarcolemma, cell membranecell membraneConnective tissue surrounds muscle Connective tissue surrounds muscle and forms a network that extends and forms a network that extends throughout the musclethroughout the muscle

SarcoplasmSarcoplasm

Cytoplasm of muscle cellCytoplasm of muscle cell

Under sarcolemmaUnder sarcolemma

Contains contractile proteins, Contains contractile proteins, enzymes, fat, and glycogen particles, enzymes, fat, and glycogen particles, nuclei, and specialized cellular nuclei, and specialized cellular organellesorganelles

ProteinsProteins

Greatest amounts:Greatest amounts:

MyosinMyosin

ActinActin

TropomysinTropomysin

Also, large amounts of myglobinAlso, large amounts of myglobin

Muscle cellsMuscle cells

Multi-nucleatedMulti-nucleated

Striated appearanceStriated appearance

Sarcoplasmic Reticulum (SR)Sarcoplasmic Reticulum (SR)

Embedded in sarcoplasmEmbedded in sarcoplasm

Network of channels and vessicles Network of channels and vessicles

Lies in parallel to the myofibrilsLies in parallel to the myofibrils

Lateral end: sac-like cistern which Lateral end: sac-like cistern which stores Ca2+stores Ca2+

Transverse Tubules (T-Tubules)Transverse Tubules (T-Tubules)Perpendicular to myofibrilPerpendicular to myofibrilT-tubule and two terminal cisternae are the T-tubule and two terminal cisternae are the region of the Z-line region of the Z-line Called a triadCalled a triadTwo triads/sarcomereTwo triads/sarcomereOne sarcomere: Z line – Z lineOne sarcomere: Z line – Z lineT-tubules open to the outside of each T-tubules open to the outside of each muscle fibermuscle fiberFunctioning as a network, spreading the AP Functioning as a network, spreading the AP from outer to inner portion of muscle fiberfrom outer to inner portion of muscle fiber

Muscle Blood supplyMuscle Blood supply

With exercise, increase in blood flow With exercise, increase in blood flow to muscleto muscle

Myosin molecule Myosin molecule

composed of 6 polypeptide chainscomposed of 6 polypeptide chains

2 heavy 2 heavy

4 light4 light

One myosin filament is made up of 200 or One myosin filament is made up of 200 or more myosin molecules more myosin molecules

Exactly 1.6 micrometers in lengthExactly 1.6 micrometers in length

Tails are in centerTails are in centerHeads coming out from the center (0.2 Heads coming out from the center (0.2 micrometers)micrometers)hinges at two pointshinges at two points– arm separates from the filament arm separates from the filament – where the head attaches to the armwhere the head attaches to the arm

filament is twisted, cross-bridges are displaced filament is twisted, cross-bridges are displaced from previous set by 120° from previous set by 120° Insures that cross-bridges extend in all Insures that cross-bridges extend in all directions from the filamentdirections from the filament

ActinActin

Primary protein in thin filamentPrimary protein in thin filament

Forms the backbone of filamentForms the backbone of filament

Approximately 3000 actin/myofibrilApproximately 3000 actin/myofibril

TroponinTroponin and and TropomyosinTropomyosin

Two other proteins in filamentTwo other proteins in filament

Two shapes of actinTwo shapes of actin

1. Globular (G-actin) polymerizes and 1. Globular (G-actin) polymerizes and unfolds intounfolds into

2. Fibrous (F-actin) double stranded in helix, 2. Fibrous (F-actin) double stranded in helix, complete revolution every 70 nanometerscomplete revolution every 70 nanometers

13 G-actin molecules in each revolution of 13 G-actin molecules in each revolution of the helical strandthe helical strand

Each G-actin has an ADP molecule Each G-actin has an ADP molecule attached to itattached to it

ADP are purported to be the active sites ADP are purported to be the active sites on the actin for cross-bridge interactionon the actin for cross-bridge interaction

Active sites are staggered, one site on Active sites are staggered, one site on total filament every 2.7 nanometerstotal filament every 2.7 nanometers

each actin is 1 micrometer longeach actin is 1 micrometer long

Filament bases Filament bases

Inserted into Z discs, other end protrudes Inserted into Z discs, other end protrudes into sarcomereinto sarcomere

In spaces between myosin moleculesIn spaces between myosin molecules

Z line Z line

Actin filaments extending from either sideActin filaments extending from either side

Into neighboring sarcomeresInto neighboring sarcomeres

Passes from myofibril to myofibrilPasses from myofibril to myofibril

Attaching the myofilaments to each other Attaching the myofilaments to each other across the muscle fiberacross the muscle fiber

SarcomereSarcomere

Basic contractile unit, from Z line to Z lineBasic contractile unit, from Z line to Z line

TropomyosinTropomyosin

Associated with the actin filamentAssociated with the actin filament

Loosely connected to the F-actin strandsLoosely connected to the F-actin strands

Wrap themselves spirally around the sides Wrap themselves spirally around the sides of the F-actin helixof the F-actin helix

At rest, tropomyosin molecules lie on top At rest, tropomyosin molecules lie on top of the actin active sites, inhibiting of the actin active sites, inhibiting interaction between actin and myosininteraction between actin and myosin

each tropomyosin covers about seven each tropomyosin covers about seven active sitesactive sites

Troponin Troponin

Protein attached near one end of each Protein attached near one end of each tropomyosin moleculetropomyosin molecule

Complex of 3 protein subunitsComplex of 3 protein subunits

1. Troponin I (TnI) strong affinity for actin1. Troponin I (TnI) strong affinity for actin

2. Troponin T (TnT) strong affinity for 2. Troponin T (TnT) strong affinity for tropomyosintropomyosin

3. Troponin C (TnC) strong affinity for 3. Troponin C (TnC) strong affinity for calciumcalcium

Troponin complex may attach the Troponin complex may attach the tropomyosin to the actintropomyosin to the actin

Each subunit plays a role in the contractile Each subunit plays a role in the contractile processprocess

Sarcomere UltrastructureSarcomere Ultrastructure

alternating light and dark bands along the alternating light and dark bands along the length of the muscle fiber give it its length of the muscle fiber give it its characteristic striated appearancecharacteristic striated appearance

Lighter area: I bandLighter area: I band

Darker zone: A bandDarker zone: A band

I band (isotropic): when light passes through this I band (isotropic): when light passes through this band, its velocity is the sameband, its velocity is the sameA band (anisotropic): light does not scatter A band (anisotropic): light does not scatter equallyequallyZ line bisects the I band and adheres to the Z line bisects the I band and adheres to the sarcolemma, adding stability to the sarcomeresarcolemma, adding stability to the sarcomereZ line has alpha actin (maintains spacing of Z line has alpha actin (maintains spacing of actin) and desmin (connects z lines of different actin) and desmin (connects z lines of different myofibrils together)myofibrils together)

Position of the actin and myosin results in an overlap of Position of the actin and myosin results in an overlap of the filaments in the sarcomere the filaments in the sarcomere Center of the A band is the H zone, a lighter area due to Center of the A band is the H zone, a lighter area due to the absence of actin filaments in this regionthe absence of actin filaments in this regionCentral part of the H zone is bisected by the M line, Central part of the H zone is bisected by the M line, which delineates the sarcomere’s centerwhich delineates the sarcomere’s centerM line is protein structures that support the arrangement M line is protein structures that support the arrangement of the myosin filamentsof the myosin filamentsalso has myomesin which provides an anchor for titin also has myomesin which provides an anchor for titin (elastic filament(elastic filamentHelps maintain centering between Z lines) and M-CK, Helps maintain centering between Z lines) and M-CK, provides ATP from CPprovides ATP from CP