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Terms in this set (116)

1. each skeletal muscle is an organ made up of several types of tissue
3. muscle tissue

4. nerve & blood supply
- nerves imp b/c is a voluntary muscle; nervous system relies on input from muscles to coordinate muscle reactions
- blood supply imp. for oxygen and nutrient coming to the muscle and removing waste products

5. connective tissue sheaths - surround muscle fibers, collections of muscle fibers, and entire muscle

i. endomysium - inner most sheath
- areolar connective tissue and surrounds each muscle fiber in a muscle

ii. perimysium - middle sheath
- surrounds each fascicle (collection/cluster of muscle fibers)
- dense irreg CT

iii. epimysiym most external sheath
- dense irreg CT
- blends with superficial or deep facia or tendons or aponeuroses which attach muscle to bone

**skeletal muscles can be though of having a belly positioned between muscle attachment sites; where muscle cells/fibers are located; connective tissue sheaths extend past the belly and serve as attachment sites
2. muscle attachments
- muscles span joints and attach to bones in at least 2 places
a. origin vs. insertion - attachment sites
i. origin - attachment to immovable or less moveable bone
ii. insertion - attachment to movable bone, ex. Biceps at elbow is insertion and biceps at shoulder is origin site

4. direct vs. indirect attachments
i. direct - epimysium fused to periosteum of bone or perichondrium of cartilage
ii. indirect - may be attached to either ropelike tendon or sheet-like aponeurosis

a) tendon - rope/cord-like

b) aponeurosis - sheet

a. general cell features
1) skeletal muscle fibers are much larger, elongated cylindrical cells - that's why called fibers

2) skeletal muscle fibers are multinucleated - up to 100 or more nuclei which are pushed to the perimeter of the cell, just under the sarcolemma (muscle fiber plasma membrane)

3) both of these features derive from events of embryonic development

1) myoblasts - mesodermal cells differentiate into myoblasts - myoblasts fuse creating a fiber w/ multiple nuclei ; some
2) syncytium
3) satellite cells - MUSCLE STEM CELLS after mucle fibera re formed, primitive myoblasts remain is muscle tissue - some ability to help thru mitosis and diffusion and limited regeneration of skeletal muscle tissue

2. sarcolemma (muscle fiber plasma membrane) encloses cell contents
a. sarcoplasm (muscle fiber cytoplasm) contains typical organelles
1) mitochondria - need lots of ATP

2) myoglobin - similar to hemoglobin
a) pigment with a high affinity for O2
b) stores & transfers O2 from the blood hemoglobin to the mitochondria

3) glycosomes - cellular inclusions in which glycogen is stored

b. other organelles are modified to support specific muscle function: myofibrils, myofilaments, & sarcomeres & T tubules
1) myofibrils are the contractile elements of the muscle fiber
- densely packed, rod-like elements that extend the length of the muscle

2) myofibrils consist primarily of two types of protein structures called myofilaments - responsible for 80% muscle fiber volume; extend entire length of muscle
a) thin filaments -composed mainly of the protein actin

b) thick filaments -composed mainly of the protein myosin

- all the myofibrils are anchored to the sarcolemma to the muscle cell by the protein dystrophin; sarcolemma is attached to endomysium which is attached to tendon or aponeurosis or direct attachment that connects the muscle to bone; collagen fibers of the endomysium will transfer tension to the attachment of the muscle; contraction causes enough tension for pulling of muscle

3) myofilaments are arranged in compartments called sarcomeres which do not extend the entire length of the muscle cells - extends from one z disk to the next
a) this is the smallest contractile/functional unit of the muscle
b) each myofibril is made up of a series of sarcomeres, positioned end to end, side by side

4) the thick and thin filaments overlap each other, producing striations - b/c alternating A and I bands

vi. description of the light & dark bands which create the striations in myofibril; bands in each adjacent sarcomere
1) A band -- the dark area (b/c thick and thin filaments overlap)

2) I band -- light area (b/c only thin filaments so a lot of light can get thru)

3) H zone -- a narrow, lighter region in the center of each A band - only thick filaments; changes in length during muscle contraction
4) M line -- a line which divides the H zone in the center vertically; myomesin proteins; anchoring site for components of sarcomere

5) Z disc or Z line -coin-shaped, but looks like a dark line through the middle of the I band
a) a single sarcomere extends from one Z disc to the next
- anchors thin filaments
- connects each myofibril to all surrounding myofibrils throughout width of the muscle fiber

b) elastic filaments, composed of the protein titin
1 extend from the Z disc to the tips of thick filaments, then through the thick filament to the M line
2 stabilize myofilaments & enhance extensibility/elasticity of muscle fibers

vii. the sarcomere in cross-section
1) each thick filament is surrounded by 6 thin filaments

2) each thin filament is surrounded by 3 thick filaments

- contributes to efficient interaction b/t thick and thin filaments

3. molecular structure of myofilaments
a. thick filaments
i. myosin - each contains 2 heavy and 4 light polypeptide chains
1 heavy chains intertwine to form a myosin tail
2 2 globular heads on the opposite side of heavy chains
3 Flexible hinge region b/t the head and tail allows heads to pivot and change orientation
4 During contraction - light chains link myosin heads to thin filaments, forming cross bridges
5 Making and breaking cross bridges is contraction of sarcomere
6 Myosin heads extend in a spiraling arrangement from the thick filament; tails are oriented towards M line and are ready to anchor the thick filament
7 Each myosin head contains a region that has ATPase function and an ATP binding site

ii. consist of a rod-like tail connected to the globular head region by a hinge region which allows the head to pivot

iii. during contraction, light chains help link the head to thin filaments, forming crossbridges

b. thin filaments
i. actin
1) polypeptide subunits of actin are called G actin (globular actin) -polymerized into long actin filaments called F actin and have a myosin-binding site called the active site
- to form a thin filament, g actin filaments link together covalently to form long fibrous F actin (filamentous actin); 2 F actin strands twist together to form a thin filament

2) nebulin - protein at core of thin filament and extends the entire length b/t 2 F action strands and helps stabilize F actin and thin filament in general

3) active site

4) regulatory proteins associated with thin filaments
a) tropomyosin - rod shaped protein spiraling thin filaments; 2 filaments; covers active sites on G actin subunits

b) troponin -consists of 3 globular subunits; complex protein; one subunit binds to tropomyosin and holds the troponin-tropomyosin complex together; one subunit binds to G actin and hold the troponin-tropomyosin complex over active sites on actin; 3rd has binding sites for calcium ion and low calcium levels are low in a resting muscle and calcium binding site is empty; when muscles contract, there's a higher number of calcium ions present in the sarcoplasm and calcium can bind to that binding site on troponin, allowing cross bridges b/t myosin and actin to form

e. transverse tubules (T tubules)
i. T tubules are tunnel-like invaginations of the sarcolemma -extend into sarcoplasm at right angles cell surface - go deep into cell's interior
- - membranous structures specially differentiated for muscle contraction activity
- tubes wrap around and pass thru myofibrils
- fluid inside in identical to extracellular fluid
- membrane continuous w sarcolemma - if action potential passes along sarcolemma, will pass down T tubules too so signal spreads quickly within the muscle cell and they can contract at the same time
ii. T tubules are located at the level of the A band-I band junction

f. sarcoplasmic reticulum (SR)
i. SR is a system of smooth ER that forms a tubular network around individual myofibrils

- membranous structures specially differentiated for muscle contraction activity
- in skeletal muscle fibers, the sarcoplasmic reticulum is organized to form a tubular network around each myofibril
- regulate sarcoplasmic Ca2+ levels - Ca ion pumps pump calcium from the sarcoplasm back into SR - can store high concentrations of Ca b/c calcium concentrating proteins called calsequestrin in the SR - contractions begin when calcium release channels open in the SR and release it into the sarcoplasm

- at the A-I band junctions on either side of T tubule, SR forms enlarged chambers -called terminal cisternae (singular = terminal cisterna) - calcium ion and concentration high here

iii. triad = a T (transverse) tubule and the 2 terminal cisternae adjacent to it - activate skeletal muscle and trigger contraction thru calcium release

1) integral proteins extending from T tubule membrane into intermembrane space act as voltage sensors
2) integral proteins of terminal cisternae (which also extend into the intermembrane space) are voltage sensors/receptors that regulate the release of Ca2+ from the SR - proteins change shape when action potential
- integral proteins protrude into intermembrane space b/t T tubule and terminal cisternae and control Ca release channels so if one of their proteins change shape so does the other and Ca is released
- SR helps end contraction process b/c has calcium ion pumps in its membrane to pump calcium back into SR from sarcoplasm