what are the characteristics of skeletal muscle tissue?
attaches to bone via tendons, striated, contracts, voluntarily controlled
what are the 3 structures of connective tissue associated w/ skeletal muscle?
epimysium, perimysium, endomysium
describe the epimysium
connective tissue layer which surrounds the entire skeletal muscle system; outter
describe the perimysium
connective tissue that surrounds the fascicle; around
what is a fascicle?
bundles of skeletal muscle fibers
describe the endomysium
connective tissue that surrounds each skeletal muscle fiber (cell) w/in a fascicle
describe skeletal muscle fiber
cellular unit of skeletal muscle
what are the 3 components of skeletal muscle fiber?
sarcolemma, transverse (T) tubules, sarcoplasm
plasma membrane of a skeletal muscle fiber
describe transverse (T) tubules
folds of sarcolemma that extend into the interior of a skeletal muscle fiber
cytoplasm of a skeletal muscle fiber
what are 3 organelles found in the sarcoplasm?
nuclei, mitochondria, sarcoplasmic reticulum (SR)
multinucleate- many nuclei in skeletal muscle fiber
what are myoblasts?
several fused precursor cells
produce ATP(energy); required for muscle contraction
describe sarcoplasmic reticulum (SR)
ER of skeletal muscle fiber; membraneous sac stores (Ca+2) ions- muscle relaxation & releases (Ca+2) ions into sarcoplasm to initiate muscle contraction
what are 2 molecules found in the sarcoplasm?
glycogen & myoglobin
storage form of glucose; break dwn into glucose to make ATP; found in liver
reddish protein that stores O2 & releases it when needed by mitochondria to generate ATP
rodlike structures; cause skeletal muscle fiber to shorten (contraction); 2 types thick filament, thin filament
what are the 2 types of contractile filaments?
thick filament, thin filament
describe thick filaments
contain the protein myosin
describe thin filaments
contain 3 types of proteins; actin, troponin, tropomyosin
repeating unit of myofibrils; consist of 5 regions- Z disc, A band, I band, H zone, M line
what is a Z disc?
composed of a-actin proteins; distance between 2 successive Z discs defines sarcomere
what is an A band?
dark area, extends length of thick filament; includes parts of thin filament that overlap thick filament
what is an I band?
light area, extends from Z discs on either side; includes part of thin filament that do not overlap thick filament
what is an H zone?
central part of A band; contains only thick filament, dark
what is the M line?
central part of H zone; consist of myomesin, interconnect neighboring thick filaments
what is titin?
long protein, extends from each half of sarcomere; connect Z disc to M line; flexible, stretch & snap back; structural support
what is dystrophin?
protein that links thin filaments to proteins in sarcolemma; pull sarcolemma causes skeletal muscle fiber to shorten; structural support
what is the main function of skeletal muscle?
cause movement of various parts of the body; pull on tendons, in turn pulls on the bone, causes movement
describe the structure of a thick filament
myosin molecules; 2 polypeptide chains that form a rodlike tail & double head; 2 binding site- actin-binding site & ATP-binding site
describe the structure of a thin filament
3 proteins- actin, tropomyosin, troponin
what is actin?
thin filament, form double stranded actin helix; myosin-binding site-myosin head can attach to cause contraction
what is tropomyosin?
rod-shaped protein, promotes skeletal muscle relaxation; covers myosin-binding sites on actin- prevents myosin from interacting with actin
what is troponin?
globular protein, promotes skeletal contraction; Ca+2 ions released into sarcoplasm, binds to troponin, pushes tropomyosin off myosin-binding sites on actin, interact(contraction)
describe sliding filament mechanism
myosin heads attach to & "walk" along the thin filaments at both ends of sarcomere, pulling the thin filament towards the M line; thin filament slides inwards & meet at center of sarcomere
what are the 4 steps of the contraction cycle?
ATP hydrolysis, attachment of myosin to actin, power stroke, detachment of myosin from actin
describe ATP hydrolysis
hydrolyzes ATP into ADP & phosphate group; energy generated stored in myosin head-energized
describe attachment of myosin to actin
energized myosin heads attach to myosin-binding site on actin; only 1 head binds at a time
what is a crossbridge?
myosin head attaches to actin during contraction cycle
describe the power stroke
after crossbridge is formed, myosin head pulls thin filament past thick filament towards center of sarcomere, generating tension(force); uses energy stored in myosin head
describe the detachment of myosin from actin
as ATP binds to the ATP-binding site on myosin head, it detaches from actin; myosin hydrolyzes newly bound ATP; (continues as long as ATP avaliable & Ca+2 level is high)
describe a motor unit
consist of a motor neuron & skeletal muscle fibers it innervates; motor neuron elicts A.P. all fibers contract together
what is a neuromuscular junction (NMJ)
site of communication between a motor neuron & skeletal muscle fiber (presynaptic cell & postsynaptic cell)
what are the 3 components of NMJ?
synaptic end bulbs of the motor neuron, motor end plate, synaptic cleft
describe synaptic end bulb of the motor neuron
contain synaptic vesicles filled w/ neurotransmitters (acetylcholine, ACh)
describe the motor end plate
sarcolemma of the skeletal muscle fiber; contains ACh-receptors that bind to ACh
describe the synaptic cleft
space between synaptic end bulb of motor neuron & motor end plate of the skeletal muscle fiber
what are the 4 ways synaptic transmission occurs at the NMJ?
release of acetylcholine, activation of ACh receptors, production of muscle action potentials, termination of ACh activity
describe release of acetylcholine
nerve a.p. at endbulb of motor neuron causes synaptic vesicles to undergo exocytosis, synaptic vesicles fuse w/ motor neuron's plasma mem., releasing ACh into synaptic cleft, diffuses across synaptic cleft
describe activation of ACh receptors
binding of ACh to receptor on motor end plate opens an ion channel in ACh receptor, small cations (Na+) flow across mem.
describe production of muscle A.P.
inflow of Na+, makes inside of muscle fiber more (+), change triggers muscle A.P., then propagates along sarcolemma into T tubule system, causing muscle fiber to contract
describe termination of ACh activity
ACh no longer released when A.P. in motor neuron ceases, ACh is broken down by enzyme acetylcholinesterase(AChE), prevents formation of another A.P.
what is acetylcholinesterase (AChE)?
enzyme which breaks down the molecules of ACh into acetyl & choline
what is excitation-contraction coupling?
sequence of events connecting muscle A.P. to muscle contractions
what are the 3 ways muscle fibers obtain ATP?
transfer of phosphate from creatine phosphate to ADP to form ATP, anaerobic glycolysis, aerobic (cellular) respiration
describe transfer of phosphate from creatine phosphate to ADP to form ATP
immediate source of ATP, 15 seconds;
what is creatine?
small molecule similar to an amino acid in structure; found in skeletal muscle fibers & cardiac muscle fibers
what is creatine kinase (CK)?
transfers 1 of the phosphate groups of ATO to creatine to form creatine phosphate when ATP is in excess
what is creatinine?
break down of creatine, forming a nitrogenous waste product
describe anaerobic glycolysis
source of ATP when a skeletal muscle fiber lacks O2 or doesnt have enough O2 to meet metabolic needs (weight-lifting)- 2 min
describe aerobic (cellular) respiration
source of ATP when skeletal muscle fiber has plenty of O2 (resting, walking, moderately running on treadmill)- several min to hr or more