WEEK 3 - Muscle Contraction Flashcards
three types of muscle tissue
- smooth muscle tissue (involuntary control, short, spindle-shaped, 1 nuclei and no striations)
- cardiac muscle tissue (involuntary control, branched, 1 nuclei and striations present)
- skeletal muscle tissue (voluntary control, long fibres, multi-nucleated and striations present)
skeletal muscle cell
skeletal muscle cell is the same thing as muscle fibre and is the same thing as myocyte
tendon: attaches muscle to the bone
after that, is a covering called epimysium which is a CT
after that is the perimysium which is wrapping little fascicles (groups of muscle fibres)
each structure is one muscle cell wrapped by endomysium (between individual muscle fibres)
skeletal muscle > fascicle > muscle fiber/myocyte> myofibril > sarcomere
the muscle fibre is a skeletal muscle cell aka myocyte
sarcolemma: plasma membrane of the cell
sarcolemma < endomysium < fascicle < perimysium < organ < epimysium
- sarcolemma has sarcoplasm (cytoplasm), a lot of mitochondria (which makes ATP and contains nuclei), and it has a myofibril which is an organelle
adaptations of skeletal muscle cells:
- myofibrils of skeletal muscle have a flat sarcoplasmic reticulum that wraps around them
- they have t-tubules which are invaginations of the sarcolemma, and disperses action potentials
sarcomere
- myofibrils are made of individual sarcomeres that align end to end this is why we see striations)
- smallest contractile unit (is what allows contractions to occur)
- inside sarcomeres are myofilaments (actin and myosin)
myofilaments
- thin filaments are actin
- thick filaments are myosin has heads that bind ATP
elastic filament: titan holds filaments in place
how does a skeletal muscle contract
- stimulation by a motor neuron
- the neuromuscular junction between neuron and muscle fibre
- generation of an action potential
neuron
- cell bodies aka soma
- axons give rise to many branches, with their own axon termini
- one axon termini of one branch, innervates only one single muscle cell
- each muscle fibre has one NMJ and is innervated by one single motor neuron
- one motor neuron can innervate a bunch of muscle cells, not a 1:1 ratio
how does innervation of a muscle cell by a neuron cause contraction
- acetylcholine is a neurotransmitter and is released from the axon terminal to the neuromuscular junction, the acetylcholine binds to the receptor on the sarcolemma = then the sodium ions rush in and cause depolarisation
- the action potential is generated and it travels down the t-tubule and activates the voltage-gated calcium channels
- calcium ions are related to the sarcoplasmic reticulum in response, goes into the sarcoplasm protein
- the calcium binds to a protein called troponin which is bound to actin, a cross-bridge forms between actin and myosin
the cross bridge cycle (actin and myosin stuck together)
- calcium increases in the sarcoplasm and binds to troponin
- tropomyosin stops myosin binding! it binds to actin, blocks the binding of myosin to actin
- calcium ions bind to troponin, troponin gets excited and changes shape, and it moves the tropomyosin off the sites where myosin needs to bind
- myosin heads that are attached to ADP and phosphate, they bind to actin and forms the cross bridge
only then the tropomyosin is off can the myosin bind to actin and forms the cross bridge!!
cross bridge cycle
- cross-bridge formation: myosin heads that are attached to ADP and phosphate, bind to actin and form the cross bridge
- power stroke: ADP and phosphate are released and the myosin head bends, since it’s attached to actin, it moves the actin toward the middle of the sarcomere = contraction
in absence of ATP, myosin heads will not detach
- starts 3-4 hrs after death
- peak being 12 hrs after postmortem
- need ATP to detach, but also need ATP to pump the calcium ions back into SR
- lack of ATP keeps the myosin stuck to actin
- lack of ATP prevents the stimulation, calcium, from going away
- cross-bridge detachment: need ATP to release myosin, if u don’t have it, myosin heads stay stuck in the power stroke so they’re constantly contracted (rigor mortis)
- ADP and phosphate popped off, ATP snuck its way in, and it is the message for myosin to let go of actin it wakens the link, myosin head detaches, breaks the cross bridge - reactivation of myosin head: reactivation of the myosin head: ATP is hydrolyzed, myosin is now ready and returns to the pre-stroke high-energy, its sticking out
as long as calcium is bound to troponin, the cycle will continue
it will also continue if ATP is available / get rid of it by hydrolysis = ADP and Pi
how do you stop the cycle?
- will stop if the calcium is pumped into the SR so that it is no longer bound to troponin
- will stop if ATP is unavailable so the cycle stops between steps 2 and 3
- will stop if enzymes (acetylcholinesterase degrades the Ach) remove Ach form the NMJ
types of muscle contractions
concentric: muscle contracts and the muscle shortens
eccentric: muscle still contracted but lengthens
isotonic: muscles change in length so actin filaments shorten and cause movement
3 pathways for regenerating ATP during muscle activity
- direct phosphorylation
- coupled reaction of CP, creatine phosphate and ADP, muscles store a lot of CP
- CP gets converted to creatine and then the P lost is added to ADP to create ATP
- energy source: CP
- oxygen is not needed
- products: - 1 ATP per CP and creatine
- duration of energy provided: 15 seconds - anaerobic pathway
- glycolysis and lactic acid formation
- energy source: glucose from carbs
any extra carbs are converted to glycogen
- glucose becomes pyretic acid
- oxygen is not needed
- products: 2 ATP per glucose, lactic acid
- duration of energy provided
- pyretic acid can’t enter the Krebs cycle because there isn’t enough oxygen present, so lactic acid is made
- aerobic pathway
aerobic respiration
energy source: glucose pyretic acid, free acids from adipose tissue, amino acids from protein catabolism
oxygen use is needed
products: 32 ATP per glucose, CO2, H2O
duration of energy provided: hours
3 pathways for regenerating ATP during muscle activity
- direct phosphorylation
- coupled reaction of CP, creatine phosphate and ADP, muscles store a lot of CP
- CP gets converted to creatine and then the P lost is added to ADP to create ATP
- energy source: CP
- oxygen is not needed
- products: - 1 ATP per CP and creatine
- duration of energy provided: 15 seconds - anaerobic pathway
- glycolysis and lactic acid formation
- energy source: glucose from carbs
any extra carbs are converted to glycogen
- glucose becomes pyretic acid
- oxygen is not needed
- products: 2 ATP per glucose, lactic acid
- duration of energy provided
- pyretic acid can’t enter the Krebs cycle because there isn’t enough oxygen present, so lactic acid is made
- aerobic pathway
aerobic respiration
energy source: glucose pyretic acid, free acids from adipose tissue, amino acids from protein catabolism
oxygen use is needed
products: 32 ATP per glucose, CO2, H2O
duration of energy provided: hours
smooth muscle cell
- found in walls of most hollow organs including respiratory, digestive, urinary, and circulatory, EXCEPT in the smallest blood vessels and the heart - the heart contains cardiac muscle!)
longitudinal layer
- fibres/muscle cell run parallel to long axis of organ
- contraction causes organs to shorten
circular layer inferior to the other
- fibres run around the circumference of organ
- contraction causes lumen of the organ to constrict
peristalsis: alternating contractions and relaxations of layers mix and squeeze substances through lumen of digestive organs
