skeletal muscle Flashcards
macroscopic to microscopic structure
muscle > muscle fascicle > muscle fibre > myofibril > sarcomere
what are sarcomeres?
- structural unit of myofibril in striated muscle
- dark and light bands due to actin and myosin
what cell types are muscles comprised of?
- muscle fibres
- vascular cells
- fibroblasts
- satellite cells
where do forces come from?
- interactions between myosin and actin forming cross- bridges
how many actin for every one myosin?
- six
what does the Z lines indicate?
- boundaries of each sarcomere
what is the m- line?
- located in middle of sarcomere
what does the I- band consist of?
- thin filaments only
- lighter due to actin
what does H zone consist of?
- thick filaments
- heavy as only myosin
what does the A band consist of?
- 2 myofilaments overlap
what happens to the bands during contraction?
- A band remains same
- I band reduced as actin filaments move towards each other
- H zone reduced
what do muscle spindles detect?
- stretch in muscles
what do muscle fibres prevent and how?
- prevents overstretching
- if contracting too much the antagonistic muscle will contract to reduce effects
what is the role of titin?
- protein that helps to stabilise and stretch sarcomeres
why is there a membrane around myofibril?
- helps with contraction of fibres
what is the sarcoplasmic reticulum?
- forms sleeve like segment around each myofibril
what is the role of mitochondria?
- helps to generate ATP
what are transverse tubules?
- lies adjacent to SR and action potentials travel down them which is important for release of calcium ions
what is a motor unit?
- motor neurone and its muscle fibres
- spinal cord> motor neurone> muscle fibres
how many fibres can one motor neuron innervate?
- many muscle fibres
why are action potentials propagated?
- to fibres to cause contraction
what does the journey of action potential depend on ?
- arrangement of fibres e.g. straight through if parallel
how are charges analysed?
- frequency can determine amount of force e.g. if brain sends more impulses than a bigger force will be formed = increased motor unit recruitment
what is the neuromuscular junction?
- synapse between a motor neuron terminal and its muscle fibres
- neuronal signals from brain/ spinal cord interact with fibres
what are the two proteins responsible for linking membrane action potential with calcium released in the cell?
- dihydropyridine (DHP) receptor (membrane)
- ryanodine receptor (sarcoplasmic reticulum)
what does action potential cause?
- depolarisation of plasma membrane
what does action potential cause?
- calcium channels open allowing calcium ions to diffuse into axon terminals and bind to proteins
what do the calcium ions cause?
synaptic vesicles containing neurotransmitters bind to membrane and release acetylcholine
what does the acetylcholine do once released?
- binds to nicotinic receptors opening ion channels ( Na+ moves in, K+ moves out)
= electrochemical gradient across membrane
how does the process at neuromuscular junction end ?
- motor end plate depolarises
- muscle fibre action potential initiated
- propagation (end plate potential)
what does every action potential in a motor neuron produce?
- a action potential in each muscle fibre
- 1:1
how long do action potentials last in skeletal muscle?
1-2 mscs before mechanical activity; mechanical delay between production of electrical activity and muscle contraction
how long does mechanical activity last following an action potential? what does it depend on?
- may last 100ms or more
- depends on availability of intracellular Ca2+
describe a relaxed muscle
- low calcium ions
- cross bridge cannot bind with actin because tropomyosin is covering binding site
what is tropomyosin?
- chain around actin filament that blocks myosin head from attaching
describe an active muscle
- high calcium ions
- calcium ions bind to troponin thus moves tropomyosin away from cross- bridge binding site
what happens once motor end plate is depolarised ?
- wave of depolarisation flows down t- tubules
what is released from sarcoplasmic reticulum /cisternae and what does it do?
- calcium ions
- binds to troponin causing a shift in tropomyosin
what happens when tropomyosin is moved?
- actin binding site is exposed allowing myosin to bind
- cross bridge forms between actin and myosin as the bridge bind, rotate and generate force
what does calcium removal restore?
- tropomyosin blocking action
where is the calcium transported back to?
- sarcoplasmic reticulum
what is the first stage of cross- bridge cycle?
- energised cross bridges bind to actin
what does the binding to actin release?
- ATP hydrolysis products from myosin, producing angular movement
what happens after ATP hydrolysis products are released?
- ATP bound to myosin breaking the link between actin and myosin
what happens to the ATP after link is broken?
- ATP bound to myosin is split energising the myosin cross bridge
what is ATPase?
- enzyme determines the speed of ATP hydrolysis and resulting sarcomere shortening velocity
what factors affect transmission?
- rate of calcium reuptake
- rate of ATP breakdown
what is frequency tension relationship?
- frequency of vibration directly proportional to tension
what do nonoverlapped areas represent?
specificity of metabolic function among the body’s three energy systems
- three overlapped portions represent generality
what is a power stroke?
- myosin pulls on actin causing actin to slide towards middle of sarcomere
what is excitation- contraction coupling?
refers to sequence of events by which an action potential in plasma membrane activates force generating mechanisms
what is the contraction time?
time interval from the beginning of tension development to peak tension
what is latent period?
- occurs after action potential before tension in muscle increases
1-2 milliseconds between stimulus and initiation of contraction
what is a muscle twitch?
- mechanical response of a fibre to a single action potential
what is summation?
- increase of muscle tension from successive action potentials
what is the sodium potassium pump?
- establishes the resting membrane potential of the cells
-70mV
what happens at rest to the pump?
- more sodium in extracellular space than when an action potential arrives
what does the arrival of the action potential cause?
- depolarisation means more sodium diffuse into intracellular space
what is repolarisation?
- restores the charge between the inside and outside of a cell membrane
what is hyperpolarisation?
- movement of a cell’s membrane potential toa more negative value
- less likely to fire an action potential
what does creatine phosphate provide?
- fast energy to form ATP from ADP
1-2 seconds
what is glycolysis? - energy metabolism
- energy from glucose in anaerobic conditions
8-10 seconds
what is oxidative phosphorylation?
- energy from glucose/ fat in aerobic conditions
2-3 days
how are glycogen and creatine phosphate stores replenished?
oxygen debt= oxygen the muscles use after exercise has finished
what is conduction failure?
- muscle fatigue caused by potassium accumulation in t- tubules
what does lactic acid build up cause?
- fatigue as functioning of proteins and mechanisms involved in Ca release and uptake are effected
what does inhibition of cross bridge cycling cause?
- fatigue through the build up of ADP in muscle fibres; preventing the release of actin
how do fuel substrates cause fatigue?
- low muscle glycogen and blood glucose
- dehydration
what is central command fatigue? when does it occur?
- failure of CNS to propagate signals from brain to motor neurones
- happens alot in heat
what are type 1 muscle fibres?
- slow oxidative
- high fatigue resistance + mitochondria
- thinner and low glycolytic
- muscle velocity 1 muscle lengths/ sec
what are type 11a muscle fibres?
- fast oxidative fibres
- highly oxidative and moderately glycolytic
- moderate contraction speed/ resistance to fatigue
- muscle velocity 2.5 muscle lengths/sec
what are type 11b muscle fibres?
- fast glycolytic
- large diameter to increase muscle force
- low fatigue resistance +mitochondria
- muscle velocity 3.5 muscle lengths/ sec
