Muscle structure and contraction Flashcards
What happens in a reflex action?
- Stimulus (heat)
- Pain receptor-stimulated
- Signal sent along sensory neuron
- Signal passed along relay neuron (message sent to brain)
- Signal sent along motor neuron
Muscles
- Effectors
- Carry out a response to a stimulus
- Bring about movement
Three types of muscle
- Cardiac
- Smooth
- Skeletal
Cardiac muscle
- Found in heart
- Involuntary
- No conscious
- Striated
Smooth muscle
- Found in walls of blood vessels and digestive systems
- Involuntary
- No conscious control
Skeletal muscle
- Found attached to the bone
- Voluntary
- Conscious control
- Striated
Contract - pulling on incompressible bone to move them
- Can’t push - must work in antagonistic pairs = opposing actions
Antagonistic examples
- Biceps and triceps
- Hamstring and quadriceps
What are skeletal muscles made of?
- Tiny muscle fibres - Myofibrils
- Made up of 2 types of myofilaments (even smaller)
- The fibres line up parallel to each other to maximise strength and increase power
Muscle is an example of tissue
- Made up of many similar cells = work together to carry out specific function
- Movement
- Separate muscle cells fuse together into muscle fibres - share nuclei and cytoplasm = SARCOPLASM
Sarcoplasm
- Mainly found around muscle fibres
- Contains many mitochondria = ATP in respiration and ER
Two myofilaments
- Actin and Myosin
- Both fibrous proteins
- Arranged in repeated units - Sacromeres
Actin
- Protein filament
- Thinner
- Made up of 2 strands twisted around each other
Myosin
- Protein filament
- Thicker
- Long rod shaped tails with bulbous heads
- Project the side
Sarcomere structure
- Myofibrils - striated - alternating light and dark coloured bands
- Light = I bands - only actin - no overlap with myosin
- Dark = A bands - overlapping actin and myosin
- End of each A band - lighter coloured area - H-Zone - myosin filaments - no overlap with actin
- Centre of I band - Z line - distance between z lines in sacromere
What do skeletal muscles contain?
- Mixture of slow and fast twitch fibres
- Proportion may change depending on location and function of muscle
Two types of muscle fibre
- Fast twitch
2. Slow twitch
Fast twitch
- Quick and powerful contraction
- Tire quickly - act for short period
- Dominant in athletes for power events
- Appear dark stained
Fast twitch muscle adaptions
- Aneraboic respiration
- Thicker + more numerous myosin filaments
- High concentration of glycogen
- Rely on glycolysis - ATP production = rapid contraction
- Lots of ATPase to synthesis ATP from ADP and Pi
- Lots of phosophocreatine - molecule rapidly generate ATP
= Lactic acid can build up
Slow twitch
- Slow, less powerful contraction
- Don’t tire quickly - act over a longer period
- More common in endurance exercise (tennis)
- White
Slow twitch muscle adaptions
- Aerobic respiration
- Lots of mitochondria to supply ATP
- High conc of myoglobin - stores o2
- Numerous blood vessels deliver o2 and glucose
- Contraction is delayed due to complex aerobic reactions
Silent filament mechanism
- Theory of muscle contraction
- Actin and myosin don’t get shorter = slide past each other = overall size of sacromere + muscle decrease
Sacromere is contracted
- Shorter I band
- Z lines move closer together
- Little/no H zone (myosin)
- A band remains the same width as the myosin filaments have not become shorter
Muscle proteins
- Actin
- Myosin
- Tropomyosin
- Troponin
Actin
Binding sites
Tropomyosin
- Forms long threads that are wound around actin filaments - prevent myosin-binding
- Muscle relaxed
Myosin
- Tails
- 2 bulbous heads at one end
- Heads fit into the binding sites on actin
Cross bridges
- Relaxed muscles - tropomyosin molecules block binding sites of the actin
- Myosin heads = resting position
- ADP + Pi bound to myosin head
- CA2+ stimulates - troponin = tropomyosin to change shape which exposes the binding site on actin
- Myosin head attach = cross-bridge + head tilts
- Moves the actin filament along = sarcomere is shorter
- ADP molecule detaches
Power stroke
- Movement of myosin
- Dragging actin = power stroke
- Myosin cross bridge binds to actin molecule
- Cross bridge bends - pulling thin myofilament inward
- Cross bridge at end of power stroke + returns to original conformation
- Cross bridge binds to more distal actin molecule = repeat
Detachment
- ATP attaches to each myosin head
= detach from actin-binding site - ATP hydrolysed by ATPase = energy for myosin head to return to original position
REPEAT
- Myosin head + ADP molecule reattaches itself further along the actin filament
Sarcolemma
- Cell membrane of a striated muscle fibre cell
Sarcoplasmic Reticulum
- System of flattened membranes
- Stores and pumps out Ca2+ ions
Sarcomeres
- Repeated units of myosin and actin in the muscle
Troponin
- Globular protein
- Moves tropomyosin so myosin can bind
Muscle relaxation
- Nerve stimulation stops
- Ca2+ are actively transported back to SR - energy from hydrolysis of ATP
- Reabsorption of Ca2+ allows tropomyosin to block the actin filament again
- Myosin heads no longer bind to actin filament = muscle no longer contracts
- Sarcomere will lengthen
ATP
- Hydrolysed to ADP + Pi = energy
- For myosin heads to return to their original position
- Reabsorption of Ca2+ into SR by active transport
ATP synthesis
- Synthesised in small amounts in anaerobic respiration
- Larger amounts in aerobic respiration - series of reactions - can be slow (not immediate)
- Generated more rapidly anaerobically using phosphocreatine
- Stored in muscle + acts as a reserve supply of phosphate which can combine with ADP to make ATP
- Phosphocreatine store - replenished using phosphate from ATP when the muscle is relaxed.
ATP synthesis formula
Phosphocreatine + ADP = creatine + ATP - energy
ATP + creatine = ADP + phosphocreatine
Neuromuscular junction
- The point where a motor neurone meets a muscle fibre
- Lots of them along a muscle - enable muscle fibres to contract simultaneously
= Muscle action fast and powerful - Action to transmit the message across the synapse - same as in a cholinergic synapse (acetylcholine)
What happens when acetylcholine diffuses to the postsynaptic membrane
- Causes SR to release its store of Ca2+ into the myofibrils
- Calcium binds to troponin on thin filament = changes shape, moving tropomyosin into the groove in the process
- Myosin cross bridges can now attach + cross bridge cycle can take place
Motor unit
- Motor fibres supplied by a single motor neurone
Small force
- Only a few units are stimulated
Large force
- A lot of units are stimulated
