Unit 6: Organisms respond to changes in their environment

• effector organs
• respond to nervous stimulation
• movement

e.g.

• cardiac muscle - heart
• smooth muscle - walls of blood vessels + gut
• involuntary contractions

BUT...

• skeletal muscle - bulk of body muscle
• attached to bone
• voluntary + conscious control of contractions

• myofibrils - tiny muscle fibres that make up muscles
• maximum strength and efficiency
• cells fuse together to form muscle fibres (bundles)
• bundles of fibres make up whole muscle

Muscle Fibres:

• share nuclei + cytoplasm = sarcoplasm
• sarcoplasm - around muscle fibre
• lots of mitochondria + ER

Myofibrils are made up of two protein filaments:

• actin - thinner + two intertwining strands

• myosin - thicker + long rod shaped cells + projecting bulbous heads

• light and dark coloured bands - striations
• I bands (isotropic) - light bands = no overlap of myosin + actin
• A bands (anisotropic) - dark bands = overlap of myosin + actin
• H - zone = centre of each A band
• Z - line = centre of each I band

• Sarcomere = distance between adjacent Z-lines
• Parallel to each other in muscle = force in one direction
• Muscle contractions - sarcomere shortens 
• Different pattern of light (I) bands and dark (a) bands

• Tropomyosin - protein - forms fibrous strand around actin

• two types - fast twitch + slow twitch
• variation - type of muscle + person

• slower
• less powerful
• longer
• endurance, e.g. long distance running
• e.g. calf muscles - constantly contracts - keep upright
• AEROBIC RESPIRATION - no build up of lactic acid
• more effective 
• shorter contractions
• lots of myoglobin - red - lots of oxygen
• lots of blood vessels - oxygen + glucose - respiration
• mitochondria - ATP

• more rapid
• powerful
• shorter 
• intense exercise, e.g. weight lifting
• e.g. biceps muscle - short bursts

• thicker + more myosin filaments
• lots of glycogen
• lots of enzymes - ANAEROBIC RESPIRATION
• rapid production of ATP
• PHOSPHOCREATINE - rapid production of ATP
• ADP + Pi -> ATP 
• energy for muscle contraction

• motor neurone meets skeletal muscle fibre
• lots of junctions along muscle
• shorter time for wave of contraction to cross muscle
• simultaneous contraction
• rapid + powerful
• essential for survival
• stimulated by action potential

• muscle fibres - 1 motor neurone - motor unit
• controls force muscle exerts
• weak force = fewer motor units stimulated
• strong force = many motor units stimulated

• reaches synaptic knob
• synaptic vesicles fuse with presynaptic membrane
• releases acetylcholine into synaptic cleft
• attaches to receptor sites on postsynaptic membrane = membrane of muscle fibre
• alters permeability to Na+ ions - enter rapidly = DEPOLARISATION

• neurotransmitter = acetylcholine
• broken down by acetylcholinerase
• muscle isn't over-stimulated
• choline + ethanoic (acetyl) acid diffuses back into neurone
• recombine to form acetylcholine
• energy (ATP) supplied by mitochondria

Similarities:

• neurotransmitters are transported by diffusion
• receptors - bind with neurotransmitter - influx of Na+
• use Na+/K+ pump to repolarise axon
• enzymes break down neurotransmitter

Differences:

Neuromuscular Junction

• excitatory
• neurones link to muscles
• motor neurones
• end of action potential
• acetylcholine binds to receptors on membrane of muscle fibre

Cholinergic Synapse

• excitatory or inhibitory
• neurones link to neurones or effectors
• motor, sensory or intermediate (relay) neurones
• new action potential - postsynaptic membrane
• acetylcholine binds to receptors on postsynaptic membrane

• movement
• muscles attached to skeleton
• incompressible bone skeleton
• muscle exerts force - bones move around joints
• the muscle does not change shape
• move part of skeleton
• one muscle can't move part in opposite directions
• another muscle = ANTAGONISTIC PAIR
• pull in opposite directions
• one contracts + one relaxes

• contraction of muscle fibre
• actin + myosin filaments slide past one another
• muscle function depends on shape of proteins

• more overlap of actin + myosin during contraction

Changes to sarcomere:

• I-band narrows
• Z-lines are closer together = shorter sarcomere
• H-zone narrows
• A-band stays the same width - length of myosin
• muscle contraction not due to shorter filaments

2 proteins:

• fibrous protein - filament - made up of lots of molecules (tail)
• globular protein - two bulbous structures (head)

• globular protein
• molecules in long + twisted chains
• helical strand

TROPOMYOSIN

• long + thin threads
• wound around actin filaments

• actin + myosin filaments slide past each other
• changes in band patterns on myofibrils

• bulbous heads of myosin form cross bridges with actin
• attach to binding sites on actin
• flex in unison - pull actin filaments along myosin filaments
• ATP - source of energy
• detach - return to original angle
• re-attach further along the actin
• continuous process

• action potential reaches many neuromuscular junctions simultaneously
• Ca2+ ion protein channels open
• Ca2+ ion diffuse into synaptic knob
• synaptic vesicles fuse with presynaptic membrane
• release acetylcholine (neurotransmitter) into synaptic cleft
• acetylcholine diffuses across synaptic cleft
• binds with receptors on the muscle cell-surface membrane
• DEPOLARISATION

WAVE OF EXCITATION

• action potential travels into fibre
• via tubules (T-tubules) - extensions of cell-surface membrane
• in cyctoplasm of muscle - sarcoplasm
• tubules - ER of muscle = sarcoplasmic reticulum
• active transport of Ca2+ ion from cytoplasm - low concentration
• Ca2+ protein channels open
• Ca2+ diffuse into muscle cytoplasm
• down concentration gradient
• Ca2+ ions cause tropomyosin to pull away 
• Ca2+ ions change tertiary structure of tropomyosin - protein!
• bindings site are no longer blocked

• ADP attach to myosin head
• binds to actin + forms a cross bridge
• attached to actin - myosin heads change angle
• pulls actin along - releases ADP
• ATP attaches to each myosin head
• myosin heads detach from actin
• Ca2+ ions activate ATPase - forms ADP + Pi via hydrolysis
• provides energy for myosin to return to its original position
• myosin head + ADP - reattaches further along actin
• cyclical process - needs a high concentration of Ca2+ in the myofibril

• myosin are joined tail to tail
• move in opposite directions
• actin moves in opposite directions
• actin filaments move towards each other
• shorter distance between Z-lines
• movement

• no nervous stimulation
• Ca2+ actively transported into endoplasmic reticulum
• energy - hydrolysis of ATP to ADP + Pi
• reabsorption of Ca2+ - tropomyosin moves back
• binding sites are blocked again
• myosin heads can't bind to actin
• no muscle contraction
• muscle relaxes
• force from antagonistic muscles pulls actin out from between myosin

• needs lots of energy
• hydrolysis of ATP to ADP + Pi

• movement of myosin heads
• reabsorption of Ca2+ ions into endoplasmic reticulum by active transport

• escape from danger - rapid response - life-saving
• most ATP is regenerated - respiration of pyruvate in mitochondria
• lots of mitochondria in muscle

ENERGY SUPPLY

• demand for oxygen is greater than the rate of its supply
• need for anaerobic respiration
• PHOSPHOCREATINE + glycolysis

• can't supply energy directly
• regenerates ATP
• stored in muscle
• reserve supply of phosphate
• ADP + Pi -> ATP
• muscle relaxes - store of phosphocreatine is replenished - Pi from ATP