UNIT 5 BIOLOGY: MUSCLE CONTRACTION

3 TYPES:

• CARDIAC MUSCLE - FOUND IN THE HEART
• SMOOTH MUSCLE - FOUND IN WALLS OF BLOOD VESSELS

BOTH UNDER INVOLUNTARY CONTROL

• SKELETAL MUSCLE - CONNECTED TO BONE - VOLUNTARY CONTROL

INDIVIDUAL MUSCLES ARE MADE UP OF MILLIONS OF TINY MUSCLE FIBRES CALLED MYOFIBRILS

TO INCREASE STRENGTH OF MUSCLE - CELLS FUSE TOGETHER TO MAKE MUSCLE FIBRES - SHARE NUCLEI AND CYTOPLASM CALLED SARCOPLASM

SARCOPLASM FOUND AROUND CIRCUMFRENCE OF THE FIBRE, WITHIN SARCOPLASM IS MITOCHONDRIA AND ENDOPLASMIC RETICULULM.

MADE UP OF TWO TYPES OF PROTEIN FILAMENT 

• ACTIN - thin, 2 strands twisted round each other 
• MYOSIN - thick, long rod shaped fibres with bulbous heads that project to the side

MYOFIBRILS - STRIPED - ALTERNATING LIGHT AND DARK BANDS #]

ISOTROPIC BAND ( I BAND) - LIGHT - MYOSIN AND ACTIN DONT OVERLAP HERE 

ANISOTROPIC BAND ( A BAND) - DARK - MYOSIN AND ACTIN OVERLAP 

AT CENTRE OF OF A BANDS - H ZONE - lighter 

AT CENTRE OF I BAND - Z LINE - distance between 2 adjacent z lines called sarcomere 

WHEN MUSCLE CONTRACTS - sarcomeres shorten and pattern of light and dark bands change 

SLOW TWITCH FIBRES - contract more slowly, less powerful contraction over a longer period, for endurance - adapted for aerobic respiration;

• large amount of myoglobin (store of oxygen) 
• supply of glycogen to provide source of metabolic energy 
• rich supply of blood vessels to deliver oxygen and glucose
• numererous mitochondria for atp

FAST TWITCH FIBRES - contract rapidly - power contractions only for a short period, for intense activities - adapted for role 

• thicker and more numerous mysoin filaments 
• high concentration of enzymes involved in anaerobic respiration
• store of PHOSPHOCREATINE -- can rapidly generate ATP FROM ADP in anaerobic conditions 

POINT WHERE MOTOR NEURONE MEETS A SKELETAL MUSCLE FIBRE 

MANY JUNCTIONS SPREAD THROUGHOUT MUSCLE - ENSURES  CONTRACTION OF MUSCLE IS RAPID AND POWERFUL 

WHEN NERVE IMPULSE RECIEVED AT JUNCTION:

• synaptic vesicles fuse with presynaptic membrane 
• releases acetyl choline 
• acetylcholine diffuses across to postsynaptic membrane altering its permeability to sodium ions as it goes - leads yo depolarisation 

WHEN A MUSCLE CONTRACTS:

• I band becomes narrower 
• z lines move closer - sacromere shorterns
• h zone becomes narrower 

A band remains same width - discounts theory that muscle contraction is due to filaments themselves shortening it must be from the sliding theory 

MYOSIN - MADE UP OF 2 TYPES 

• FIBROUS PROTEIN ARRANGED INTO A FILAMENT MADE OF SERVERAL HUNDRED MOLECULES - TAIL
• GLOBULAR PROTEIN - TWO BULBOUS SECTIONS AT THE END

ACTIN - GLOBULAR PROTEIN - MOLECULSE ARE ARRANGED INTO LONG CHAINS THAT TWIST AROUND EACH OTHER TO FORM A HELICAL STRAND 

TROPMYOSIN - FORMS LONG THIN HEADS WHICH ARE WOUND AROUND ACTIN FILAMENTS 

• action potential reaches neuromuscular junction, causing calcium ions to open  - influx into synaptic knob
• calcium ions cause the synaptic vesicales to fuse with presynaptic membrne and release acetly choline into cleft
• acetylcholine diffuses across and binds with receptors on post synaptic - depolarisation 
• action potential travels deep into fibre of muscle through a series of tubules
• tubules are in contact with endoplasmic reticulum - which has absorbed calcium ions from the cytoplasm of the muscle 
• action potential opens calcium ion channels on endoplasmic reticulum - ions flood into cytoplasm
• calcium ions cause tropomyosin molecules that were blocking the binding sites on the actin filament to pull away 
• mysoin head can now bind to the binding site on actin filament 
• myosin heads change angle once attatched - moving actin filament along as it does so and releasing ADP
• ATP molecule binds to myosin head causing it to detatch

• calicum ons then activate enzyme ATPASE which hydrolses the ATP to ADP - this provides energy for mysoin head to return to original posotion 
• myosin head carries on doing this but further across filament 
• when it stops - calcium ions are activley transported back into endoplasmic reticulum using ATP hydrolyses 
• meaning tropmyosin can block actin filament again 
• cant bind so muscle relaxes 

SUPPLIED BY HYDROLYSIS OF ATP TO ADP AND (P) AND INORGANIC PHOSPHATE

ENERGY IS NEEDED FOR

• movement of myosin heads
• reabsorbtion of calcium ions into endoplasmic reticulum through active transport
• IN ANAEROBIC RESPIRATION A CHEMICAL CALLED PHOSPHOCREATINE IS USED TO GENERATE ATP RAPIDLY
• ITS STOREED IN THE MUSCLE AND ACTS A RESERVE SUPPLY OF PHOSPHATE WHICH IS AVALIABLE TO COMBINE WITH ADP TO MAKE ATP
• STORE REPLENISHED USING PHOSPHATE FROM ATP WHEN MUSCLE IS RELAXED