Muscle Contraction

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Types of muscle

  • Smooth muscle- contracts without conscious control, found in walls of internal organs
  • Cardiac muscle- contracts without conscious control, but is only found in the heart
  • Skeletal muscle- contracts with conscious control, the muscle we use to move

Skeletal Muscle

  • Made up of long cells- muscle fibres
  • The cell membrane of muscle fibre cells is called the sarcolemma
  • Bits of the sarcolemma fold inwards across the muscle fibre and stick to the sarcoplasm (a muscle cell's cytoplasm), these folds are called transverse (T) tubules and they help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre
  • A network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm- stores and releases calcium ions that are needed for muscle contraction
  • Lots of mitochondria to provide the ATP needed for muscle contraction
  • Multinucleate (contains many nuclei)
  • Has lots of long cylindrical organelles called myofibrils- made up of proteins and are highly specialised for contraction
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Myofibrils

  • Contain bundles of thick and thin myofilaments that move past each other to make muscles contract
  • The thick myofilaments are made of the protein myosin
  • The thin myofilaments are made of the protein actin
  • Made up of dark and light bands- Dark bands are made up of myosin and actin and are called A bands, the light bands contain only actin and are called I bands
  • Made up of many short units called sarcomeres- the end of each sarcomere is marked with a Z-line, in the middle is an M-line, and arounf the M-line is the H-zone which contains only myosin filaments

The sliding filament theory

  • Where the myosin and actin filaments slide over one another to make the sarcomeres contract
  • A-band stays the same length
  • I-band gets shorter
  • H-zones get shorter
  • z-lines get shorter
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Myosin and actin filaments

Myosin filaments

They have globular heads that are hinged so that thhey can move back and forth. Each myosin head has a binding site for actin and a binding site for ATP

Actin filaments

They have binding sites for myosin heads, called actin-myosin binding sites, two other proteins called tropomyosin and troponin are found between actin filaments. These proteins are attached to each other and they help myofilaments move past each other

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1. Muscle contraction

Arrival of an action potential

  • When an action potential arrives it depolarises the sarcolemma
  • The depolarisation spreads down the T-tubules to the sarcoplasmic reticulum causing stored calcium ions to be released into the sarcoplasm
  • The calcium ions bind to troponin, causing it to change shape, this displaces the tropomyosin exposing the actin-myosin binding filament on the actin filament
  • The myosin head binds to the site forming an actin-myosin cross bridge

Movement of the actin filament

  • The calcium ions activate the enzyme ATPase, which breaks down ATP into ADP+Pi to provide the energy needed to move the myosin head to side, pulling the actin filament along

Breaking of the cross bridge

  • ATP provides the energy to break the actin-myosin cross bridge
  • The myosin head returns to its starting position and reattaches to another binding site
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2. Muscle contraction

Return to resting state

  • Calcium ions leave their binding sites on the troponin and are moved by active transport back in the sarcoplasmic reticulum
  • The troponin molecules return to their shapes, causing the tropomyosin to block the actin-myosin binding sites again
  • The actin filaments slide back to thei relaxed position, which lengthens the sarcomere
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Energy for muscle contraction

Aerobic respiration

  • Most ATP is produced this way via oxidative phosphorylation in the cells's mitochondria
  • Good for long periods of low-intensity exercise due to it requiring a good supply of oxygen

Anaerobic respiration

  • ATP is made rapidly by glycolysis with the end product being pyruvate which is converted into lactate
  • Lactate causes muscle fatigue and so thi process is only good for short periods of hard exercise

ATP-phosphocreatine (PCr) system

  • ATP is made by phosphorylating ADP- adding a phosphate group taken from PCr
  • PCr is stpred inside cells and the ATP-PCr system generated ATP very quickly
  • PCr runs out after a few seconds so its used duirng short bursts of vigorous exercise
  • Anaerobic and alactic- doesn't form lactate
  • ADP+PCr--> ATP+Cr (creatine)
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Slow twitch and fast twitch muscle fibres

Slow twitch

  • Contract slowly and work over a long period of time without tiring- good for endurance
  • High proportions are found in the muscles used for posture e.g. calves
  • Aerobic respiration
  • Lots of mitochondria and blood vessels to supply the muscles with oxygen- mitochondria normally found at the edge of muscle fibres, so that there's a short diffusion pathway for oxygen from the blood vessels
  • Rich in myoglobin- a protein that stores protein

Fast twitch

  • Contract very quickly and get tired very quickly- good for short bursts of speed/power
  • High proportions are found in muscles you use for fast movement e.g. eyes and arms
  • Anaerobic respiration using glycogen
  • Have stores of PCr
  • Don't have many mitochondria, blood vessels or myoglobin
  • High concentration of enzymes needed for anaerobic respiration
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