Muscle Contraction

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Microscopic Structure Of Skeletal Muscles

Each muscle is made up of myofibrils. Myofibrils are made up of two types of protein filament:

Actin- Which is thinner and consists of two strands twisted around one another

Myosin- Which is thicker and consists of long rod-shaped fibres with bulbous heads that project to the side

Myofibrils appear striped due to their alternating light-cloured and dark-colouredbands

Light bands- Isotropic bands -The actin myosin filaments do not overlap in this region

Dark bands- Anistropic bands -The actin and myosin overlap in this region

The centre of each antiotropic band is a lighter-coloured region- H-zone

At the centre of each isotropic band is a line called the Z-line

The distance between Z-lines are called the sacromere- when the muscle contract these sacromeres shorten and the pattern of light and dark bands change

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Types Of Muscle Fibres

Slow Twitch Fibres- Contract slowly and provide less powerful contractions over a longer period. They are adapted to endurance work. They are more common in muscles used to keep the body upright. They are adapted for aerobic respiration to avoid a build up of lactic acid, or an individual would function less effectively. These adaptions include having:

  • A large store of myoglobin- stores oxygen
  • A supply of glycogen to provide a source of metabolic energy
  • A rich supply of blood vessels to deliver oxygen and glucose 
  • Numerous mitochondria to produce ATP

Fast Twitch Fibres- Contract rapidy and produce powerful contractions but only over a short period. Adapted to intense exercise. More common in muscles which need to do short bursts of intesnse activity. Fast twitch fibres are adapted to their role by having:

  • Thicker and more numerous myosin filaments
  • A high concentration of enzymes invloved in anaerobic respiration
  • A store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anearobic conditions and so provide energy for muscle contration
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Neuromuscular Junctions

The point where a motor neurone meets a skeletal muscle fibre

There are many junctions- ensure that contraction of a muscle is rapid and powerful

If there was only one- The muscles would not contract simultaneously and movement would be slow

All muscle fibres supplied by a single motor neurone and act together as a single functional unit- know as A Motor Unit

When a nerve impulse is recieved- The synaptic vesicles fuse with the presynaptic membrane and release acetylchlorine

The acetylchlorine diffuses to the postsynaptic membrane altering its permeablilty to Na+ ions which flood in depolarising the membrane

The acetylchlorine is broken down by acetylchlorinesterase to chlorine and ethanoic acid which diffuse back into the neurone- ensure that the muscle is not over-stimulated  

Acetylchlorine is reformed using energy from the mitochondria 

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Evidence For The Sliding Filament Mechanism

When a muscle contracts the following changes occur to the sacromere:

  • The I-band becomes narrower 
  • The Z-lines move closer together- The sacromere shortens
  • The H-zone becomes narrower 
  • The A-band remains the same width.

The theory that muscle contaction is due to the filaments shortening is disproved by the fact that when the mucles contact the myosin filaments have not become shorter 

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Muscle Filaments

Myosin is made up of two types of protein:

  • A fibrous protein arranged into a filament made up of several hundred molecules (tail)
  • A globular protein formed into two bulbous structures at one end (head)

Actin is a globular protein- molecules arranged into long chains that are twisted aound one another to form a helical strand

Tropomyosin forms long thin threads that are wound around actin filaments 

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The Sliding Filament Mechanism

The bulbous heads of the myosin filaments form cross-bridges with the actin filaments 

They attach themselves to the binding sites on the actin filaments and then flex together to pull the actin filaments along the myosin filaments 

They then detach and use ATP as a source of energy return to their original angle and re-attach themselves further along the actin filaments

Theis is repeated 100 times a second 

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Muscle Stimulation

An action potential reaches many neuromuscular junctions simultaneously causing calcium ion channels to open and calcium ions to move into the synaptic knob 

The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release their acetylchlorine into the synaptic cleft 

Acetylchlorine diffuses across the synaptic cleft and binds with receptors on the postsynaptic membrane on the postsynaptic membrane causing it to depolarise 

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Muscle Contaction

The action potential travels deep into the fibre through a system of tubules that branch throughout the cytoplasm of the muscle 

The tubules are in contact with the endoplasmic reticulum of the muscle which has absorbed calcium ions from the cytoplasm of the muscle 

The action potential opens the calcium ion channels on the endoplasmic reticulum and calcium ions flood into the muscle cytoplasm down a diffusion gradient 

The calcium ions cause the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away

The ADP molecule attached to the myosin heads change their angle, pulling the actin filament along and releasing a molecule of ADP

An ATP molecule attches to each myosin head, causing it to become detached

The calcium ions activate the enzyme ATPase- hydrolyses the ATP to ADP provides energy for the myosin head to return to its original position

The myosin head re-attaches itself further along the actin filament and the cycle is repeated 

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Muscle Relaxation

When nervous stimulation ceases calcium are actively transported back into the endoplasmic reticulum using energy from the hydrolysis

This reabsorbtion of the calcium ions allows tropomyosin to block the actin filament again

Myosin heads are now unable to bind to actin filaments and concentration ceases

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Energy Supply During Muscle C

Most ATP regenerated from ADP during the respiration of pyruvate in the mitochondria 

The process requires oxygen and in a very active muscle oxygen is rapidly used up and it takes times for the blood supply to replenish it

A means of rapidly generating ATP anearobically is also required- This is achieved using a chemical phosphocreatine 

Phosphocreatine cannot supply energy directly to the muscle, so instead it regerates ATP

Phosphocreatine is stored in muscle and acts as a reserve supply of phosphate, which is available immediately to combine with ADP and so re-form ATP

The phosphocreatine store is replenished using phosphate from ATP when the muscle is relaxed

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