Muscle Contraction

Skeletal muscle is attatched to bone and acts under voluntary, conscious control.

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

• Actin: thinner and consists of two strands twisted around one another
• Myosin: thicker and consists of long rod-shaped fibres with bulbous head

Myofibrils appear striped due to their alternating light and dark-coloured bands. The light bands are called isotropic bands (I bands). They are lighter because the myosin and actin filaments don't overlap. The dark bands are called anisotropic bands (A bands). They appear darker because the actin and myosin filaments overlap.

At the centre of each I band is a line called the Z-line. The distance between adjacent Z-lines is called a sacromere. When a muscle contracts, the sacromeres shorten and the pattern of light and dark bands changes. At the centre of each A band is a lighter colour region, called the H-zone.

Tropomyosin: forms a fibrous strans around the actin filament

Slow-twitch muscle fibres (aerobic)

Fast-twitch muscle fibres (anaerobic)

A neuromuscular junction is the point where a motor neurone meets a skeletal muscle fibre.

Whena nerve impulse is recieved at the neuromuscular junction, the synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine. The acetylcholine diffuses to the postsynaptic membrane, altering its permeability to sodium ions, which enter rapidly, depolarising the membrane.

Acetylcholine is broken down by acetylcholinesterase to ensure that the muscle is not over-stimulated. The resulting choline and ethanoic acid diffuse back into the neurone, where they are recomined to form acetylcholine using energy from mitochondria.

A contraction of a skeletal muscle involves the actin and myosin filaments to slide past eachother: sliding filament mechanism.

Changes to the sacromere:

• the I-band becomes narrower
• the Z-lines move closer together (the sacromere shortens)
• the H-zone becomes narrower

The bulbous heads of the myosin filaments form cross-bridges with the actin filaments by attatching themselves to binding sites on the actin filaments and then flexing and pulling the actin filaments along the myosin filaments. 

They then become detatched and use ATP to return to their origional angle and re-attatch themselves further along the actin filaments.

Myosin 

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 whose molecules are arranged into long chains that are twisted around one another to form a helical strand

Tropomyosin

Forms long thin threads that are wound around actin filaments

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 acetylcholine into the synaptic cleft
  
• acetylcholine diffuses across the synaptic cleft and binds with receptors on the postsynaptic membrane, causing it to depolarise

Muscle Relaxation

• When nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP
• this reabsorption fo the calcium ions allows tropomyosin to block the actin filament again
• myosin heads are unable to bind to actin filaments and contraction ceases

• the action potential travels deep into the fibre through a system of tubules (T-tubules) that branch throughout the sacroplasm
• the tubules are in contact with the endoplasmic reticulum of the muscle which has actively absorbed calcium ions from the sarcoplasm
• the action potential opens the calcium channels on the ER and calcium ions flood into the sarcoplasm 
• the calcium ions cause tropomyosin molecules, that were blocking the binding sites, on the actin filament to pull away
• the ADP molecules attatched to the myosin head means they can bind to the actin filament and form a cross-bridge
• once attatched to the actin filament, the myosin heads change their angle, pulling the actin filament along and releasing a molecule of ADP
• an ATP molecule attatches to each myosin head, causing it to become detatched from the actin filament
• the calcium ions then activate the enzyme ATPase, which hydrolyses ATP to ADP, providing energy for the myosin head to return to its origional position
• the myosin head, once more with an ADP molecule, reattatches itself further along the actin filament and the cycle is repeated as long as nervous stimulation of the muscle continues

• tropomyosin molecule prevents myosin head from attatching to the binding site on the actin molecule
• calcium ions released from the ER cause the tropomyosin molecule to pull away from the binding sites on the actin molecule
• myosin head attatches to the binding site on the actin filament
• myosin head changes angle, moving the actin filament along, ADP is released
• ATP molecule fixes to myosin head, causing it to detatch from the actin filament
• hydrolysis of ATP to ADP by ATPase provides energy for the myosin head to return to its normal position
• myosin head reattatches to a binding site further along the actin filament and the cycle is repeated

The energy for muscle contraction is supplied by the hydrolysis of ATP to ADP and Pi.

The energy is needed for:

• the movement of the myosin heads
• the reabsorption of calcium ions into the ER by active transport

Most ATP is regenerated from ADP during the respiration of pyruvate in the mitochondria. However, this process requires oxygen. In very active muscles, oxygen is rapidly used up and it takes time for the blood supply to replenish it.

Therefore, phosphocreatine is used to rapidly generate ATP anaerobically.

Phosphocreatine cannot supply energy directly to the muscle, so instead it generates ATP. It is stored in muscle and acts as a reserve supply of phosphate, which is available immediately to combine with ADP and re-form ATP. The store is replenished using phosphate from ATP when the muscle is relaxed.