Skeletal Muscles

There are 3 types of muscles in the body:

• Cardiac= only in heart
• Smooth= found in walls of blood vessels
• Skeletal= attached to bone and only type of muscle under conscious control

Muscles are made up of myofibrils.

If cells of muscles were joined together from one end of cell to another, the point between cells would be a point of weakness. Because of this muscle cells are fused together into muscle fibres. Cells of same myofibrils share the same nucleas and cytoplasm (sarcoplasm). Within these, there are many mitochondia and endoplasmic reticulum (sarcoplasmic reticulum)

Myofibrils made up of 2 types of protein filaments:

• actin- thinner, consists of 2 strands twisted around each other
• myosin- thincker, made up of long rod shaped fibres with bulbous heads projecting outwards

Myofibrils have coloured bands:

• Isotrophic band (I)- appear lighter since only consists of actin, so no overlap
• Anisotrophic band (A)- appear darker since this is where actin and myosin overlap
• H zone- region in the centre of the sacromere that is lighter iin colour since there is only myosin
• Z line- lies at the centre of the I bands

Slow twitch

• Contract more slowly, less powerful.
• Endurance activities (eg. Marathon). Aerobic activities.
• Large stores of myoglobin
• Supply of glycogen
• Rich supply of blood vessels
• Lots of mitochondria

Fast Twitch

• Contracts more rapidly, more powerful.
• Intense exercise (eg. 100m) Anaerobic activities
• Have thicker, a more myosin fillaments
• Have higher number of enzymes used for anaerobic respiration
• Large store of phosphocreatine to provide phosphate for ATP

Many neuromuscular junctions are spread through the muscle for simultaneous contraction

Each muscle fibre has one motor neuron associated with it. The muscle fire and neuroone make up one motor unit

When only slight force needed, only a few motor units are stimulated

When a nerve impulse reaches the neuromuscular junction, the synaptic vesicles join with the presynaptic membrane and release aceytlcholine. This diffuses across to the postsynaptic membrane and stimulates sodium ions to enter.

The acetylcholine is then broken down by acetylcholinesterase and then diffuses back into the presynaptic neurone

Dutring muscle contraction, actin and myosin slide past each other.

When a muscle contracts, the following changes occur to the sarcomere:

• The I band becomes narrower
• The Z lines move closer together
• The H zone becomes narrower
• The A band stays the same as the band is determind by the mysoin length

Myosin is made up of 2 different types of proteins:

• Fibrous protien arranged into the filament as the tail
• A globular protien that forms a head

Actin is globular protien with molecules that are arranged into 2 chanins that twist around each other in a helical manner

Tropomyosin forms long, thin stnads that wind around the actin molecule

There are 3 stages of contraction: stimulation, contraction, relaxation

When an action potential reaches the neuromuscular junctions, calcium ion channels open and calcium ions move into the synaptic knob

The calcium ions cause the synaptic vesicles to move to the presynaptic membrane and fuse with it, releasing acetylcholine

The neurotransmiter diffuses across the synaptic cleft an binds with receptors on the sodium voltage gated channels on the postsynaptic membrane causing it to depolarise

The action potential moves through the fibres by travelling through T- tubules that branch through the sarcoplasm

The action potential moves through the tubules until it reaches the sarcoplasmic reticulum and opens calcium channels

Calcium ions diffuse out into the muscle. Calcium ions cause tropomyosin to change shape so that the binding sites on the actin filament are exposed

Thhe myosin head forms cross bridges with actin by binding with the receptor site, helped by an ADP molecule attached to its head

Once the cross bridge is formed, the myosin head changes direction and slides the actin across, losing ADP

Calcium ions activate ATPase which hydrolyses ATP and releases energy that allows myosin head to resume its orginal shape

The mysoin head now has new ADP molecule that will allow it to bind with a new receptor site along the actin filament

When the muscle isn't being stimulated, the sarcoplasmic reticulum moves calcium ions back in by active transport

The removal of calcium ions means that tropomyosin can establish its orginal postition, covering the myosin head binding sites

Energy is needed for the movement of myosin heads and the active transport of calcium ions.

ATP often needs to be generated anaerobically, phosphocreatine provides immediate phosphate molecules to combine with ADP to form ATP