STRUCTURE OF SKELETAL MUSCLE

Neuromuscular junction - the point where a motor neurone meets a skeletal muscle fibre

Muscles are effector organs that respond to nervous stimulation by contracting and bringing about movement.

There are three types of muscle:

cardiac muscle: found in the heart
smooth muscle: found in the walls of blood vessels and the gut
skeletal muscle: makes up the bulk of body muscle in vertebrates and is attached to bone

Cardiac and smooth muscle are not under conscious control whereas skeletal muscle is. Cardiac and skeletal muscle is striated whereas smooth muscle isn't.
Individual muscles are made up of muscle fibres called myofibrils which when they are arranged together are very strong. The myofibrils have multi-nuclei and a type of cytoplasm called sarcoplasm that contains a lot of mitochondria and endoplasmic reticulum.

Myofibrils are made up of two types of protein filaments:

actin: thinner and consists of 2 strands twisted around each other
myosin: thicker and consists of long rod-shaped fibres with heads that come out at the side

Myofibrils are striped because they contain alternating light-coloured and dark-coloured bands. The light bands are called the I-bands and they are lighter because the actin and the myosin filaments don't overlap. The dark bands are called the A-bands and they look darker because the actin and myosin filaments overlap.

At the centre of each A band is a lighter coloured section called the H-zone. At the centre of each I-band there's a Z-line and the distance between adjacent Z-lines is called a sarcomere.
When a muscle contracts, the sarcomere will shorten.

Two other important proteins in a muscle are:

tropomyosin: which forms a fibrous strand around the actin filament
troponin: a globular protein that's involved in muscle contraction

There are two types of muscle fibre and the proportions of these vary from muscle to muscle:

1) slow-twitch fibres: contract more slowly and provide less powerful contractions over a longer period. They are therefore adapted for endurance work, like prolonged exercise and so are often found in muscles like calf muscles in humans which need to constantly contract to keep the body upright. They are adapted for aerobic respiration and some of the adaptions include

lots of mitochondria to produce ATP
a large store of myoglobin which stores oxygen
a supply of glycogen to provide a source of metabolic energy
lots of blood vessels to deliver oxygen and glucose

2) fast-twitch fibres: contract more quickly and produce powerful contractions but only for a short period. They are therefore adapted for intense exercise like weight lifting and so are found in muscles which need to do short bursts of intense activity, like the biceps in the upper arm. Adaptations in fast-twitch muscle fibres include:

thicker and more numerous myosin filaments
a high concentration of enzymes involved in anaerobic respiration
a store of phosphocreatine, a molecule which can rapidly generate ATP from ADP and Pi in anaerobic conditions and so provide energy for muscle contractions

CONTRACTION OF SKELETAL MUSCLE

The process that brings about the contraction of the muscle fibre involves the actin and myosin filaments sliding past each other and is therefore called the sliding-filament mechanism.

When a muscle contracts, changes happen to the sarcomere:

the I-band becomes narrower
the Z-lines move closer together and the sarcomere shortens
the H-zone becomes narrower

The A-band stays the same width because the width of this band is determined by the length of the myosin filaments.

-Myosin is made up of two types of protein:

a fibrous protein arranged into a filament made up of several hundred molecules (the tail)
a globular protein formed into two spherical structures at one end (the 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

SLIDING FILAMENT THEORY

The spherical heads of the myosin filaments form cross-bridges with the actin filaments by attaching themselves to the binding sites on the actin filaments. They then flex in unison and pull the actin filaments along the myosin filaments. They then become detached and, using ATP as a source of energy, return to their original angle and re-attach themselves further along the actin filaments. This process is repeated up to 100 times a second. Also known as a ratchet mechanism.

Muscle stimulation:

an action potential reaches many neuromuscular junctions at the same time, 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 contraction:

the action potential travels deep into the fibre through a system of T-tubules that branch throughout the sarcoplasm of the muscle
the T-tubules are in contact with the endoplasmic reticulum of the muscle (the sarcoplasmic reticulum) which has actively absorbed calcium ions from the sarcoplasm
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 means that they are now in a state to bind to the actin filament and form a cross-bridge
once attached to the actin filament, the myosin heads change their angle, pulling the actin filament along as they do so and releasing a molecule of ADP
an ATP molecule attaches to each myosin head causing it to become detached from the actin filament
the calcium ions then activate the enzyme ATP-ase which hydrolyses the ATP to ADP and Pi. This hydrolysis provides the energy for the myosin head to return to its original position
the myosin head, again with an attached ADP molecule then reattaches itself further along the actin filament and the cycle is repeated as long as nervous stimulation of the muscle continues

Muscle relaxation:

when nervous stimulation stops, the calcium ions are actively transported back into the sarcoplasmic reticulum using energy from the hydrolysis of ATP
this reabsorption of calcium ions allows tropomyosin to block the actin filament again
myosin heads now can't bind to the actin filaments and contraction stops-the muscle relaxes

Energy supply during muscle contraction

Energy is supplied by the hydrolysis of ATP to ADP and Pi. The energy that is released is needed for:

the movement of myosin heads
the reabsorption of calcium ions into the endoplasmic reticulum by active transpor

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MUSCLE CONTRACTION