11.1 Microscopic structure of skeletal muscle
Each muscle fibre is made of myofibrils which in turn are made of two types of protein filament:
- Actin - Thinner & consists of two strands twisted around each other
- Myosin - Thicker & made of long rod-shaped fibres with bulbous heads that project to the side
Myofibrils appear striped due to alternating light and dark bands:
- Light bands - isotropic (or I bands) look light as the actin & myosin do not overlap. At the centre of each I band is the Z line. The distance between adjacent Z lines is called a sarcomere. When the muscle contracts, the sarcomeres shorten & the pattern of light and dark bands changes
- Dark bands - anisotropic (or A bands) look dark as actin and myosin overlap. At the centre of each A band is a lighter coloured region called the H zone
Two other important proteins in muscle:
- Tropomyosin - Forms a fibrous strand around the actin filament
- A globular protein, troponin - which is involved in muscle contraction
11.1 Types of muscle fibre
There are two types of muscle fibre, proportions vary from muscle to muscle & person to person:
- Slow-twitch fibres - contract slower & provide less powerful contractions over a longer period. They're less adapted to endurance work, more common in muscles like the calf which contract constantly to keep the body upright. They're adapted for anaerobic respiration in order to avoid a build up of lactic acid, which would make them less efficient. The adaptations include having: myoglobin (red molecule that stores O, makes fibres red), glycogen fpr metabolic energy, lots of blood vessels to deliver O & Glucose & lots of mitochondria to produce ATP
- Fast-twitch fibres - Contract quicker, producing powerful contractions but only for a short time. Adapted to intense exercise, more common in muscles that need short bursts of intense activity (e.g. biceps). Fast-twitch fibres are adapted to their role by having: more myosin filaments that are thicker, lots of enzymes used in anaerobic respiration & phosphocreatine (a molecule that can quickly make ATP from ADP in anaerobic conditions, giving energy for muscle contraction
11.1 Neuromuscular junctions (NM)
This is the point where a motor neurone meets a skeletal muscle fibre - many along the muscle. They allow quick muscle contraction when stimulated by action potentials.
All muscle fibres supplied by a single motor neurone act as a single unit & are known as a motor unit. This arrangement gives control over the force exerted by the muscle. If only slight force is needed, fewer units are stimulated, if a greater force is required, more motor units are stimulated.
When a nerve impulse reaches the NM junction, synaptic vesicles fuse with the presnaptic membrane & release acetylcholine. The acetylcholine diffuses to the post synaptic membrane, altering its peermeability to sodium ions (Na+), which enter rapidly, depolarising the membrane.
The acetylcholine is then broken down by acetylcholinesterase, this ensures the muscle is not over stimulated. The resulting choline & ethanoic acid (acetyl) diffuse back into the neurone, where they are recombined to form acetylcholine using energy from the mitochondria found there.
11.2 Evidence for sliding filament mechanism
If the sliding filament mechanism is correct, actin & myosin overlap more in a contracted muscle than a relaxed one. When a muscle contracts, the following changes occur to a sarcomere:
- The I band becomes narrower
- The Z lines move closer together (in other words, the sarcomere shortens)
- The H zone becomes narrower
The A band stays the same width. As the width is due to the length of the myosin filaments, the myosin filaments have not become shorter. This discounts the theory that muscle contraction is due to the filaments shortening.
Three main proteins involved in the process:
- Myosin - Made of two types of protein: A fibrous one arranged into a filament of hundereds of molecules, (the tail) & a globular protein formed into two bulbous structures at one end (the head)
- Actin - globular protein made of long chains twisted around each other, forming a helical strand
- Tropomyosin - Forms long threads wound around actin filaments
11.2 Sliding filament mechanism (1 of 3)
- An action potential reaches many neuromuscular junctions simultaneously, causing Calcium ion channels to open & Calcium ions to move into synaptic knob
- Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane & release acetylcholine into synaptic cleft
- Acetylcholine diffuses across synaptic cleft and binds with receptors on the postsynaptic membrane, causing it to depolarise.
11.2 Sliding filament mechanism (2 of 3)
- The AP travels deep into the fibre through T-tubule system that branches throughout sarcoplasm
- The Tubules are in contact with ER of muscle which absorbed Ca ions from cytoplasm of muscle
- The AP opens Ca ion channels on ER & Ca ions flood into sarcoplasm down gradient
- The Ca ions cause tropomyosin (that was blocking binding sites) to pull away
- The ADP molecule attached to the myosin heads binds to actin filament & forms cross bridge
- Once attached to the actin filament, the myosin heads change angle, pulling the actin filament along & releasing an ADP molecule
- An ATP molecule attaches to each myosin head, detaching it from the actin filament
- The Ca ions activate the enzyme ATPase, which hydrolyses the ATP to ADP. This hydrolysis provides the energy for the myosin head to return to its original position.
- The myosin head, once more with an ADP molecule attached, then reattaches itself further along the actin filament - the cycle is repeated as long as nervous stimulation of the muscle continues
11.2 Sliding filament mechanism (3 of 3)
- When nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic reticulum using energy gained from the hydrolysis of ATP
- This reabsorption of the calcium ions allows tropomyosin to block the actin filament again
- Myosin heads are now unable to bind to actin filaments and contraction ceases, i.e. the muscle relaxes
11.2 Energy supply during muscle contraction
Muscle contraction requires considerable energy. This is supplied by the hydrolysis of ATP to ADP and Pi. The energy released is needed for:
- The movement of the myosin heads
- The reabsorption of Calcium ions into the endoplasmic reticulum by active transport
In an active muscle, obviously there is great demand for ATP. In some circumstances, the ability of muscles to work intensely can be life saving.
Most ATP is regenerated from ADP during respiration of pyruvate in mitochondria,(plentiful in muscles). However, this needs oxygen. In a very active muscle, oxygen is quickly used up & it takes time for the blood to replenish it. Therefore, a way of making ATP anaerobically, is needed.
This is achieved using phosphocreatine. It can't supply energy directly to the muscle, so instead it regenerates ATP, which can. PC is stored in muscle & acts as a reserve of phosphate which reforms ATP by binding to ADP.
The PC store is replenished using phosphate from ATP when the muscle is relaxed.