Muscle mechanics

There are three basic types of muscle
- Skeletal muscle
- Cardiac muscle
- Smooth muscle
The muscle types need different properties to perform these different operations

Skeletal muscle is striated muscle due to its microscopic appearance
Under voluntary control
Muscle fibres are organised into motor units
When a single nerve enters a muscle it splits and activates several muscle cells
A nerve and the muscle cells it activates is called a motor unit
When the nerve fires the whole motor unit is stimulated and the muscle cells contract together
- Muscles with large motor units have coarse movements
- Muscles with small motor units give fine, graded movements
Muscle cells have short refractory period

End-plate potential is generated by single motor neurone action potential
This is enough to depolarise the muscle and initiate a muscle action potential
This is propagated in both directions along the sarcolemma and T tubule system
The T tubular system communicates with sarcoplasmic reticulum
Voltage-gated type calcium channels open
Releases calcium into the sarcoplasm and around the myofibrils
Interaction of calcium with troponin C results in muscle contraction
Muscle relaxation occurs when calcium is pumped back into the sarcoplasmic reticulum

A single nerve impulse produces a muscle twitch
Single stimuli usually release enough acetylcholine to produce action potentials in the muscle membranes
This will cause the muscle to contract after a short delay
A simple twitch usually only generates about 20-30% of the maximum tension
The muscle starts to relax before the maximum tension is reached
Muscle contractions can be added to produce more force
If a second stimulus is given before a muscle relaxes the muscle will shorten further
This process is known as summation
If many stimuli are given very close together the muscle will go into continuous contraction called tetanus
Tetanus gives a maximum tension several times higher than a simple twitch
Another way to increase the force of contraction is to recruit more motor units
Muscle produces the greatest isometric tension at intermediate lengths
At rest many of the body's muscles are close to their optimum lengths

Within skeletal muscles there are different types of muscle fibres
The relative proportions of the different types varies between muscle and individuals
Type 1 or red fibres
- Have many mitochondria
- Also contain myoglobin
- Contract slowly but resist fatigue
Type 2 or white fibres
- Contain few mitochondria
- Rely on glycolysis to supply energy
- Contract rapidly but fatigue quickly

The basic unit of muscle contraction is the sarcomere
The striated appearance of skeletal muscle is due to the alignment of molecular bands and lines
The most prominent are the A and I bands and the Z line
The unit between 2 Z lines is called the sarcomere
When muscle contracts the sarcomere shortens and the Z lines move closer together

When muscle contracts protein filaments slide together
Electron microscopy combined with chemical experiments show that muscle is composed of 2 contractile proteins
- Thin filaments - actin, attached to Z line, found in both A and I bands
- Thick filaments - myosin, found in A band
When muscle contracts the actin filaments slide into the A band, overlapping with myosin
When this happens
- The Z lines move closer together
- The I band becomes shorter
- The A band stays at the same length
Actin and myosin connect through crossbridges
The more crossbridges the more tension
ATP is required for both contraction and relaxation of muscle
It is required for the sliding of the filaments which is accomplished by a bending movement of the myosin heads
It is also required for the separation of actin and myosin which relaxes the muscle
A sudden inflow of Ca is the trigger for muscle contraction
In the resting state the protein tropomyosin winds around actin and covers the myosin binding sites
The Ca binds to a second protein, troponin
This causes the tropomyosin to be pulled to the side, exposing the myosin binding sites
With the sites exposed muscle will contract if ATP is present