A2/A-level Biology - Muscle Movement (2)

• There are three types of muscle:

1. Skeletal muscle (also known as striated muscle or voluntary muscle)
2. Smooth muscle (also known as involuntary muscle)
3. Cardiac muscle

• All muscle types are composed of cells that are elongated to form fibres, but the three types above differ significantly in many areas of their structure.
• The autonomic nervous system is responsible for the control of all involuntary muscle, that includes smooth muscle and cardiac muscle; and the somatic nervous system is responsible for the control of all voluntary muscle – voluntary muscle is also called striated or skeletal muscle.

• All smooth muscle in the body is not under voluntary control.
• There are many different types of smooth muscle, and it is found all over the body.
• Smooth muscle can be found surrounding alveoli, the iris of the eye, the walls of the intestine and the walls of arteries and arterioles. 
• Examples of functions of smooth muscle include:
  - In the walls of the intestine, bundles of smooth muscle are responsible for peristalsis (moving food along the tract)
  - In the iris of the eye, groups of the muscle control the intensity of light entering the eye, by contracting their radial bundles of muscle to dilate the pupil, or contracting their circular bundles of muscle to constrict the pupil

• Cardiac muscle is myogenic, meaning some of the muscle fibres from atrial and ventricular muscle can contract without receiving a nerve impulse, although nerves from the autonomic nervous system constantly send impulses to the cardiac muscle to regulate heart rate.
• The contraction and relaxation of the cardiac muscle is repetitive and continuous throughout your entire life.
• The muscle always contracts powerfully, unless there is a medical problem, and cardiac muscle does not fatigue. 

• The action of voluntary muscles leads to the movement of the skeleton at the joints.
• Each fibre of skeletal muscle is surrounded by a membrane called the sarcolemma which contains the muscular cytoplasm (known as the sarcoplasm).
• The fibres make up many sarcomeres (one sarcomere is the smallest contractile unit of a muscle).
• These muscle cells are rich in mitochondria, and have their own specialised endoplasmic reticulum, called sarcoplasmic reticulum, which has rich calcium stores for stimulating contracting at the neuromuscular junction.
• The striated bands of skeletal muscle are separated and given names.

Do you still remember the Sliding Filament Theory of Contraction in A2/A-level Biology?

• The muscle bands are filled primarily with two proteins:
  - Actin (yellow) and Myosin (red)
• There are various ‘zones’ which give the muscle its banded appearance.

• There are two components of the sliding filament model.
• The thin filament consists of two thin strands of actin, a globular protein, coiled around each other.
• Surrounding them is another rod-shaped protein called tropomyosin which coils around the actin.
• The role of tropomyosin is to reinforce the actin strands.
• Attached to each molecule of tropomyosin is a molecule of troponin – a complex consisting of three polypeptides: one binding to actin, one binding to tropomyosin, and one binding to calcium ions. 
• The thick filament consists simply of bundles of the protein myosin.
• There are molecules of myosin along the thin strand which consist of two tails pointing out of the strand at opposite ends, and at each end of the tail is a myosin head.  

(i) the I-band consists solely of the thin filament (mainly actin), and is on either end of the sarcomere
(ii) the H-zone consists solely of the thick filament in the centre of the sarcomere (just myosin)
(iii) the A-band comprises both the thin and the thick filaments (the entire sarcomere excluding the I-band)
(iv) the M-line is found at the centre of the sarcomere, through the centre of the A-band and H-zone
(v) the Z-line is found at either end of the sarcomere, and separates adjacent sarcomeres (also sometimes known as Z-discs as they are really disc-shaped proteins in the myofibrils which hold the thin filaments in place)

During muscle contraction, this is how the sliding filament theory explains contraction:

• The binding sites on actin for the myosin heads are covered up by the molecule tropomyosin, but when calcium ions enter the muscle cell and bind to the binding site on troponin, they shift shape slightly, revealing the binding site for the myosin heads on the actin.
• When calcium ions are not present, the muscle cannot contract, but as calcium moves into the cell and binds to troponin, myosin is able to bind to the actin molecules.
• After calcium moves into the cell and binds to troponin, revealing the actin binding sites, the myosin heads attach to the surrounding actin (which they have a slight affinity for), forming a cross-bridge.
• The myosin heads all have a molecule of ADP and a phosphate attached to them when not active.
• When the heads bind to the actin, they eject the ADP and phosphate, which releases energy, and they use this energy to bend their heads towards the thin filaments and pull the thin filament towards them (this is the power stroke).
• Then a molecule of ATP replaces the one lost before it on the myosin head, which breaks down the cross-bridge, and the ATP is hydrolysed to give an ADP and a phosphate – returning the myosin back to its original state – and this causes the myosin head to pull back down, releasing the thin filament, which slides back into place.
• As long as there is still calcium present in the muscle cell, the actin binding sites will remain open, and so this cycle can continue for as long as the calcium remains (another resource the cell must have for this to happen is ATP to keep replenishing the lost molecules of ADP and phosphate groups).

• You can see what happens to the I-band and the H-zone (they both shorten in size as the thin filaments are pulled in by the thick filaments), but notice that the A-band remains the same length, as the length of the thick filament does not change. 

• In order for the contraction to stop, the calcium ions must be removed from the muscle cell via active transport.
• This is another use for ATP in the muscle cell. Once the calcium break away from the troponin, and exit the cell, the tropomyosin shifts back over the actin to cover the binding sites: this prevents myosin heads from forming cross-bridges there.
• It is important to note that during the power stroke, it is not the hydrolysis of ATP that releases the energy needed for the stroke, but actually the release of ADP and phosphate which generates the energy for the power stroke. 
• ATP is stored quite well in muscle cells, as they have such high energy requirements.
• ATP is obtainable from aerobic respiration and anaerobic respiration (although the latter has a much lower net ATP production), and different amounts of ATP are generated depending on the respiratory substrate used. 

That's the end of the topic!

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