Muscles (SNAB Topic 7)

Muscles come in pairs, as they can only pull in one direction. Two are needed to co-ordinate movement, an extensor, and a flexor. They make an antagonistic pair.

Then there's joints:

Joints are surrounded by:

• Muscles
• Tendons (which join muscles to bone)
• Ligaments (which join bone to bone)
• A fibrous capsule and
• A synovial membrane which secretes...
• Synovial fluid (a lubricant that prevents joint friction)

The joint itself is protected by:

• Cartilage (which absorbs synov. fluid)
• And pads of cartilage which are added for additional protection.

There are also types of joint e.g ball and socket - the hip

1. Muscles are made of muscle fibres (duh!). These are formed from multinucleate cells).

2. Muscle fibres consist of myofibrils, and myofibrils are made up of contractile units called sarcomeres.

3. Sarcomeres are made up of two types of protein molecules, a thin thilament known as ACTIN and a thicker one known as MYOSIN. These two proteins are VERY important so you must remember there names and the difference between the two!

When trying to remember the order of these, think of the structure a bit like a russian doll e.g. myosin and actin are inside the sarcomeres, which are inside the myofibrils, which are inside the muscle fibres ... and so on! I find it best to repeat stuff aloud, even if I sound like a dunce.

For muscles to contract, the Actin slides between the Myosin, shortening the sarcomere and hence the muscle.

---              --- A.

----------------- M.

---              --- A.

(above) Before contracting...

-------   ------- A.

----------------- M.

-------   ------- A.

(above) After contracting. Can you see how the Actin has slid over the Myosin?

Actin is associated with two other protein molecules, TROPONIN and TROPOMYOSIN.

Myosin, however has no assistance as it acts like a sort of catapult during the reaction and it's head's are shaped a bit like a nine iron golf clubs. These heads protrude off it's surface and change orientation when the muscles shorten, moving the Actin.

This process, which I shall now talk about in more detail is known as the SLIDING FILAMENT THEORY.

1. A nerve impulse arrives at a neuromuscular junction.

2. Ca2+ (calcium) ions are released from the sarcoplasmic reticulum.

3. These ions then diffuse through the sarcoplasm (a bit like cytoplasm) and movement is initiated. But how?

1. Ca2+ ions attach to the troponin molecule, causing it to move.

2. As a result the tropomyosin on the actin filament shifts position, exposing a bunch of myosin binding sites.

3. Myosin heads bind with the binding sites on the actin, forming cross-bridges. 

4. When these heads bind, ADP + Pi (Adenosine Diphosphate and Phosphate) is released from them.

5. The myosin changes shape, causing the heads to nod forward. This makes the attached actin move over it and the muscle contracts.

6. From the chemicals the heads released, ATP (Adenosine Triphosphate) is formed and it reattaches to them. This energy causes the heads to detach.

7. ATPase (an enzyme) on the myosin heads then hydrolyses ATP back into ADP+Pi, causing a change in the heads' shape, returning them to upright. The muscle than relaxes. THIS CYCLE REPEATS AT EVERY MUSCLE CONTRACTION.

NOTE: When ATP is absent the cross-bridges remain - you know this stiffening as RIGOR MORTIS

Once the muscle has relaxed, it is no longer nerve stimulated, so Ca2+ ions are actively transported/pumped into the sarcoplasm using ATP.

Also.. the troponin and tropomyosin return to their original positions, blocking the myosin binding sites on the actin.

TADAH! This is the muscles and joints bit OVER! Well Done ( :