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11.2 Movement

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11.2.1 Human movement. Human movement is produced by the skeletal

acting as simple lever machines. The physics of a lever system can be directly compared to that of a limb.

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In general terms the muscles and bones of the spine (red) are force magnifiers. This force is used to stabilize the skeleton and provide a stable platform (red) for the movement of the limbs. Such lever produce very little range of movement but a great deal of force.

The muscles and bones of the limbs are generally arranged into 3rd class levers and in such a way to become distance magnifiers. The reason for this is to provide range of movement for the limb rather than strength.

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The image illustrates the concept of 'range of movement'. Red = strength Blue = range of motion

These simple ideas of machines can be applied to the skeletal system and human movement.

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11.2.2 Joint structure

A. Humerus (upper arm) bone. B. Synovial membrane that encloses the joint

capsule and produces synovial fluid. C. Synovial fluid (reduces friction and

absorbs pressure). D. Ulna (radius) the levers in the flexion and

extension of the arm. E. Cartilage (red) living tissue that reduces

the friction at joints. F. Ligaments that connect bone to bone and

produce stability at the joint.

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Antagonistic Pairs:

To produce movement at a joint muscles work in pairs.

Muscles can only actively contract and shorten. They cannot actively lengthen.

One muscles bends the limb at the joint (flexor) which in the elbow is the biceps.

One muscles straightens the limb at the joint (extensor) which in the elbow is the triceps.

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11.2.3 Elbow joint structure.

1. Humerus forms the shoulder joint also the origin for each of the two biceps tendons

2. Biceps (flexor) muscle provides force for an arm flexion (bending). As the main muscle it is known as the agonist.

3. Biceps insertion on the radius of the forearm

4. Elbow joint which is the fulcrum or pivot for arm movement

5. 5. Ulna one of two levers of the forearm 6. Technically in a flexion like this the

Biceps performs a concentric contraction.

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Elbow Joint

6. Triceps muscle is the extensor whose contraction straightens the arm.

7. Elbow joint which is also the pivot (fulcrum)for this movement.

It should be noted that the description of movement is fairly complex. A true Triceps extension takes place against gravity.

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Exercise: Bend your arm in a flexion. Point your elbow

upwards vertically. Raise your hand vertically above your head. This is a true concentric contraction of the Triceps

Pick up a heavy object in concentric Biceps flexion. Now lower and straighten your arm. You should feel your Biceps contracted but Triceps relaxed. That an eccentric contraction of the Biceps This just shows how complex movement can be!

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11.2.4 Movement at the hip and knee joint.

Comparison of movement at the hip and knee joint:

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Knee Joint: The knee joint is an

example of a hinge joint. The pivot is the knee joint. The lever is the tibia and

fibula of the lower leg. A knee extension is

powered by the quadriceps muscles.

A knee flexion is powered by the hamstring muscles.

Movement is one plane only.

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The Hip Joint:

Rotation is in all planes and axis of movement.

The lever is the femur and the fulcrum is the hip joint.

The effort is provided by the muscles of quadriceps, hamstring and gluteus

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The Shoulder A ball and socket joint. The humerus is the lever. The shoulder (scapula and

clavicle) form the pivot joint.

Force is provided by the deltoids, trapezius and pectorals.

Movement is in all planes.

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11.2.5 Striated muscle structure.1. Tendon connecting muscle to

bone. These are non-elastic structures which transmit the contractile force to the bond.

2. The muscle is surrounded by a membrane which forms the tendons at its ends.

3.  Muscle bundle which contains a number of muscle cells(4) (Fibres) bound together. These are the strands we see in cooked meat. The plasma membrane of a muscle cell is called the sarcolemma and the membrane reticulum is called the sarcoplasmic reticulum.

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Striated Muscle Structure

4. The muscle fibre (Cell) is multinucleated

There are many parallel protein structures inside called myofibrils.

Myofibrils are combinations of two filaments of protein called actin and myosin.

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Actin and MyosinThe filaments overlap to give a distinct

banding pattern when seen with an electron microscope.

This shows the arrangement of actin and myosin filaments in a myofibril

The thick myosin filaments overlap with the thinner actin filaments.

Myofibril cross section: a) Actin only b) Myosin only c) Myosin attachment region adds

stability d) Actin and myosin overlap in cross

sections

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11.2.6 Structure of a sarcomere

A sarcomere is a repeating unit of the muscle myofibrils.

defined by the distance between two Z lines

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Note: large number of mitochondria Diagonal myofibrils

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11.2.7 Mechanism of muscle contraction.1. Action potential arrives at the end of a

motor neuron, at neuromuscular junction.

2. Motor neuron releases the neurotransmitter acetylcholine(Ach)

3. Ach binds to receptor protein opening Na+ channels

4. Na+ enters the muscle cell (initiation of action potential.

5. Aaction potential spreads rapidly through the large muscle cell by invaginations in the sarcolemma called T-tubules.

6. This causes the sarcoplasmic reticulum to release its store of Ca2+ into the myofibrils.

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6. Myosin filaments have cross bridge lateral extensions.

7. Cross bridges include an ATPase which can oxidise ATP and release energy.

8.The cross bridges can link across to the parallel actin filaments.

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9. Actin polymer is associated with tropomyosin that occupies the binding sites to which myosin binds in a contraction.

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10. When relaxed the tropomyosin sits on the outside of the actin blocking the binding sites.

11. Myosin cannot cross bridges with actin until the tropomyosin moves into the groove.

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12. The calcium binds to troponin on the thin filament, which changes shape, moving tropomyosin into the groove in the process.

13. Myosin cross bridges can now attach and the cross bridge cycle can take place.

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Cross Bridge Cycle: The energy for the cycle is produced by the ATPase section of the cross bridge

structure. This energy temporarily changes the shape of the cross bridge which is now attached to the actin. The two slide relative to each other giving an overall shortening

1. The cross bridge swings out from the myosin and attaches to the thin filament.2. The cross bridge changes shape and rotates through 45°, causing the filaments

to slide. The energy from ATP splitting is used for this step, and the products (ADP + Pi) are released.

3. A new ATP molecule binds to myosin and the cross bridge detaches from the thin filament.

4. The cross bridge changes back to its original shape, while detached (so as not to push the filaments back again). It is now ready to start a new cycle, but further along the thin filament.

5. This model is for one myosin molecule cross bridging to one actin. Looking at some of the diagrams we can see that there must be many cross bridges formed.

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Cross Bridge Cycle

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11.2.8 Electron micrographs of muscle fibre contraction.

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These show that each sarcomere gets shorter (Z-Z) when the muscle contracts, so the whole muscle gets shorter.

But the dark band, which represents the thick filament, does not change in length.

This shows that the filaments don’t contract themselves, but instead they slide past each other.