Download Muscles and movement - bio-ib-basu

Muscles and movement:
11.2.1 Human movement.
11.2.2 Joint structure and antagonistic muscle pairs.
11.2.3 Elbow joint structure.
11.2.4 Movement at the hip and knee joint.
11.2.5 Striated muscle structure.
11.2.6 Structure of a sarcomere
11.2.7 Mechanism of muscle contraction.
11.2.8 Electron micrographs of muscle fibre contraction.
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.
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.
The image illustrates the concept of 'range of movement'
discussed above.
These simple ideas of machines can be applied to the
skeletal system and human movement.
1
11.2.2 Joint structure and antagonistic muscle pairs.
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.
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.
2
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. Ulna one of two levers of the forearm
Technically in a flexion like this the Biceps performs a
concentric contraction.
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.
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!
11.2.4 Movement at the hip and knee joint.
Comparison of movement at the hip and knee joint:
3
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.
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.
4
The shoulder is 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.
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.
4. The muscle fibre Cell)shown here and above is
multinucleated
There are many parallel protein structures inside
called myofibrils.
Myofibrils are combinations of two filaments of protein called actin and myosin.
The filaments of actin and myosin overlap to give a distinct banding pattern when seen
with an electron microscope.
5
This model show the arrangement of the actin and myosin filaments in a myofibril
Note how 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
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
Note:
large number of mitochondria
Diagonal myofibrils
Sarcoplasmic reticulum
6
11.2.7 Mechanism of muscle contraction.
1. An action potential arrives at the end of
a motor neuron, at the neuromuscular
junction.
2. This causes the release of the
neurotransmitter acetylcholine.
3 This initiates an action potential in the
muscle cell membrane.
4. This action potential is carried quickly
throughout the large muscle cell by
invaginations in the cell membrane called
T-tubules.
5. The action potential causes the sarcoplasmic reticulum (large membrane vesicles) to
release its store of calcium into the myofibrils.
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.
9. Actin polymer is associated with tropomyosin that occupies the binding sites to
which myosin binds in a contraction.
7
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.
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.
Cross Bridge Cycle:
The energy for the cycle is produced by the ATPase section of the crossbridge structure.
This energy temporarily changes the shape of the crossbridge which is now attached to
the actin polymer. The two slide relative to each other giving an overall shortening
1. The cross bridge swings out from the thick filament 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 “power stroke” 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.
This model is for one myosin molecule cross bridging to one actin. Looking at some of
the diagrams above we can see that there must be many cross bridges formed but not
quite together.
8
11.2.8 Electron micrographs of muscle fibre contraction.
If electron micrographs of a relaxed and contracted myofibril are compared it can be
seen that:
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.
9