Notes
DPHL Biology
11.3 Nerves, Muscles and Movement
Methods of locomotion
A skeleton is any firm structure that gives mechanical support to the body and protection to the softer parts of
the organism. They vary greatly between different organisms and are used in different ways to provide
locomotion.
Muscles and bones.
When muscles contract they provide the force needed
for locomotion. Muscles can only contract so they
work in pairs (antagonistic pairs) just like the bicep
and tricep muscles shown in the diagram below.
Elbow Joint
Nerves stimulate the contraction of muscles, they
control which muscles, when and how strongly.
Bones provide an anchor for muscles and act as
levers, may act to change direction of the
movement.
Elbow joint
Ligaments - bone to bone
Tendons – bone to muscle
Bones – anchorage for muscle
Capsule - seals joint
Synovial fluid lubricates the joint
Cartilage – smooth , tough tissue that lubricates the joint
Muscle under the electron-microscope
Muscles are made of bundles of smaller fibres called myofibrils. Skeletal muscle appears striped due to the
structure of myofibrils. A myofibril is made of many repeated units called sacromeres. A sarcomere is the
fundamental unit of action of a muscle fibre and is the region between two dark lines called Z lines. The dark
bands consist of thick and thin filaments (actin and myosin), except central region where only thick filaments
occur (myosin). The light band only contains thin filaments (actin). Z line formed by proteins connecting actin
filaments.
How muscles contract – the sliding filament theory
When muscles contract – sarcomere shortens, I band gets shorter, A band remains the same length, H zone
gets narrower, Z lines move closer together. Note that the actual length of the actin and myosin filaments
remains the same, it is just that they slide past each other.
The mechanics of contraction
Myosin filaments are rod shaped with a globular end (myosin head). The head can form a cross bridge with
actin. When attached to actin, the myosin head can change shape and slide the actin further along the myosin.
These bridges can be formed and broken down up to 100 times a minute in a muscle contraction. Calcium ions
are required for cross bridges to form and the breakdown of ATP provides the energy needed by the ratchet
mechanism. Contraction is initiated by a nerve impulse which triggers the availability of calcium ions and
ATP.
Muscle contraction animation
The diagram below describe the arrangements of actin and myosin. Please note that actin filaments are made of
chains of globular actin molecules. Myosin chins instead have characteristic globular heads and long tails.
At rest actin and myosin are prevented from contacting each other by two other proteins: tropomyosin and the
Ca++ binding protein troponin. Upon stimulation, Ca++ is released from internal stores and binds to troponin
which induces a conformational change of tropomyosin and allows actin-myosin interaction.
The myosin head can bind hydrolise ATP (loss of one phosphate)to ADP. This gives energy to the myosin head
to bind to actin and to bend pushing the acting filaments along. ADP is then resubstituted with ATP and actin
and myosin come apart. Hydrolisis of ATP will start another binding and sliding cycle.pushing the actin
filament along and resulting in contraction of the cell
The Myosin Cross-bridge Cycle.
A. ATP binding to a cleft at the back of the head causes a conformation which cannot bind actin.
B. As the ATP is hydrolysed, the head swings back about 5nm to the cocked position the ADP and Pi remain
bound.
C+D. The force generating stages. When the Pi leaves the myosin, the head binds the actin and the power
stroke is released as the head bind actin. ADP is released to continue the cycle. At this stage the head in bound
to actin in the rigor or tightly bound state.
Just like rowing!
Questions
DPHL Biology
11.3 Nerves, Muscles and Movement
1. What is a sarcomere?
2. What is the role of :
i.
Calcium
ii.
Tropomyosin
iii.
Acetylcholine
iv.
ATP
In the sliding filament theory of muscle action
3. (a)
Identify the structures labelled I and II on the diagram of the elbow joint below.
[Source: R Allen and T Greenwood, (2001) Advanced Biology 2, Student Resource
and Activity Manual, 3rd edition, Biozone International Limited, page 98]
(2)
(b)
Explain how the action of the muscles is co-ordinated at this joint by the nervous system.
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(3)
(c)
State and describe one injury that could occur in this joint.
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(2)
3. The electron micrograph shows a longitudinal section through part of a striated muscle.
a. The diagram shows the A band, the I band and the H zone. Which one or more of these:
i.
contains actin but not myosin
ii.
shortens when the muscle contracts
iii.
What is labeled m in the electron micrograph
b. Draw a diagram to show what the sarcomere would like if the muscle was contracted..