Untitled Lesdraftson

Title: B3.3: Muscle and motility (HL only)

Keywords: Actin, myosin, troponin, synovial joints

LI: To be able to describe and explain how the contraction of muscles results

in movement

Starter:

Many muscles work in antagonistic
pairs e.g. the bicep and tricep.
Suggest what this means.

Title: B3.3: Muscle and motility (HL only)

Keywords: Actin, myosin, troponin, synovial joints

LI: To be able to describe and explain how the contraction of muscles results

in movement

Answer:

In an antagonistic muscle pair as one
muscle contracts the other muscle
relaxes or lengthens.

What

you need
to know
in this
unit?

•B3.3.1: Recognise the concept of movement in
different species.

•B3.3.2: Outline the structure of a sarcomere.

•B3.3.3: Explain how a sarcomere contracts.

•B3.3.4: Outline the role of titin and antagonistic
muscles in muscle relaxation.

•B3.3.5: Describe the structure and function of motor
units in skeletal muscles.

•B3.3.6: Outline the role of the skeleton as anchorage
for muscles and as levers.

•B3.3.7: Explain the role of the different components of
a joint.

•B3.3.8: Compare the range of motion of a joint.

•B3.3.9: Explain the antagonistic action of the internal
and external intercostal muscles.

•B3.3.10: Give examples of reasons for locomotion.

•B3.3.11: Describe adaptations for swimming in marine
mammals.

Guiding
question

How do muscles contract and
cause movement?

What are the benefits to
animals of having muscle
tissue?

Movement as a universal feature of living

organisms

Movement is a universal feature of
living organisms. Whether an
organism
is multicellular or unicellular, complex
or simple, motile or sessile, they will
demonstrate movement in some form.
Q. What do we mean by movement
and why is it important?

Movement as a universal feature of living

organisms

Movement is the change in position or
location of an organism or body part
relative to its surroundings. It can
be voluntary or involuntary and can occur
in response to internal or external stimuli.

Movement allows organisms to interact
with their environment, obtain resources,
find mates, respond to threats and engage
in social behaviours.

Muscle contraction

Muscles are composed of cells arranged to form fibres. These fibres
can contract to become shorter, which produces a force. There are 3
different types of muscle:

• Smooth muscle
• Cardiac muscle
• Skeletal muscle

In this unit we will be focusing on skeletal muscle.

The structure of a sarcomere

Within muscle cells are
thread-like structures
called myofibrils, which are
composed of repeating units
called sarcomeres.
Sarcomeres are composed of two
types of protein filaments: thick
filaments (myosin) and thin
filaments (actin), the alternating
arrangement of which gives
skeletal muscle its striated look.

Structure of the sarcomere

Task: Using the image
describe the structure
of the sarcomere.
Specifically, what forms
the:
• A band
• H zone
• I band

Structure of the sarcomere

• The A band is a region of the

sarcomere contains Myosin and a
region which overlaps with Actin
(darker region).

• The H-Zone is the middle of the

sarcomere which only contains
myosin.

• The I band is a region of the

sarcomere which only contains Actin
(lighter region)

Other structure of the sarcomere

• Z discs, they organise and

anchor the actin filaments. The
length between Z-discs is known
as the sarcomere length.

• In the centre, you have the

M-Line, this organises and
anchors the myosin.

The sliding filament model of contraction

The force achieved during muscle
contraction is a result of the
simultaneous contraction of all the
sarcomeres in that muscle. The sliding
filament theory explains the contraction
of a sarcomere.

According to this theory, when a muscle
is stimulated to contract, actin filaments
slide over the myosin filaments towards
the centre of the sarcomere

The sliding filament model of contraction

This results in:
• Z discs being pulled closer

together, shortening the sarcomere
and resulting in the overall
shortening of the muscle fibre

• The H bands and I bands decrease

in length as actin is pulled inwards,
overlapping more myosin and
reducing the area where only
myosin or actin is present.

Structure of myosin

Myosin myofilaments possess
head groups which contain
bindings sites for actin and
ATP.
Myosin heads are a globular
shape that are hinged, allowing
them to move back and forth and
enabling it to slide the actin
filaments closer towards it

Structure of Actin

A molecule of tropomyosin wraps
around each thin filament (actin). This
is held in place by another polypeptide
called troponin. The tropomyosin
prevents the myosin head binding to
the actin.

Each troponin molecule consists of 3
polypeptides. One polypeptide binds
to the actin, one to the tropomyosin
and one to calcium ions

What causes the filaments to slide over each other?

Stage 1

Tropomyosin

molecule

prevents

myosin head from attaching to the
binding site on the actin molecule

What causes the filaments to slide over each other?

Stage 2

When a muscle is stimulated by a motor
neuron, calcium ions are released from
the sarcoplasmic reticulum, a
specialised endoplasmic reticulum that
is found in the sarcoplasm (cytoplasm)
of muscle cells

Calcium ions bind to the troponin cause
the tropomyosin molecule to move
away from the binding sites on the actin
molecule.

What causes the filaments to slide over each other?

Stage 3

Myosin head now
attaches to the binding
site on the actin
filament, forming a
cross-bridge.

What causes the filaments to slide over each other?

Stage 4

The head of the myosin
changes angle, moving
the actin filament along
as it does so (power
stroke). The ADP
molecule is released

What causes the filaments to slide over each other?

Stage 5

ATP molecule fixes to
the myosin head,
causing it to detach
from the actin filament.

What causes the filaments to slide over each other?

Stage 6

Hydrolysis of ATP to
ADP by myosin
provides the energy for
the myosin head to
resume it’s normal
position

What causes the filaments to slide over each other?

Stage 7

The head of myosin
reattaches to a binding
site further along the
actin filament, forming
another cross-bridge,
and the cycle is repeated

Further reading: Rigor mortis

When an individual dies,
their muscles can become
very stiff, in a process
known as Rigor Mortis.

Following what we have
learnt, suggest why the
muscles become stiff after
death.

Further reading: Rigor mortis

Once we die, we can no
longer respire and
regenerate ATP.
This means the
cross-bridge between the
actin and myosin cannot
be broken. Resulting in
muscle stiffness.

The role of titin in muscle relaxation

Titin is an immense (giant), elastic protein, one of the largest
proteins known to science. Whereas the median human
protein length is 375 amino acids, human titin is composed of
over 34 000 amino acids.
It connects the Z discs and the M lines and providing support
and stability to the muscle fibres.

The role of titin in muscle relaxation

Titin acts as a molecular spring. When a
sarcomere is stretched, titin is also
stretched, storing potential energy that is
released when the stretching force is
released.
This release of energy helps to return the
sarcomere to its original length, allowing
the muscle to resume normal function.

The role of antagonistic muscles in muscle

relaxation

When a muscle contracts, it shortens
and pulls on its attachment points,
causing movement of a bone or other
body part. A muscle can only contract
and generate force in one direction.
Because of this, some muscles are
found in antagonistic pairs, such as the
biceps and triceps.

When one muscle in the antagonistic
pair contracts, the other relaxes.

The role of antagonistic muscles in muscle

relaxation

Task: Research other antagonist
muscle pairs in the body.
For each:
- List the muscles involved
- The movement they cause

Skeletal muscle units

Depending on the variety of strengths of
contraction a muscle needs it will contain either a
few muscle cells or a great many muscle cells in
one motor unit.
For example leg muscles need to be able to allow
you to move from a slow amble to a fast sprint
and to help control your posture whereas feet
muscles need to aid in maintaining your posture
and move the position of the foot to allow you to
walk.
A cluster of muscle fibres being innervated by
one motor neurone = a motor unit

Control of contraction

As previously mentioned, in order to
initiate contraction of a muscle, Ca2+
must be released. In the case of
skeletal muscles, this is controlled by
Motor neurons.
In the case of skeletal muscle, the
axon terminal of a motor neuron
connects to individual muscle fibres
via a specialised synapse called
the neuromuscular junction.

Research task: Control of contraction

Research how motor
neurons can increase the
Ca2+ concentration in
sarcomeres which leads
to the contraction of
skeletal muscle.

Bones and joints

Bones and joints

The adult human body has
206 bones which form the
human skeleton.
Task: What is the role of
the human skeleton?

Bones and joints

Answers:
• Protects major organs
• Produce blood cells
• Support
• Movement

Skeleton: Anchorage for muscles and lever

Although skeletons are unable to
move by themselves, they act as
anchorage points for muscles, and
because of their rigidity, the bones in
a skeleton act as levers.

Levers are rigid structures that act
around a fulcrum in response to an
applied force. Think of a door and a
hinge.

Skeleton: Anchorage for muscles and lever

The bone acts as the lever, the
joint as the fulcrum and the
force is generated by the
contraction of a muscle that is
attached to the bone.

Exoskeletons

Exoskeletons made of the complex
polysaccharide chitin and located on the
exterior of the body.
Because the exoskeleton is located outside the
body, it must be periodically shed and replaced
as the animal grows.
The exoskeleton acts as an attachment site for
muscles. When the muscles contract, they pull
on the tendons which in turn pull on the
exoskeleton and produce movement.

Endoskeletons

Vertebrates, animals with a backbone
(spinal column), have an endoskeleton,
an internal structure made of bone and
cartilage.
The endoskeleton provides support and
protection for the body's internal organs,
and acts as anchorage sites for muscles.
Muscles contract to generate the force
necessary to move the bone they are
attached to and allow movement of the
body.

Movement at a synovial joint

Joints are the articulating
surfaces between two or more
bones. Synovial joints are
joints that are enclosed in a joint
capsule, where bones are
separated by a fluid-filled
cavity, allowing free movement
between the bones.

Movement at a synovial joint

Task:
Research the following
components of the synovial
joint. For each explain the
structure and function:
• Cartilage
• Synovial fluid
• Ligament
• Tendon

Movement at a synovial joint

Answers:
Cartilage: covers the end of the bones in
a synovial joint, acting as a cushion to
absorb shock. Cartilage has a smooth
surface, which helps to facilitate the
smooth movement of bones over each
other.
Synovial fluid: Within the cavity
is synovial fluid, which acts as a lubricant
to reduce friction between bones.

Movement at a synovial joint

Answers:
Ligament: The bones in a synovial joint are
connected by ligaments. They are strong,
flexible bands of connective tissue that provide
stability to the joint and prevent excess
movement or dislocation of the joint.
Tendon: muscles are attached to bones
via tendons, strong fibrous bands of connective
tissue. When a muscle contracts, it exerts a
pulling force on the tendon, and hence on the
bone, which causes the movement of that bone
relative to the rest of the joint.

Joint example: Hip joint

The hip joint is a synovial joint that connects the femur bone in the
thigh to the pelvis bone. It is a ball and socket joint – the rounded head
of the femur fits into the cup-like socket of the pelvis.
The joint allows for a wide range of movement:
• Flexion
• Extension
• Abduction
• Adduction
• Rotation
• Circumduction

Task: For Each range of
movement, explain what It
means and act it out.

Range of motion of a joint

The range of motion (ROM) in a joint refers to the type and amount
of movement that is possible at that joint. Joints with a high ROM
allow for a wider range of movement and often multiple planes of
motion, allowing for greater flexibility and mobility e.g. the hip and
elbow joint

Factors affecting your ROM

ROM is determined by:
• The joint’s anatomical structure,
• The surrounding muscles, ligaments, tendons
• The presence of other tissue that can facilitate or limit movement.
• Age
• Injury or disease
• Regular stretching

Measuring your ROM

To measure the amount of movement of a joint, the joint angle is
measured. This is the angle at which a bone can move relative to
its resting position; a manual instrument called goniometer can be
used.

Measuring your ROM

The intercostal muscles

The intercostal muscles are a group of muscles located between
the ribs and the thoracic cavity that are involved in breathing
(ventilation). There are two main groups of intercostal muscles:

• The external intercostal muscles are the most superficial

(closest to the surface of the body). They run in a downward
and forward direction, diagonally towards the centre of the
chest. When they contract the rib cage is lifted up and out

The intercostal muscles

The intercostal muscles are a group of muscles located between
the ribs and the thoracic cavity that are involved in breathing
(ventilation). There are two main groups of intercostal muscles:

• The internal intercostal muscles are deeper and run in an

upward and forward direction towards the centre of the chest,
perpendicular to the external intercostal muscles. When they
contract the rib cage moves down and in.

The intercostal muscles

There is also a third layer, called the innermost intercostal
muscles, which are responsible for assisting in forced expiration
during heavy breathing.

The intercostal muscles

Antagonistic action of intercostal muscles

Because of the different orientation of muscle fibres in the
internal and external layers of intercostal muscles, when they
contract they move the rib cage in opposite directions.

When one layer contracts, the other layer is stretched, storing
potential energy in the sarcomere protein titin. When contraction
of sarcomeres ends, the release of the potential energy stored in
titin helps to return the sarcomere to its original length.

Reasons for
locomotion in

animals

Locomotion is the ability of
an organism to move its
whole body from one place
to another – motile
organisms can locomote but
sessile organisms cannot.
Task: Why do organisms
locomote?

Reasons for locomotion in animals

• Foraging for food: the act of searching for and collecting

food.

• Searching for a mate: Most animals reproduce sexually, which

requires the fusion of male and female gametes.

• Escaping danger: Escape behaviours are usually fast and

robust, enabling the animal to swiftly avoid threat

• Migrations: refers to the large-scale seasonal movement of an

animal group from one place to another

Adaptations for swimming in marine mammals

Mammals are a diverse
group
of endothermic animals
that breathe air through
lungs, give birth to live
young, produce milk to
feed their young and have
hair or fur at some point
in their life.

Adaptations for swimming in marine mammals

Marine mammals, such as
whales, dolphins, seals, sea
lions and manatees, have
evolved a range of
adaptations for swimming
in their aquatic
environments.
Task: Research the
adaptations marine
mammals have and create
a fact profile of a
particular marine
mammal.

Adaptations for swimming in marine mammals

• Streamlined bodies that minimise drag, allowing for efficient

movement through the water.

• Adaptations of forelimbs to form flippers that are positioned on

the sides of their bodies which are usually long and narrow

• Adaptation of the tail to form a fat and wide fluke.
• Changes of the airways to allow periodic breathing between

dives. Whales and dolphins have a specialised nostril called a
blowhole.

Adaptations for swimming in marine mammals

• Marine mammals have higher concentrations of the protein

myoglobin in their muscle tissues. Myoglobin has a high
affinity for oxygen, allowing marine mammals to store more
oxygen and dive for extended periods of time.

• Many marine mammals have larger lungs relative to their body

size and more capillaries surrounding their alveoli.

• Many marine mammals have higher volumes of blood relative

to their body size, allowing them to carry more oxygen.

Adaptations for swimming in marine mammals

• The large size of many marine mammals minimises their

surface area-to-volume ratio which reduces heat loss in cooler
ocean waters.

Summary

• Movement is a change in position or location of an organism or body

part relative to its surroundings. All organisms, whether motile or
sessile, exhibit movement.

• Sarcomeres are the contractile units of skeletal muscles, and they are

made of repeating arrangements of the contractile proteins, actin and
myosin. During muscle contraction, actin filaments slide over the
myosin filament, shortening the sarcomere.

• Sarcomeres also contain an elastic protein called titin, which

prevents overstretching of a sarcomere, and acts to return a
sarcomere to its original length following muscle contraction.

Summary

• Antagonistic muscles are pairs of muscle groups that work in

opposing action. The biceps and triceps are an example of an
antagonistic pair that work around the elbow joint to allow
flexion and extension of the elbow. The internal and external
intercostal muscles are antagonistic muscle pairs that work to
move the ribs to facilitate breathing.

• There are two types of skeletons, exoskeletons and

endoskeletons. Skeletons act as anchoring points for muscles
and they work as levers to allow movement.

Summary

• Synovial joints are the joints between two or more articulating

bones. They contain synovial fluid inside a fluid capsule and
typically allow a wide range of motion. The hip joint is an
example of a synovial joint.

• The range of motion in a joint refers to the type and amount of

movement that is possible at that joint. The amount of
movement is also referred to as a joint angle, and can be
measured manually using a goniometer, or digitally using
computer analysis.

Summary

• Locomotion is the ability of an organism to move its whole

body from one place to another. Locomotion is necessary for
foraging, escaping danger, finding a mate and migration.

• Marine mammals evolved on land and then returned to the

oceans around 50–60 million years ago. Adaptations of marine
mammals to their aquatic environment include a streamlined
shape, adaptation of the forelimbs into flippers, and then tail
into a fluke, and specialised airways.

Checklist

• Recognise the concept of movement in different species.
• Outline the structure of a sarcomere.
• Explain how a sarcomere contracts.
• Outline the role of titin and antagonistic muscles in muscle

relaxation.

• Describe the structure and function of motor units in

skeletal muscles.

Checklist

• Outline the role of the skeleton as anchorage for muscles

and as levers.

• Explain the role of the different components of a joint.
• Compare the range of motion of a joint.
• Explain the antagonistic action of the internal and external

intercostal muscles.

• Give examples of reasons for locomotion.
• Describe adaptations for swimming in marine mammals.