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Muscles
Syllabus 3.5.3
Toole pages 184 - 191
Aims:
1. Identify and describe the 3 types of muscle in
humans.
2. Label a diagram showing the gross and
microscopic structure of skeletal muscle.
3. Describe how actin and myosin is arranged
within a myofibril.
4. Identify and describe the 2 types of muscle
fibre.
Muscles as effectors.
• Muscles are effector
organs.
• They respond to
nervous stimulation
by contracting, which
brings about
movement.
Types of muscles
There are three types of muscle in the body.
1.
2.
3.
Smooth
Cardiac
Skeletal
Smooth muscle
•
•
•
•
Found in the wall of the gut, blood vessels and various cavities.
Unstriated (Does not have strips running across them).
Involuntary.
Contracts and fatigues slowly.
Cardiac muscle
•
•
•
•
Found exclusively in the heart.
Striated (Has strips running across them).
Involuntary.
Contracts myogenically, so is able to contract without
receiving impulses from nerves.
• Contracts rhythmically without fatigue.
Skeletal muscle
•
•
•
•
Attached to the skeleton.
Striated (Has strips running across them).
Voluntary.
Contracts and fatigues rapidly.
The gross and microscopic structure of
skeletal muscle
•
Individual muscles are made up of millions of tiny muscle fibres called myofibrils.
•
The myofibrils are lined up parallel to each other in order to maximise their strength and give
maximum force.
•
Myofibrils are composed of two types of protein filament:
1.
2.
Actin
Myosin
Actin
•
This is a thinner filament.
•
It consists of two globular protein strands twisted around one
another.
•
It contains two important proteins:
1. Tropomyosin
•
These are long thin fibrous strand that are wound around the actin
filaments.
2. Troponin
•
Involved in muscle contraction.
Myosin
• This is a thicker
filament.
• It has a long rod
shaped tail
composed of
several hundred
fibrous proteins
arranged into a
filament.
• It has 2 bulbous
heads that
project to the
side, which are
composed of a
globular protein.
The fine structure of muscle
•
Parallel myofibrils can be seen with characteristic pattern of alternating dark
and light bands.
• The diagrams to the left show
a short length of a single
myofibril.
• The region of myofibril from
one Z line to the next is called
a sarcomere.
• A sarcomere is the basic unit
of a myofibril.
• When a muscle contracts the
sarcomeres shorten and the
pattern of light and dark bands
change.
• The whole myofibril consists of
a long chain of such units
placed end to end.
• Myofibrils appear
stripped due to their
alternating light coloured
and dark coloured
bands.
• The light bands are
called isotropic bands
(I bands).
• They appear light
because actin and
myosin filaments do not
overlap in this region.
• The dark bands are
called anisotropic
bands (A bands).
• They appear darker
because the actin and
myosin filaments
overlap in this region.
The bands explained
• At the centre of
each anisotropic
band is a lighter
coloured region
called the H-zone.
• Running across the
middle of the Hzone is a dark Mline.
• At the centre of
each isotropic band
is a line called the
Z-line.
The bands explained
Types of muscle fibres
•
There are two main types of muscle
fibres in the human body.
1. Slow twitch fibres
2. Fast twitch fibres
•
They are distinguished by how quickly
they contract.
Slow twitch fibres
• These fibres are predominate in muscles that are involved with
sustained but relatively low levels of activity (maintenance of posture
and long distance running).
• In these activities the oxygen supply keeps pace with the demand
for ATP, so aerobic respiration can provide the necessary energy
and anaerobic respiration is not necessary.
• There are large numbers of mitochondria, as this is the site of
aerobic respiration.
• These fibres have a good blood supply and contains lots of
myoglobin, which serves as an oxygen store.
• The high myoglobin and blood content gives slow twitch fibres a
brownish red colour (‘red muscles’).
Fast twitch fibres
•
These fibres are the exact opposite.
•
They are adapted for sudden bursts of maximum activity, such as sprinting,
throwing, jumping and lifting.
•
These activities consume ATP at such a rate that the supply of oxygen for
aerobic respiration can not keep up with demand.
•
The energy need is therefore met by very high rates of anaerobic
respiration.
•
These fibres have few mitochondria, but a large quantity of the enzymes
required for anaerobic respiration.
•
The myoglobin content is low and the blood supply is unexceptional.
•
Muscles with a higher proportion of fast twitch fibres are therefore ‘white
muscles’.