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. Smooth2. Cardiac3. 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. Actin2. 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.

The bands explained• 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 H-zone is a dark M-line.

• At the centre of each isotropic band is a line called the Z-line.

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’.