13.9 Voluntary And Involunatry Muscles

Question 1

What are the three types of muscle in the body?

Answer 1

• skeletal muscle (voluntary muscle)
• involuntary muscle (smooth muscle)
• cardiac muscle (heart muscle)

Question 2

Describe skeletal muscle
- fibre appearance (under microscope)
- structure
- function
- contraction speed

Answer 2

• skeletal muscle contraction is controlled consciously
• it’s made up of many muscle fibres — these are multinucleated and can be centimetres long
• some muscle fibres contract very quickly - they’re used for speed and strength but fatigue quickly
• some muscle fibres contract slowly and fatigue slowly — used for endurance and posture
• fibres have cross striations, fibres are tubular and multinucleated, fibres are regularly arranged so muscle contracts in one direction

Question 3

Describe involunatry muscle
- fibre appearance (under microscope)
- structure
- function
- contraction speed

Answer 3

• involuntary muscle contraction is controlled unconsciously
• it’s also called smooth muscle because it is non-striated (doesn’t have striped appearance)
• found in the walls of hallow internal organs e.g. the gut, the blood vessels
  Your gut smooth muscles contract to move food along (peristalsis) and your blood vessel smooth muscles contract to reduce blood flow
• each muscle fibre is uninucleate
• the muscle fibres are spindle-shaped with pointed ends, and they’re only about 0.2mm long
• the muscle fibres contract slowly, and dont fatigue
  Muscle fibres have no regular arrangement (different cells can contract in different directions)

Question 4

Describe cardiac muscle
- fibre appearance (under microscope)
- structure
- function
- contraction speed

Answer 4

• cardiac muscle is myogenic
• found in the walls of your heart and its function is to pump blood around the body
• its made of muscle fibres connected by intercalated discs, which have low electrical resistance so nerve impulses pass easily between cells
• the muscle fibres are branched, to allow never impulses to spread quickly through the whole muscle, and interconnect resulting in simultaneous contraction
• each cardiac muscle fibre is uninucleate
• the muscle fibres are shaped like cylinders and they’re about 0.1 mm long
• cardiac muscle does show striations but they are much fainter than those in skeletal muscle
• the muscle fibres contract rhythmically and don’t fatigue

Question 5

Describe and explain the structure of skeletal muscle

Answer 5

Skeletal muscle is made up of large bundles of long cells, called muscle fibres
Th cell membrane of muscle fibre cells is the sarcolemma.

Bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (muscle cell’s cytoplasm)
- these folds are called transverse (T) tubules and they help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre

A network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm
- the sarcoplasmic reticulum stores and releases calcium ions that are needed for muscle contraction

• muscle fibres have lots of mitochondria to provide the ATP that’s needed for muscle contraction
• they are multinucleated, and have lots of long cylindrical organelles called myofibrils
  Myofibrils are made up proteins and are highly specialised for contraction

Question 6

Describe the structure of myofibrils

Answer 6

• contain bundles of thick and thin myofilaments that move past each other to make muscles contract
• the thick myofilaments are made of the protein myosin and the thin myofilaments are made of the protein actin

(Myofibril under the microscope: dark bands contain thick myosin filaments and some overlapping actin filaments — these are called A-bands
Light bands contain think actin filaments only — these are called I-bands)

A myofibril is made up of many short units called sarcomeres
- The ends of each sarcomere are marked with a Z-line
- In the middle of each sarcomere is an M-line (the M-line is in the middle of the myosin filaments)
- Around the M-ling is the H-zone. The H-zone only contains myosin filaments.

When a muscle contracts the sarcomere shortens, and H-zone decreases