A-Level Biology - 3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors

How do muscles act?

Muscles act in antagonistic pairs against an incompressible skeleton

Describe the structure of skeletal muscle

made up of bundles of muscle cells (muscle fibres)

muscle fibres are elongated cell containing many nuclei

sarcoplasm contains myofibrils formed from contractile proteins

many mitochondria in the sarcoplasm produce ATP to release energy for muscle contraction

Name the subcellular structures of a muscle fibre

cell surface membrane = *sarcolemma*

cytoplasm = *sarcoplasm*

endoplasmic reticulum = *sarcoplasmic reticulum*

Describe the structure of a myofibril

protein filaments in myofibrils:

*myosin* (thick filaments)

*actin* (thin filaments)

Myofibrils can be divided into sections called *sarcomeres*

sarcomeres shorten during muscle contraction as myosin and actin filaments slide past each other

Name the features of a sarcomere

Z-line

M-line

A band

H zone

I band

What is the Z-line?

The boundary between sarcomeres

Actin filaments attach here

What is the M-line?

middle of sarcomere

Myosin filaments attach here

What is the A band?

region of the sarcomere across which myosin filaments extend

What is the H zone?

region containing only myosin filaments

What is the I band?

region containing only actin filaments

What happens to the sarcomere during contraction?

Z-lines get closer together

Myosin filaments don't change in length, so the A band remains same size

H zone shrinks during contraction as actin and myosin fires overlap more

I band shrinks during contraction as actin and myosin fires overlap more

Describe the roles of calcium ions and ATP in the contraction of a myofibril

1) Sarcolemma is depolarised, so calcium ions diffuse into myofibrils from sarcoplasmic reticulum

2. Calcium ions cause movement of tropomyosin on actin

3) Causes exposure of the binding sites on the actin

4) Myosin heads attach to binding sites on actin (forms actinomyosin bridges)

5) Release of Pi and ADP from myosin heads cause them to bend, pulling actin molecules (power-stroke)

6) Attachment of a new ATP molecule to each myosin head causes myosin heads to detach from actin sites

7) Hydrolysis of ATP on myosin heads causes them to bend back to original position (re-cocked)

Describe what happens in a myofibril when the electrical impulse stops

contraction stops when the impulse stops: calcium ions actively transported back into sarcoplasmic reticulum

actin moves back and muscle can return to its relaxed length

What is the role of ATP in myosin contraction?

1) Reaction with ATP breaks binding of myosin to actin

2) Provides energy to move myosin head

What is phosphocreatine?

a molecule stored by muscles that can be used for the rapid production of ATP over short periods

Describe the role of ATP and phosphocreatine in muscle contraction

supply of ATP is required for muscle contraction:

-to break actinomyosin bridges

-to move the myosin head

-so actin filaments are moved inwards

-for active transport of calcium ions

a phosphate ion from phosphocreatine is transferred to ADP to rapidly generate ATP anaerobically

ADP + phosphocreatine → ATP + creatine

Name two types of muscle fibre

fast and slow skeletal muscle fibres

What are fast muscle fibres most suited to?

short bursts of high-intensity activity

Give the properties of fast muscle fibres

higher concentration of glycogen and phosphocreatine

contract rapidly

ATP supplied mostly from anaerobic respiration

Fatigues rapidly due to high lactate production rate

fewer capillaries and mitochondria

Give the location of fast muscle fibres

limbs of animals that flee predators or hunt prey at high speeds

human eyelid muscles

biceps and triceps in the upper arms of humans

What are slow muscle fibres most suited to?

sustained, low intensity activities

Give the properties of slow muscle fibres

lower concentration of glycogen and phosphocreatine

contract slower

ATP supplied mostly from aerobic respiration

Fatigues more slowly due to reduced lactate formation

more capillaries and mitochondria

Give the location of slow muscle fibres

limbs of animals that migrate or stalk prey over long distances

human back and leg muscles

Explain how the difference in glycogen concentrations is related to the different properties of fast and slow muscle fibres

1) Fast fibres used during short-term/intense exercise

2) Slow fibres used during longer-term exercise

3) Creatine used to form phosphocreatine, which combines with ADP to form ATP

4) Glucose stored as glycogen

5) Glycogen hydrolysed to glucose for respiration