Skeletal muscles are stimulated to contract by nerves and act as effectors

Sarcolemma

Plasma membrane of muscle fibre

Sarcoplasm

Cytoplasm of a muscle fibre

Sarcoplasmic reticulum and role

• Specialised type of smooth endoplasmic reticulum found in the sarcoplasm

• Stores many Ca²⁺ ions

T-tubule and role

• Inward folds of the sarcolemma that extend deep in the sarcoplasm

• Allow action potential to reach sarcoplasmic recticulum

What are the 2 key types of proteins that form a sarcomere?

• Myosin (thick filament)

• Actin (thin filament)

I band → Is it variable or constant?

• Part of actin that doesn’t overlap with myosin

• Variable

A band → Is it variable or constant?

• Total width of myosin

• Constant

H band → Is it variable or constant?

• Part of myosin that doesn’t overlap with actin

• Variable

Z line → Is it variable or constant?

• Parameters of one sarcomere

• Variable

Describe the process leading up to the release of Ca²⁺

1. Action potential from motor neurone

2. Depolarises the sarcolemma

3. Depolarisation spreads down T-tubules the the sarcoplasmic reticulum

4. Sarcoplasmic reticulum releases stored Ca²⁺ into sarcoplasm

Outline the sliding filament theory from the release of Ca²⁺ ions into the sarcoplasm

1. Calcium ions bind to troponin, causing displacement of tropomyosin

2. This exposes myosin binding sites on actin

3. Myosin head binds to actin forming actin-myosin cross bridge

4. Myosin head completes power stroke, pulling actin towards centre of sarcomere

5. ATP binds to myosin head, making it detach from actin

6. Ca ²⁺ activates ATP hydrolase, so ATP is hydrolysed into ATP + Pi

7. The energy released from the hydrolysis of ATP is used to convert myosin heads into “cocked” state (original position)

8. Cycle repeats causing actin to slide along myosin, shortening the sarcomere

What are the 2 types of muscle fibre?

• Slow twitch

• Fast twitch

Outline the differences in contraction between slow and fast twitch muscle fibres

Slow twitch: Contract more slowly, providing less powerful contractions over a longer period of time

Fast twitch: Contract rapidly, providing more powerful contractions over a shorter period of time

Outline the differences in mitochondria between slow and fast twitch muscle fibres

Slow twitch: High number to produce ATP

Fast twitch: Smaller number of mitochondria

Outline the differences in myoglobin between slow and fast twitch muscle fibres

Slow twitch: Large store of myoglobin to store oxygen (gives red colour to fibres)

Fast twitch: Low myoglobin content (gives white colour to fibres)

Outline the differences in location between slow and fast twitch muscle fibres

Slow twitch: Deeply situated in muscle

Fast twitch: On the surface of muscle

Outline the differences in fibres between slow and fast twitch muscle fibres

Slow twitch: Lower number of fibres in motor unit

Fast twitch: Higher number of fibres in motor unit, therefore muscle stronger

Outline the differences in adaptations between slow and fast twitch muscle fibres

Slow twitch: Adapted for aerobic respiration to avoid build up of lactic acid

Fast twitch: Adapted for short bursts of intense exercise (mostly anaerobic)

Outline the differences in glycogen between slow and fast twitch muscle fibres

Slow twitch: A supply of glycogen to provide a source of metabolic energy

Fast twitch: Larger amounts of glycogen

Outline the differences in phosphocreatine between slow and fast twitch muscle fibres

Slow twitch: Low concentration of PCr

Fat twitch: High concentration of Pcr

What is the 3rd way skeletal muscles can produce ATP?

The ATP-phosphocreatine system

What is the role of PCr in providing energy during muscle contraction?

• PCr provides the phosphate

• To phosphorylate ADP to produce ATP

What is the equation that shows the hydrolysis of PCr forming ATP?

ADP + PCr ⇌ ATP + Cr