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

How do muscles work in antagonistic pairs?

• The agonist muscle contracts, pulling on the bone and producing force

• The antagonist muscle relaxes

• This happens against the incompressible skeleton

• So the muscle transmits force to the bone

What is the advantages of muscles working in antagonistic pairs?

• A second muscle is required to reverse the movement caused by the first muscle

• As muscles can only pull

• So both muscles contract to maintain posture and stabilise joints

What is a skeletal muscle?

The muscle that contracts to move bones

Describe and label the gross and microscopic structure of a skeletal muscle

• Made of many bundles of muscle fibres packaged together

• Attached to bones by tendons

Muscle fibres contain:

• Inward folding sarcolemma to form transverse tubules

• Sarcoplasm

• Multiple nuclei

• Many myofibrils

• Sarcoplasmic reticulum

• Many mitochondria

Describe and label the ultrastructure of a myofibril

• Many protein filaments are arranged in a parallel pattern

• Containing myosin (thick filament) and actin (thin filament)

• Proteins are arranged in functional units called sarcomeres

• Z disc: ends of sarcomeres where actin filaments attatch

• M line: central point of sarcomeres where myosin filaments attach

• H zone: region only containing myosin filaments

• A band: region where myosin filaments extend

• I band: region containing only actin filaments

Why do sarcomeres form banding patterns?

• I bands contain only actin filaments so they are light

• H bands contain only myosin filaments so they are darker than I bands

• A bands contain overlapping myosin and actin filaments so they are the darkest

What is the overall process of muscle contraction?

• Myosin heads slide actin filaments along myosin, causing the sarcomere to contract

• Simultaneous contracton of many sarcomeres cayses myofibrils and muscle fibres to contract

• When sarcomeres contract, H zones get shorter, I band gets shorter, A band stays the same and Z lines get closer

What is the role of actin in myofibril contraction?

• Calcium ions bind to tropomyosin, causing it to move

• Binding sites on actin are exposed

• This allows the myosin head with ADP attached to bind to binding sites on actin

• Actomyosin crossbridge is formed

• Myosin heads change angle, pulling actin along myosin using energy from ATP hydrolysis

• New ATP binds to myosin head, causing it to detach from binding site

• Myosin attaches to a different binding site further along actin

• Process is repeated as long as calcium ion concentration is high

What is the role of myosin in myofibril contraction?

• The myosin head with ADP attached binds to binding sites on actin

• Actomyosin crossbridge is formed

• Myosin heads change angle, pulling actin along myosin using energy from ATP hydrolysis

• New ATP binds to myosin head, causing it to detach from binding site

• Hydrolysis of ATP by ATP hydrolase, activated by calcium ions, releases energy for myosin heads to return to original position

• Myosin attaches to a different binding site further along actin

• Process is repeated as long as calcium ion conc

What is the role of calcium ions in myofibril contraction?

• Depolarisation spreads down sarcolemma via t tubules

• So calcium ions release from sarcoplasmic reticulum

• Calcium ions diffuse to myofibrils

• Calcium ions bind to tropomyosin, causing it to move

• Binding sites on actin are exposed

• This allows the myosin head with ADP attached to bind to binding sites on actin

• Actomyosin crossbridge is formed

• Hydrolysis of ATP by ATP hydrolase, activated by calcium ions, releases energy for myosin heads to return to original position

What is the role of tropomyosin in myofibril contraction?

• Calcium ions bind to tropomyosin, causing it to move

• Binding sites on actin are exposed

• This allows the myosin head with ADP attached to bind to binding sites on actin

• Actinomyosin crossbridge is formed

What is the role of ATP in myofibril contraction?

• Myosin heads change angle, pulling actin along myosin using energy from ATP hydrolysis

• New ATP binds to myosin head, causing it to detach from binding site

• Hydrolysis of ATP by ATP hydrolase, activated by calcium ions, releases energy for myosin heads to return to original position

What is the process of muscle relaxation?

• Calcium ions are actively transported back into the endoplasmic reticulum using energy from ATP

• Tropomyosin moves back to block the myosin binding site on actin again, so no actinomyosin cross bridges can be formed

What is the role of phosphocreatine in muscle contraction and when is it used?

• Acts as a source of inorganic phosphate

• So rapidly phosphorylates ADP to regenerate ATP

• Short lasting so only used in short bursts of vigorous exercise

• Aerobic and alactic process

What is the role of ATP in muscle contraction?

• Provides energy to reset the position of myosin heads for contraction to repeat

• Provides energy for active transport for calcium ions to return to the sarcoplasmic reticulum after contraction

• Breaks the actin-myosin cross bridge by attaching to the myosin head and causing it to detach from the actin filament

Compare the structure, location and general properties of slow and fast skeletal muscle fibres

Structure

• Slow twitch has high conc of myoglobin which stores oxygen for aerobic respiration, fast twitch has low levels of myoglobin

• Slow twitch has many mitochondria so has a high rate of aerobic respiration, fast twitch has lots of glycogen that is hydrolysed to provide glucose for glycolysis/aerobic respiration

• Slow twitch has many capillaries so has a high concentration of oxygen and glucose for aerobic respiration

• Fast twitch has a high concentration of enzymes for anaerobic respiration

• Fast twitch stores phosphocreatine

Location

• Slow twitch has a high proportion in muscles used for posture, fast twitch has a high proportion in muscles used for fast movement such as biceps and eyelids

General Properties

• Slow twitch are specialised for slow, sustained contractions, fast twitch are specialised for brief, intensive contractions

• Slow twitch obtain ATP from aerobic respiration, fast twitch obtain ATP from anaerobic respiration

• Slow twitch release energy slowly and so fatigue slowly, fast twitch obtain release energy quickly and so fatigue quickly due to high lactate concentration