skeletal muscles

how do muscles work?

work in antagonistic pairs → pull in opp. directions eg. biceps/triceps

• one muscle contracts (agonist) pulling on bone

• one relaxes (antagonist)

skeleton is incompressible so muscle can transmit force to bone

advantage of muscles working in antagonistic pairs?

second muscle required to reverse movement caused by the first, and contraction of both helps maintain posture

gross and microscopic structure of skeletal muscle?

made of bundles of muscle fibres packed together

attached to bones by tendons

what do muscle fibres contain?

• sarcolemma - cell membrane folded inwards to form T tubules

• sarcoplasm

• multiple nuclei

• many myofibrils

• sarcoplasmic reticulum

• many mitochondria

ultrastructure of a myofibril?

made of 2 types of long parallel protein filaments

• myosin - thick

• actin - thin

arranged in

bonding pattern seen in myofibrils?

I bands - light bands, only thin actin

A-bands - dark bands, only thick myosin + some actin

• H zone - only myosin

• darkest region contains overlapping actin + myosin

muscle contraction overview?

myosin heads slide actin along myosin contracting sarcomere

simultaneous contraction of many sarcomeres causes myofibrils + muscle fibres to contract

when sarcomeres contract:

• H zones shorten

• I band shortens

• A band stays same

• Z lines get closer

muscle contraction process?

• depolarisation spreads down sarcolemma via T tubules causing Ca2+ release from sarcoplasmic reticulum, which diffuse to myofibrils

• Ca+ bind to tropomyosin, causing it to move → exposing binding sites on actin

• allowing myosin head w/ ADP attached to bind to sites on actin → forming actinomyosin cross bridge

• myosin head changes angle, pulling actin along myosin (ADP released), via energy from ATP hydrolysis

• energy used by myosin heads to return to og. position

• myosin reattaches to dif binding site further along actin, process repeats as long as Ca2+ conc. is high

muscle relaxation?

Ca2+ actively transported back into endoplasmic reticulum via energy from ATP

tropomyosin moves back to block myosin binding site on actin again → no actinomyosin cross bridges

role of phosphocreatine in muscle contraction?

source of Pi phosphorylates ADP to regenerate ATP

• ADP + phosphocreatine → ATP + creatine

runs out after few seconds → used in short bursts of vigorous exercise

anaerobic + alactic

properties of slow twitch muscle fibres?

specialised for slow contractions

produce ATP slowly from aerobic respiration

fatigues slowly

location of slow twitch muscle fibres?

high proportion in muscles used for posture eg. back, calves

legs of long distance runners

structure of slow twitch muscle fibres?

• high conc. of myoglobin → stores O2 for aerobic respiration

• many mitochondria → respiration

• many capillaries → high conc. of O2/glucose for resp + prevent lactic acid build up causing fatigue

general properties of fast twitch muscle fibres?

specialised for brief intensive contractions eg sprinting

produce ATP less quickly from mostly anaerobic resp.

fatigues quickly due to high lactate conc.

location of fast twitch muscle fibres?

high proportion in muscles used for fast movement eg biceps, eyelids

legs of sprinters

structure of fast twitch muscle fibres?

• low levels of myoglobin

• lots of glycogen → hydrolysed into glucose for glycolysis/aerobic respiration, which is insufficient so large amts. req.

• high conc. of enzymes involved om anaerobic resp.

• store of phosphocreatine