3.6.3 Skeletal Muscles

antagonistic pairs

muscles can only pull so they work in pairs to move bones around joints - pull in opposite directions

-agonist contracts while antagonist relaxes

gross structure of skeletal muscle

myofibrils = muscle cells fused together to form bundles of parallel muscle fibres

microscopic structure of skeletal muscle

-myofibrils divided into sarcomeres

-sarcoplasm = muscle cell’s cytoplasm with lots of mitochondria + glycogen granules and sarcoplasmic reticulum

-sarcolemma = cell membrane which folds inwards and sticks into sarcoplasm to form T-tubules (transverse)

roles of different features

-mitochondria + glycogen granules = provide ATP + hydrolysed into glucose for respiration

-sarcoplasmic reticulum = stores and releases Ca2+ ions 

-T-tubules = help to spread electrical impulses to sarcoplasmic reticulum

myofibrils structure

-myosin = thick filament
-actin = thin filament

-Z-line = boundary between sarcomeres

-M-line = middle of filaments

-I-band = only actin = light

-A-band = overlap of actin + myosin = dark

-H-zone = only myosin

sliding filament theory 

-myosin + actin filaments slide over one another to make sarcomeres contract so myofibrils + muscles contract

evidence that supports theory

-H-zone narrows

-I-band narrows

-Z-lines get closer

-A-band remains the same

STAGES of muscle contraction 1) stimulation of muscle

same process as synapse but at neuromuscular junction instead always with acetylcholine from motor neurone to skeletal muscle cell

STAGES of muscle contraction 2) arrival of action potential

-sarcolemma is depolarised + action potential travels down T-tubules to sarcoplasmic reticulum

-causes voltage-gated Ca²⁺ channels to open in reticulum so releases Ca²⁺ ions into sarcoplasm

-Ca²⁺ ions bind to troponin attached to tropomyosin causing conformational change of troponin

-so tropomyosin is removed and myosin binding sites are exposed

STAGES of muscle contraction 3) binding of myosin head

-myosin head in low energy configuration so ATP binds to myosin head + is then hydrolysed causing head to bend into high energy configuration

-myosin pulled back into position and binds to actin binding site to form actin-myosin cross bridge

STAGES of muscle contraction 4) movement of actin filament

-Ca²⁺ ions also activate enzyme ATP hydrolase which hydrolyses ATP into ADP + Pi for energy

-power stroke = causes head to move and pull actin filament over myosin

-actin moves towards centre of sarcomere

-head is now in low energy configuration

STAGES of muscle contraction 4) breaking of cross bridge

ADP + Pi dissociate but another ATP molecule binds to myosin head and breaks cross bridge

-myosin head detaches from actin + returns to original position

-myosin reattaches to different binding site further along

STAGES of muscle contraction 5) muscle relaxing

-Ca²⁺ ions leave binding sites and are actively transported back into sarcoplasmic reticulum through Ca²⁺ ATPase pump

-tropomyosin moves back and blocks actin-myosin binding sites

energy for muscle contraction

1) aerobic respiration - generates ATP, for long periods of low exercise

2) anaerobic respiration - makes ATP rapidly by glycolysis, for short periods of hard exercise

3) ATP-phosphocreatine system 

phosphocreatine (PCr)

-PCr reacts with ADP and adds a phosphate group to produce ATP + creatine

-PCr is stored inside cells and generates ATP very quickly, for short bursts of vigorous exercise

-anaerobic respiration

-some creatine get broken down into creatinine which is removed via kidneys

ADP + PCr → ATP + Cr

slow twitch muscle fibres

-found in sites of sustained contraction/posture e.g. calves

-long duration contraction - work for long time

-energy is released slowly through aerobic respiration to prevent lactate buildup

PROPERTIES of slow twitch muscle fibres

-glycogen = many ends can be hydrolysed to release glucose for respiration

-contain myoglobin = higher affinity for oxygen at lower partial pressures, reddish colour

-many mitochondria + blood vessels = high supply of ATP + oxygen

-mitochondria found near edge = short diffusion pathway for oxygen

fast twitch muscle fibres

-found in sites of rapid, powerful contraction/fast movement

-short-term contraction - get tired quickly

-energy is released quickly through anaerobic respiration using glycogen

PROPERTIES of fast twitch muscle fibres

-large store of PCr = energy can be generated very quickly

-more + thicker myosin filaments

-few mitochondria + blood vessels = get fatigued easily

-less myoglobin = pale colour