Skeletal muscles

Muscles act as antagonistic pairs against an incompressible skeleton

• muscles work in antagonistic pairs

  • one muscle contract (agonist) → pulls on bone/produces force

  • one muscle relaxes (antagonist)

  • e.g. biceps and triceps in the arms

• attached to bones by tendons

• ligaments attached from one bone to the other

• skeletal muscle is incompressible so muscle can transmit force to bone

• advantages of skeletal muscles being arranged in antagonistic pairs

  • muscles can only contract/pull

  • 2nd muscle required to reverse the movement caused by 1st

  • help maintain posture → contraction of both muscles

Gross and microscopic structure of skeletal muscle

• muscle made up of bundles of muscle fibres (muscle cell) packaged together

• muscle cells contain

  • Cell membrane = sarcolemma

  • Cytoplasm = sarcoplasm

  • Myofibrils made up of two proteins, actin and myosin

  • shared nuclei

  • lots of sarcoplasmic reticulum

Ultrastructure of a myofibril

• myofibril made up of many sarcomeres which are made up of partly overlapping myosin and actin filaments (proteins)

• A sarcomere consists of:

  • Ends - Z line

  • Middle - M line

  • H zone - around M line which contains only myosin

• Myosin filaments are thicker than thinner actin filaments

• this causes a banding pattern to be seen in a relaxed myofibril under an electron microscope

  • I - bands → light bands containing only thick actin filaments

  • A - bands → dark bands containing thick myosin filaments and some overlapping actin

Muscle contraction

• Myosin heads slide actin past/along myosin causing the sarcomere to contract

• simultaneous contraction of lots of sarcomeres causes myofibrils and muscle fibres to contract

• when sarcomeres contract (shorten):

  • H zones shorter

  • I band shorter

  • A band same

  • Z lines closer

The roles of actin, myosin, calcium ions and ATP in myofibril contraction

1. Action potential/depolarisation spreads down the T-tubule causing the release of calcium ions from the sarcoplasmic reticulum, which diffuse through the sarcoplasm to the myofibril

2. calcium ions bind to tropomyosin, causing it to move as it changes shape, exposing the myosin binding site on actin

3. so myosin heads, with ADP attached, attach to binding sites forming an actinomyosin crossbridge

4. Myosin heads move/change angle, pulling actin along myosin (ADP released)

5. ATP binds to myosin head causing it to detach from actin binding site/break crossbridge

6. the hydrolysis of ATP by ATPase (which is activated by calcium ions) releases energy e.g. for myosin heads move back to original position

7. Myosin reattaches to a different binding site further along actin filament (the cycle is repeated as long as calcium ions are present)

Slow twitch muscle fibres

• specialised for slow, sustained contractions (endurance)

• endurance activities e.g. maintaining posture, long distance running

• located in muscles that give posture and in leg muscles of long distance runners

• aerobic respiration produces ATP (oxidative phosphorylation) to release energy slowly

• high levels of myoglobin (red coloured protein that stores oxygen) makes them a reddish colour → store large amount of oxygen in muscle for aerobic respiration

• many mitochondria (site of aerobic respiration) → high rate of aerobic respiration

• many capillaries → short diffusion pathway/large SA → supply high conc. of oxygen/glucose for aerobic respiration and to prevent build-up of lactic acid causing muscle fatigue

Fast twitch muscle fibres

• specialised for producing rapid, intense contraction of short duration

• shot bursts of speed and power e.g. sprinting

• located in the legs of sprinters

• anaerobic respiration produces ATP to release energy quickly

• low levels of myoglobin makes them into a whitish colour → anaerobic respiration doesn’t need oxygen

• lots of glycogen → hydrolysed to lots of glucose → used during glycolysis (anaerobic respiration) which is inefficient, yielding only 2 ATP per glucose molecule

• higher conc. of enzymes involved in anaerobic respiration (in cytoplasm) → high rate of anaerobic respiration

• store phosphocreatine which rapidly generates ATP from ADP by providing phosphate

• muscles can get fatigued quickly because of high amounts of lactate

Role of phosphocreatine in muscle contraction

• phosphocreatine stored inside cells

• rapidly makes ATP by phosphorylating ADP (adding phosphate group from PCr)

• PCr runs out after a few seconds so its used in short bursts of vigorous exercise

• anaerobic and alactic