6.3 skeletal muscles

describe how muscles work

in antagonistic pairs - pull in opposite directions

• one muscle contracts, pulling on bone

• one muscle relaxes

skeleton is incompressible so muscle can transmit force to bone

what is the advantage of antagonistic pairs

second muscle reverses movement caused by first

contraction of both muscles helps maintain posture

describe the gross and microscopic structure of skeletal muscle

made of many bundles of muscle fibres packaged together

attached to bones by tendons

muscle fibres contain:

• sarcolemma (CSM) which folds inwards to form transverse (T) tubules

• sarcoplasm (cytoplasm)
  multiple nuclei

• many myofibrils

• sarcoplasmic reticulum (ER)

• many mitochondria

describe the ultrastructure of a myofibril

made of two types of long protein filaments, arranged in parallel

• myosin - thick filament

• actin - thin filament

arranged in functional units called sarcomeres

• ends - Z line / disc

• middle - M line

• H zone - contains only 1 myosin

explain the banding pattern to be seen in myofibrils

I bands - light bands containing only thin actin filaments

A bands - dark bands containing only thick myosin filaments

• H zone contains only myosin

• darkest region contains overlapping actin and myosin

give an overview of muscle contraction

myosin heads slide actin along mysoin causing the sacromere to contract

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

when sarcomeres contract:

• H zones get shorter

• I band gets shorter

• A band stays the same

• Z lines get closer

what are the roles of actin, myosin, calcium ions and ATP in myofibril contraction

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

2. calcium ions bind to tropomyosin causing it to move - exposing binding sites of actin

3. allowing myosin head, with ADP attached, to bind to binding sites on actin - forming an actinomyosin crossbridge

4. myosin head changes angle, pulling actin along myosin, using energy from ATP hydrolysis

5. new ATP binds to myosin head causing it to detach from binding site

6. hydrolysis of ATP by ATP hydrolase releases energy for myosin heads to return to original position

7. myosin reattaches to a different binding site further along actin

8. process is repeated as long as calcium ion conc. is high

what happens during muscle contraction

1. Ca²⁺ actively transported back into endoplasmic reticulum using energy from ATP

2. tropomyosin moves back to block myosin binding site on actin again - no actinomyosin cross bridges

describe the role of phosphocreatine in muscle contraction

a source of inorganic phospahte - rapidly phosphorylated ADP to regenerate ATP

• ADP + phosphocreatine —> ATP + creatine

runs out after a few seconds - used in short bursts of vigourous exercise

anaerobic and alactic

general properties of a slow