27: Muscle Contraction Flashcards
Describe the structure of Skeletal Muscles:
- made up of bundles of muscle fibres, surrounded by connective tissue
- each muscle fibre is a single cell - multinucleated
- contain many mitochondria
- made up of many myofibrils composed of many sarcomeres
What 2 Protein Filaments are the Myofibrils made up from?
What do these make up?
- actin + myosin
- sarcomere
Describe Actin Properties:
- thin filament protein
- made up of 2 helical strands of globular actin molecules
- associated with tropomyosin which is a fibrous protein wrapped around the actin filament
Describe Myosin properties:
- thick filament protein
- consists of a tail and a globular head
- head can protrude in all directions to form actinomyosin bridges with the thin actin in muscle contraction
Describe the 5 Segments of the Sarcomere:
Z line: the anchor of the thin actin filaments, the gap between them is the sarcomere
M line: the anchor of the thick myosin filaments
I band: contains thin actin filaments only
H zone: contains thick myosin filaments only
A band: contains dark regions where thin and thick filaments overlap
Describe the appearance of the Sarcomere when contracted:
Z line: shorter distance between them
M line: no change
H zone: narrower
I band: narrower
A band: dark areas of A band wider, but OVERALL length is same width
Describe the Muscular Stimulation:
- action potential at the synaptic knob opens calcium ions, allowing Ca2+ to diffuse in
- this causes synaptic vessels to fuse with the presynaptic membrane, allowing ACh to be released by exocytosis
- ACh diffuses across synaptic cleft, binding to receptors on the membrane of the muscle cell (sarcolemma)
- this opens sodium ion channels, causing Na+ to diffuse into the sarcolemma
- action potential generated, passes through muscle fibres via T Tubules, that branch across the sarcoplasm of muscle fibres
- action potential causes calcium ion channels on the sarcoplasmic reticulum (ER in muscle cells), to open allowing Ca2+ to diffuse into the sarcoplasm
- the Ca2+ causes tropomyosin molecules that were blocking the binding sites on actin filament to move away
Describe the Sliding Filament Theory:
- action potential causes calcium ion channels on the sarcoplasmic reticulum (ER in muscle cells), to open allowing Ca2+ to diffuse into the myofibrils
- the Ca2+ causes tropomyosin molecules that were blocking the binding sites on actin filament to move away
- this causes exposure of the binding sites on the actin
- this allows the myosin head to attach to the binding site on the actin (forming an actinomyosin bridge)
- once attached to the actin filament, the myosin head changes it angle, pulling filament along (power stroke) releasing ADP + Pi
- this allows the actin filament to be pulled in, hence shortening the sarcomere, and in turn the muscle fibres
- new ATP binds to each myosin head
- enzyme ATP hydrolase (activated by Ca2+), hydrolyses ATP -> ADP + Pi, allowing myosin head to detach and return to original position
Describe Muscle Relaxation:
- Ca2+ are actively transported back into the sarcoplasmic reticulum (ER) from the myofibrils
- this allows tropomyosin to rebind and block actin filament
- so contraction ceases, and muscle relaxes
Describe the Role of Phosphocreatine in Muscle Contraction:
- stored in muscles
- it breaks down to form Creatine + Phosphate and releases energy
- Phosphate + energy releases used to reform ATP (ADP + Pi -> ATP)
- phosphocreatine cannot directly supply energy to the muscles but instead regenerates ATP
Describe the Features of Slow-Twitch Fibres:
- slow rate of contraction
- many more mitochondria, providing ATP, via aerobic respiration, for sustained contraction
- contains significantly more myoglobin than fast twitch fibres (this has a higher affinity for oxygen than haemoglobin for aerobic respiration
-more capillaries
Describe the Features of Fast-Twitch Fibres:
- fast rate of contraction
- mainly anaerobic respiration (glycolysis)
- fewer capillaries
- rapid, powerful contractions
