Skeletal and Smooth Muscle

Function - transform chemical energy to mechanical energy

• Skeletal
• Smooth
• Cardiac

Thick myosin filament

Thin actin filament

On contraction:

• Sarcomere shortened
• H zone disappear
• I band narrows

• Sarcomere - basic muscle unit, can shorten/elongate
• Sarcomere defined by 2 Z discs/lines and contains actin and myosin
• Overlapping actin and myosin slide in and out
• Myosin pulls actin from both sides 
• Amount tension developed is proportional to overlap of actin and myosin

• ATP hydrolysis energises myosin head upwards
• Cross bridge formation occurs
• ADP dissociates from myosin
• Bond between actin and myosin strengthens
• Power stroke - myosin moves actin forward
• ATP binds to myosin head
• Cross bridge dissociation

• Motor unit - group of skeletal muscle cells activated by 1 alpha motor neurone
• Equal amount of force produced by each muscle cell
• Relative muscle contraction force dependent on number of motor units activated

Muscle twitch - muscle contraction and relaxation

Summation - second stimulus applied before previous relaxation is complete

Tetanus - multiple stimuli given before muscle relaxation

Maximum contraction force - force observed during tetanus phase

Isotonic contraction - force by changing muscle length

Concentric contraction - force by shortening muscle length

Eccentric comtraction - force by elongating muscle length

Isometric contraction - force without changing length of muscle e.g. withstand heavy objects

Short-duration Excersise

• ATP stored in muscle used first
• ATP formed by direct phosphorylation - ADP + Pi
• Glycogen stored broken down to glucose - oxidation to ATP in anaerobic pathway

Prolonged-duration Excersise

• ATP generated by breakdown of several nutrient energy fuels by aerobic pathways
• Muscles can fatigue - tetanus tapers off and rest period occurs
• Longer period of exercise = shorter tetanus length

Low Ca2+ concentration

• 2 out of 4 troponin sites occupied
• Troponin inhibits myosin head binding

High Ca2+ concentration -

• All 4 troponin sites occupied
• Troponin changes conformation
• Tropomyosin slides away
• Myosin head binds to actin
• Cross-bridge formation

• Action potential arrives at neuromuscular junction
• Voltage-gated Ca2+ channels open
• Acetylcholine released from presynaptic membrane
• Nicotonic receptor activated
• Postsynaptic membrane depolarises
• Muscle action potential spreads along sarcolemma
• Action potential invades T tubules
• Depolarisation releases Ca2+ from sarcoplasmic reticulum
• Ca2+ binds to tropnin complex
• Cross-bridge cycle inhitiated

• Spindle-like structure
• Each cell connected to single autonomic nerve fibre
• Non-overlapping actin filaments - network of cells

• Ca2+ ions enter cytosol from outside cell or SR
• Ca2+ binds to and activates calmodulin
• Activated calmodulin activates myosin light chain kinase enzymes
• Enzyme catalyses phosphate transfer to myosin heads
• Myosin head ATPases activated

• No troponin in smooth muscle - myosin head activation by phosphorylation
• Cross bridge activity ends when phosphate is removed by phosphorylase enzymes and Ca2+ levels fall