Lecture 2: Muscle Contraction Flashcards
What are the 3 muscle types?
- smooth muscle (involuntary control, autonomic nervous system)
- cardiac muscle (autonomic nervous system + circulating chemicals)
- skeletal muscle (voluntary control)
Skeletal muscles are…?
- under voluntary control from somatic nervous system
- usually attached to bones
- contract to bring about movement
Outline macroscopic structure of skeletal muscles.
bone –> tendon –> muscle surrounded by epimysium –> fascicles bounded by perimysium –> muscle fibre (myofibre) surrounded by endomysium
Outline microscopic structure of skeletal muscles
muscle fibre –> covered by plasma membrane called sarcolemma –> t-tubules tunnel into centre –> cytoplasm called sarcoplasm contains myoglobin + mitochondria –> network of fluid filled tubules = sarcoplasmic reticulum –> composed of myofibrils
Facts about myofibrils?
- 1 to 2 micrometres in diameter
- extend along entire length of myofibres
- composed of 2 main types of protein: ACTIN and MYOSIN
What gives skeletal muscle it’s striated (striped) appearance?
Myofilaments of light and dark bands
Don’t extend along length of myofibre
Overlap and are arranged in compartments called sarcomeres
What are the dense protein areas that separate sarcomeres called?
Z-discs
What are the dark bands called and what are they made of?
A bands (thick - made of myosin, also some overlapping actin)
What are the light bands called and what are they made of?
I band (thin - made of actin)
What do the myosin and actin filaments do?
They overlap
Structure of myosin?
- 2 globular heads
- single tail formed by 2 alpha helices
- tails of several hundred molecules form 1 filament
Structure of actin?
- actin molecules twisted into helix
- each molecule has myosin binding site
- filaments also contain troponin and tropomyosin
- troponin complex covers the binding site for myosin
What is the sliding filament theory of muscle contraction?
- during contraction I band became shorter (thin, actin band)
- A-band remained the same length (thick, myosin)
- H-zone narrowed or disappeared
What is the H-zone?
The middle of the A band where there is only myosin (thick filaments)
How is muscle contraction initiated (steps)?
1) AP opens VGCaCs
2) Ca2+ enters pre-synaptic terminal
3) Ca2+ triggers exocytosis of vesicles
4) ACh diffuses across cleft
5) Binds to ACh receptors, induces APs in muscle
6) local currents flow from depolarized region + adjacent region, AP spread along surface of muscle fibre membrane
7) ACh broken down by acetylcholine esterase - muscle fibre response to that molecules of ACh ceases
How is muscle contraction activated after the muscle has received an AP?
1) AP propagates along surface membrane into T-tubules
2) Dihydropyridine (DHP) receptor in T-tubule membrane: senses change in voltage and changes shape of protein linked to ryanodine receptor
3) opens ryanodine receptor Ca2+ channel in the sarcoplasmic reticulum (SR)
4) Ca released from SR into space around filaments
5) Ca binds to troponin and tropomyosin moves
6) allows crossbridges to attach to actin
7) Ca is actively transported into SR continuously while APs continue. ATP driven pump
Outline steps in excitation contraction coupling.
1) in presence of Ca –> movement of troponin from tropomyosin chain
2) exposes myosin binding site on surface of actin chain
3) ‘charged’ myosin heads bind to exposed site on actin filament
4) binding and discharge of ADP causes myosin head to pivot (POWER STROKE) –> pulls actin filament towards centre of sarcomere
5) ATP binding –> releases myosin head from actin chain
6) ATP hydrolysis –> provides energy to recharge myosin head
Which protein filament does the pulling during muscle contraction?
myosin
Which neurons control muscle contraction?
Voluntary neural control from upper motor neurons in brain and lower motor neurons in brainstem
What is a motor unit?
name given to single motor neuron together with all the muscle fibres that it innervates
SMALLEST FUNCTIONAL UNIT OF MUSCLE CONTRACTION
What does stimulation of 1 motor unit cause?
contraction of all muscle fibres in that unit
What are the 3 types of motor unit?
- slow (S, type I)
- fast, fatigue resistant (FR, type IIA)
- fast, fatiguable (FF, type IIB)
Key features of slow motor units?
- smallest diameter cell bodies
- small dendritic trees
- thinnest axons
- slowest conduction velocity
Key features of FR type IIA motor units?
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
Key features of FF type IIB motor units?
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
How are muscle fibre types distributed throughout muscle?
randomly distributed - muscles have diff proportions of slow + fast twitch muscles
What muscle fibre characteristic is associated with high amounts of S (type 1 motor units), high FR type IIA and low FF, type IIB?
myoglobin content
What colour is associated with each type of motor unit?
Slow - red
FR, type IIA - pink
FF, type IIB - white
What aerobic capacities are associated with each type of motor unit?
Slow - high
FR, type IIA - moderate
FR, type IIB - low
What anaerobic capacities are associated with each type of motor unit?
Slow - low
FR, type IIA - high
FR, type IIB - high
How are motor unit types classified?
By amount of tension generated, speed of contraction and fatiguability of motor neuron
Properties of Type I motor units?
- slow twitch
- low force
- fatigue resistant
Properties of Type IIA motor units?
- fast twitch
- moderate force
- fatigue resistant
Properties of Type IIB motor units?
- fast twitch
- high force
- high fatigue
Which 2 mechanisms does the brain use to regulate the force that a muscle can produce?
recruitment and rate coding
What is recruitment?
- motor units are not randomly recruited, there is an order
- governed by ‘size principle’ smaller units are recruited first - generally slow twitch units
- as more force is required, more units are recruited - allows fine control under which low force levels are required
What is rate coding?
- motor unit can fire at range of frequencies - slow units fire at a lower frequency
- as firing rate increases, force produced by unit increases
- summation occurs when units fire at frequency too fast to allow muscle to relax between arriving APs
How is muscle force regulated?
by the number of motor units recruited
What are the effects of neurotrophic factors?
- type of growth factor
- prevent neuronal death
- promote growth of neurons after injury
- motor unit + fibre characteristics dependent on nerve which innervates them
- if fast + slow twitch muscle are cross innervated, slow one becomes fast and vice versa
- motor neuron has some effects on properties of muscle fibres it innervates
What are the 3 main types of muscle contraction?
- concentric (muscle gets shorter)
- eccentric (muscle gets longer)
- isometric (muscle stays the same length)
What may cause muscle fibres to change from type IIB to IIA?
most common following training
What may cause type I to change to type II muscle fibres
possible in cases of severe deconditioning or spinal cord injury - microgravity during spaceflight results in shift from slow to fast muscle fibre types
What effect does ageing have on muscle fibre types?
- loss of type I and II fibres but also preferential loss of type II fibres
- results in larger proportion of type I fibres in aged muscle
