5.3: Muscle microstructure and contraction Flashcards
3 muscle types
Smooth muscle
Cardiac muscle
Skeletal muscle
Control of smooth muscle
Under involuntary control from autonomic nervous system
Control of cardiac muscle
Can Contract autonomously
Under influence of autonomic nervous system and Circulating chemicals
Control of skeletal muscles
Under voluntary control, usually attached to bones and contract to bring about movement
Structure of skeletal muscles from macroscopic to microscopic
Bone
Tendon
Muscle
Fascicles
Myofibre
Myofibril
Myofilaments
Fascicles are
Bundles of muscle fibres (myofibres)
structure of myofibres
Covered by plasma membrane- sarcolemma
T-tubes tunnel into centre
Sarcoplasm - myoglobin and mitochondria present
Sarcoplasmic reticulum
Composed of myofibrils
What two proteins make up skeletal muscle myofibrils
Actin and myosin
Characteristics of myofilaments
Light and dark bands - striated appearance
Do not extend along length of myofibres
Overlap and arranged in sarcomeres
Structure of myofilaments
Dense protein Z discs separate sarcomeres
Dark band- A band (thick myosin)
Light band - I band (thick - actin)
Myosin and actin filaments overlap
Structure of myosin
Two globular heads
Single tail formed by two alpha helices
Tails of 100s molecules form one filament
Structure of actin
Twisted into helix
Each molecule has myosin binding site - exposed as Ca binds
Filaments contain tropomyosin and Troponin
Sliding filament theory
During contraction:
I band shortens
A band remains the same
H-zone narrows
Initiation of muscle contraction
1) Action potential opens voltage gated Ca2+ Channels
2) Ca enters pre-synaptic terminal
3) Ca triggers exocytosis of vesicles
4) ACH diffuses across synaptic cleft and binds to ACH receptors , inducing action potential in muscle
5) neurone depolarised, AP spreads along surface of muscle fibre membrane
6) ACH broken down by acetylcholine esterase - causes muscle fibre to cease
Activation of muscle contraction
1) AP spreads along muscle sarcolemma and into T-tubules
2) Dihydropyridine receptor in T-tubule membrane, senses voltage and changes shape of protein linked to ryanodine receptor
3) opens ryanodine receptor Ca2+ channel in SR, resleasing Ca2+ into space around filaments
4) Ca2+ binds to troponin and tropomyosin moves
5) actin myosin cross bridges form
5) Ca2+ actively transported into SR continuously, while AP continue . ATP driven pump
Excitation contraction coupling
1) in presence of Ca2+ troponin moves from tropomyosin chain
2) movement exposes myosin binding site on actin filament
3) charges myosin heads bind
4) power stroke : discharge of ADP causing myosin head to cock, pulling actin filament towards centre of sarcomeres
5) ATP binding releases myosin head from actin chain
6) ATP hydrolysis provides energy to recharge myosin head
What protein filament does the pulling during muscle contraction
Myosin
Neural control of muscle contraction
Upper motor neurons in brain
Lower motor neurons in brain stem or spinal cord
Voluntary neural control from upper and lower motor neurons
What is the name for a single motor neurone together with all muscle fibres that it innervates
Motor unit
Stimulation of one motor unit causes
Contraction of all muscle fibres in that unit
How many muscle fibres does each motor neuron supply
600
How many motor neurones do humans have
420,000 motor neurones
250 million skeletal muscle fibres
3 types of motor unit
Slow (S, type I)
Fast, fatigue resistant (FR, type IIA)
Fast fatiguable (FF, type IIB)
Characteristics of Slow motor unit
Smallest diameter cell bodies
Small dendritic trees
Thinnest axons
Slowest conduction velocity
Characteristics of fast motor unit
Larger diameter cell bodies
Larger dendritic trees
Thicker axons
Faster conduction velocity
Distribution of muscle fibre types
Randomly distributed throughout muscle
Muscles have different proportions of slow and fast twitch muscles
What are motor unit types classified by
Amount of tension generated, speed of contraction and fatiguability of motor unit
Two mechanisms by which brain regulates force a single muscle can produce
Recruitment and rate coding
In muscle recruitment
Motor units recruited in order
Governed by size principle, smaller first
More force required more units recruited, allowing fine control under which low force levels are required
In muscle Rate cording
Motor unit can fire at range of frequencies, slow units fire at lower frequency
Firing rate increase force produced by unit increases
Summation occurs as units fire at frequency too fast to allow muscle to relax between arriving APs
Neurotrophic factors are
Type of growth factor
2 functions of neurotrophic factors
Prevent neuronal death
Promote growth of neurons after injury
What are motor unit and fibre characteristics dependent on
Nerve that innervates them
What occurs if a fast and slow twitch muscle are cross innervated
Slow muscle becomes fast and vice versa
What does the motor neuron have some effect on
Properties of muscle fibres it innervates
3 types of muscle contraction
Concentric
Eccentric
Isometric
In isometric contraction
Muscle does not change length during contraction
In concentric contraction
Contraction when muscle shortens
In eccentric contraction
Contraction where muscle is longer
What can change properties of muscle fibres
Training, type IIB and IIA most commonly
What can change slow muscle into fast muscle
NOT training !
Unless severe reconditioning or spinal cord injury
Microgravity from space results in shift of muscle types
What effect does aging have on muscle types
Loss of type I and II fibres
Preferential loss of type II fibres
Results in larger proportion of type I fibres in aged muscle - evidence from slower contraction times
