Muscle function Flashcards
What are the 3 types of muscles?
Smooth muscle: involuntary control from autonomic nervous system
Cardiac muscle: can contract autonomously. Under control of ANS and circulating chemicals
Striated muscle: under voluntary control, usually attached to bones
What is the macrostructure of muscles?
Bone -> tendon -> muscle surrounded by epimysium -> fascicles bound by perimysium -> muscle fibre (myofibre) surrounded my endomyseum
What is the microstructure of muscles?
Muscle fibre -> covered by membrane called sarcolemma -> t-tubules tunnel into centre -> cytoplasm called sarcoplasm contains myoglobin and mitochondria -> network of fluid filled tubules called sarcoplasmic reticulum -> compound of myofibrils
What are myofibrils?
1-2 micrometres in diameter
Extend along entire length of myofibres
What are the 2 types of myofibrils?
Actin: molecules twisted into helix, each with myosin binding site. Also have troponin and tropomyosin. Troponin is binding site for myosin
Myosin: 2 globular heads, single tail formed by 2 alpha helices, tails of several hundred molecules form 1 filament
What are myofilaments?
Skeletal muscles is striated/ striped
There are myofilaments of light and dark bands
They don’t extend along the length of the myofibre
They overlap and are arranged in compartments called sarcomeres
What are the types of myofilaments?
Z- Disks: dense protein areas that separate sarcomeres (1 sarcomere is between 2 Z discs)
A bands: dark bands, thick, made of myosin, also some overlapping actin
I bands: light bands, thin, made of actin
H Zone: middle of A band where theres only myosin
M line: middle of H zone and middle of sarcomere
What is the sliding filament theory?
I band becomes shorter
A band remains the same length
H zone narrows or disappears
How is muscle contraction initiated?
- AP opens VGCCs
- Ca2+ enters pre-synaptic terminal
- Ca2+ triggers exocytosis of vesicles
- ACh diffuses across cleft
- ACh binds to ACh receptor and induces AP in muscle
- Local currents flow from depolarised region to adjacent region, AP spreads along surface of muscle fibre membrane
How is muscle contraction activated after initiation?
- AP propagates along surface membrane into t-tubules
- DHP (dihydropyridine) in t-tubule membrane senses change in voltage and changes shape of protein linked to ryanodine receptor
- Opens ryanodine receptor Ca2+ channel in SR
- Ca2+ is released from SR into space around filaments
- Ca2+ binds to troponin and tropomyosin moves to expose actin binding site
- Allows cross bridges of myosin to attach to actin
- Ca2+ actively transported into SR continuously whilst APs continue- ATP driven pump
What happens in excitation-contraction coupling?
- In presence on Ca2+, movement of troponin by tropomyosin
- Exposed myosin binding site on surface of actin chain
- Charged myosin head binds to exposed site on actin filament
- Binding and discharge of ADP causes myosin head to pivot (power stroke). This pulls actin filament towards centre of sarcomere
- ATP binding- released myosin head from actin chain
- ATP hydrolysis- provides energy to recharge myosin head
What are the 3 types of muscle contraction?
Concentric- muscle gets shorter whilst producing force- happens when you’re raising weight during bicep curl
Eccentric- muscle gets longer whilst producing force
Isometric- muscle stays same length
What is a motor unit?
Single motor neurone and all the muscle fibres it innervates
Stimulation causes contraction of all its fibres
Only 1 motor unit can innervate a given muscle fibre
How do you increase the specificity of a contraction?
The higher the innervation ratio (no. of fibres innervated by a single motor neurone) the more specific the contraction
What are the difference types of motor units?
Slow motor unit (S, type I)
Fast fatigue resistant motor unit (type IIA)
Fast fatiguable motor unit (type IIB)
How is the slow motor unit structured and what are features of them?
Small diameter cell bodies
Small dendritic trees
Thinnest axons
Slowest conduction velocity
How is the fast motor unit structured?
Large diameter cell bodies
Large dendritic trees
Thicker axons
Faster conduction velocity
What are features of the slow motor unit?
High myoglobin content
Red in colour
High aerobic capacity,
Low anaerobic capacity
What are features of the fast fatigue resistant motor unit?
High myoglobin content
Pink in colour
Moderate aerobic capacity
High anaerobic capacity
What are features of fast fatiguable motor unit?
Low myoglobin content
White in colour
Low aerobic capacity
High anaerobic capacity
What kind of twitch fibres are found in muscles?
Muscle fibres are randomly distributed with different proportions of fast and slow twitch fibres
Slow twitch fibres contract slowly but keep going for a long time
Fast twitch fibres contract quickly but fatigue quickly
How is muscle force regulated?
Muscle uses recruitment and rate encoding to regulate force that muscle can produce
What is the effect of recruitment?
Allows fine control (low recruitment) as well as power (high recruitment)
Governed by size principe- smaller units recruited first (generally slow twitch). As more force is required, more units are recruited
What is the effect of rate coding?
Higher firing rate= more power
Motor units can fire at range of frequencies- slow units fire at lower frequency
Motor neurone firing rate increases leading to increase in force by motor unit
Summation happens when units fire too fast to relax in between arriving APs
What is the effect of neurotrophic factors?
Neurotrophic factors = type of growth factor
Prevent neuronal death and promote growth of neurones after injury
Controls motor unit and fibre characteristic
How does the plasticity of muscle fibres change?
Training: muscle fibres can change from type IIB to type IIA following training
Deconditioning/ spinal cord injury: changes from type I to type II may be possible with sever deconditioning or spinal cord injury
Ageing: loss of type I and II fibres but preferential loss of type II
