Muscle Structure And Adaptation Flashcards
What do muscles form from in the embryo?
→ Somites (paraxial mesoderm)
What does the sclerotome consist of?
→ Bone
→ Ribs
→ Cartilage
What is the myotome?
→ Muscle precursors
What is the syndetome?
→ Tendons
What does the notochord do?
→ Induces the mesoderm to become myogenic by inducing myogenic regulatory factors
What do paracrine factors induce?
→ Myf5 and MyoD (myogenic transcription factors)
Describe myogenesis
→ Notochord induces mesoderm to become myogenic
→ Paracrine factors induce Myf5 and MyoD which leads to myogenic commitment
→ myoblasts proliferate
→ Exit the cell cycle due to myogenin expression
→Structural proteins are expressed and myotubes form
→ Myotubes align and fuse parallel to each other
→ Biphasic muscle development : primary and secondary
→ Satellite cells : regeneration and post natal growth
What are the structural proteins expressed when myotubes form?
→ Actin and myosin
What can embryonic fibre number be affected by?
→ temperature
→ hormones
→ Nutrition
→ Innervation
What happens to muscle mass after birth?
→ Increase in fibre size (hypertrophy)
What are satellite cells?
→ Muscle stem cells
What are muscle stem cells?
→ Undifferentiated muscle precursors that are self-renewing
→return to quiescence when not needed
Why are muscle cells multinucleated?
→ They are big and long cells
→ huge production of structural proteins
→ this needs to occur along the length of the muscle fibre
What do myosin isoforms dictate?
→ Different chemomechanical transduction
→ ATP hydrolysis
→ Shortening velocity
What do troponin and tropomyosin isoforms dictate?
→ Determine sensitivity to Ca2+
What do titin isoforms dictate?
→ Elastic properties
What is an example of a type I fibre?
→ Back extensor muscle
What are type I fibres used for?
→ Posture
What are the properties of type I fibres?
→ High mitochondrial content - oxidative phosphorylation
→ Heavily vascularised
→ Sustained contraction
→ Abundant myoglobin
→small diameter
What are the properties of type II fibres?
→ High force contraction
→Fatigue easily
→ Less mitochondria - glycolytic respiration
→ Poorly vascularised
→large diameter
What ratio of muscle fibres do untrained individuals have?
→ 50:50 fast to slow
What ratio of muscle fibres do long distance runners have?
→ 60-70% slow
What ratio of muscle fibres do sprinters have?
→ 80% fast
What are properties of powerlifters muscles?
→ Hypertrophied
→ Highly glycolytic
→ Fatigue easily
→ High muscle to total body mass ratio
→ muscle size interferes with locomotion
What is the order that MHC can split ATP in from fastest to slowest?
→ 2B
→ 2A
→ 2X
→ 1
→ embryonic is the slowest
What is MHCexoc?
→ extraocular myosin heavy chain
→movements of the eyeball and upper eyelid
What does testosterone inhibit?
→ inhibits adipogenesis
What are the three adverse effects that synthetic anabolic steroids can have?
→ Increase BP
→ affect the heart
→ Shrinkage of testicles
What can be used for muscle mass loss?
→ testosterone
How does muscle recover from a minor injury such as a sprain?
→ Damaged muscle fibres necrose
→ There is an inflammatory response and neutrophils and macrophages respond to the damaged muscle fibre
→ There is an increase in satellite cell proliferation
→ they fuse with muscle fibres and regenerate them
How does muscle recover from laceration?
→ incomplete laceration
→ fibrotic tissue forms
What is sarcopenia?
→ age related loss of muscle mass
→Atrophy of muscle fibres- associated with type I fibres to express Type II phenotype
What is loss of muscle mass associated with?
gain in fat mass
→ decreased satellite cell number and recruitment
→Anabolic resistance – reduced protein synthesis in response to hormonal stimulation or resistance exercise
Which heavy chains do type I and type II fibres express?
→Slow twitch fibres (type I) express type I myosin heavy chain
→Fast twitch fibres (type II) express type II MHC
What type of fibres are present in the eye muscle(lateral rectus)?
mainly type II fast twitch
What types of fibres are present in calf muscle(gastrocnemius)?
mixed type I & II
What type of fibres are present in soleus(calf)?
more type 1 slow twitch
What does the motor unit consist of?
→a motor neuron and the set of muscle fibres within a muscle that it innervates
What is the range of motor unit sizes?
→10 to many 100s of muscle fibres
What is neurone pool?
→A muscle may be innervated by 10s to 100s of motor MNs
Describe incomplete tetanus
→the muscle goes through quick cycles of contraction with a short relaxation phase for each
What is complete tetanus?
→If the stimulus frequency is so high that the relaxation phase disappears completely, contractions become continuous
What happens during tetanus?
→the concentration of Ca++ ions in the sarcoplasm allows virtually all of the sarcomeres to form cross-bridges and shorten, so that a contraction can continue uninterrupted
Describe the difference in firing rates in fast and slow motor units
→Fast motor units need higher firing rates to generate tetanic forces than slow motor units
→Slow motor units are recruited first, followed by fast units for higher levels of force generation
What is the difference between spatial and temporal summation?
→Spatial summation occurs when multiple presynaptic neurones together release enough neurotransmitter to exceed the threshold of the postsynaptic neurone.
→Temporal summation occurs when one presynaptic neurone releases neurotransmitter many times over a period of time. Time dependent
What is muscle hyperplasia?
→Increase in muscle mass due to formation of new muscle fibres
What is the difference between hyperplasia and hypertrophy?
→Hyperplasia refers to the process where cells in an organ or tissue increase in number
→Hypertrophy is when these cells in an organ or tissue increase in size
How is smooth muscle tone achieved?
→Driven by reflex arcs from muscle spindles (sectioning dorsal roots abolishes resting tone)
→activation of a few motor units at a time
Compare hypertonia and hypotonia
→hypotonia- absence of the low-level contractions that lead to muscle tone
→hypertonia- excessive muscle tone. present with muscle rigidity (as seen in Parkinson’s disease)
