Flashcards in Session 8 Deck (55):
What are the 3 types of muscle?
Which types of muscle are striated/non-striated?
- Striated: skeletal; cardiac
- Non-striated: smooth
What is the differences in the types of muscle in terms of morphology?
- Skeletal: Long parallel cylinders; multiple peripheral nuclei; strait ions
- Cardiac: Short, branched cylinders; single (or 2) central nucleus; striations
- Smooth: Spindle-shaped; tapering ends; single central nucleus; no striations
What are the differences in the types of muscle in terms of connections?
- Skeletal: Fasicle bundles; tendons
- Cardiac: Junctions join cells end to end
- Smooth: Connective tissue; gap and desmosome-type junctions
What are the differences in the types of muscle in terms of control?
- Skeletal: Somatic motor neurone (voluntary control)
- Cardiac: Autonomic modulation (involuntary control); intrinsic rhythm
- Smooth: Autonomic (involuntary); intrinsic activity; local stimuli
What difference are there in the types of muscle in terms of power?
- Skeletal: Rapid; forceful
- Cardiac: Lifelong variable rhythm
- Smooth: Slow, sustained and rhythmic
How does skeletal muscle develop?
- Myoblasts develop from multipotent myogenic stem cells from the mesoderm
- Myoblasts fuse to form a primary myotube with a chain of multiple central nuclei
- Centrally positioned nuclei are displaced to the cell periphery by newly synthesised actin and myosin microfilaments
What types of skeletal muscle fibres are there?
What are some differences between red and white muscle fibres?
- Diameter: Red-smaller; white-larger
- Vascularisation: Red-rich; white-poor
- Mitochondria: Red-numerous; white-few
- Contractions: Red-slow, repetitive, weak; white-faster, stronger
- Fatigue: Red-slowly; white-rapidly
What are the different layers of connective tissue surrounding muscle?
- Endomysium (surrounds a cell/fibre)
- Perimysium (surrounds a fasicle)
- Epimysium (surrounds whole muscle)
What does nuclei of skeletal muscle look like in transverse and longitudinal sections?
- Transverse: Peripheral
- Longitudinal: Rows
What do muscle fibres contain?
- Myofibrils (made up of actin and myosin microfilaments)
What is the thin filament in a skeletal muscle cell?
What is the thick filament in skeletal muscle cells?
Describe the structure of a sarcoma re
- M line is within the H band, which is within the A band
- Z line is within the I band
- H band contains only myosin
- A band is the length of the myosin including overlapping actin
- I band is only actin
What is the thin myofilament made up of?
Describe the binding of the Troponin molecule in actin
- Has 3 binding sites
- TnI to actin
- TnC to calcium
- TnT with tropomysosin
Describe the structure of thick filaments
- Each filament contains many myosin molecules
- Mysoin is a rod-like structure from which 2 heads protrude
Describe the structure of thin filaments
- Actin filament forms a helix
- Tropomysosin molecules coil around to reinforce
- Troponin complex is attached to each Tropomysosin molecule
What is the role of ionic calcium in contraction?
- Ionic calcium binds to TnC of Troponin
- Causes a conformational change
- Moves Tropomysosin away from actin binding sites
- Myosin heads can now bind to actin and begin contraction
What are the stages of contraction?
- Stage 1 Attachment: Mysoin head is tightly bound to actin molecule
- Stage 2 Release: ATP binds to the myosin head, causing it to uncouple from the actin filament
- Stage 3 Bending: Hydrolysis of the ATP causes uncoupled myosin head to bend and advance a short distance (5nm)
- Stage 4 Force Generation: Myosin head binds weakly to actin causing release of inorganic phosphate which strengthens binding, causes power stroke which returns myosin head to former position
- Stage 5 Reattachment: ATP binds to myosin head causing detachment from actin
What happens at a neuromuscular junction?
- Small terminal swelling of the axon that contains vesicles of acetylcholine
- Nerve impulses cause the release of acetylcholine which binds to receptors on the Sarcolemma
- This initiates an action potential which propagates along the muscle
What are the stages leading to contraction of skeletal muscle?
- Initiation: nerve impulse along motor neuron axon arrives at neuromuscular junction
- Impulse prompts release of acetylcholine (Ach) into synaptic cleft causing local depolarisation of the sarcolemma
- Voltage gated Na+ channels open allowing Na+ into cell
- General depolarisation spreads over sarcolemma and into T tubules
- Voltage sensor proteins of the T tubule membrane changes their conformation
- Gated Ca2+ release channels of adjacent terminal cisternae are activated by this conformation change
- Ca2+ is rapidly released from the terminal cisternae into the sarcoplasm
- Ca2+ binds to the TnC subunit of Troponin
- The contraction cycle is initiated and Ca2+ returns to the terminal cisternae of the sarcoplasmic reticulum
What are the distinguishing features of cardiac muscle?
- Centrally positioned nuclei (1/2 per cell)
- Intercalated discs (for electrical and mechanical coupling with adjacent cells)
- Adherens-type junctions (anchor cells and actin)
- Gap junctions (electrical coupling)
What is the difference between skeletal and cardiac muscle in terms of Myofibrils?
- Skeletal: Distinct myofibrils
- Cardiac: Myofibrils are absent, instead the myofilaments actin and myosin form continuous masses in the cytoplasm
Where do the T tubules lie in skeletal and cardiac muscle?
- Skeletal: junction of A and I bands
- Cardiac: Z line
Do skeletal and cardiac muscle have triads or dials of T tubule and sarcoplasmic reticulum?
- Skeletal: Triads
- Cardiac: Diads
What is the structure of Purkinje fibres?
- Large cells with:
~ Abundant glycogen
~ Sparse myofilaments
~ Extensive gap junction sites
What is the function of Purkinje fibres?
- Transmit action potentials to the ventricles from the atrioventricular node
- Conduct action potentials rapidly compared to regular cardiac muscle
- Allows ventricles to contract synchronously
What are the features of smooth muscle?
- Spindle-shaped (fusion) with central nucleus
- Not striated, no star omers or T tubules
- Contraction still relies on actin-myosin interactions
- Contraction is slower, more sustained and requires less ATP
- May stay contracted for days
- Capable of being stretched
- Responds to stimuli from nerve signals, hormones, drugs or local concentrations of blood gases
- Forms sheets, bundles or layers containing thousands of cells
- Thick and thin filaments are arranged diagonally within the cell, spiralling so that smooth muscle contracts in a twisting way
Can skeletal muscle repair itself?
- Cells cannot divide
- Tissue can regenerate by mitosis activity of satellite cells, so that hyperplasia follows muscle injury
- Satellite cells can also fuse with existing muscle cells to increase mass (skeletal muscle hypertrophy)
Can cardiac muscle repair itself?
- Incapable of regeneration
- Fibroblasts invade, divide and lay down scar tissue following damage
Can smooth muscle repair itself?
- Cells retain mitosis ability and can form new smooth muscle cells
- Particularly evident in pregnant uterus where muscle wall becomes thicker by hypertrophy (swelling) and hyperplasia (mitosis) of individual cells
Describe the remodelling of muscles
- Is continual
- Contractile proteins are replaced in 2 weeks
- Atophy: muscle wastes away when destruction of proteins is more than replacement
- Hypertrophy: muscle cells increase in size when replacement of proteins is more than destruction
What is the difference between hypertrophy and hyperplasia?
- Hypertrophy: increase in cell size
- Hyperplasia: increase in cell numbers
What is the effect of exercise on skeletal muscle?
- Sarcoplasmic reticulum swells
- Increased volume of mitochondria
- Increased z band width
- Increased ATP synthase
- Increased density of T tubule systems
- Increase in number of contractile proteins
- Little evidence for hyperplasia
How does high resistance exercise affect skeletal muscles?
- Stimulates contractile protein synthesis, fatter muscle fibres, larger muscle
- Increased muscle mass and strength and may lead to hypertrophy with the help of myosatellite cells
How does endurance exercise affect skeletal muscle?
- Increased endurance without hypertrophy
- Stimulates synthesis of mitochondrial proteins, vascular changes allowing for greater oxygen utilisation, shift to oxidative metabolism of lipids
What is disuse atrophy?
- Occur with bed rest, limb immobilisation, sedentary behaviour
- Causes loss of protein, which leads to reduced fibre diameter, which leads to loss of power
What happens to skeletal muscles as we age?
- Atrophy with age from age 30 onwards
- Loss of 50% muscle mass by 80 (sarcopenia)
What is denervation atrophy?
- Also called neurogenic muscular atrophy
- Muscle no longer receives contractile signals that are required to maintain normal size
- Signs of lower motor neuron lesions: weakness, flaccidity, muscle atrophy with spontaneous twitching, degeneration 10-14 days after injury
- Innervation past 3 months has a low chance of recovery, not possible after 2 years
- Muscle fibres are replaced by fibrous and fatty tissue
- Fibrous tissue leads to contractures and as muscle shortens leads to debilitating/disfiguring contractures (needs daily stretching)
How can muscle length be adjusted?
- Sustained stretching
- Addition of sacromeres; changes in neurology (pain, stretch receptors and stretch reflex); viscose lactic properties (connective tissue alignment)
- Reduced muscle length if immobilised
What stops acetylcholine?
- At high motor neuron firing rates, ACh release decreases
- Only 25% of Ach receptors need to be occupied for an action potential to be triggered
What is myasthenia gravis?
- Autoimmune destruction of end-plate ACh receptors
- Loss of junctional folds at end-plate
- Widening of synaptic cleft
- Crisis point when it affects respiratory muscles
What are the symptoms of myasthenia gravis?
- Fatigability and sudden falling due to reduced ACh release
- Drooping upper eyelids
- Double Vision
- Affected by General state of health, fatigue and emotion, symptoms fluctuate
What is the treatment for myasthenia gravis?
- Acetylcholine inhibitors eg pyridostigmine
- Immune suppressants
- Plasmapheresis: removal of harmful antibodies from patients serum
- Ice on eyelids decreases Acetylcholinesterase activity
How is neuromuscular transmission disrupted in botulism poisoning?
- Toxins block ACh release
(Botox cosmetic treatment)
How is neuromuscular transmission disrupted in organophosphate poisoning?
- Irreversibly inhibits acetycholinesterase
- ACh remains in receptors and muscles stay contracted
What are muscular dystrophies?
- Genetic faults that cause the absence or reduced synthesis of specific proteins that normally anchor actin filaments to the sarcolemma
- in absence causes muscle fibres cells can tear themselves apart when contracting
What causes Duchenne muscular dystrophy?
- There is a complete lack of dystrophin
- Muscle fibres tear themselves apart on contraction
- Enzyme creatine phosphokinase liberated into serum
- Calcium enters cell causing necrosis
- Pseudohypertrophy (swelling) before fat and connective tissue replace muscle tissue
What are the signs and symptoms of Duchenne muscular dystrophy?
- Early onset - Gower's sign (hands on knees to generate strength)
- Contractures (imbalance between agonist and antagonist muscle)
- Steroid therapy (prenisolone)
- Ataluren drug trials in humans, ribosomal interaction to produce dystrophin
What is malignant hyperthermia?
- A rare, autosomal dominant disorder
- Causes a life threatening reaction to certain drugs used for general anaesthesia
How do general anaesthetic drugs work normally?
- Are volatile anaesthetic agents and the neuromuscular blocking agent succinylcholine
- Succinylcholine inhibits action of ACh, acting non-competitively on muscle-type nicotinic receptors
- Is degraded by butyrylcholinesterase much more slowly than the degradation of ACh by Acetylcholinesterase
How do anaesthetic drugs cause malignant hyperthermia?
- In susceptible individuals, drugs can induce a drastic and uncontrolled increase in skeletal muscle oxidative metabolism
- Overwhelms body's capacity to supply O2, remove CO2 and regulate body metabolism
- Leads to circulatory collapse and death if not treated quickly