Muscle and function Flashcards
Describe different types of muscle fibres, their means of contraction, and the trophic effects of nerves on muscles
Three types of skeletal muscle fibres:
- Type 1: slow twitch -> very fatigue resistant and capable of producing repeated, low-level contractions. Produces large amounts of ATP through aerobic metabolism.
- Type 2a: fast twitch oxidative -> produce fast, strong muscle contractions but prone to fatigue. Produces ATP at a fast rate via anaerobic and aerobic metabolism.
- Type 2b: Fast twitch glycolytic -> Produce short bursts of power but fatigue quickly. Produces ATP slowly via anaerobic metabolism
Describe skeletal muscle type’s structural features
Cell length: 1mm - 1m
Cell diameter: 10-100 um
Long parallel cylinders, multiple peripheral nuclei, striations.
CT components: epimysium, perimysium, and endomysium
t-tubules: triad with 2 terminal cisternae, 2 per sarcomere.
No cell-cell junctions
Describe cardiac muscle’s structural features
Cell length: 50-100 um
Cell diameter: 10-20 um
Short branched cylinders, single central nucleus and striations.
Only has endomysium for CT.
T-tubules is a diad with small terminal cisternae with 1 per sarcomere. Cell-cell junctions are intercalated discs
Describe smooth muscle’s structural features
Cell length: 50-100 um,
cell diameter: 5-10 um
Spindle-shaped, tapering ends, single central nucleus
CT: endomysium, with sheaths
No T-tubules, but well developed sarcoplasmic reticulum.
Gap-junctions present
Describe skeletal muscle’s functions
- Voluntary innervation
- SOMATIC efferent innervation
- Type 1 and 2 fibres contraction
- Regulation of contraction: Calcium causes tropomyosin movement and exposes myosin-binding sites on actin
Describe cardiac muscle’s functions
- involuntary innervation
- Autonomic nervous system innervation
- Rhythmic contraction, via conductive system
- Regulation of contraction: calcium causes tropomyosin movement and exposes myosin-binding sites on actin
Describe smooth muscle’s functions
- Involuntary innervation
- autonomic nervous system innervation
- Slow, partial, rhythmic and spontaneous contraction
- Regulation of contraction: by phosphorylation of myosin light chain kinase in presence of calcium - calmodulin
Describe skeletal muscle’s growth and regeneration
No mitosis.
Hypertrophy is response to demand
Limited regeneration
Describe cardiac muscle’s growth and regeneration
No mitosis in normal conditions.
Hypertrophies in response to demand.
No regeneration in normal conditions.
Describe smooth muscle’s growth and regeneration
Mitosis is present.
Hypertrophy and hyperplasia in response to demand.
Regeneration is present.
Describe organization of skeletal muscles (smallest to largest):
- Sarcomere: actin and myosin arranged
- Myofibril: sarcomeres lined up in a line
- Fibre (muscle cell): bundles of myofilament in ONE muscle cell
- Fascicles: Bundles of fibres
- Muscle: bundles of fascicles
Describe the different types of muscle connective tissues
- Epimysium -> dense irregular connective tissue sheath around entire muscle
- Perimysium -> Septa of epimysium that separates muscle into fascicles
- Endomysium -> loose connective tissue around each muscle fibre (muscle cell)
List the components of muscle fibres
- Sarcoplasm -> Cytoplasm of muscle fibres
- Sarcolemma -> Plasma membrane of muscle cells
- Sarcoplasmic reticulum (sER) -> networks around myofibrils; stores contain calcium
- T-tubules -> Invaginations of sarcolemma; transmit action potential deep into muscle
- Terminal cisterna -> sER rings adjacent to T-tubules; storages of calcium inside the muscle fibre
Describe the sarcomere (contractile unit)
- Thin actin filaments joined together at z-lines
- Thick myosin filaments in between actin filaments
- During contraction, myosin filaments in between actin filaments to bring z-lines closer
Describe the motor unit
A single motor neuron and all the skeletal muscle fibres that are innervated by its axonal terminals.
- Action potential transfer across neuromuscular junction:
1. Motor neuron axon terminal releases acetylcholine (ACh)
2. ACh diffuses across synapse
3. ACh binds with nicotinic receptors on muscle membrane
4. Muscle membrane depolarizes
Describe contractile proteins
Actinomyosin -> complex of parallel threads of actin and myosin that slide past each other during contraction
Describe regulatory proteins:
- Tropomyosin -> protein that coils around actin filament and covers myosin binding sites.
- Troponin -> protein that binds tropomyosin
- When Calcium binds to troponin, the tropomyosin’s conformation changes and myosin can bind with actin
Describe cytoskeletal proteins:
- Titin -> very large protein responsible for the passive elasticity of muscles; connects M-disk to Z-line and recoils muscles after stretching and prevents overstretching
- Desmin -> Protein of a specific type of intermediate filament that regulates architecture of sarcomeres
Describe the cross-bridge cycle
- Cross-bridge cycle -> repeated cycle of myosin heads on thick filaments binding with thin filaments and producing force.
- Steps:
1. calcium binds to troponin, tropomyosin changes shape, and exposes myosin binding site
2. Myosin head binds to actin; ADP released
3. Power stroke pulls z-line together
4. ATP binds to myosin; myosin head releases actin
5. ATP -> ADP causes myosin to reset conformation
6. Go to step 2
Describe pathology of cross-bridges
- Mutations in contractile proteins are often lethal or lead to muscle damage
- Hypertrophic cardiomyopathy - faulty contractile proteins in heart lead to enlargement of heart muscles to compensate
Describe Excitation-contraction coupling
Excitation-contraction coupling -> link between the action potential of the sarcoplasm to the calcium release and muscle contraction by the muscle fibre.
Steps:
- Action potential propagates along sarcoplasm and into T-tubules
- Action potential depolarizes voltage sensory on T-tubules
- Voltage sensors on T-tubules are coupled with adjacent calcium channels on the sarcoplasmic reticulum
- Calcium channels release calcium into sarcoplasm
Define ‘foot process’
T-tubule voltage sensor and adjacent sarcoplasmic reticulum calcium channel
Describe myotonia
Myotonia: failure to relax at the end of voluntary contraction.
- Abnormal ion channels cause hyperexcitability
- Voluntary contraction leads to extended forces
Describe myasthenia gravis
Autoimmune disease leading to weakening of voluntary muscle contraction
- Antibodies block nicotinic ACh receptors on muscle end plate
- Muscle depolarization due to single ACh vesicle release decrease
- AP doesn’t initiate on muscle
