15.7 - Structure Of Skeletal Muscle Flashcards
What are muscles, and what is their primary function?
Muscles are effector organs that respond to nervous stimulation by contracting, enabling movement.
Name the three types of muscles and where they are found.
Cardiac muscle: Found exclusively in the heart.
Smooth muscle: Found in the walls of blood vessels and the gut.
Skeletal muscle: Makes up the bulk of body muscle, attached to bones, and operates under voluntary control.
Which types of muscles are under involuntary control?
Cardiac and smooth muscles operate involuntarily, and their contractions are largely unconscious.
What structural advantage do skeletal muscles have compared to individual cells joined end-to-end?
Skeletal muscle fibres are formed by the fusion of individual cells, preventing weak points and increasing contraction efficiency.
What is the sarcoplasm, and what does it contain?
The sarcoplasm is the cytoplasm of muscle fibres, found around their circumference. It contains large concentrations of mitochondria and endoplasmic reticulum.
How are skeletal muscles organized for maximum force?
They are composed of millions of myofibrils, arranged parallel to maximize strength, and bundled into progressively larger units.
What are the two types of protein filaments in myofibrils, and how do they differ?
1) Actin: Thin, with two strands twisted around one another.
2) Myosin: Thick, with long rod-shaped tails and bulbous heads projecting to the sides.
Why do myofibrils appear striped?
Due to alternating light (I bands) and dark (A bands) regions.
I bands: Light because thick and thin filaments do not overlap.
A bands: Dark because thick and thin filaments overlap.
Define the following terms
- H-zone
- Z-zone
- Sarcomere
H-zone: Lighter region in the center of the A band.
Z-line: Line at the center of the I band.
Sarcomere: Distance between adjacent Z-lines; the functional unit of muscle contraction.
What is the role of tropomyosin in muscle fibres?
Tropomyosin is a fibrous protein that winds around actin filaments, playing a role in muscle contraction.
What are the two types of muscle fibres?
Slow-twitch fibres: Adapted for endurance work and aerobic respiration.
Fast-twitch fibres: Adapted for short, intense bursts of activity and anaerobic respiration.
What adaptations do slow-twitch fibres have for endurance?
- Large store of myoglobin.
- Rich supply of blood vessels for oxygen and glucose.
- Numerous mitochondria for ATP production
What adaptations do fast-twitch fibres have for intense activity?
- Thicker and more numerous myosin filaments.
- High glycogen concentration.
- High concentration of enzymes for anaerobic respiration.
- Store of phosphocreatine for rapid ATP generation.
What is a neuromuscular junction?
The point where a motor neurone meets a skeletal muscle fibre, allowing rapid and coordinated contraction.
Why are there many neuromuscular junctions in a muscle?
To ensure simultaneous stimulation and contraction of muscle fibres for rapid, powerful movements.
What is a motor unit?
A group of muscle fibres supplied by a single motor neurone, acting together as a functional unit.
How is acetylcholine involved in neuromuscular junction function?
- Released from synaptic vesicles at the presynaptic membrane.
- Diffuses across the synaptic cleft.
- Binds to receptors on the muscle fibre membrane, increasing its sodium ion permeability and causing depolarisation.
How is overstimulation of a muscle prevented?
Acetylcholine is broken down by acetylcholinesterase into choline and ethanoic acid, which are reabsorbed and recombined in the neurone.
List similarities between neuromuscular junctions and cholinergic synapses.
- Both use neurotransmitter diffusion.
- Both have receptors that trigger sodium ion influx upon binding.
- Both repolarize via the sodium-potassium pump.
- Both use enzymes to break down neurotransmitters.
List key differences between neuromuscular junctions and cholinergic synapses.
- Neuromuscular junctions are only excitatory, while cholinergic synapses can be excitatory or inhibitory.
- Neuromuscular junctions link neurones to muscles; cholinergic synapses link neurones to neurones or other effectors.
- Action potentials end at neuromuscular junctions but may propagate in cholinergic synapses.
- Neuromuscular junctions involve only motor neurones, whereas cholinergic synapses can involve sensory, motor, and intermediate neurones.
- Acetylcholine binds to muscle fibre receptors in neuromuscular junctions, while it binds to postsynaptic membrane receptors in cholinergic synapses.
