15.7: Structure of skeletal muscle Flashcards
Muscles
Effector organs that respond to nervous stimulation by contracting and so bring about movement.
Three types of muscle
smooth, cardiac and skeletal
Cardiac muscle
In the heart
Smooth muscle
In the walls of blood vessels and the gut
Skeletal muscle
Makes up the bulk of the body in vertebrates, attached to the bone, under conscious control.
Which types of muscle aren’t under conscious control
cardiac and smooth
Myofibrils
Tiny muscle fibres that make up individual muscle and are extremely powerful collectively.
Sarcoplasm
Nuclei and cytoplasm which are shared by muscle fibres.
Muscle fibres made from
separate muscle cells fused together
Where is the sarcoplasm mostly found
around the circumference of the fibre and contains large conc.s of mitochondria and ER
Myofibrils are made up of two types of protein filament
actin, myosin
Actin
Thinner and consists of two strands twisted around one another
Myosin
Thicker and consists of long rod shaped tails with bulbous heads that project to the side
Why do mircofibrils appear striped
due to alternating light and dark coloured bands
Light bands
I bands,
appear lighter because the thick and thin filaments don’t overlap in this region
Dark bands
A bands,
appear darker because the thick and the thin filaments overlap in this region
H-zone
A lighter-coloured region at the centre of each A-band.
Z-line
A line at the centre of each I band.
Sacromere
Distance between adjacent z-lines. When a muscle contracts, sacromeres shorten and the pattern of bands changes.
Tropomyosin
An important protein found in muscle, which forms a fibrous strand around the actin filament.
Two types of muscle fibre
Slow twitch and fast twitch
Slow-twitch muscle fibres
Contract more slowly than fast-twitch and provide less powerful contractions but over a longer period of time. Adapted to endurance work and for aerobic respiration to avoid lactic acid build up.
Fast-twitch muscle fibres
Contract more rapidly and produce powerful contractions but only for a short period. Adapted to intense exercise.
Adaptations of fast-twitch muscles
4
- Thicker and more myosin filaments
- High conc. of glycogen
- High conc. of enzymes involved in anaerobic respiration which provides ATP rapidly
- store of phosphocreatine, so provide energy for muscular contractions
Phosphocreatine
A molecule that rapidly generate ATP from ADP in anaerobic conditions and so provides for muscular contractions.
(A store in fast-twitch muscle fibres)
Neuromuscular junctions
Point where a motor neurone meets a skeletal muscle fibre (many junctions along a muscle)
Motor unit
All muscle fibres supplied by a single unit act together as a single functional unit.
Motor unit
Role in muscle exertion
Gives control over the force the muscle exerts, the intensity of the force needed- number of units stimulated.
What happens when a nerve impulse is received at the neuromuscular junction?
Synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine.
Acetylcholine then diffuses to the postsynaptic membrane and alters its permeability to Na+ ions which enter rapidly and depolarise the membrane.
At neuromuscular junction how it ensured the muscle isn’t overstimulated?
Acetylcholine is broken down by acteylcholinesterase to ethanoic acid and choline which then diffuse back into the neurone, where energy from mitochondria recombine acetylcholine.
Similarities between neuromuscular junctions and a synapse?
4
- have neurotransmitters that are transported by diffusion
- have receptors that cause an influx of Na+ ions when bonded to neurotransmitter
- use a Na-K pump to repolarise the axon
- use enzymes to break down the neurotransmitter
Difference between neuromuscular junction and a cholinergic synapse
Excitatory/inhibitory?
neuromuscular junction: only excitatory
cholinergic synapse: excitatory or inhibitory
Difference between neuromuscular junction and a cholinergic synapse
Neurones involved
neuromuscular junction: only motor
cholinergic synapse: motor, sensory and intermediate may be involved
Difference between neuromuscular junction and a cholinergic synapse
What does it link?
neuromuscular junction: neurones-muscles
cholinergic synapse: neurones-neurones or neurones- other effector organs
Difference between neuromuscular junction and a cholinergic synapse
Involvement with action potentials
neuromuscular junction:
Action potential ends here
cholinergic synapse: new action potential may be produced on postsynaptic neurone
Difference between neuromuscular junction and a cholinergic synapse
Where does acetylcholine bind to receptors on?
neuromuscular junction: binds to receptors on membrane of muscle fibre
cholinergic synapse:
Binds to receptors on membrane of postsynaptic neurone
