Muscles Flashcards
What are the types of muscle?
Cardiac muscle is found only in the heart.
Smooth muscle is found in the walls of blood vessles and the gut.
Neither of these is under concious control.
Skeletal muscle is attached to bone, and acts under voluntary, concious control.
What are myofibrils?
Individual muscles are made up of tiny muscle fibres called myofibrils.
Alone, they produce almost no force, but collectively can be extremely powerful.
The myofibrils are arranged to give maximum force.
Muscle is composed of smaller units bundled into progressively larger ones.
Why is the structure of muscles not regular?
If muscle was individual cells joined end to end, the junction inbetween would be a point of weakness that would reduce the overall strength.
What is the structure of the muscle?
The separate cells have become fused together into muscle fibres.
The muscle fibres share a nuclei and cytoplasm, the sarcoplasm, mainly found around the circumference of the fibre.
There is a large concentration of mitochondria and endoplasmic reticulum here.
What are myofibrils made up of?
Two types of protein filament:
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.
What are the myofibril bands?
Myofibrils appear striped due to alternating light-coloured and dark-coloured bands.
The light bands are I-bands (isotropic), and are lighter because the thick and thin filaments don’t overlap here.
The dark bands are A bands (anisotropic), and the thick and thin bands overlap here.
What is the structure inside the myofibril bands?
At the centre of each A band is a lighter coloured region the H-zone.
At the centre of each I band is the Z-line.
The distance between adjacent Z-lines is the sarcomere.
When a muscle contracts, the sarcomere shortens, and the pattern of light and dark bands changes.
What is tropomyosin?
An important protein in the muscle which forms a fibrous strand around the actin filament.
What are slow-twitch muscle fibres?
Slower, less powerful contractions but over a longer period that fast twitch.
Adapted to endurance work.
Adapted for aerobic respiration to avoid lactic acid build up, which would effect functionality, and prevent long-duration contraction.
How are slow twitch fibres adapted for aerobic respiration?
A large store of myoglobin (red molecule that stores oxygen).
A rich supply of blood vessels to deliver oxygen and glucose.
Numerous mitochondria to produce ATP.
What are fast twitch muscle fibres?
Contract more rapidly and powerfully, but for a short period.
Adapted for intense exercise - weight lifting.
How are fast twitch fibres adapted for anaerobic activity?
Thicker and more numerous myosin filaments.
A high concentration of glycogen.
A high concentration of enzymes involved in anaerobic respiration, which provides ATP rapidly.
A store of phosphocreatine, which rapidly regenerates ATP in anaerobic conditions.
What are neuromuscular junctions?
The point where a motor neurone meets a skeletal muscle fibre.
There are many junctions along the muscle, to ensure the muscle fibres contract simultaneously and the movement not slow.
What is a motor unit?
All muscle fibres supplied by a single motor neurone act together as a single functional unit.
This arrangement gives control over the force that the muscle exerts.
For small force, only a few units are stimulated.
How does a nerve impulse travel in a neuromuscular junction?
When a nerve impulse is recieved, the synaptic vesicles fuse with the presynaptic membrane, and release acetylcholine.
The acetylcholine diffuses to the postsynaptic membrane (of the muscle fibre), altering its permeability to sodium ions, which enter rapidly, depolarising the membrane.
How does acetylcholinesterase work?
The acetylcholine is broken down by acetylcholinesterase to ensure the muscle is not over-stimulated.
The resulting choline and ethanoic acid diffuse back into the neurone, where they are recombined to form acetylcholine using energy from ATP.
What are the similarities of the neuromuscular junction and a synapse?
They both have neurotransmitters that are transported by diffusion.
Both have receptors, that on binding with the neurotransmitter, cause an influx of sodium ions.
Both use a sodium-potassium pump to repolarise the axon.
Both use enzymes to breakdown the neurotransmitter.
What are the characteristics of the neuromuscular junction?
Only excitory.
Only links muscles to neurones.
Only motor neurones are involved.
The action potential ends here (end of a neural pathway).
Acetylcholine binds to receptors on membrane of muscle fibre.
What are the characteristics of the cholinergic synapse?
May be excitory or inhibitory.
Links neurones to neurones, or neurones to other effectors.
Motor, sensory and intermediate neurones may be involved.
A new action potential may be produced along another neurone.
Acetylcholine binds to receptors on the membrane of post-synaptic neurone.
What are antagonistic pairs?
The skeletal muscle pairs pull in opposite directions and when one is contracted the other is relaxed.
What is the evidence of the sliding filament mechanism?
When a muscle contracts, in the sarcomere:
The I-band becomes narrower.
The Z-lines move close together / the sarcomere shortens.
The H-zone becomes narrower.
What is the evidence that discounts against other mechanisms?
The A-band remains the same width.
As the width of this is determined by the length of the myosin filaments, it followed that the myosin filaments have not shortened.
This discounts the theory that muscle contraction is due to the filaments themselves shortening.
What is myosin?
Made up of two types of protein:
A fibrous protein arranged into a filament made up of several hundred molecules (tail).
A globular protein formed into two bulbous structures at one end (head).
What is actin?
A globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand.
Tropomyosin forms long thin threads that are wound around actin filaments.
