Flashcards in Muscle Contraction Deck (53):
Effector organs that respond to nervous stimulation by contracting and so bring about movement.
What are the three types of contracting muscle in the body and where are they found?
Not under our control:
Cardiac muscle- found exclusively in the heart.
Smooth muscle- Found in walls of blood vessels and the gut.
Under conscious control:
Skeletal muscle-makes up bulk of body muscle in vertebrates. It is attached to bone and acts under voluntary, conscious control.
Describe the structure of a muscle.
Individual muscles are made up of millions of tiny muscle fibres called myofibrils. The myofibrils are arranged in order to maximise strength.
Muscle is made up of smaller units bundled into progressively larger ones.
Separate cells are fused together into muscle fibres. These muscle fibres share a nuclei and also cytoplasm, called sarcoplasm, which is mostly found around the circumference of the fibre. Within the sarcoplasm is a large concentration of mitochondria and endoplasmic reticulum
What are the two types of protein filament that myofibrils are made up of?
ACTIN: which is thinner and consists of two strands twisted around one another.
MYOSIN: which is thicker and consists of long rod-shaped fibres with bulbous heads that project to the side.
Why do myofibrils appear striped?
Due to alternating light coloured and dark coloured bands.
What are the light bands in myofibrils and why do they appear lighter?
ISOTROPIC BANDS (I-bands)
Appear lighter because the actin and myosin filaments do not overlap in this region.
What are the dark bands and why do they appear dark?
ANISOTROPIC BANDS (A-bands)
Appear darker because the actin and myosin filaments overlap in this region.
What is the lighter coloured region in the centre of each A-band?
What is at the centre of each I-band?
What is the distance between adjacent Z-lines?
What happens when a muscle contracts?
Sarcomeres shorten and the pattern of light and dark bands change.
What are two other important proteins found in the muscle?
-TROPOMYOSIN- which forms fibrous strand around the actin filament.
-TROPONIN- a globular protein involved in muscle contraction.
What is in the centre of each sarcomere (h-zone)?
What are the two types of muscle fibres?
-Sow twitch fibres
-Fast twitch fibres
Describe the contraction of slow twitch fibres.
Contract more slowly and provide less powerful contractions over a longer period.
What are slow twitch fibres adapted to?
Endurance work, such as running a marathon.
In what muscles are slow twitch fibres more common in?
calf muscles, which need to contract constantly to maintain the body in an upright position.
How are slow twitch fibres adapted to their role?
adapted for aerobic respiration in order to avoid a build up of lactic acid, which would cause them to function less effectively.
-Have a large store of myoglobin (bright red molecule that stores oxygen, which accounts for red colour of slow twitch fibres).
-A supply of glycogen to provide a source of metabolic energy.
-A rich supply of blood vessels to deliver oxygen and glucose.
-Numerous mitochondria to produce ATP.
Describe the contraction of fast twitch fibres.
Contract more rapidly and produce powerful contractions but only for a short period.
What are fast twitch fibres adapted to?
Intense exercise, such as weight lifting.
Where are fast twitch fibres more common?
Muscle which need to do short bursts of intense activity, like the biceps.
How are fast twitch fibres adapted to their role?
-Thicker and more numerous myosin filaments.
-A high conc. of enzymes involved in anaerobic respiration.
-A store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction.
What is a neuromuscular junction?
Is the point where a motor neurone meets a skeletal muscle fibre. There are many such junctions along the muscle.
What would happen if there was only one neuromuscular junction?
It would take time for a wave of contraction to travel across the muscle, in which case, not all the fibres would contract simultaneously and the movement would be slow.
What is rapid muscular contraction frequently essential for?
What does the many neuromuscular junctions across a muscle ensure?
Ensures that contraction is rapid and powerful when it is simultaneously stimulated by action potentials.
What is a motor unit?
All muscle fibres supplied by a single motor neurone act together.
What does the arrangement of a motor unit ensure?
Gives control over the force that the muscle exerts.
If only a slight force is needed, only a few units are stimulated.
if greater force is required, a larger number of units are stimulated.
What happens when a nerve impulse is received at the neuromuscular junction?
The synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine.
The acetylcholine diffuses to the postsynaptic membrane, altering its permeability to sodium ions (Na+), which enter rapidly, depolarising the membrane.
What is done to ensure that the muscle is not over stimulated?
The acetylcholine is broken down by acetylcholinesterase. The resulting choline and ethanoic acid (acetyl) diffuse back into the neurone, where they are recombined to form acetylcholine using energy provided by the mitochondria found there.
Why is it called the sliding filament mechanism?
Process involves the actin and myosin filaments sliding past one another.
What will be the case if the sliding filament mechanism is correct?
There will be more overlap of actin and myosin in a contracted muscle than in a relaxed one.
What changes occur to the sarcomere when the muscle contracts?
-The I-band becomes narrower.
-The Z-lines move closer together or, in other words, the sarcomere shortens.
-The H-zone becomes narrower.
What discounts the theory that muscle contraction is due to the filaments themselves shortening?
The A band remains the same width. As the width of this band is determined by the length of the myosin filaments, it follows that the myosin filaments have not become shorter.
What two types of protein make up myosin?
-A fibrous protein arranged into a filament made up of several hundred molecules (the tail).
-A globular protein formed into two bulbous structures at one end (the head).
Describe the structure of myosin?
-Myosin filaments have globular heads that are hinged, so they can move back and forth
-Each myosin head has a binding site for actin and a binding site for ATP.
-Actin filaments have binding sites for myosin heads, called myosin-actin binding sites.
Describe the structure of actin?
Actin is a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand. Has a binding site for myosin heads- myosin-actin binding site.
Two other proteins called tropomyosin and troponin are found between actin filaments. These proteins are attached to each other and they help myofilaments move past each other,
Describe the structure of myosin?
The heads of the myosin molecule protrude, while the tails wrap around one another to form the filament.
Describe the structure of actin?
Actin is a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand.
Describe the structure of tropomyosin.
Forms long, thin threads that are wound around actin filaments.
Describe the process of muscle contraction (2nd step of sliding filament mechanism).
-The action potential travels deep into the fibre through a system of tubules (T-tubules) that branch throughout the cytoplasm of the muscle (sarcoplasm).
-The tubules are in contact with the endoplasmic reticulum of the muscle (sarcoplasmic reticulum) which has actively absorbed calcium ions from the cytoplasm of the muscle.
-The action potential opens the calcium ion channels on the endoplasmic reticulum and calcium ions flood into the muscle cytoplasm down a diffusion gradient.
-The calcium ions bind to troponin, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament.
-This exposes the binding site, which allows the myosin head to bind.
-The ADP molecules attached to the myosin heads means they are now in a state to bind to the actin filament and form a cross-bridge.
-Once attached to the actin filament, the myosin heads change their angle, pulling the actin filament along as they do so and releasing a molecule of ADP.
-An ATP molecule attaches to each myosin head, causing it to become detached from the actin filament.
-The calcium ions then activate the enzyme ATPase, which hydrolyses the ATP to ADP. The hydrolysis of ATP to ADP provides the energy for the myosin head to return to its original position.
-The myosin head, once more with an attached ADP molecule, then reattaches itself further along the actin filament and the cycle is repeated as long as nervous stimulation of the muscle continues.
Describe the process of muscle stimulation. (3rd stage of sliding filament mechanism)
-When nervous stimulation ceases, calcium ions leave their binding sites on the troponin molecules and are moved by active transport back into sarcoplasmic reticulum.
-The troponin molecules return to their original shape, pulling the attached tropomyosin molecules with them. This means the tropomyosin molecules block the actin-myosin binding sites.
-Myosin heads are now unable to bind to actin filaments and contraction ceases.
-The actin filaments slide back into relaxed position.
Describe the sliding filament mechanism (theory).
The bulbous heads of the myosin filaments form cross bridges with the actin filaments. They do this by attaching themselves to binding sites on the actin filaments, and then flexing in unison, pulling the actin filaments along the myosin filaments. They then become detached and, using ATP as a source of energy, return to their original angle and re-attach themselves further along the actin filaments. This process is repeated up to 100 times a second.
Describe the process of muscle stimulation (1st step of sliding filament mechanism).
-An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion channels to open and calcium ions to move into the synaptic knob.
-The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release their acetylcholine into the synaptic cleft.
-Acetylcholine diffuses across the synaptic cleft and binds with receptors on the postsynaptic membrane, causing it to depolarise.
Where does the energy from ATP come from?
Hydrolysis of ATP to ADP and inorganic phosphate.
What is the energy released by ATP needed for in muscle contraction?
-Movement of the myosin heads.
-The reabsorption of calcium ions into the endoplasmic reticulum by active transport.
Where does the ATP come from in muscle contraction?
Most ATP is regenerated from ADP during respiration of pyruvate in the mitochondria, which are particularly plentiful in the muscle.
However this process requires oxygen . In a particularly oxygen is rapidly used up and it takes time for the blood supply to replenish it.
Therefore a means of rapidly regenerating ATP anaerobically is also required. This is achieved using a chemical called phosphocreatine.
Cannot supply energy directly to the muscle, so instead it regenerates ATP, which can. Phosphocreatine is stored in muscle and acts as a reserve supply of phosphate, which is available immediately to combine with ADP and so reform ATP. The phosphocreatine store is replenished using phosphate from ATP when the muscle is relaxed.
The cell membrane of muscle fibre cells. Bits of it fold inwards across the muscle fibre and stick into the sarcoplasm (transverse (T) tubules).
Transverse (T) tubules
Bits of sarcolemma that fold inwards across the sarcoplasm. They help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre.
A network of internal membranes, runs throughout the sarcoplasm.
The sarcoplasmic reticulum stores and releases calcium ions.
Lots of mitochondria to provide ATP the ATP that's needed for the muscle contraction.
They are multinucleate.
Lots of long, cylindrical organelles called myofibrils.