Muscles Flashcards
What coordinates muscular movement
The CNS receives sensory information and decides what kind of response is needed. If it is movement the CNS send signals along neurones to tell skeletal muscles to contract
What is skeletal muscle composed of
Large bundles of cells called muscle fibres
What is the cell membrane of a muscle fibre
Sarcolemma
What are the infolds of the sarcolemma of a muscle fibre
Transverse or T tubules
What do T tubules do
Help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre
What is the network of internal membranes in the sarcoplasm
Sarcoplasmic reticulum
What does the sarcoplasmic reticulum do
Stores and releases calcium ions that are needed for muscle contraction
Why do muscle fibres have lots of mitochondria
To provide ATP for muscle contraction
What are myofibrils
Long cylindrical organelles made up of proteins specialised for contraction. Several hundred to thousands of myofibrils are in muscle fibres
What do myofibrils contain
Thick myofilaments - myosin
Thin myofilaments - actin
What do dark bands contain and what are they called
Thick myosin filaments called the A-bands
What do light bands contain and what are they called
Thin actin filaments called the I-bands
What is a myofibril made up of
Short units called sarcomeres
What is the Z-line
The ends of sarcomeres
What is the M-line
The middle of each sarcomere and myosin filament
What is the H-zone
Section of the sarcomere that only contains myosin filaments
How do sarcomeres contract
Myosin and actin slide over one another (the myofilaments don’t contract)
What does the simultaneous contraction of lots of sarcomeres result in
The myofibrils and muscle fibres contract.
When do sarcomeres return to their original length
When the muscle relaxes
During contraction, what happens to the A-band
Stays the same length
During contraction, what happens to the I-band
Gets shorter
During contraction, what happens to the H-zone
Gets shorter
During contraction, what happens to the sarcomere length
Gets shorter
What do myosin filaments have
- Have globular heads that are hinged so can move back
- Each myosin head has a binding site for actin and a binding site for ATP
What do actin filaments have
- Binding sites for myosin heads, called actin-myosin binding sites
- Two other proteins called tropomyosin and troponin are found between actin filaments
What do the proteins of tropomyosin and troponin do
They are attached to each other and help myofilaments move past each other
What does the binding site of a resting muscle look like and what does it mean
The actin-myosin binding site is blocked by tropomyosin which is held in place by troponin. This means myofilaments can’t slide past each other because the myosin heads can’t bind to the actin-myosin binding site on the actin filaments
What are the stages involved in forming an actin-myosin cross bridge (1st stage of contraction)
- An action potential from a motor neurone stimulates a muscle cell, depolarising the sarcolemma.
- Depolarisation spreads down the T-tubules to the sarcoplasmic reticulum causing it to release calcium ions into the sarcoplasm
- Calcium ions bind to troponin causing a change in its tertiary structure
- This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament
- Exposing the binding site allowing the myosin head to bind
- The bond formed when a myosin head binds to an actin filament is an actin myosin cross bridge
What are the stages after forming an actin-myosin cross bridge to the power stroke (2nd stage of contraction)
- Calcium ions activate the enzyme ATPase which breaks down ATP into ADP+P, to provide the energy needed for muscle contraction
- The energy released from ATP moves the myosin head, which pulls the actin filament along in a rowing action known as the power stroke
What are the stages after the power stroke to muscle contraction (3rd stage of contraction)
- ATP also provides the energy to break the actin-myosin cross bridge, so the myosin head detaches form the actin filament after it is moved.
- The myosin head then reattaches to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated
- Many new cross bridges form and break very rapidly pulling the actin filament along, shortening the sarcomere, causing the muscle to contract
- This will continue as long as calcium ions are present and bound to troponin
What happens when the muscle stops being stimulated
- Calcium ions leave their binding sites on the troponin molecules and are moved by active transport back into the sarcoplasmic reticulum
- The troponin molecules return to their original shape, pulling the attached tropomyosin molecules with them, blocking the actin-myosin binding sites.
- Muscles aren’t contracted because no myosin heads are attached to actin filaments
- The actin filaments slide back to their relaxed position, which lengthens the sarcomere
How is energy generated during aerobic respiration and at what intensity
It is generated through oxidative phosphorylation in the cell’s mitochondria which occurs with oxygen so long periods of low intensity exercise
What type of respiration is glycolysis used for and why
Anaerobic respiration for short periods of hard exercise such as a 400m sprint. The end product of glycolysis is pyruvate which is converted to lactate by lactate fermentation. This can build up in the muscles and cause muscular fatigue
What is a neuromuscular junction
A synapse between a motor neurone and a muscle cell
What do neuromuscular junctions use as the neurotransmitter
Acetylcholine
What are the receptors called that acetylcholine bind to
Nicotinic cholinergic receptors
How do neuromuscalar junctions work
In the same way as synapses whereby they release neurotransmitter which triggers depolarisation (once a threshold is met) in the post synaptic cell, which causes the muscle to contract.
What is used to break down acetylcholine and when
Acetylcholinesterase is stored in clefts on the postsynaptic membrane and released to break it down after use.
What may interrupt the binding or affect how acetylcholine binds and what impact does this have
A chemical (e.g a drug) may block or affect how the neurotransmitter binds. This may prevent the action potential from being passed on to the muscle so it will not contract.
This can be fatal if it affects the muscles involved in breathing as will prevent ventilation occurring
What are the features of skeletal muscle (4)
It is made up of many muscle fibres that are multinucleated
It is striated (dark and light bands)
Different types of muscle fibres
Contraction happens voluntarily/consciously
What are the features of involuntary/smooth muscle (5)
Contraction happens involuntarily/unconsciously
It is not striated - smooth
Found in the walls of hollow internal organs such as the gut and blood vessels
Uninucleated
Contract slowly and do not fatigue
What are the features of cardiac muscle (7)
Contracts on its own - myogenic. But rate of contraction is involuntary and controlled by the autonomic nervous system
Found in the walls of the heart
Made from muscle fibres with intercalated discs = low electrical resistance so nerve impulses pass easily between cells
Branched to allow nerve impulses to spread quickly through the whole muscle
Uninucleated
Faintly striated - not as strong as skeletal muscle
Fibres contract rhythmically and do not fatigue
What can be used to monitor muscle fatigue
Electrodes placed on the skin to detect electrical signals that cause muscle contraction. The procedure is called electromyography which generates an electromyogram
