Skeletal muscles are stimulated to contract by nerves and act as effectors Flashcards
Which muscle is found in the heart?
- cardiac
Which muscle is involuntary?
- cardiac
Which muscle is found most in the body?
- skeletal
Which muscle is found along arteries and veins?
- smooth
Which muscle is also involuntary?
- smooth
Which muscle is really thick?
- cardiac
Which muscle is able to be controlled when it relaxes and contracts?
- skeletal
Which muscle is often around the joints?
- skeletal
Which muscle contracts in a different way to the other two?
- skeletal
Which muscle is usually found in pairs, working antagonistically?
- skeletal
Structure of the smooth muscle
- cells are spindle shaped with one central nucleus
- contractile filaments irregularly arranged
Why are the contractile filaments irregularly arranged?
- so that the contraction is not in one direction as in skeletal muscle
Structure of cardiac muscle
- cells branch and connect with each other
- specialised striated muscle that does not fatigue
Why do cells in cardiac muscle branch and connect with each other?
- to assist the passage of nerve impulses through the muscle
Structure of skeletal muscle
- muscle fibres
- myofibril
- protein filaments
- fascicles
Muscle fibres
- cells that make up skeletal muscle
- they have fused together and share nuclei and cytoplasm
Fascicles
- bundles of muscle fibres
Myofibril
- bundle of protein filaments wrapped in endomysium (found in muscle fibres) that make up the muscle fibres
Protein filaments
- when bundled together and wrapped in endomysium, make up myofibril
Actin
- thinner, globular protein with twisted chains
Tropomyosin
- forms long thing threads around actin filaments with troponin proteins present
Troponin
- proteins present in tropomyosin wound around d actin filaments
Myosin
- thicker fibre made of two proteins (fibrous protein arranged into a filament made up of several hundred molecules, the tail) and (globular protein which forms a bulbous head which protrudes from filament)
Sarcomere
- section of myofibril that appears banded due to filaments present
Light band
- I (isotopic) band - filaments don’t overlap
Dark band
- A (anisotropic) band - filaments do overlap
Neuromuscular junction
- part where a motor neurone meets (synapses with) a skeletal muscle fibre.
What does multiple junctions along the length of a muscle allow for?
- rapid and powerful contraction when simultaneously stimulated by action potentials
What do all muscle fibres supplied by a single motor neurone act as?
- a single functional motor unit
What does the number of units stimulated depend on?
- the force required
Sarcolemma
- term for the cell membrane that encloses each muscle cell
Sarcoplasm
- cytoplasm of muscle cell
What is the epimysium?
- outer layer of muscle
What is the perimysium?
- inner layer of muscle
What is the fasciculus?
- giant muscle bundle
What is tendon?
- joins muscle to bone
What is the Z zone?
- light band section
What is the H zone?
- dark filaments only section
What is the M zone?
- middle of the dark band
How does the body prevent overstimulation of the muscle as this neuromuscular junction?
- breakdown of the acetylcholine neurotransmitter
When the muscle contracts (actin fibres move inwards) what would happen to the length of the I band, A band, H zone, Z lines and sarcomere length?
- All decrease except nothing happens to A band
Describe muscle stimulation
- action potential reaches neuromuscular junction, causing calcium ion channels to open
- allowing calcium ions to move into the axon terminal
- these ions cause the synaptic vesicles to fuse with the pre synaptic membrane and release molecules or neurotransmitter into the synaptic cleft
- diffuses across the gap and binds with receptors on the post synaptic membrane causing it to depolarise
- The action potential travels deep into the muscle fibre through a system of T –tubules that branch through the sarcoplasm
- tubules are in contact with the endoplasmic reticulum of the muscle fibre (sarcoplasmic reticulum) which has actively absorbed calcium ions from the sarcoplasm in the muscle fibre
- action potential opens the calcium ion channels in the sarcoplasmic reticulum and calcium ions diffuse back into the sarcoplasm down a diffusion gradient
Muscle contraction first stages
- calcium ions are released from sarcolemma after stimulation from the T system
- calcium ions bind to the troponin and it changes shape
- troponin displaces tropomyosin and exposes the myosin binding sites that were previously blocked
Muscle contraction 1
- The bulbous heads of the myosin attach to the binding sites on the actin filaments to form a cross-bridge – they have ADP attached to
them which puts them in a state allowing them to do this
Muscle contraction 2
- the myosin heads change position to achieve a lower energy state and slide the actin filaments past the stationary myosin – we say that it ‘pushes the actin along with a power stroke’– the
molecule of ADP is also released from the head in the process
Muscle contraction 3
- ATP binds to the bulbous heads of myosin and causes it to become detached
Muscle contraction 4
- Calcium then activates the enzyme ATPase which hydrolyses the ATP back to ADP. This provides the energy to “re-cock” the heads back into their normal position (recovery stroke), and the ADP stays attached to the myosin head – the cycle can begin again
Muscle relaxation
- When nervous stimulation ceases, calcium ions are actively transported back into the sarcoplasmic reticulum using energy from the hydrolysis of ATP
- Troponin reverts to its normal shape allowing tropomyosin to re-block the actin filament binding site. Myosin heads are now unable to bind to actin filaments and the muscle relaxes - actin filaments slide back to their original position
How might ATP production change compared to if you walked or sprinted and why might you want to respire anaerobically?
- ATP production increases
- anaerobic respiration quicker way of making ATP than aerobic respiration
Why can intense exercise rapidly fatigue muscles?
- lactate build up
Why after exercise is lactate oxidised back into pyruvate to be aerobically respired?
- make sure that the chemical energy locked up in lactate is not wasted
Anaerobic respiration in muscles produces lactate (also known as lactic acid). Lactate lowers pH. Explain how this could lead to muscle fatigue and soreness.
- lower pH increases concentration of H+ ions
- H+ ions can alter hydrogen bonds and ionic bonds in proteins
- changing tertiary structure
- actin, myosin and tropomyosin could be less effective
- active site of ATPase could change shape meaning it is not complementary to its substrate
- preventing enzyme substrate complexes forming
- less ATP regenerated
What does phosphocreatine do?
- regenerates ATP
How does phosphocreatine regenerate ATP?
- chemical that is stored in muscle and acts as a reserve supply of phosphates
- During exercise, phosphocreatine phosphorylates ADP to ATP, so more ATP becomes available
- at rest, ATP phosphorylates creatine back to phosphocreatine
What are the two types of muscle fibres?
- slow twitch and fast twitch
What do fast twitch fibres mainly use and why?
- anaerobic respiration
- quicker process for producing ATP than aerobic respiration so fast fibres are better for short bursts of intense exercise
What do slow twitch fibres mainly use and why?
- aerobic respiration
- yields more ATP per glucose molecules so slow fibres are better for endurance
What are the main differences between slow and fast fibres?
- slow fibres have many capillaries whereas fast fibres have fewer capillaries
- slow fibres have higher myoglobin concentration whereas fast fibres have lower myoglobin concentrations
- slow fibres have lower phosphocreatine concentration whereas fast fibres have higher phosphocreatine concentration
- slow fibres have lower glycogen concentration whereas fast fibres have higher concentration of glycogen
Why does a slow fibre have more capillaries than a fast fibre does?
- provides lots of oxygen for the slow fibres so they can maintain a high rate of aerobic respiration
Why do fast fibres have more phosphocreatine than slow fibres?
- to help provide ATP quick enough for short bursts of intense exercise
Why do fast fibres contain higher glycogen concentrations than slow fibres?
- Fast fibres contract quicker than slow fibres
- Fast fibres need to produce ATP quicker
- Fast fibres mainly use anaerobic respiration
- Less ATP per glucose is produced using anaerobic respiration
- Fast fibres need to respire a lot of glucose
- Large glycogen stores are needed in fast fibres to release enough glucose via hydrolysis
Differences between myoglobin and haemoglobin
- myoglobin contains one polypeptide chain whereas haemoglobin contains four polypeptide chains
- myoglobin contains one haem group whereas haemoglobin contains four haem groups
- myoglobin has oxygen store in muscle fibres whereas hameoglobin carries oxygen in red blood cells
- myoglobin has no cooperative oxygen binding whereas haemoglobin has cooperative oxygen binding
What is cooperative oxygen binding?
- one oxygen binding makes it easier for the next one to bind
Using the graph suggest why is myoglobin an excellent oxygen store?
- Myoglobin has an extremely high affinity for oxygen (greater than haemoglobin)
- it gives oxygen up at very low partial pressures
- when it does release oxygen, it does so rapidly
- this will help prolong aerobic respiration
- and delay anaerobic respiration
- and in turn, delay lactate production
