Organisms Response To The Environment: Nervous Coordinations And Muscles

Question 1

Give three ways the movement of ions is controlled across the axon membrane

Answer 1

• phospholipid bilayer prevents sodium and potassium ions simply diffusing across it
• channel proteins allow the sodium and potassium ions to pass through the phospholipid bilayer by facilitated diffusion
• some carrier proteins can actively transport potassium ions in and sodium ions out of the axon as it functions as a sodium potassium pump

Question 2

What are the events that occur in the axon membrane that allow it to reach resting potential?

Answer 2

1. Sodium potassium exchange pump requires ATP as it pumps the ions against their concentration gradient
2. 3 sodium ions actively transported out and 2 potassium ions in.
3. There are more sodium ions in the tissue fluid outside the axon than in the cytoplasm so am electrochemical gradient is formed. K+ can then diffuse in and Na+ diffuses out however the membrane is more permeable to k+ hence the potential difference is established
4. The membrane is polarised as resting potential of -70mV is reached (-ve on the inside)
5. Membrane is impermeable to sodium ions as voltage activated sodium ion channels are closed due to a change in the tertiary structure of the protein

Question 3

What is action potential?

Answer 3

A self propagating process overall causing depolarisation by opening all of the sodium ion channels

Question 4

Give the events of action potential

Answer 4

1. Membrane depolarises when sodium ions diffuse in rapidly through open channels as the channels are highly permeable to sodium ions
2. Membrane potential changes to +40mV
3. local circuit created with the neighbouring resting potential further along the axon
4. Na+ diffuse through the axoplasm along the circuit into the region ahead
5. The membrane potential in the region ahead is reduced and made less negative
6. At threshold of -55mV, voltage gated sodium ion channels open causing depolarisation of the next region

Question 5

Give the steps of repolarisation

Answer 5

1. At +40mV, voltage activated potassium ion channels open
2. The potassium ions diffuse out rapidly
3. The membrane becomes hyper polarised at -80mV as its now lower than the resting potential
4. The refractory period takes place which is the time taken to reset polarisation

Question 6

What happens during the refractory period?

Answer 6

Once an action potential has been created in any region of an axon, the inward movement of sodium ions is prevented as the sodium voltage-gated channels are closed so it’s impossible for a further action potential to be generated as the sodium potassium exchange pump resets the resting potential

Question 7

What are the three roles of the refractory period?

Answer 7

• action potentials are propagated in one direction only as action potentials can only pass from an active region to a resting region and action can’t be propagated in a refractory region
• it produces discrete impulses as the refractory period prevents action potentials being formed immediately behind the first so they are always separated
• limits the number of action potentials as they are separated so only a certain number can pass along the axon in a given time so limits the strength of the stimulus that can be detected

Question 8

How can organisms perceive the size of a stimulus?

Answer 8

• By the number of impulses passing in a given time as the larger the impulse the more stimuluses generated
• by having different neurones with different threshold values

Question 9

What is the all or nothing principle?

Answer 9

There is a certain level of stimulus being the threshold value that needs to be reached in order to trigger an action potential

Question 10

How does an action potential pass along a myelinated axon?

Answer 10

Through saltotory conduction

1. sensory and motor neurones are myelinated by being tightly surrounded by Schwann cells
2. The myelin insulates sections of the axon against ion movement
3. This greatly extends local circuits so depolarisation only occurs at the nodes of ranvier so propagation of nerve impulses through action potentials is much faster than in unmyelinated neurones

Question 11

What are the factors that affect the speed of conductance of an action potential?

Answer 11

• myelinated neurones the action potential travels at 100ms-1 compared to unmyelinated at 0.5ms-1 as a result of saltatory conduction
• thickness of of the unmyelinated neurone as a thicker neurone reduces diffusion resistance for sodium ions along local circuits hence speeds up the action potential

Question 12

Give the steps of synaptic transmission in cholinergic synapses

Answer 12

1. action potential arrives at the end of the neurone to the synaptic knob, depolarising the presynaptic membrane causing voltage gated ca2+ ion channels to open
2. Influx of ca2+ triggers synaptic vesicles to fuse with the presynaptic membrane and release the neurotransmitter acetylcholine into the synaptic cleft by exocytosis
3. Acetyl choline diffuses across the synaptic gap and binds with protein receptors in the post synaptic membrane
4. Protein receptors change shape and open a hydrophilic pore increasing membrane permeability to Na+ ions
5. If enough receptors channels open, resting potential may reach threshold of -55mV causing opening of voltage gated Na+ ion channels causing depolarisation of post synaptic membrane and an action potential is generated
6. Enzyme acetylcholine esterase forms enzyme substrate complexes with acetylcholine hydrolysing the ester bond forming acetate and choline preventing further binding of the neurotransmitter to the protein receptors
7. The choline is actively transported back over the presynaptic membrane and combines with acetyl CoA from the mitochondria enable acetylcholine to reform

Question 13

What are the two features of a synapse?

Answer 13

• its unidirectional so transmission can only pass from the pre synaptic neurone to the post synaptic neurone because the protein receptors are only present on the post synaptic neurone
• they filter put low level stimuli as low frequency of action potentials lead to insufficient concentrations of neurotransmitter released to trigger a new action potential in a post synaptic membrane hence the stimulus can undergo summation

Question 14

What are the two types of summation?

Answer 14

• Spatial summation = multiple Presynaptic neurones release enough neurotransmitter to exceed threshold of post synaptic neurone so a new action potential is triggered
• temporal summation = where a single presynaptic neurone releases neurotransmitter at a higher frequency so the concentration of neurotransmitter exceeds the threshold value of the postsynaptic membrane triggering a new action potential

Question 15

How does synaptic inhibition in terms of GABA receptors work?

Answer 15

1. Some separate neurones can release the inhibitory neurotransmitter GABA which bind to protein GABA receptors in the post synaptic membrane
2. This causes chloride ion channels to open causing cl- to move into the postsynaptic neurone by facilitated diffusion and potassium ion protein channels to open causing k+ to move out of the post synaptic neurone
3. This causes the inside of the postsynaptic membrane to become more negative and the outside to become more positive
4. The post synaptic membrane hyperpolarises from -65mV to -80mV making it harder for a new action potential to be created and other neurones to cause depolarisation as a larger influx of sodium ions is needed

Question 16

Give the similarity between neurotransmitters in the neuromuscular junction and the cholinergic synapse

Answer 16

It’s unidirectional due the protein receptors only being present on the post synaptic membrane

Question 17

Give the 4 differences between neurotransmitters in the neuromuscular junction NJ and the cholinergic synapse CS

Answer 17

• NJ Is only excitatory but CS could be excitatory or inhibitory
• NJ connects motor neurone to muscles whole CS connects two neurones which could be sensory, relay or motor
• NJ it’s self is the endpoint of the action potential while in CS a new action potential is generated in the next neurone
• in NJ acetylcholine binds to protein receptors on the muscle fibre membranes while is CS, acetylcholine binds to receptors on the post synaptic membrane

Question 18

What does a muscle fibre consist of?

Answer 18

Many myofibrils which are made up of myosin and actin

Question 19

What’s a sacromere? What does it contain?

Answer 19

Sections of myofibrils that shorten in muscle contraction. They contain two Z plates 3 myosin filaments, and 4 actin filaments on either side

Question 20

In a sacromere, 
What is the I band?
What is the A band?
What is the H zone?
What happens to each of these during muscle contraction? Include z plates

Answer 20

1. Sections of the sacromere of actin only
2. Sections of the sacromere where the actin and myosin overlap and myosin only
3. Sections of the sacromere if myosin only

During muscle contraction:
Z plates come together
I bands shorten as actin is pulled over the myosin
H zone shortens as actin overlaps the myosin more
A band stays the same

Question 21

Describe the structure of myosin

Answer 21

It consists of two types of protein:

• fibrous protein arranged into a filament made up of several hundred molecules
• a globular protein formed into two bulbous structures, one at each end

Question 22

Describe the structure of actin

Answer 22

A globular protein that contains multiple molecules arranged into long chains that are twisted around each other to form a helical strand

Question 23

What does it mean that muscles are antagonistic

Answer 23

They act in pairs so they pull in opposite directions and when one is contracted the other is relaxed

Question 24

Give the 8 features of fast twitch muscle fibres

Answer 24

• perform strength work
• suited for rapid movements
• come useful for sprinters
• undergo anaerobic respiration
• their blood vessels are fewer
• energy release is in small amounts and quickly
• they have a smaller number of mitochondrian
• they have a smaller amount of myoglobin

Question 25

Give the 8 features of slow twitch fibres

Answer 25

• help stamina
• suited for endurance activities
• useful for marathon runners
• undergo aerobic respration
• have many blood vessels
• release large amounts of energy slowlu
• have more mitochondria
• more myeoglobin

Question 26

What are the relative strengths, contactions and endurance of slow titch, fast twitch A and fast twitch B fibres

Answer 26

Strength: slow twitch-low, fast twitch A- medium, fast twitch B -high
Contraction speed: slow twitch- slow, fast twitch A- medium, fast twitch B- fast
Endurance: slow twitch- high, fast twitch A- medium, fast twitch B, low

Question 27

What makes slow twitch muscle fibres more red than fast twitch muscle fibres?

Answer 27

The slow twitch muscle fibres contain more myoglobin and haemoglobin so are undergoing more aerobic respiration

Question 28

Compare the release of ATP with fast and slow twitch fibres

Answer 28

Slow twitch: ATP is released is in larger quantities of per glucose but the process of aeriobic respration is much slower

Fast twitch: much less ATP is released per glucose but they undergo anaerobic respiration so the process is much faster