6.2 - Nervous coordination Flashcards
Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non myelinated axon. (3)
- Myelination provides electrical insulation
- In a myelinated neurone saltatory conduction happens
- In myelinated neurones depolarisation occurs only at nodes of Ranvier
/When one node is stimulated the neurone’s cytoplasm conducts enough electrical charge to depolarise the next node causing the impulse to ‘jump’ from node to node - In non-myelinated depolarisation occurs along whole/length (of the axon)
‘accept action potentials for depolarisation’
Explain how a resting potential is maintained across the axon membrane in a neurone. (3)
- 3 Sodium ions are actively transported out and 2 potassium ions in
- The neurone’s membrane is less permeable to sodium ions than potassium ions
- The neurone’s membrane is more permeable to potassium ions compared to sodium ions
This means more potassium ions leave the neurone than sodium ions entering - There is a higher concentration of potassium ions inside the neurone and a higher concentration of sodium ions outside the neurone
/Potassium ions diffuse out of the neurone due to the concentration gradient
/Sodium ions diffuse in due to the concentration gradient and sodium ions can’t diffuse in (due to alternative explanation) - This means less sodium ions enter the neurone compared to the amount of potassium ions leaving the neurone
‘less permeable to sodium ions’ = ‘sodium gates/channels are closed’ (alternative explanation)
A scientist investigated the effect of inhibitors on neurones. She added a respiratory inhibitor to a neurone. The resting potential of the neurone changed from –70 mV to 0 mV. (3)
- No/less ATP produced
- No/less active transport / Sodium/potassium pump inhibited
‘accept Na+ not/fewer moved out and K+ not/fewer moved in’ - This means that the electrochemical gradient not maintained
/ Facilitated diffusion of ions causes change to 0 mV
/ This results in the same concentration of sodium and potassium ions either side of membrane
/ No net movement of sodium and potassium ions
‘accept reaches electrical equilibrium/balance’
‘accept concentration gradient of sodium and potassium ions not maintained’
Describe the sequence of events involved in transmission across a cholinergic synapse. Do not include details on the breakdown of acetylcholine in your answer. (5)
- Depolarisation of presynaptic membrane
‘accept action potential for depolarisation’ - Calcium channels open and calcium ions enter synaptic knob
- Calcium ions cause the synaptic vesicles to move/fuse with the presynaptic membrane and release acetylcholine/neurotransmitter
- Acetylcholine/neurotransmitter diffuses across the synaptic cleft
- Acetylcholine attaches to receptors on the postsynaptic membrane
- Sodium ions enter postsynaptic neurone leading to depolarisation
‘accept action potential or generator potential for depolarisation’
Dopamine is a neurotransmitter released in some synapses in the brain. The transmission of dopamine is similar to that of acetylcholine. Dopamine stimulates the production of nerve impulses in postsynaptic neurones. Describe how. Do not include in your answer the events leading to the release of dopamine and the events following production of nerve impulses at postsynaptic neurones. (3)
- Dopamine diffuses across synapse
- Dopamine attaches to receptors on postsynaptic membrane
- This stimulates entry of sodium ions and depolarisation/action potential
‘accept generator potential for action potential’
Dopamine has a role in numerous processes in the brain including pain relief. The release of dopamine can be stimulated by chemicals called endorphins produced in the brain. Endorphins attach to opioid receptors on presynaptic neurones that release dopamine. Morphine is a drug that has a similar structure to endorphins and can provide pain relief. Explain how. (2)
- Morphine attaches to opioid receptors
‘reject reference to active site’ - More dopamine released to provide pain relief
‘reject receptors release dopamine’
GABA is a neurotransmitter released in some inhibitory synapses in the brain. GABA causes negatively charged chloride ions to enter postsynaptic neurones. Explain how this inhibits postsynaptic neurones. (3)
- Inside of postsynaptic neurone becomes more negative/hyperpolarisation/inhibitory postsynaptic potential
‘accept -75mV or any value below this as equivalent to more negative’
‘accept decrease in charge’ - More sodium ions required to reach threshold
/ Not enough sodium ions enter to reach threshold - This makes depolarisation/action potential less likely
When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob. Describe how. (3)
- Nerve impulse / depolarisation of membrane causes calcium ion channels (proteins) to open
- Calcium ions enter by facilitated diffusion
- This causes the synaptic vesicles to fuse with presynaptic membrane
Identify and explain one key factor which contributes to establishing and maintaining a membrane resting potential of about -70 mV and explain how this is achieved. (2)
- The active transport of sodium ions out of the axon and potassium ions into the axon
- 3 sodium ions are actively transported out of the axon compared to 2 potassium ions which are transported in
- Differential membrane permeability
/ The cell surface membrane is less permeable to sodium ions than potassium ions
This means more potassium ions diffuse out of the axon compared to sodium ions diffusing in
Explain how the refractory period ensures the unidirectional flow of a nervous impulse in a neurone. (2)
- Sodium channels are closed during hyperpolarisation
This means that depolarisation cannot occur in that region again until the resting potential has been restored - As a result the membrane can only be depolarised ahead of an action potential
List three features of a neurone that can affect how quickly an impulse is transmitted along that neurone. (3)
- Diameter of the axon
- Myelination / the presence of Schwann cells
- Temperature
Suggest why an axon with a greater diameter is able to transmit an action potential more quickly than an axon with a smaller diameter. (2)
- A larger diameter means the axon membrane has a greater surface area for the diffusion of ions so the rate of diffusion is faster
- Faster diffusion leads to a faster rate of depolarisation / action potential propagation
- A larger diameter means a greater volume of cytoplasm containing ions so there is less electrical resistance
- Less electrical resistance means the action potential can move through to the next section of the axon more quickly
Describe how spatial summation results in the propagation of a new action potential in the postsynaptic neurone. (2)
- Summation leads to the release of sufficient neurotransmitter molecules to reach the threshold level required to trigger an action potential
- Several/multiple presynaptic neurones release neurotransmitter molecules at the same time which exceeds the threshold value
State why it is important that neurotransmitters, such as acetylcholine, are broken down and removed from the synapse. (2)
- They will continually bind to the receptors on the postsynaptic membrane
- This will result in a continuous state of depolarisation of the postsynaptic membrane / the postsynaptic membrane will remain depolarised
Tubocurarine chloride is a plant-derived poison used by indigenous South Americans to coat the tips of hunting arrows and darts. It is a muscle relaxant with a similar shape to acetylcholine. Its presence prevents the generation of an action potential in the post-synaptic neurone. Explain how the presence of tubocurarine chloride in the synapse will prevent the generation of an action potential. (3)
- Tubocurarine chloride binds to / blocks the acetylcholine receptors on the postsynaptic membrane/neurone
- Acetylcholine cannot bind to the receptors
- Sodium/Na+ channels cannot open
- Sodium/Na+ cannot enter the postsynaptic neurone
- Depolarisation does not occur / an action potential cannot be initiated (as the threshold isn’t reached)
