Explain how a resting potential is maintained across the axon membrane in a neurone.
- Higher concentration of potassium ions inside and higher concentration of sodium ions outside (the neurone) OR potassium ions diffuse out
- (Membrane) more permeable to potassium ions (leaving than sodium ions entering) OR (Membrane) less permeable to sodium ions (entering than potassium ions leaving); Accept for ‘less permeable to sodium ions’ is ‘impermeable to sodium ions’ or ‘sodium gates/channels are closed’ (alternative explanation).
- Sodium ions (actively) transported out and potassium ions in;
Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon.
- Myelination provides electrical insulation
- (In myelinated) saltatory (conduction) OR (In myelinated) depolarisation at nodes (of Ranvier);
- In non-myelinated depolarisation occurs along whole/length (of axon);
Describe the sequence of events involved in transmission across a cholinergic synapse. Do not include details on the breakdown of acetylcholine in your answer.
- Depolarisation of presynaptic membrane; Accept action potential for depolarisation.
- Calcium channels open and calcium ions enter (synaptic knob); Accept Ca2+.
- (Calcium ions cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter
- Acetylcholine/neurotransmitter diffuses across (synaptic cleft)
- (Acetylcholine attaches) to receptors on the postsynaptic membrane;
- Sodium ions enter (postsynaptic neurone) leading to depolarisation
Describe how the influx negatively charged ions can inhibit postsynaptic neurones.
- (Inside of postsynaptic) neurone becomes more negative/hyperpolarisation/inhibitory postsynaptic potential; Ignore K+ Accept -75mV or any value below this as equivalent to more negative Accept ‘decrease in charge’
- More sodium ions required (to reach threshold) OR Not enough sodium ions enter (to reach threshold); Accept Na+ for sodium ions
- For depolarisation/action potential; Context must covey idea that depolarisation / action potential is less likely
Explain how blocking the calcium ion channels
at some synapses can reduce impulses at the
post synaptic membrane.
- No/fewer calcium ions enter synaptic knob OR No/less calcium enter synaptic knob via calcium ion channels;
- No/fewer synaptic vesicles move to/fuse with presynaptic membrane and no/less glutamate is released;
- No/less neurotransmitter diffuses across (synaptic cleft);
- No/less (neurotransmitter attaches) to receptors on the postsynaptic membrane;
- No/fewer sodium ions enter (postsynaptic neurone) so no/ fewer impulses (sent to brain);
what happens during synaptic transmission?
- Action potential at axon terminal, depolarizes, pre S mem more permeable to Ca2+, ion channels open, ions diffuse in
- vesicle fuses with pre synaptic membrane, releases neurotransmitter (acetylcholine) in exocytosis
- neurotransmitter diffuse across synaptic cleft
- binds to receptors on post S mem, Na+ channels open and ions diffuse in
- Depolarisation, if threshold reached, action potential will generate
what is a neuromuscular junction?
excitatory cholinergic synapse between motor neuron and muscle cell
post s mem has more acetylcholine, increase chance of Na+ channels opening and depolarisation
post s mem has lots of folds storing acetylcholinesterase
what are similarities between neuromuscular junction and cholinergic synapse (4)
neurotransmitters diffuse across
NT bind to post s mem receptors so Na+ diffuse in
axon return to resting potential due to NaK pump
neurotransmitters broken down by enzymes in cleft
what are differences between neuromuscular junctions and cholinergic synapses (5)
CS- excitatory and inhibitory, NJ only excitatory
CS- another action pot may be generated along different neurone, NJ action pot ends here
CS- links neurones to neurones or effector, NJ link neurone to muscle
CS- intermediate, motor and sensory neurones involved NJ, only motor neurones
CS- acetylcholine bind to (receptors) postSN mem, NJ, bind to muscle fibre mem
what and why is unidirectionality of synapses?
impulse can only travel in one direction
- vesicles only in pre synaptic knob
-receptors only on post synaptic neurone
what is spatial summation?
several impulses from diff presynaptic neuron release NT, binds, enough Na+ enter, threshold reached
what is temporal summation?
several quick impulses sent from same presynaptic neuron, enough NT build up to bind to post S mem, reach threshold
acetylcholine stay in cleft ( too much to break down), inc depolarisation
