Topic 6.2 - Nervous Coordination Flashcards
What is resting potential?
The inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside)
Explain how a resting potential is established across the axon membrane in a neurone (3)
1) Na+ / K+ pump actively transports:
- (3) Na+ out of axon AND (2) K+ into axon
2) Creating an electrochemical gradient:
- Higher K+ concentration inside AND higher Na+ concentration outside
3) Differential membrane permeability:
- More permeable to K+ -> move out by facilitated diffusion
- Less permeable to Na+ (closed channels)
How does a stimulus lead to the generation of an action potential (2)
1) Na+ channels open; membrane permeability to Na+ increases
2) Na+ diffuse into axon down electrochemical gradient (causing depolarisation)
How does depolarisation lead to the generation of an action potential (3)
1) If threshold potential is reached, an action potential is generated
2) As more voltage-gated Na+ channels open (positive feedback effect)
3) So more Na+ diffuse in rapidly
How does depolarisation lead to the generation of an action potential (2)
1) Voltage-gated Na+ channels close
2) Voltage-gated K+ channels open -> K+ diffuse out of axon
How does hyperpolarisation lead to the generation of an action potential
K+ channels slow to close so there’s a slight overshoot -> too many K+ diffuse out
How does resting potential lead to the generation of an action potential?
Restored by Na+ / K+ pump
Describe the all or nothing principle (2)
1) For an action potential to be produced, depolarisation must exceed threshold potential
2) Action potentials produced are always the same magnitude / size / peak at the same potential
- Bigger stimuli instead increase frequency of action potentials
Explain how the passage of an action potential along non-myelinated axons results in nerve impulses (2)
1) Action potential passes as a wave of depolarisation
2) Influx of Na+ in one region increases permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open so adjoining region depolarises
Explain how the passage of an action potential along myelinated axons results in nerve impulses (4)
1) Myelination provides electrical insulation
2) Depolarisation of axon at nodes of Ranvier only
3) Resulting in saltatory conduction
4) So there is no need for depolarisation along whole length of axon
Suggest how damage to the myelin sheath can lead to slow responses and / or jerky movement (2)
1) Less / no saltatory conduction -> depolarisation occurs along whole length of axon
- So nerve impulses take longer to reach neuromuscular junction -> delay in muscle contraction
2) Ions / depolarisation may pass / leak to other neurones
- Causing wrong muscle fibres to contract
Describe the nature of the refractory period (2)
1) Time taken to restore axon to resting potential when no further action potential can be generated
2) As Na+ channels are closed / inactive / will not open
Explain the importance of the refractory period (3)
1) Ensures discrete impulses are produced (action potentials don’t overlap)
2) Limits the frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)
- Higher intensity stimulus causes higher frequency of action potentials
- But only up to certain intensity
3) Also ensures action potentials travel in one direction -> can’t be propagated in a refractory region
How does myelination affect speed of conductance (2)
1) Depolarisation at Nodes of Ranvier only -> saltatory conduction
2) Impulse doesn’t travel / depolarise whole length of axon
How does axon diameter affect speed of conductance
Bigger diameter means less resistance to flow of ions in cytoplasm
How does temperature affect speed of conductance
1) Increases rate of diffusion of Na+ and K+ as there is more kinetic energy
2) But proteins / enzymes could denature at a certain temperature
What are Cholinergic synapses?
Synapses that use the neurotransmitter acetylcholine (ACh)
Describe transmission across a Cholinergic synapse at the pre-synaptic neurone (2)
1) Depolarisation of the pre-synaptic membrane causes opening of voltage-gated Ca2+ channels
- Ca2+ diffuse into pre-synaptic neurone / knob
2) Causing vesicles containing ACh to move and fuse with the pre-synaptic membrane
- Releasing ACh into the synaptic cleft (by exocytosis)
Describe transmission across a Cholinergic synapse at the post-synaptic neurone (2)
1) ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane
2) Causing Na+ channels to open
- Na+ diffuse into post-synaptic knob causing depolarisation
- If threshold is met, an action potential is initiated
Explain what happens to acetylcholine after synaptic transmission (3)
1) It is hydrolysed by acetylcholinesterase
2) Products are reabsorbed by the presynaptic neurone
3) To stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation
Explain how synapses result in unidirectional nerve impulses (2)
1) Neurotransmitter only made in / produced from pre-synaptic neurone
2) Receptors only on post-synaptic membrane
Explain summation by synapses (3)
1) Addition of a number of impulses converging on a single post-synaptic neurone
2) Causing rapid buildup of neurotransmitter
3) So threshold more likely to be reached to generate an action potential
Describe spatial summation (2)
1) Many pre-synaptic neurones share one synaptic cleft / post-synaptic neurone
2) Collectively release sufficient neurotransmitter to reach threshold to trigger an action potential
Describe temporal summation (2)
1) One pre-synaptic neurone releases neurotransmitter many times over a short time
2) Sufficient neurotransmitter to reach threshold to trigger an action potential
