[3.6] Organisms Respond to Changes in their Internal & External Environments Flashcards
Describe the structure of a myelinated motor neurone.
Describe resting potential.
- Inside of axon has a negative charge relative to outside (-70mV).
- i.e. more positive ions outside compared to inside.
Explain how a resting potential is established across the axon membrane in a neurone.
- Na⁺/K⁺ pump actively transports:
- 3 Na⁺ out of axon AND 2 K⁺ into axon.
- Creating an electrochemical gradient:
- Higher K⁺ concentration inside AND higher Na⁺ concentration outside.
- Differential membrane permeability:
- More permeable to K⁺ -> move out by facilitated diffusion.
- Less permeable to Na⁺ (closed channels).
Explain how changes in membrane permeability lead to deplorisaiton and the generation of an action potential.
- Stimulus.
- Na⁺ channels open; membrane permeability to Na⁺ increases.
- Na⁺ diffusion into axon down electrochemical gradient (causing depolarisation).
- Depolarisation.
- If threshold potential is reached, an action potential is generated.
- As more voltage-gated Na⁺ channels open (positive feedback effect).
- So more Na⁺ diffuse in rapidly.
- Repolarisation.
- Voltage-gated Na⁺ channels close.
- Voltage-gated K⁺ channels open; K⁺ diffuse out of axon.
- Hyperpolorisation.
- K⁺ channels slow to close so there’s a slight overshoot = too many K⁺ diffuse out.
- Resting potential.
- Restored by Na⁺/K⁺ pump.
Draw and label a graph showing an action potential.
Describe the all-or-nothing principle.
- For an action potential to be produced, depolarisation must exceed threshold potential.
- Action potentials produced are always same magnitude / size / peak at same potential.
- Bigger stimuli instead increase frequency of action potentials.
Describe the nature of the refractory period.
- Time taken to restore axon to resting potential when no further action potential can be generated.
- As Na⁺ channels are closed / inactive / will not open.
Explain how the passage of an action potential along non-myelinated and myelinated axons result in nerve impulses.
NON-MYELINATED AXON
- Action potential passes as a wave of depolarisation.
- Influx of Na⁺ in one region in increases permeability of adjoining region to Na⁺ by causing voltage-gated Na⁺ channels to open so adjoining region depolarises.
MYELINATED AXON
- Myelination provides electrical insulation.
- Depolarisation of axon at nodes of Ranvier only resulting in saltatory conduction (local currents circuits).
- 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.
- Less / no saltatory conduction; depolarisation occurs along whole length of axon.
- So nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction.
- Ions / depolarisation may pass / leak to other neurones.
- Causing wrong muscle fibres to contract.
Describe the factors that affect speed of conductance.
Myelination
- Depolarisation at nodes of Ranvier only resulting in -> saltatory conduction.
- Impulse doesn’t travel / depolarise whole length of axon.
Axon diameter
- Bigger diameter means less resistance to flow of ions in cytoplasm.
Temperature
- Increases rate of diffusion of Na⁺ and K⁺ as more kinetic energy.
- But proteins / enzymes could denature at a certain temperature.
Describe the structure of a synapse.
What are cholinergic synapses?
- Synapses that use the neurotransmitter acetylcholine (ACh).
Describe transmission across a cholinergic synapse.
At pre-synaptic neurone
- Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca²⁺ channels.
- Ca²⁺ diffuse into pre-synaptic neurone / knob.
- Causing vesicles containing ACh to move and fuse with pre-synaptic membrane.
- Releasing ACh into the synaptic cleft (by exocytosis).
At post-synaptic neurone
- ACh diffuses across synaptic cleft to bind to specific **receptors* on post-synaptic membrane.
- 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.,
- It is hydrolysed by acetylcholinesterase.
- Products are reabsorbed by the presynatpic neurone.
- To stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation.
Describe summation by synapses.
- Addition of a number of impulses converging on a single post-synaptic neurone.
- Causing rapid build-up of neurotransmitter (NT).
- So threshold more likely to be reached to generate an action potential.
Describe temporal transmission.
- One pre-synaptic neurone releases neurotransmitter many times over a short time.
- Sufficient neurotransmitter to reach threshold to trigger an action potential.
