6.2.1 Nerve impulses Flashcards
What is the structure of the cell body?
Contains the organelles found in a typical animal cell, including the nucleus.
What is the axon?
A long, conductive fibre that carries the nerve impulse along the motor neuron.
What do Schwann cells do?
Wrap around the axon to form the myelin sheath.
What are the nodes of Ranvier?
Gaps in the myelin sheath.
What is the resting potential?
The difference in electrical charge between the inside and outside of a neuron’s cell membrane, when a neuron is NOT conducting an impulse.
What is the resting potential of a neuron?
-70mV
Why is the resting potential of a neuron negative?
There are more positive ions, Na+ and K+, outside compared to inside - so inside is more negative.
What is the resting potential maintained by?
Sodium potassium pump involving active transport and ATP.
What does the sodium potassium pump do?
Moves 2 K+ ions in and 3 Na+.
What is an action potential?
When a neuron’s voltage increases beyond a certain point from the resting potential, it generates a nerve impulse.
What is depolarisation (increase in voltage) due to?
The neurone membrane becoming more permeable to Na+ ions.
Describe the ion channels during a resting potential.
- Some K+ ion channels are permanently open and others are closed.
- Voltage gated Na+ ion channels are closed.
- K+ ions are moving out of axon.
What causes the start of depolarisation?
A stimulus provides enough energy to cause voltage-gated Na+ ion channels in axon membrane to open.
What do ions do at the start of depolarisation?
- Na+ ions diffuse into axon, at the same time as K+ ions diffusing out.
- Causes a slight increase in voltage.
What happens if the increase in voltage is above -55mV?
This provides enough energy to enable more Na+ ion channels to open.
What happens during depolarisation?
- More Na+ ions diffuse in.
- Sharp increase in voltage.
What does depolarisation always peak at?
+40mV
What happens to the Na+ ion channels at +40mV?
They close.
What happens during repolarisation?
- K+ ion channels open, meaning that twice the amount of K+ ions diffuse out.
What does the K+ ion channels opening during repolarisation cause?
A decrease in voltage - becomes more negative.
Where does depolarisation happen?
Nodes of Ranvier.
What does the voltage reaching +40mV cause?
The next part of the axon to start depolarisation.
What is hyperpolarisation?
Overshoot of voltage beyond resting potential - refractory period.
What is the all or nothing principle?
If the depolarisation does not exceed -55mV, then an action potential + impulse are not produced.
What is a refractory period?
- After an action potential has been generated, the membrane enters a refractory period.
- Where it cannot be stimulated because Na+ channels cannot be opened.
Why is the refractory period important?
- Ensures that an action potential occurs in one direction only.
- Ensures that action potentials are separated from one another.
- Limits the number of impulses.
What factors affect the speed of action potentials?
- Myelination & saltatory conduction.
- Axon diameter.
- Temperature.
What is saltatory conduction?
Action potential jumps from one node of Ranvier to the next.
How does axon diameter affect speed of conduction?
- The wider the diameter, the faster the speed of conduction.
How does temperature increase the speed of conduction?
- A higher temperature increases the speed of conduction:
1. Ions diffuse faster -> action potentials are generated by ions moving (via facilitated diffusion + active transport).
2. Enzymes involved in ATP work faster -> there is more ATP for active transport in the Na+/K+ pump.
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.
- Membrane more permeable to potassium ions.
- 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 insulation.
- Saltatory conduction (in myelinated).
- In non-myelinated depolarisation occurs along whole/length (of axon).
Suggest two advantages of simple reflexes.
- Rapid.
- Protect against damage to body tissues.
