Nerve impulses Flashcards
Describe the structure of a myelinated motor neuron.
Cell body - proteins and neurotransmitter chemicals are made here. contains organelles from a typical animal cell
dendrites - carry action potentials to surrounding cells
axon - conductive long fibre that carried nervous impulses along the motor neuron
schwann cells - wrap around the axon to form the myelin sheath.
nodes of Ranvier - gaps between myelin sheath
What is resting potential?
The difference in electrical charge inside and outside of the neuron when a neuron is not conducting an impulse.
Why is the resting potential -70mV?
There are more positive ions (Na+ and K+) outside compared to inside hence the inside of the neuron is more negative (-70mV)
What maintains the resting potential of -70mV?
Sodium-potassium pump
How many K+ and Na+ ions does the pump move out and in?
2 K+ in
3 Na+ out
Explain how a resting potential is maintained across the axon membrane in
a neurone.
Higher concentration of sodium ions outside the neurone and highee concentration of potassium ions inside the neurone. The membrane is more permeable to K+ leaving than Na+ entering. This means the sodium potassium pump actively transports 3 Na+ out and 2K+ in
What is action potential?
When the neurons voltage increases beyond a set point from the resting potential which generates a nervous impulse.
What causes the increase in voltage or depolarisation?
neurons membrane becoming more permeable to Na+
Describe action potential.
- A resting potential of -70mV is maintained by the sodium potassium pump. At rest, the voltage-gated sodium ion channels are closed and there are more Na+ outside the cell than inside. There are more K+ inside the cell than outside. This creates an electrochemical gradient since the membrane itself is negative.
- A stimulus provides enough energy to open the voltage-gated sodium ion channels in the axon membrane. This causes Na+ ions diffusing into axon and K+ diffusing out.
- At the threshold (-55mV) more energy is provided to open more voltage-gated sodium ion channels to open. This means the membrane is depolarising due to more Na+ diffusing into the membrane.
- Action potential peaks at +40mV because the voltage-gated sodium ion channels will close at this peak
- Voltage-gated potassium ion channels open which allows K+ to diffuse out of the neurone in order to return the membane potential to its negative resting value. More positive ions are outside than inside.
- When this continues to happen, the membrane potential may become more negative than the resting potential for a while because the K+ channels are slow to close (refractory period)
What happens if depolarisation does not exceed -55mV?
No action potential or impulse is produced.
What effect do bigger stimuli have?
Increase the frequency of action potentials.
Any stimulus that does trigger depolarisation to -55mV will always peak at the same maximum voltage. Why is this an advantage?
It ensures that animals only respond to large enough stimuli instead of responding to every single change in environment which would overwhelm them.
Explain what the refractory period is.
After the action potential is generated, the membrane enters the refractory period of -80mV where it cannot be stimulated because the sodium channels are recovering and cannnot be opened
Why is the refractory period important?
- Ensures that an action potential is not immediately generated after another and that each one is separate.
- Ensures that action potential travels forward in one direction which stops the action potential from spreading out in 2 directions which would prevent a response to a stimulus.
- Limits the number of impulse transmissions which prevents over reaction to stimulus and reduces overwhelming the senses
