Nerve Impulses EXAM PRACTICE
Resting potential Reflexes are described to be rapid and automatic response, explain how and why? (4 marks)
Rapid - Only involves 3 neurons / myelination / few synapses as synaptic transmission is slow / saltatory conduction /
Automatic - Does not necessarily involve passage to brain / only spinal cord / same pathway used each time / higher brain centres not involved
1. Sodium potassium ion pump, actively transports 3 Na+ out the neuron and 2 K+
/ no thinking
in per ATP molecule hydrolysed
2. This means there is a higher positive charge outside the neuron compared to
inside the neuron
3. At the same time the neuron membrane has a higher permeability to K+ as How is the strength of a stimulus encoded?
there are more K+ leak channels
4. So K+ can leak back out the neurone establishing the positive charge outside ● By the frequency of action potentials
the neuron further ● The higher the frequency of action potentials the stronger the stimulus
Resting potential is -65mV The passage of action potentials along an unmyelinated axon:
Note that the voltage/potential difference is measured in terms of the inside the neuron
relative to the outside of the neuron ● Sodium ions diffuse into the axon causing depolarisation
Action potentials ● Sodium ions diffuse sideways down their electrochemical gradient producing local current
● The positive charge accumulates on the next patch and they reach threshold
● So voltage gated Na+ ion channels open in the next patch causing depolarisation and more local currents
1. The resting potential of the neuron membrane is -65mV
● Action potential only goes forward due to the refractory period in the previous patch
2. A stimulus e.g. the Pacinian Corpuscle causes sodium ions to diffuse into the
● The refractory period occurs when the membrane is at +40mV and is being repolarised
neuron through stretch mediated sodium ion channels depolarize the neuron
● During this time voltage gated sodium ion channels are closed and cannot be opened
membrane causing a generator potential (depolarising generator potential)
● A new action potential can only occur when the membrane potential is returned to resting potential through voltage gated
3. If the generator potential reaches threshold approximately -55mV then this
K+ ion channels being open and the sodium potassium ion pump.
results in the opening of the voltage gated sodium ion channels (POSITIVE
FEEDBACK)
4. Na+ rapidly diffuse (influx) down their electrochemical gradient into the neuron Importance of refractory period:
membrane depolarising it further
5. If the membrane potential reaches +40 mV then this results in the closing of ● Keep the propagation of nerve impulse in one direction
the voltage gated sodium ion channels and the opening of the voltage gated ● Keep nerve impulses discrete - resting potential must be returned before another action potential can be reached as they
potassium ion channels cannot merge
6. The K+ rapidly diffuse down their electrochemical gradient out of the neuron ● To keep and upper limit of impulse transmission as action potentials cannot merge - also note that there is a minimum
this causes the inside of the neuron to become negative again - this process is time between action potentials
called repolarisation
7. So many potassium ions leave the neuron causing the membrane to be hyper
polarised
8. This triggers the voltage gated potassium ion channels to close
9. The resting potential is restored by the Na+/K+ pump and the leak channels
Resting potential Reflexes are described to be rapid and automatic response, explain how and why? (4 marks)
Rapid - Only involves 3 neurons / myelination / few synapses as synaptic transmission is slow / saltatory conduction /
Automatic - Does not necessarily involve passage to brain / only spinal cord / same pathway used each time / higher brain centres not involved
1. Sodium potassium ion pump, actively transports 3 Na+ out the neuron and 2 K+
/ no thinking
in per ATP molecule hydrolysed
2. This means there is a higher positive charge outside the neuron compared to
inside the neuron
3. At the same time the neuron membrane has a higher permeability to K+ as How is the strength of a stimulus encoded?
there are more K+ leak channels
4. So K+ can leak back out the neurone establishing the positive charge outside ● By the frequency of action potentials
the neuron further ● The higher the frequency of action potentials the stronger the stimulus
Resting potential is -65mV The passage of action potentials along an unmyelinated axon:
Note that the voltage/potential difference is measured in terms of the inside the neuron
relative to the outside of the neuron ● Sodium ions diffuse into the axon causing depolarisation
Action potentials ● Sodium ions diffuse sideways down their electrochemical gradient producing local current
● The positive charge accumulates on the next patch and they reach threshold
● So voltage gated Na+ ion channels open in the next patch causing depolarisation and more local currents
1. The resting potential of the neuron membrane is -65mV
● Action potential only goes forward due to the refractory period in the previous patch
2. A stimulus e.g. the Pacinian Corpuscle causes sodium ions to diffuse into the
● The refractory period occurs when the membrane is at +40mV and is being repolarised
neuron through stretch mediated sodium ion channels depolarize the neuron
● During this time voltage gated sodium ion channels are closed and cannot be opened
membrane causing a generator potential (depolarising generator potential)
● A new action potential can only occur when the membrane potential is returned to resting potential through voltage gated
3. If the generator potential reaches threshold approximately -55mV then this
K+ ion channels being open and the sodium potassium ion pump.
results in the opening of the voltage gated sodium ion channels (POSITIVE
FEEDBACK)
4. Na+ rapidly diffuse (influx) down their electrochemical gradient into the neuron Importance of refractory period:
membrane depolarising it further
5. If the membrane potential reaches +40 mV then this results in the closing of ● Keep the propagation of nerve impulse in one direction
the voltage gated sodium ion channels and the opening of the voltage gated ● Keep nerve impulses discrete - resting potential must be returned before another action potential can be reached as they
potassium ion channels cannot merge
6. The K+ rapidly diffuse down their electrochemical gradient out of the neuron ● To keep and upper limit of impulse transmission as action potentials cannot merge - also note that there is a minimum
this causes the inside of the neuron to become negative again - this process is time between action potentials
called repolarisation
7. So many potassium ions leave the neuron causing the membrane to be hyper
polarised
8. This triggers the voltage gated potassium ion channels to close
9. The resting potential is restored by the Na+/K+ pump and the leak channels
