Nerve impulses Flashcards
What is the overall charge inside the axon?
Negative.
In the sodium-potassium pump, what number of each ion (Na+ and K+) are pumped, and in which direction?
3 Na+ OUT of the axon and 2 K+ IN to the axon.
What is used to power active transport?
ATP.
What does the sodium potassium pump do?
Hydrolyses ATP to pump 3 NA+ ions out of the neurone for 2 K+ ions in.
Why do K+ ions diffuse out more rapidly than Na+ ions?
The nerve cell membrane is 25 times more permeable to potassium ions than sodium ions because it has more K+ ion channel proteins than Na+ ion channel proteins.
What is passive movement of ions by concentration gradient?
The movement of ions as they diffuse down their concentration gradient - from high concentration to low concentration.
What is passive movement of ions by electrical potential?
The movement of ions by attraction to the charge opposite to the one they have.
What causes the resting potential?
The inside of the axon membrane is negatively charged compared to the outside. This causes a potential difference across the axon membrane.
What is the resting potential of a nerve? (give units)
-70 mV.
What word can be used to describe a neuron at resting potential (more negative inside than outside)?
Polarised.
What word can be used to describe a neuron that is excited?
Depolarised.
Name 3 ways in which depolarisation may be achieved:
- A stimulus arriving at a receptor cell (e.g. vibration of hair cell in the ear)
- A chemical fitting into a receptor site
(e. g. a neurotransmitter) - A nerve impulse travelling down a neuron.
What word can be used to describe how a nerve impulse progresses along neurons?
Self-propagating.
What is an action potential?
The action potential is the state of the neuron membrane when a nerve impulse passes by.
What is the significance of voltage gated Na+ channels in an action potential?
- Small change in membrane voltage depolarises membrane enough to open some voltage-gated Na+ channels
- As more sodium ions enter the cell, more sodium channels open meaning yet more sodium ions enter
- This means a small change in the membrane permeability to Na+ means a big change in membrane potential.
Why can action potentials be described as “all-or-nothing” responses?
Nerve impulses all look the same; there are no big ones and little ones. This is because membrane potential either reaches the threshold to cause an action potential or not.
What are the stages of an action potential in chronological order? (5 stages)
1) Stimulus
2) Depolarisation
3) Repolarisation
4) Hyperpolarisation (refractory period)
5) Return to resting potential
What are the respective voltages for: 1) Resting potential 2) Threshold 3) Repolarisation 4) Hyperpolarisation (refractory period)? (with units)
1) -70 mV
2) -55 mV
3) +35 mV
4) -90 mV
At the peak of the action potential, after depolarisation, what happens to start repolarisation?
Na+ channels close and K+ voltage-gated channels open, K+ floods out of neuron.
What happens during hyperpolarisation?
The membrane potential falls below the resting potential of –70mV. During this period no impulses can pass along that part of the membrane. This is called the refractory period.
How is resting potential restored after hyperpolarisation?
Gradual active pumping of the ions (K+ in and Na+ out) restores the resting potential.
What happens in unmyelinated neurons?
Localised electrical currents are set up and the action potential is propagated along the neuron.
What makes up a myelin sheath? (2)
Schwann cells and nodes of Ranvier.
What can be found at the nodes of Ranvier?
Voltage-gated sodium channels.
What word describes how impulses are propagated in myelinated neurons?
Saltatory propagation.
Describe saltatory propagation in myelinated neurons.
- The influx of sodium ions at one node creates just enough depolarisation to reach the threshold of the next node of Ranvier.
- In this way, the action potential jumps from one node to the next (1-3mm in length)
- The net effect is much faster propagation of the nerve impulse than is possible in unmyelinated neurons.
What are the speeds of a myelinated and unmyelinated neuron? (include units)
Myelinated: 90 m/s
Unmyelinated: 30 m/s
Name and briefly describe the 3 factors affecting the speed of an action potential.
1) Myelin sheath - acts as an electrical insulator, allows action potential to jump from one Node of Ranvier to the next
2) Diameter of the axon – greater diameter = faster conductance (due to less leakage).
3) Temperature - higher temp = faster nerve impulse (rate of diffusion is faster, rate of enzyme activity is higher)
Name 3 reasons why there is a refractory period.
- Action potentials travel in one direction only.
- Produces discrete impulses.
- Limits the frequency of impulses.
Briefly describe the 2 ways that we detect the size of a stimulus.
- The number of impulses in a given time – the larger the stimulus, the more impulses generated. Thus it is frequency of the impulses that is important not their size.
- By having neurons with different threshold values – the brain interprets the number and type of neurons and thereby determines its size.
