15.4 Speed of nerve impulse Flashcards
What is a nerve impulse
The transmission of an action potential along the axon of a neurone
What 3 factors determine the speed of a nerve impulse
- Myelin Sheath
- Diameter of axon
- Temperature
How does the myelin sheath affect the speed of a nerve impulse
Increases speed
Myelin sheath acts as an electrical insulator which prevents an action potential from forming in the part of axon covered by myelin.
However it can jump from one node of Ranvier to another (saltatory conduction)
This increases the speed at which it travels
How does diameter of axon affect speed of a nerve impulse
Greater the diameter of axon, the faster it can conduct.
This is due to less leakage of ions from a large axon
How does temperature of axon affect the speed of nerve impulse
It affects rate of diffusion of ions so therefore the higher the temperature the faster the nerve impulse.
Energy for active transport for the sodium potassium pump comes from respiration which is controlled by enzymes. These function at higher temperatures, however not too high as this will denature the enzymes.
What is a threshold value of nerve impulses
A certain level of stimulus that triggers an action potential
Below the threshold value there is no action potential so no impulse is generated. So even if there is a strong stimulus, it won’t generate an action potential..
So any stimulus above the threshold will generate a nerve impulse
All action potentials are around the same size despite the size of the stimulus.
How can organisms perceive the size of a stimulus?
Two ways
- Number of impulses travelling in a given time. The larger the stimulus, the more impulses that are generated in a given time
- By having different neurones with different threshold values. The brain interprets the number and type of neurones that pass impulses as a result of stimulus
What is the refractory period
Once an action potential has been generated in axon, there is a period where inward movement of sodium ions is prevented.
This is because sodium voltage-gated channels are closed.
So this means all action potentials can only reach the same value and can’t go above it so size of the peaks is the same
How would the graph look between a weak stimulus and strong stimulus
- Weak stimulus: Threshold value reached but there isn’t much frequency of peaks.
- Strong stimulus: Size of peak is the same as for the weak but they are more frequent
How does refractory period make action potentials travel in one direction
- It ensures that action potentials are propagated in one direction only.
Action potentials can only travel from active region to resting region, so can’t be propagated in a in a refractory period.
It prevents potentials spreading out in both directions.
So they can only move in one direction
How does refractory period produce discrete impulses
Due to refractory period, a new action potential can’t be formed immediately behind the first one
This ensures that action potentials are separate from each other
How does refractory period limit number of action potentials
As action potentials are separated from one another this limits the number of action potentials that can pass along the axon at a given time.
So limits strength of stimulus that can be detected.
