Nervous Transmission
Resting Potential
- When a neuron is not actively transmitting an impulse it is said to be resting.
- Potential difference across axon membrane is resting potential when a neuron is in resting state.
- The axon is said to be polarised as there is a potential difference - usually -70mV.
- Resting potential occurs because of the movement of sodium and potassium ions across a membrane.
- Unequal active transport of sodium ions out and potassium ions in, via a sodium-potassium pump (3Na+ out, 2k+ in)
- This creates an electrochemical gradient where ions move across the membrane by facilitated diffusion.
- Voltage-gated sodium ion channels are closed so allow no movement, but more gated potassium channels are open so allow more diffusion of potassium than sodium.
- This means the inside of the membrane has less positive charge than the outside of the membrane.
- Action potential is the changes in potential difference caused by an influx of sodium ions creating a response, by temporarily reversing the charge of the axon.
- A nerve impulse is an example of positive feedback.
All or Nothing Principle
- An impulse has to surpass a threshold value before a wave of action potential can be passed along.
- This is to stop us being super-sensitive to every stimulus, as it could be more problematic than helpful.
- No matter how large the stimulus is, the same level of action potential will be triggered.
- However, stronger impulses lead to significantly more frequent stimulations.
- A nerve impulse is an action potential which begins at one end and is propagated along the axon.
- The depolarisation of the previous region of the axon triggers the depolarisation of the subsequent region of the axon, so the action potential moves along the axon like a mexican wave.
- Once sodium ions enter the axon, they are attracted to the negative area further along.
- The refractory period is the time where the axon cannot be excited again, so voltage gated ions remain closed preventing movement into the axon.
- This means that impulses cannot be propagated backwards, and that impulses do not overlap.
- Myelinated axons transfer electrical impulses much faster than non-myelinated axons because depolarisation can only occur at the nodes of ranvier where the axon is not insulated.
- Localised circuits occur between adjacent nodes, as action potential 'jumps' from one node to the other.
- This is much quicker than a wave of depolarisation along the whole length of the axon.
- It is also more energy efficient, as it reduces the amount of ATP required, as energy is required for active transport during repolarisation, as there are less instances where it is required.
- Axons with larger diameter increase rate of saltatory conduction as the ions face less resistance.
- Higher temperature conditions increase rate of saltatory conduction as ions diffuse quicker at higher temperatures.
- However above 40 degrees the transport proteins denature so there can be no change in potentials at all
- An oscilloscope can be used to measure action potential.