Nervous Transmission Flashcards
What is resting potential?
Resting Potential: If a neurone isn’t transmitting an impulse. The p.d. across the membrane (difference in charge in and out the axon) is the resting potential
- Typically the outside is more +ve charged than the inside of the axon
- Membrane is now polarised and has a p.d. -70mV
- Resting potential occurs due to the movement of sodium and potassium ions across the axon membrane
- Plasma membrane prevents the ions from diffusing across the membrane so channel proteins are required
- They have gates and must remain open, some are open all the time
How is resting potential created?
- Na+ are actively transported out of the axon and K+ ions are actively transported in to the axon via an intrinsic protein called the sodium/potassium pump
- For every 3 sodium ions pumped out, 2 potassium ions are pumped in
- This means there is a higher [Na+] out the membrane than in the axon cytoplasm.
- Sodium ions will diffuse down the electrochemical gradient whereas potassium ions diffuse out the axon
- Most of the gated Na+ ions are closed but most of the K+ ion channels are open
- This means K+ ions can diffuse out the axon and there are more +ve charged ions outside the axon. This creates a resting potential across the membrane of -70mV (inside -ve relative to the outside)
Define:
Depolarisation:
Repolarisation:
V-Gated Ion Channels:
Depolarisation: Change in potential difference from -ve to +ve charge across the neurone membrane
Repolarisation: Change in potential difference from +ve back to -ve across a neurone membrane
Voltage-gated Ions Channel: Intrinsic membranes that form ion channels activated by electrical membrane potential near the channel.
How is an action potential generated?
- The neurone begins with a resting potential - no impulse transmission
- Some K+ channels open (the non V-gate ones) but all V-Gate N+are closed
- The stimulus energy triggers some of the Na+ V-gates to open, increasing the Na+ membrane permeability. Na+ ions diffuse into the axon down the electrochemical gradient. Decreasing the negativity of the inside of the neurone
- This charge change in turn causes more sodium ion channels to open, more sodium ions diffuse into the neurone. Positive feedback
- As the p.d. reaches +40mV, Na+V-gates close and the K+ V-gates open. Na+ can no longer enter the axon and the membrane is more permeable to K+.
- K+ ions diffuse out the axon down the electrochemical gradient, reducing the charge of the axon to be more negative than the outside.
- Initially, lots of K+ diffuse out making the axon more negative than at the resting state - hyperpolarisation
- K+ V-gates close and the Na/K pump causes more Na+ to move out the cell and K+ moves in returning the axon to resting potential (repolarisation)
How is an action potential propagated?
- Initially, the axon is at resting potential. A graded potential (stimulus) reaches the axon hillock, triggering an action potential
- At this point outside the axon, the [Na+] is high and [K+] in the axoplasm is high. (Overall the outside is more positive relative to the inside)
- During the action potential, V-gated Na+ channels open, Na+ ions diffuse down the electrochemical gradient into the axon. The membrane is now depolarised.
- The action potential generates a local current stimulating the depolarisation of the adjacent (forward) section of the membrane. Causing the V-gated Na+ channels to open.
- Behind this, the previously depolarised region, the V-gated Na+ channels close and the K+ V-gated channels open (K+ ions leave down the electrochemical gradient) - repolarisation occurring
- The action potential is propagated along each section of the membrane, with each prior section repolarising to the resting potential, ready to receive another stimulus
Why is an action potential only propagated in one direction?
This is due to the refractory period
- After a section of the membrane undergoes an action potential, there is a period of time where it cannot be excited again - the refractory period - regardless of stimulus size
- This means, the V-gated Na+ channels are closed, so Na+ cannot diffuse in
- This is the absolute refractory period and occurs until hyperpolarisation
There is also a relative refractory period
This occurs during the undershoot section, the section of the membrane can be depolarised but only if the stimulus is large enough.
What is saltatory conduction?
In a myelinated axon electrical impulses are transferred faster than in unmyelinated axons
Depolarisation can only occur in Nodes of Ranvier, the localised circuit of current is longer than in unmyelinated axons
The action potential jumps from one node to another - saltatory conduction
- It is faster because every time an ion channel opens and ions move in, time is taken, so the fewer places this must occur, the quicker the propagation of the action potential
- Saltatory conduction is also more energy efficient as repolarisation requires ATP, the less repolarisation needed the fewer molecules of ATP required.
What are the factors affecting the speed of action potential propagation?
Axon Diameter - the longer the diameter, the faster the impulse, due to less resistance to the flow of ions into the cytoplasm - in comparison to smaller axon diameters
Myelination
Temperature - higher temperature, faster nervous impulse, ions diffuse faster at higher temperatures. Up to roughly 40 Celcius as beyond that the protein channels denature
What is all or nothing principle?
- Nervous impulses are ‘all or nothing’
- There is a threshold value (-55mV), regardless of how strong the stimulus is, so long as it exceeds the threshold value, an action potential will be triggered - which will always be the same size
- The stronger the stimulus the more frequent the action potential (the action potential is always the same size ~ 30mV)
