Lecture 21 - Nerve cells and Excitability - Action potentials Flashcards
How do neurons communicate? How do they receive and process signals?
Neurons can communicate with each other via their dendrites and axons.
Incoming signals are received at the dendrites, the cell body processes the information and the axon carries the information along to the synaptic terminals, which transmit the information
The signals sent from the cell body/ soma to the synaptic terminals along the axon is called….
action potentials
Define an action potential?
a change in the voltage across the membrane of the neuron due to the flow of certain ions into and out of the neuron.
How are action potentials initiated?
- The neuron is stimulated at the dendrites. Excitatory signals at dendrites open ligand-gated sodium ion channels and allow sodium to flow into the cell, neutralising some of the negstive charge inside of the cell so the membrane becomes less negative. This is depolarisation, the cell membrane becomes less polarised.
- The influx of sodium diffuses inside the neuron and produces a current which travels towards the axon hillock- the trigger zone where action potentials start because they contain lots of voltage gated ion channels
If stimulation is strong then the signal…
is transmitted along the entire axon in an action potential
Two properties of action potentials
- very large
- rapid/transient
What is the ‘all or none’ law?
- If the neuron depolarisation exceeds the threshold potential of -55mv, action potential will fire
- The size of these action potentials is always the same size
- The neuron either does not reach the threshold and there is no action potential fired OR it does reach the threshold and an action potential is fired
What controls the opening/close of voltage gated ion channels?
voltage dependent - they only open when the membrane potential reaches a certain value
If threshold is reached, which voltage gated ion channels open? What is the result of this? When do they close?
sodium ion channels.
sodium ions move down concentration gradient into the cell making the membrane less negative.
eventually they close because of the change in charge across the membrane.
When membranes become +33 mV, which voltage gated ion channels open?
potassium voltage gated ion channels open wide and K+ ions move down the concentration gradient and out of the cell.
Name the 5 stages of the action potential
hyperpolarisation
depolarisation
overshoot
repolarisation
hyperpolarisation/undershoot
- Hyperpolarisation
the initial increase of the membrane potential to the value of the threshold, from -70mV to -55mV
- Depolarisation
- Once threshold is reached, -55mV, the VGNC open very quickly
- the channels open wide to allow lots of Na+ ions to enter cell membrane by diffusion
- the large influx of Na+ ions causes the cell membrane become positively charged - the membrane is depolarised. This means that the membrane potential moves to less negative values as the inside of the cell membrane is electropositive
- as the membrane becomes more positive, more VGNC open- positive feedback.
- when the voltage is +33mV, the voltage gated potassium ion channels open. These open slowly during depolarisation and remain open during this stage
- the VGNC will begin to become inactivated
- Overshoot
The inside of the cell keeps getting more electropositive until the potential gets closer to the equilibrium of sodium ions which is +61mV. The peak of action potential is about +40mV.
- Repolarisation
The membrane potential moves back to RMP. Sodium channels now begin to close. By this time, the slow VGKC should be fully opened. The K+ ions rush out of the cell down the concentration gradient
The voltage returns to its original RMP. The VGKC will slowly begin to close during this stage
- Hyperpolarisation
Because the potassium ion channels are slow to close, the potassium ions leave the cell for a little too long, resulting in negative overshooting called hyperpolarisation/undershoot.
The potential moves away from the RMP in a negative direction
How do we go from hyperpolarisation to the RMP?
the VGKC finally close and the potassium leak channels and sodium/potassium ion pumps restore the RMP
What is the refractory period?
- During and shortly after an action potential is generated, it is impossible/ difficult to stimulate that part of the membrane to fire again. This is the refractory period.
What are the two types of refractory period?
absolute and relative refractory periods.
What is the absolute refractory period?
lasts from the start of an action potential to the point the voltage first returns to the resting membrane value. The voltage gated sodium ion channels are inactivated during this time and cannot be activated again until the membrane is repolarised and RMP restored.
What is the relative refractory period?
the membrane potential is hyperpolarised by voltage gated potassium ion channels. An action potential can be generated if the stimulus is strong enough to overcome the hyperpolarisation and reach the threshold value of -55mv.
- The relative refractory period lasts until the end of hyperpolarisation
It is hard to depolarise the membrane during the relative refractory period. Why?
During this time some of the potassium channels are still open and the membrane is much more negatively charged than the RMP.
so a strong signal is required to induce another AP.
What is action potential propagation?
the movement of an action potential along the axon.
How does propagation work?
- During AP, there is sodium ion influx at one point of the membrane in the axon.
- Sodium ions diffuse along the axon membrane.
- The adjacent membrane therefore becomes more positively charged - depolarised and an action potential is generated at this point of the membrane.