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.
Why does propagation only move in one direction?
- Sodium ions can diffuse in both directions along the axon, however, because of refractory periods, propagation only occurs in ONE DIRECTION. The part of the membrane which has just fired is inactive.
An action potential generated at the axon hillock travels down the axon to the nerve terminal and not towards the cell body. Why?
there are higher concentration of voltage gated ion channels in the axon than in the cell body.
The velocity of propagation is dependent upon two things…
large axon diameter and myelin
How does a large axon diameter increase velocity of propagation?
- large axons = less resistance to local current because the ions have more space to travel so it is more likely that they will keep moving in the right direction – remember axons are part of the cell so contain proteins, vesicles, etc which can get in the way of ions. Therefore, larger the diameter, more space, less chance they will bump into these proteins and vesicles and things so the faster the ions move so the faster the propagation
Why does myelin sheath increase the velocity of propagation?
electrically insulates areas of axon and so allows for saltatory conduction to occur.
What is saltatory conduction?
The propagation of action potentials along myelinated axons from one node of Ranvier to another, increasing the conduction velocity of action potentials
it decreases time spent passing messages and also conserves energy.
Explain how saltatory conduction works
- an action potential is initiated at node A.
- The action potential cannot propagate actively because of the myelin which obstructs the voltage gated ion channels.
- Instead, the action potential at one node spreads along the internodal region of the axon passively.
- As the potential spreads, it gets smaller.
- Eventually the potential will reach then next node, node B
- At this node, a new action potential is initiated as the current opens up voltage gated sodium ions at the node.
- The node acts as a relay station that renews the weak signal flowing passively
Give two demyelination diseases.
- Guillain barre syndrome = destruction of Schwann cells in the PNS
- Multiple sclerosis = loss of oligodendrocytes in the brain and spinal column.
If the myelin of sensory nerves/ afferent fibres degenerate, what is the effect?
cause numbness or tingling- sensations are not travelling the way that they should
When efferent motor nerves are demyelinated, what is the effect?
this can cause weakness because the brain is spending a lot of energy but it still cannot move the affected limbs
Why are libs especially effected by the demyelination of nerves
because they have the longest nerves. The longer the nerve, the more myelin that can be destroyed
What is a graded potential?
changes in the membrane potential that are confined to a small region of the membrane
usually produced when some specific change in the cells environment acts on specialised region of the membrane.
Why are they called graded potentials?
the magnitude of the potential can vary with the strength of stimulus.
If the initiating event is not strong enough to induce depolarisation of the membrane to reach the threshold then what happens to the graded potential?
graded potential dies out and no action potential is generated.
The local current of graded potentials is decremental. What does this mean?
the flow of charge decreases as distance from site of origin of the graded potential increases.
Graded potentials can be depolarising or hyperpolarising. Explain
some stimuli are not excitatory and do not take membrane towards the threshold. Instead, they can take the membrane further from the threshold- inhibitory postsynaptic potential. The membrane is hyperpolarised.
What is temporal summation?
graded potentials decay with time and distance. Because they decay with time, they may be separated by enough time to have no effect on each other. But, if they happen close to each other, their effects may be additive- temporal summation.
What is spatial summation?
Because graded potentials decay with distance they may be separated by enough distance to have no effect on each other. If they occur near the same part of the membrane their effects may be additive – we call this spatial summation.
If two depolarisations of the same size occur at the same time and place then…
then a depolarisation twice the size will happen. The same happens for hyperpolarisations.
What happens if excitatory and inhibitory potentials of the same size occur at same time and place?
They may cancel each other out and the membrane potential may not change.
