Module 2 - Lecture 1 - Action Potential & Propagation - Review Questions Flashcards
What are the properties of a neuron?
- It is excitable tissue that is polarized which means that neurons can be stimulated to produce action potentials. 2. It is conductive tissue which means that neurons can propagate action potentials. 3. Single cells 4. Linked in networks to convey information
Know the 3 parts and general function of a neuron.
The general function of neurons is electrically excitable cells in the nervous system that function to process and transmit information. The 3 parts include: 1. Dendrites = receive information to modify a neurons membrane potential 2. Cell body = necessary for metabolic processes and DNA is stored in the cell body and contains information for synthesis of enzymes. 3. Axon = conveys the information from one point to another of the neuron in the form of an action potential.
What is the purpose of the neuron and action potential?
The purpose of the neuron is to convey information from one point to another in the nervous system or to target organs. The action potential is an all or nothing, stereotyped change in the voltage across the nerve cell membrane that is going to propagate along the axon to transmit that information.
What does it mean that an action potential is “all or nothing”?
There are no big or small action potentials in one nerve cell - all action potentials are the same size; the potential change in the nerve cell membranes either can occur fully or it doesn’t. Therefore, the neuron either does not reach the threshold or full action potential is fired - this is the “ALL OR NONE” principle.
What Ions are inside and what are outside of a neuron at rest?
A neuron at rest has a high concentration of potassium (K+) ions inside the cell, and sodium (Na+) ions outside the cell which represents the concentration gradient.
Understand chemical gradients… What are electrochemical gradients?
First of all, a chemical gradient refers to the concentration gradient of an ion or molecule. The concentration gradient may exist across a biological membrane, where the concentration is higher on one side of the membrane compared to the other side.
An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane (diffusion = flow from high concentration to low concentration of the SAME ion), and the electrical gradient, or difference in charge across a membrane (A negative and positive charge will attract each other. However, two similar charges will repel each other.)
Explain the resting membrane potential. What causes it? Is it positive or negative?
The resting membrane potential of a cell is defined as the electrical potential difference across the plasma membrane when that cell is in a non-excited state.
Because the potassium is leaking out and the positive charge is going outside. It has a flow of positive electrodes outside of the neuron, that makes the inside region appear negative.
The resting membrane potential is caused by the uneven distribution of ions between the inside and the outside of the cell, and by the different permeability of the membrane to different types of ions.
The resting membrane potential is typically around -70 mV but it ranges from -40 mV to -90 mV.
Know the 3 sources of an action potential and how it interacts with the threshold.
The 3 sources of an action potential include:
Receptor potentials – due to the activation of sensory neurons by external stimuli such as light, sound, or heat.
How it interacts with threshold = the idea that we can have a stimuli, like the pressure on the skin and that is able to cause the electrical change in the neuron membrane using a mechanoreceptor = open up the ion channels that let in positive sodium and allows for an action potential to occur.
Synaptic potentials – at the synapses between neurons, special chemicals released between neurons causing a membrane potential change
The idea that in the event of an end plate potential, the ligand gated channels that are opening up will allow for an action potential to occur.
Electrical Stimulation – positive or negative current from electrical stimulus can cause a membrane potential change
In general, action potentials are the result of a surge of positive ions into the cell above a certain threshold.
What ion channels are responsible for the Raising Phase (Depolarization) of an AP?
The voltage-gated sodium channels are responsible for the depolarization phase of an action potential. Upon reaching threshold, sodium rushes into the cell and this results in a rapid voltage change.
HOWEVER…Both the sodium and potassium channels open when there is a depolarization to the membrane potential. To understand… the following image illustrates the two different cycles to explain each ion channels reaction.
What factors are responsible for repolarization?
The factors that are responsible for repolarization is a combination of inactive sodium channels and the conductance of potassium channels.
Sodium channels have an inactivation mechanism (configuration change of the protein) that stops the flow of Na+ ions to penetrate the membrane which is initiated by prolonged depolarization. This inactivation mechanism will remain until the neuron returns to a negative resting membrane potential and when that happens the protein will go back to an activated state = it will go back to it’s CLOSED state since it is negative in the inside and below the threshold.
KEY CONCEPT = Na+ channels have an inactivation mechanism that stops the flow of Na+ ions. This mechanism is initiated by prolonged depolarization.
At the same time that sodium channels are inactivating, potassium channels are starting to open up. Potassium does not have any inactivation mechanism associated with the protein. BUT just like the potassium channels are slow to open, they are also slow to close…which results in repolarization and brief hyperpolarization.
KEY CONCEPT = K+ channels are slower to close and allow k+ to leave the cell thereby allowing the inside of the cell to become negative again.
Explain the mechanisms to why membrane potential enters Hyperpolarization.
The combination of inactive Na channels along with conductance of K out of the cell results in brief hyperpolarization. The K channels, because these channels are slow to close, “overshoot” and expel more positive ions than needed causing the inside of the neuron to become more negative than it was before depolarization.
Know the mechanisms and function of Absolute and Relative Refractory
The mechanism of an absolute refractory period is when the sodium channels become inactivated because of prolonged depolarization and therefore during that time period new AP cannot be produced.
The function of this absolute refractory period is this limits how frequently AP can occur and prevents AP from traveling “backward” towards its origin.
The mechanism of a relative refractory period is simply due to the delay in potassium channels closing, you now need a higher depolarization to reach the threshold for an action potential to occur. To be clear, the relative refractory period is the time where a second AP can only be produced by a larger depolarization.
Can provide information about the importance of a signal as the nervous system can overcome the hyperpolarized state and increase the frequency of the signal (rather than the amplitude which remains constant).
Understand the similarities and differences between voltage-gated sodium and potassium channels
Because their opening is influenced by membrane potential, both channel types are voltage-gated. For each channel, depolarization increases the probability of channel opening, whereas hyperpolarization closes them.
Both channel types must have a voltage sensor that detects the potential across the membrane.
There is a difference in ion selectivity and the kinetic properties of the gating of the two channels differ = further depolarization inactivates the sodium channels because they have an inactivation mechanism but the potassium channels do not.
Depolarization is caused by the fast positive cycle of the sodium channels, however, the hyperpolarization is caused by the slow negative cycle of the potassium channels.
Understand the implication of relative refractory period for a large depolarization event
The nervous system can overcome the relative refractory period with large depolarization events to increase the frequency of the action potentials.
What is the function of active transporters, how does it contribute to the membrane potential?
Active transporters move Na and K against their concentration gradient.
Critical in removing Na+ after an action potential to restore the RMP.
