Topic 2: Action potentials Flashcards
What is an action potential? (spike, nerve impulse, discharge)
Rapid reversal, for an instant, of the negatively interior of neuronal membrane. Inside of membrane becomes positively charged in relation to outside
What is the code used by neurons to transfer information from one location to another?
Frequency and pattern of action potentials
What is oscilloscope used for?
Study action potential, records voltage as it changes over time
What are the components of action potential shown on oscilloscope?
-Rising phase
-Overshoot
-Falling phase
-Undershoot (or after-hyperpolarization)
What is the rising phase characterised by?
Rapid depolarization of membrane –> change in membrane continues till Vm reaches peak of approx. 40mV.
-when inside of the membrane has a negative electrical potential there is a large driving force on Na+. Therefore, Na+ rushes into the cell through the open sodium channels, causing membrane to rapidly depolarise
What is the potential difference (voltage) of neuronal membrane at rest?
-65mV
What is the overshoot characterised by?
-The part of the AP where the inside of the neuron is positively charged with respect to the outside (i.e., the part above 0mV)
-because the relative permeability of the membrane greatly favours sodium, the membrane potential goes to a value close to ENa, which is greater than 0mV
What is the falling phase characterised by?
-The rapid repolarisation until the inside of the membrane is actually more negative than the resting potential
- The behaviour of 2 types of channels contributes to the falling phase, first the voltage-gated sodium channels inactivate. Second, the voltage-gated potassium channels finally open (triggered to do so 1 msec earlier by the depolarisation of the membrane).
-There is a great driving force on K+ when the membrane is strongly depolarised. Therefore, K+ rushes out of the cell through the open channels, causing the membrane potential to become negative again
What is the undershoot (or after-hyperpolarisation) characterised by?
-The part of AP where membrane potential dips lower (more negative) than the resting membrane potential
-The open voltage-gated potassium channels add to the resting potassium membrane permeability. The membrane potential goes toward Ek, causing a hyperpolarisation relative to the resting membrane potential until the voltage-gated channels close again
What happens after the undershoot phase?
Gradual restoration of resting potential, from beginning to end the action potential lasts about 2 msec
What is a generator potential?
A graded response to stimuli, or graded depolarisation, could produce AP if reaches threshold.
When is an action potential triggered?
Depolarisation of membrane beyond what is called threshold
What does the rate of action potential generation, or firing frequency depend on?
size of continuous depolarising current
What is the limit to the rate at which a neuron can generate AP?
about 1000Hz (1000 impulses per second)
Once AP is initiated how long is it impossible to initiate another AP? And what is this period called?
about 1 msec, absolute refractory period
What is the relative refractory period?
-Can be relatively difficult to initiate another AP after the end of absolute refractory period, the amount of current required to depolarise the neuron to AP threshold is elevated above normal
-The membrane potential stays hyperpolarised until the voltage-gated potassium channels close. Therefore, more depolarising current is required to bring the membrane potential to threshold
What is optogenetics?
introduces into neuron foreign genes that express membrane ion channels that open in response to light
What is channelrhodopsin-2 (ChR2)
-photopigment, introduced into mammalian cells, encodes a light-sensitive cation channel that is permeable to Na+ and Ca2+.
- channel opens rapidly in response to blue light –> in neurons the inward flow of cations is sufficient to produce depolarisation beyond threshold
Depolarisation of cell during the action potential is caused by what?
influx of sodium ions across the membrane
repolarisation of the cell during the action potential is caused by what?
Efflux of potassium ions across the membrane
What is the role of the pumps (such as sodium-potassium pumps)?
Work continuously to establish and maintain concentration gradients
What does intracellular recording require?
impaling neuron or axon with a microelectrode.
What does intracellular recording measure?
The potential difference between the tip of the intracellular electrode and another electrode placed in the solution bathing the neuron (electrically continuous with earth, and thus called ground)
What is the intracellular electrode filled with?
concentrated salt solution (often KCL) having high electrical conductivity
What is intracellular electrode connected to and what is the potential difference displayed on?
Connected to an amplifier that compare potential difference between electrode and ground. Potential difference can be displayed using oscilloscope