Lecture 14 Flashcards
Review from last lecture
-The % available curve shows us how RMP influences the number of Nav channels that will open if a depolarization were to take place.
-The depolarization from RMP will activate the Nav and Kv channels according to the Na and K conductance curves (approximates the percent of channels that get activated)
What happens to Nav channels during an action potential
They become inactivated
What happens during the undershoot/afterhyperpolarization
During the afterhyperpolarization/undershoot (which is driven by Kv channels) the negative Vm promotes recovery from activation of Nav channels
The time required for this recovery, such that we can get another AP is known as the refractory period.
What are the two types of refractory periods?
absolute and relative
Explain the absolute refractory period.
This is the period of time where no amount of stimulation can generate a second AP and this is due to the excessive inactivation of Nav channels.
Explain the relative refractory period.
This is the period where we can generate a second AP but it depends on the strength of stimulation after the first AP. At first we need a strong stimulus to generate the AP because we have fewer Nav channels available. But as more Nav channels recover, the less depolarization we need to generate the second AP
Why are refractory periods important?
In the nervous system, information is often encoded by AP frequency, stronger signal = higher frequency. Example, retinal ganglion cells, nociceptive neurons, sensory coding.
We also control our muscles this way, higher frequency = stronger contraction
More on absolute refractory periods
the absolute refractory period determines the maximum AP frequency of a neuron.
Given the strongest stimulus possible, how fast can AP’s fire in succession? Fore example, a 1 ms refractory period causes the max frequency to be1000 APs/sec, or 1000 Hz
More on relative refractory periods
faster (shorter) RRPs have a higher AP frequency for a given stimulus
Neurons with short RRPs are more responsive to a given stimulus
slower (longer) RRPs have a lower AP frequency for a given stimulus
The in-vivo situation is much more complex.
There are many types of Nav and Kv channels in vertebrates
-humans have 9 Nav channel genes
-humans have 40 Kv channel genes
-they each have different activation and inactivation properties
-functional differences can affect the AP threshold and refractory period
Many different combinations of Nav and Kv channels can be expressed in different neurons
There are also many different ion channels. The electrical properties of a neuron depend on the numbers, types, and localization of all the different of ion channels that are expressed at the cell membrane.
Leak ion channels can modulate AP frequency
Many neurons respond to changes in the RMP by altering the AP frequency. For example, in retinal ganglion cells, increasing injection of depolarizing currents raises the AP frequency. This means that ion channels that affect the RMP also affect the frequency of APs
How do Na+ leak channels modulate RMP?
Increasing Na+ conductance depolarizes the RMP (increases the AP in a tonically firing neuron)
Decreases Na+ conductance hyperpolarizes the RMP
How do K+ leak channels modulate RMP?
Increasing K+ conductance hyperpolarizes the RMP, decreases the AP frequency
Decreases K+ conductance depolarizes the RMP
Analogy for RMP
Think of RMP as a dial that can be controlled by turning leak conductances up or down
