Week 5. Giant Fiber - AP - Surgery
Contents
Week 5. Giant Fiber - AP - Surgery¶
Experimental Requirements¶
In these experiments, we need to more precisely and reliably measure action potential events from the giant fibers. Therefore, we will use a more invasive measurement technique. The measurements will still be extracellular, but a single-ended rather than a differential amplifier can be used. See the lab manual for more detail on the electrode and stimulation configuration.
In this lab, you will learn the surgery to access the ventral nerve cord for suction electrode recording. With this preparation, you will perform experiments to determine the rheobase, chronaxie and refractory period for earthworm giant fiber action potentials. Depending on how the surgery goes, you may get to some or none, or all of the experiments this week. Any that are not complete this week will be completed next week.
Excitability¶
Action potentials are evoked by a depolarization inside relative to outside on a patch of membrane. The relationship between action potential generation (“excitability”) and axon/membrane anatomy is a fundamental feature of neurons. This is ultimately a feature of recruitability and it effects how neural circuits function.
Comprehensive knowledge of the basic principles of cellular excitability is also fundamental to the education of medical students, because the measurement of neuronal excitability and signal conduction represents an essential part of electrodiagnostic testing in clinical practice. In addition, it is important to understand the principles of electrical excitability in nerve and muscle cells for the appropriate use of stimulation devices like deep brain electrodes, cardiac pace-makers, and defibrillators.
Strength-Duration Relationships¶
Stimulus voltages that don’t elicit an action potential are called subthreshold, stimulus voltages that do elicit an action potential are called suprathreshold. But threshold itself is not all-or-none. In some neurons, we can elicit an action potential with a lower stimulus amplitude if the stimulus duration is longer. A longer stimulus duration enables charge to build up that can ultimately elicit an action potential. However, there is a minimum stimulus voltage needed, even at infinite stimulus durations.
The rheobase is defined as the minimum stimulus voltage that can elicit an action potential with very long durations of the stimulus. This metric is one of the foundational parameters recorded in the Allen Institute’s Cell Types database. You can explore example data in this database to see how it relates to other anatomical and electrophysiological properties.
The chronaxie is defined as the stimulus duration at 2x the rheobase amplitude. Chronaxie is a useful measure of the excitability of a nerve — the most excitable nerves have the smallest chronaxie.
We can obtain both of these measurements of excitability (rheobase and chronaxie) by obtaining a strength-duration curve for action potential stimulation.
Refractory Period¶
The refractory period is the amount of time needed before a neuron can fire a second action potential. This is caused in part by the inactivation of sodium channels after an action potential — it takes time for them to close, to then be reopened by a depolarizing stimulus. This period is called the absolute refractory period. After an action potential, the membrane of the axon is also hyperpolarized, due to the slowness of K+ channels closing. So, there is a period of time where the neuron requires more voltage to fire an action potential. This period is called the relative refractory period.
We can obtain an estimate of the refractory periods of a neuron by delivering pairs of stimulus pulses (“paired pulses”) with different inter-pulse intervals. In this dataset, the relative refractory period is defined by the range of inter-pulse-intervals for which the action potential amplitude is reduced by more than 10%. The absolute refractory period is defined by the range of inter-pulse-intervals for which the second pulse fails to elicit an action potential at all.