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Module 1: Neuronal Biophysics

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The resting membrane potential of a cell is about -60mV on average. When the resting membrane potential changes from -60mV to -55mV, depolarization happens. As depolarization continues, the cell has to reach a threshold. The threshold is about 15mV to 20mV and with that, you get a spike of 100 millivolts that lasts for approximately 1 millisecond. This is the action potential, it is “all-or-nothing”, that is, only when you reach the threshold will the cell “spike”.

When the nerve membrane is depolarized by an outward flow of current so that it becomes less negative inside the cell, Sodium permeability rises immediately, and Na+ ions rush into the cell via their concentration gradient. This depolarization can occur by:
1. An outward current produced by an applied cathode.
2. The adjacent active region is invaded by an Action Potential.

Once sodium enters, further polarization occurs, allowing an acceleration of sodium entry, making the cell positive inside. When it reaches the threshold, an Action Potential occurs. When the action potential reaches its peak, sodium conductance falls, the sodium channel closes stopping the flow of sodium, and sodium permeability falls. This is called sodium inactivation. At the peak of the spike, potassium conductance rises and the membrane potential repolarizes, not before becoming more negative than it was before. This is called hyperpolarization.

In most cells, each action potential is initiated in the initial portion of the axon, known as the axon initial segment. This initial segment of the axon has the lowers threshold for action potential generation because it typically contains a moderately high density of Na+ channels and it is a small compartment that is easily depolarized by the in-rush of Na+ ions.

Absolute Refractory Period: Immediately after the generation of an action potential, another action potential cannot be generated regardless of the amount of current injected into the axon. Also known as Na+ inactivation.

Relative Refractory Period: Occurs during the action potential after-hyperpolarization. It follows the absolute refractory period. Characterized by a requirement for the increased ionic current to generate another action potential and results from persistence of the outward K+ current. The practical implication of refractory periods is that action potentials don’t “reverberate” between the soma and the axon terminals.

It is mathematically intractable to apply cable theory to complex branching dendrites. But in the 1960s, Wilfred Rall solved this problem by developing computational compartmental models. These models have provided a theoretical basis for dendritic function. Combined with mathematical models for the generation of synaptic potentials and action potentials, they can provide a complete theoretical description of neuronal activity.

EEG measurement systems consist of the following:
• Electrodes (either dry or wet)
• Amplifiers with filters (Signal conditioning circuit to amplify the signal and remove artefacts)
• Digital Oscilloscope (Analyses the signal).

Signal conditioners are required in order to amplify and make signals compatible with recording devices such as displays, recorders or A/D converters. However, the acquired signal will be of very low magnitude and contains artefacts. Thus, it is required to amplify and remove unwanted/noisy signal to improve the signal to noise ratio of the signal.

Cleanrooms are facilities ordinarily utilized for scientific research, chip manufacturing, and industrial production of micro-fabricated devices as well as pharmaceutical agents. Cleanrooms are used to control particle count, contaminants, and relative humidity to achieve more efficiency in fabrication of devices with more repeatability.