1.2 Broad overview of electrical events in neurons.
The wonderful behavioral complexity generated by the nervous system depends critically on very basic electrical and chemical events that underlie the ability of the nerve cells to communicate with each other and with the organs such as the muscles and glands that produce behavior. These events involve flows of ions such as Na and K across that cell membrane of neurons that lead to change in the electrical potential difference across the membrane. These electrical events in turn trigger release of chemicals by the neurons that affect other neurons and are the primary means of communication between neurons. In order to understand the nervous system we must have a fundamental understanding of these electrical and chemical events.
Some of the essential electrical and chemical events that underlie nervous system function can be seen in one of the most basic experiments performed by neuroscientists. The experiment involves recording the electrical potential difference across the membrane of a nerve cell in an intact animal. This is done by impaling the neuron with a glass microelectrode that is produced by heating the middle of a glass tube and pulling hard on the two ends to produce a very fine tip (typically a fraction of one micron in size) that can penetrate the cell membrane of a small neuron without destroying it. The electrode is filled with saline solution and the potential across the membrane is measured in comparison to a reference, or ground, located outside of the cell by using an amplifier. (If you are not familiar with the concepts of potential difference, current, resistance, or Ohms law, which are critical for understanding electrical events in neurons, then go to digression on potential difference).
The following two questions assess your basic knowledge of potential difference and Ohms law. If you can answer them, you may be able to skip the Ohms law digression. Still, you might want to play with the Ohms law simulator there to solidify your knowledge as Ohms law figures heavily in the following material.
Question 1.1 Test of your understanding of Ohms law.
This is a query question. The capital {query}.
This is a {query}. MC: What is Ohms Law? a. Force = Mass time accelaration *b. Voltage = current times resistance c. Energy = mass times the speed of light squaredQuestion 1.2 Ohms law question 2
MC: In a circuit with a battery of 5 volts and a resistor with a resistance of magnitude X, doubling the resistance will *a. double the current flow b. cut the current flow in half c. increase the voltage across the battery d. decrease the voltage across the batteryDigression: Primer on Potential Difference
If you are not already comfortable with potential difference (voltage) and current, and in particular with Ohms law, then this Primer will give you the essential background. An understanding of Ohms law and the relationship between voltage, current, and resistance is critical for understanding electrical activity in neurons, so if you have any doubts, review it here!
Most of us are familiar with potential difference or voltage from our use of batteries. A potential difference (voltage) represents a force that has the ability to move charged particles. In batteries and wires, electrons are carrying the charge. In neurons, it is charged ions such as Na and K.
Lets review the relationships between voltage, current and resistance using a simple example.
Suppose we have a battery. They typically have two terminals, one positive and the other negative.
If we connect a wire from the positive to the negative terminal, a current will flow through the wire from one terminal to the other. By convention, this is considered to be a positive charge flowing from the positive terminal to the negative one (even though it is really carried by electrons flowing in the opposite direction). If we think of the battery as a force pushing charge, then the more force, or the bigger the potential difference between the battery terminals, the bigger the current that will flow. A 5 volt battery would cause more current flow through the same wire than a one volt battery. The current through the wire is affected not only by the voltage of the battery, but also by how easily the current can flow from one end of the battery to the other. If the wire has a small diameter, then it will be hard for the current to flow through it, and the current flow through the wire will be smaller than it would be through a thicker wire placed between the terminals of the same battery (just as it is easier for water to flow through a large as opposed to a small diameter tube). The opposition to current flow is called resistance. A smaller diameter wire thus has a higher resistance than a large one.
The relationship between the voltage (V), the current (I), and the resistance (R) is described by Ohms law:
V=IR
This law involves only multiplication, so do not be thrown by its lawly status in Physics. Ohms law says that the voltage (V) is equal to the current (I) times the resistance(R).
The relationships between the three quantities is easier to understand if we re-arrange the terms:
I=V/R
Now we can see that the current flow through the wire depends on the force pushing the current (V) and the resistance of the wire. When the force (V) is higher then so is the current. Increasing the resistance, lowers the current flow through the wire. This makes sense because if there is more resistance to flow, then the flow (current) will be less.
These relationships are evident in the following simulation of Ohms law:
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