One of the most important developments in neuroscience is the development of the microelectrode in the 1940s and 1950s. The microelectrode is a device that is so small that it can penetrate a single neuron in the mammalian central nervous system without destroying the cell. Once in the cell, it can either record the electrical activity there or even stimulate the cell by carrying electrical current to the cell from an electrical source at the command of the scientist. Thus, this method allows the recording of the behavior of single neurons in the mammalian brain. It was first used in the sensory systems by Kuffler (1953). In this way, it can be used to determine what kind of stimuli that particular cell responds to.
In this illustration, you can try your hand at an experiment, measuring the response of one cell from the eye, a ganglion cell (see Chapter 3). You will be able to move a dot around the screen to see how it changes the firing rate of the cell. Without being stimulated, cells have a background firing rate. Light presented to the area of the retina that the cell responds to, the receptive field, can both increase and decrease the firing rate of the cell. See if you can find a pattern to how the cell responds to the light.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
On the Illustration tab, you can stimulate a region of the retina and see the effect on the single cell you are recording from.
On the Illustration tab, you can adjust these parameters:
Screen Area: the black area on the screen to the left is a screen that the
eye of your animal is seeing. The receptive field of the
cell you are recording from is responding to some part of this screen.
Click on or touch the screen to move the dot around and see how the cell responds by looking at the graph
to the right.
Excite Resp (+): if the firing rate goes above the horizontal blue line, the cell is said to be excited. You can place a green plus mark at that location by pressing this button.
Inhib Resp (-): if the firing rate goes below the horizontal blue line, the cell is said to be inhibited. You can place a red minus sign at that location by pressing this button.
Show Cell: after you have placed enough marks on the screen, you can show the shape of this receptive field. The green shape is the excitatory area, and the red shape indicates the inhibitory area.
Remove Last Mark: press to remove the last mark, and then sequentially the previous marks, if you added any marks by mistake.
Dot Size: after you have placed enough marks on the screen, you can change the dot size. What happens when you stimulate the entire receptive field—that is, have light fill the entire receptive field?
Pressing this button restores the settings to their default values. It also gets you a new cell, which might have a different receptive field.