Overview   Lab & Research     Rane Lab     My Research       -Neurons       -Ion Channels       -Patch Clamp       -Growth Factors       -PC12 Cells       -Summary   Pseudoscience   Evolution &   Creationism	The Patch Clamp Before delving into the realm of the patch clamp, we must first examine the electrical properties of cell membranes. If you're like me, words such as "voltage", "current", "conductance", and "electrical potential" make your spine quiver and breath run short. If so, you might consider skipping this section and going directly to growth factors. The membranes of all cells, including neurons, are coated with thin layers of electrical charge: An excess of positive charge rests on the exterior of the cell membrane and an excess of negative charge resides in the interior. The separation of charge generates an electrical potential (or force) known as the membrane potential, which, in a resting neuron, is generally between -70 and -90 millivolts (mV). This importance of this potential is that it is a key force in determining how ions flow across the membrane. I should also note that other families of ion channels play the leading role in maintaining this potential, but since my research didn't revolve around those families, we can safely ignore them. External factors influence the membrane potential (e.g. voltage), either depolarizing it to more positive potentials or hyperpolarizing to more negative potentials. Changes in membrane potential subsequently alter the activity of voltage-dependent ion channels. For example, sodium and calcium channels are typically closed when the cell is at rest--that is, when the cell membrane is approximately -90 mV. If the membrane is depolarized to approximately 0-10 mV, then these channels open, allowing positive ions to flow into the cell. This flow of ions generates an electrical current that we can measure. The recording device that I used to measure ion channel currents is called the whole cell patch clamp, and is shown in the diagram below: Very briefly, the patch clamp consists of an electrode inside a glass pipette. The pipette, which contains a salt solution resembling the fluid normally found within the cell, is lowered to the cell membrane where a tight seal is formed. When a little suction is applied to the pipette, the "patch" of membrane within the pipette ruptures, permitting access to the whole cell. The electrode, which is connected to specialized circuitry, can then be used to measure the currents passing through the ion channels of the cell. Furthermore, we can use our electrical circuitry to "clamp" the membrane potential to any voltage that we desire: very handy when measuring the activity of voltage-dependent channels. For example, suppose we bathe a neuron in a recording solution that allows only sodium ions to enter the neuron. We first clamp the neuron at its normal potential: -90 mV. As shown in the diagram to the right, when the neuron is at rest, there is no current flow in or out of the cell (e.g. the current trace is a straight line). But when the voltage is "stepped" to 10 mV, we see a sharp deflection in current, the result of sodium channels suddenly opening (activating) and allowing sodium ions to flow into the neuron. Now, you might expect that this channel activation would continue throughout the voltage pulse, but notice that there is a sharp drop in current well before the voltage pulse ends. This is due to the subsequent and permanent closing of sodium channels after they have been activated, a process known as inactivation that is prevalent in many channel types and is an excellent example of how complicated the actions of these channels can be. So, now you know a bit about neurons, ion channels, and how ion channels underlie the electrical signaling of neurons. You have learned about how ion channels and neuronal activites are measured. Let's continue and see how this activity can be regulated by another family of proteins known as growth factors.
Mind 'Scapes, its pages and contents are © 1998 by Michael D. Hilborn, President of the Biggles 2000 Time-Dimensional Corporation. You are free to copy and use the original artwork on these pages, although I would appreciate it if you ask me first.