Despite the enormous complexity of the brain, it is possible to obtain an understanding of its function by paying attention to two major details:
- First, the ways in which individual neurons, the components of the nervous system, are wired together to generate behavior.
- Second, the biophysical, biochemical, and electrophysiological properties of the individual neurons.
A good place to begin is with the components of the nervous system and how the electrical properties of the neurons endow nerve cells with the ability to process and transmit information.
Introduction to the Action Potential
Figure 1.1 |
Important insights into the nature of electrical signals used by nerve cells were obtained more than 50 years ago. Electrodes were placed on the surface of an optic nerve of an invertebrate eye. (By placing electrodes on the surface of a nerve, it is possible to obtain an indication of the changes in membrane potential that are occurring between the outside and inside of the nerve cell.) Then 1-sec duration flashes of light of varied intensities were presented to the eye; first dim light, then brighter lights. Very dim lights produced no changes in the activity, but brighter lights produced small repetitive spike-like events. These spike-like events are called action potentials, nerve impulses, or sometimes simply spikes. Action potentials are the basic events the nerve cells use to transmit information from one place to another.
Features of Action Potentials
The recordings in the figure above illustrate three very important features of nerve action potentials. First, the nerve action potential has a short duration (about 1 msec). Second, nerve action potentials are elicited in an all-or-nothing fashion. Third, nerve cells code the intensity of information by the frequency of action potentials. When the intensity of the stimulus is increased, the size of the action potential does not become larger. Rather, the frequency or the number of action potentials increases. In general, the greater the intensity of a stimulus, (whether it be a light stimulus to a photoreceptor, a mechanical stimulus to the skin, or a stretch to a muscle receptor) the greater the number of action potentials elicited. Similarly, for the motor system, the greater the number of action potentials in a motor neuron, the greater the intensity of the contraction of a muscle that is innervated by that motor neuron.
Action potentials are of great importance to the functioning of the brain since they propagate information in the nervous system to the central nervous system and propagate commands initiated in the central nervous system to the periphery. Consequently, it is necessary to understand thoroughly their properties. To answer the questions of how action potentials are initiated and propagated, we need to record the potential between the inside and outside of nerve cells using intracellular recording techniques.
Intracellular Recordings from Neurons
Figure 1.2 |
The potential difference across a nerve cell membrane can be measured with a microelectrode whose tip is so small (about a micron) that it can penetrate the cell without producing any damage. When the electrode is in the bath (the extracellular medium) there is no potential recorded because the bath is isopotential. If the microelectrode is carefully inserted into the cell, there is a sharp change in potential. The reading of the voltmeter instantaneously changes from 0 mV, to reading a potential difference of -60 mV inside the cell with respect to the outside. The potential that is recorded when a living cell is impaled with a microelectrode is called the resting potential, and varies from cell to cell. Here it is shown to be -60 mV, but can range between -80 mV and -40 mV, depending on the particular type of nerve cell. In the absence of any stimulation, the resting potential is generally constant.
It is also possible to record and study the action potential. Here the cell has already been impaled with one microelectrode (the recording electrode) which is connected to a voltmeter. The electrode records a resting potential of -60 mV. The cell has also been impaled with a second electrode called the stimulating electrode. This electrode is connected to a battery and a device that can monitor the amount of current that flows through the electrode. Changes in membrane potential are produced by closing the switch and by systematically changing both the size and polarity of the battery. If the negative pole of the battery is connected to the inside of the cell, there will be an instantaneous change in the amount of current that flows through the stimulating electrode, and the membrane potential becomes transiently more negative. This result should not be surprising. The negative pole of the battery makes the inside of the cell more negative than it was before. A change in potential that increases the polarized state of a membrane is called a hyperpolarization. The cell is more polarized than it was normally. Use yet a larger battery and the potential becomes even larger. The resultant hyperpolarizations are graded functions of the size of the stimuli used to produce them.
Figure 1.3 |
When the positive pole of the battery is connected to the electrode, the potential of the cell becomes more positive when the switch is closed. Such potentials are called depolarizations. The polarized state of the membrane is decreased. A larger battery produces even larger depolarizations. Again, the magnitude of the response is proportional to the size of the stimulus. However, an unusual event occurs when the size of the depolarization reaches a level of membrane potential called the threshold. A totally new type of signal is initiated; the action potential. Note that if the size of the battery is increased even more, the amplitude of the action potential is the same as the previous one. If the suprathreshold stimulus was long enough, however, a train of action potentials would be elicited. The action potentials would continue to fire as long as the stimulus continues, with the frequency of firing being proportional to the magnitude of the stimulus.
Action potentials are not only initiated in an all-or-nothing fashion, but they are also propagated in an all-or-nothing fashion. An action potential initiated in the cell body of a motor neuron in the spinal cord will propagate in an undecremented fashion all the way to the synaptic terminals of that motor neuron.
Components of the Action Potentials
The action potential consists of several components. The threshold is the value of the membrane potential which, if surpassed, leads to the all-or-nothing initiation of an action potential. The initial or rising phase of the action potential is called the upstroke. The region of the action potential between the 0 mV level and the peak amplitude is the overshoot. The return of the membrane potential to the resting potential is called the repolarization phase. There is also a phase of the action potential during which time the membrane potential is actually more negative than the resting potential. This phase of the action potential is called the undershoot or the hyperpolarizing afterpotential.
