Neuron physiology and action potentials

Neurons are specialised in their physiology to receive, sort out and pass on information. The signals that neurons deal with involve small changes in the electrical voltage between the inside and outside of the cell, and integration is the process by which these voltage changes are combined together to determine the neuron's output signal. This is essentially how neurons make decisions.

The most conspicuous of these voltage changes are action potentials, which are the signals that neurons use to transmit information along long axons. Each action potential lasts slightly less than a millisecond (1 ms) at a particular location along the axon and travels along the axon at a speed that varies from less than 1 m/s to nearly 100m/s depending on the girth of the axon. Action potentials may be recorded from outside an axon by using fine wires as electrodes, as shown in Fig. 2.5a. Although the voltage change between the inside and outside of the neuron is about a tenth of a volt, the signal that the electrodes outside the cell pick up is much smaller, so considerable amplification is needed to display and measure the action potential. Each action potential is conducted rapidly along the axon and passes the two electrodes in succession. As it passes the first electrode, the latter will become positive with respect to the second electrode, and as the action potential passes the second electrode, the situation is rapidly reversed. Consequently, the output signal that the amplifier delivers has an S-shaped waveform when displayed on the oscilloscope (Fig. 2.5b). On a slower time scale, action potentials appear as stick-like departures from the baseline, and because of this appearance they are commonly called spikes.

A more precise method of recording a spike at one location along an axon is to use an intracellular electrode. This consists of a glass capillary tube that is drawn out to a very fine point and filled with a conducting salt solution such as potassium acetate. The electrode is connected to an amplifier by way of a silver wire placed into the electrode (Fig. 2.6a). When the tip of the electrode is inserted through the membrane of the cell, the salt solution is in electrical contact with the inside of the cell, and the signal recorded measures the difference in electrical potential, or voltage, between the

Action Potential Neurons

Figure2.5 Activity in the axon of a neuron, recorded with extracellular electrodes. (a) The recording method shown schematically. (b) The typical appearance of action potentials when recorded by this method and displayed on an oscilloscope. The oscilloscope is a sensitive voltmeter, and displays changes in voltage recorded with time.

Figure2.5 Activity in the axon of a neuron, recorded with extracellular electrodes. (a) The recording method shown schematically. (b) The typical appearance of action potentials when recorded by this method and displayed on an oscilloscope. The oscilloscope is a sensitive voltmeter, and displays changes in voltage recorded with time.

inside and the outside of the axon. The voltage recorded by an intracellular electrode is called the membrane potential. When the tip of the electrode first enters the cytoplasm, it usually records a voltage of 60-80 mV across the membrane, with the inside of the cell negative with respect to the outside. This is called the resting potential.

The cell membrane of a resting cell is thus polarised: there is a standing electrical voltage across the cell membrane. A reduction in electrical potential across the membrane, bringing the intracellular voltage closer to the extracellular, is called a depolarisation. Most depolarising events are excitatory because they increase the likelihood that the neuron will generate an action potential. An increase in the membrane potential from the resting value is called a hyperpolarisation, and the effect of this is inhibitory as it counteracts any depolarisation. Much of the process of integration involves

Record Resting Potential

Figure 2.6 Activity in a neuron recorded with an intracellular electrode. In this method (a), one electrode is used to inject current pulses of different magnitudes into the neuron and a second is used to record the voltage, or potential, across the membrane. In (b), the typical appearance on an oscilloscope of responses by a neuron to pulses of current is shown.

Figure 2.6 Activity in a neuron recorded with an intracellular electrode. In this method (a), one electrode is used to inject current pulses of different magnitudes into the neuron and a second is used to record the voltage, or potential, across the membrane. In (b), the typical appearance on an oscilloscope of responses by a neuron to pulses of current is shown.

an interplay between depolarising and hyperpolarising membrane potentials.

A second intracellular electrode may be used to stimulate the neuron with pulses of electrical current, as shown in Fig. 2.6. Negative pulses of current hyperpolarise the neuron, producing downward deflections in the voltage recording trace. Positive pulses of current depolarise the neuron, causing upward deflections in the voltage trace. If the depolarising current pulses are small, the neuron's response is passive, and the size of the voltage change is proportional to the size of the current stimulus. However, when the depolarising potential reaches a critical threshold value, the neuron responds actively by producing a spike. Membrane potential changes rapidly, reaching a peak when the inside of the neuron is about 40 mV pos itive in relation to the outside, and then returns to its resting level, often after a transient hyperpolarisation. A second spike cannot be initiated until a short time after the first one is complete, this time being called the refractory period.

When a spike occurs at one location, it depolarises the membrane for some distance away. This depolarisation acts as the stimulus for new spikes, but, because of the refractory period, only in the length of axon that has not recently produced a spike. A spike is therefore a stereotyped event, in which the membrane potential swings rapidly between resting potential to about 40 mV positive and back. The amplitudes of spikes in extracellular recordings appear to vary from axon to axon, but this is because the size of the extracellular signals picked up by the electrodes depends on the diameters of the axons and how far away the axons are from the electrodes.

A difference in the voltage inside and outside a cell is the consequence of two features of cell physiology. The first is that charged ions are distributed unevenly between the cytoplasm and extracellular fluid, and the second is that the membrane contains pores that, when open, allow particular ions to pass through. The resting potential arises because potassium ions are more concentrated in the cell's cytoplasm than in the extracellular fluid, and pores that are open in the resting cell membrane allow these ions (but not others such as sodium or chloride) to pass through readily. Driven by the concentration gradient, potassium tends to flow out of the cell. As each potassium ion flows out of the cell, however, it carries with it a positive charge, making it successively more difficult for subsequent potassium ions to leave. A balance is established between the concentration gradient that tends to push potassium outwards and the electrical gradient that tends to retain it inside the cell. The electrical gradient at the balance point is the resting potential.

The pores that allow ions to pass through the membrane are usually formed by specialised protein molecules that aggregate in particular formations. These proteins and their associated pore are called a channel. The resting membrane potential is therefore determined by the action of one kind of potassium channel. During a spike, two other types of channel, a sodium channel and another type of potassium channel, come into play. These two channel types have pores that are closed in the resting cell, but tend to open when the neuron is depolarised to the threshold voltage and beyond. Electrical excitation of the cell causes the proteins to alter their shape so that the central pore opens, and these channels are called voltage sensitive. The voltage-sensitive sodium channels open more quickly than the potassium channels so that at first sodium ions, which are more concentrated outside the neuron than inside, tend to pass into the cell. This continues until the electrical gradient across the membrane balances the concentration gradient for sodium ions, which occurs when the inside of the cell is about 40 mV more positive than the extracellular fluid. This is why the intracellular voltage reaches +40 mV at the peak of a spike. The spike is a brief event because the sodium channels do not remain open for very long, and voltage-sensitive potassium channels open at about the same time as the sodium channels close, so that potassium now leaves the neuron and the membrane repolarises towards resting potential.

The number of ions that flow during a spike is only a small proportion of the total number of ions in the cytoplasm and extracellular fluid. Nevertheless, the neuron needs to keep its batteries topped up by maintaining a difference in concentrations of sodium and potassium between its inside and outside. It does this by means of pumps, proteins in the membrane that consume metabolic energy to transport ions against concentration gradients.

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Essentials of Human Physiology

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