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One way investigators can record an action potential is by using a microelectrode without penetrating the axon. This is done by placing two electrodes on the surface of the axon and comparing the electrical charge at each point. When there is an action potential, the electrode located there would yield a somewhat negative signal compared to the electrode where there is no action potential. As the action potential is conducted to the second electrode, the polarity of the recording reverses. This technique is used clinically to assess nerve function.

Peripheral nerves and many central pathways consist of a population of axons with various diameters. Some of these axons are myelinated, while others are not. As a result, action potentials travel at different velocities in individual axons. When recording from such a nerve with external electrodes, there is not a single synchronous peak but a series of peaks that vary in time and magnitude. This variation reflects the conduction velocity and number of axons in each velocity group. This recording is called a compound action potential.

The clinical value of such a recording is its ability, in certain disease states, to reveal the dysfunction of a particular group of axons associated with specific functions. Additionally, the non-invasive nature of the technique allows it to be performed with skin surface electrodes.

Receptor or generator potentials can transform environmental stimuli into patterns of action potentials that are conducted into the central nervous system (CNS) through one or more axons. For this to occur, the stimulus must produce receptor potentials that are large enough to change the spiking levels of one or more primary afferent fibers connected to the receptor. Weaker intensities of stimulation can produce subthreshold receptor potentials, but they do not change the activity of central sensory neurons and thus go undetected. The weakest stimulus that can be reliably detected is defined as the stimulus threshold.

Sensory transduction can be evoked by mechanical, thermal, chemical, or other forms of energy in the environment. The types of information used by a particular organism depend on its set of sensory receptors. While humans cannot sense electrical or magnetic fields, other animals have the ability to do so. For example, many fish possess electroreceptors, and some fish and birds use the earth's magnetic field for orientation during migration.

The transduction process varies depending on the type of environmental stimulus being detected. Fig. 5.13 provides three examples of how stimuli can alter the membrane properties of specific sensory receptors that transduce such stimuli. The chemoreceptor, used for taste and smell, responds when a chemical stimulant reacts with receptor molecules on the plasma membrane of the sensory receptor. The binding of the chemical stimulant to the receptor molecule opens an ion channel, allowing an influx of an ionic current that depolarizes the sensory receptor cell.

In the case of a mechanoreceptor, such as those found in the skin, the ion channel opens in response to the application of a mechanical force along the membrane. This mechanical force causes an influx of current, leading to the depolarization of the sensory receptor.

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Investigatorscanrecordanactionpotentialwithamicroelectrodewithoutpenetratingtheaxonbyplacingtwospacedelectrodesonitssurfaceandcomparingtheelectricalchargeateachpoint.Anelectrodelocatedwherethereisanactionpotentialwouldyieldasomewhatnegativesignalincomparisontoanelectrodewherethereisnoactionpotential.Astheactionpotentialisconductedtothesecondelectrode,thepolarityoftherecordingreverses.Thistechniqueisusedclinicallytoassessnervefunction.Peripheralnervesandmanycentralpathwaysconsistofapopulationofaxons ofvariousdiameters(Fig.5.12 );someoftheaxonsaremyelinated,andsomearenot.Consequently,actionpotentialstravelatdifferentvelocitiesintheindividualaxons.Asaresult,arecordingfromsuchanervewithexternalelectrodesdoesnotshowasinglesynchronouspeakbutaseriesofpeaksthatvaryintime(whichreflectstheconductionvelocityofgroupsofaxons)andinmagnitude(whichreflectsthenumberofaxonsineachvelocitygroup).Thisiscalledacompound action potential.Theclinicalvalueofsucharecordingisitsability,incertaindiseasestates,torevealthedysfunctionofaparticulargroupofaxonsassociatedwithspecificfunctions,aswellasthenoninvasivenatureofthetechniquebecauseitcanbeperformedwithskinsurfaceelectrodes( receptor or generator potentials. The receptor potentials can then be transformed into patterns of action potentials that are conducted over one or more axons into the CNS. In order for this to happen, the stimulus must produce receptor potentials that are large enough to change the spiking levels of one or more primary afferent fibers that are connected to the receptor. Weaker intensities of stimulation can produce subthreshold receptor potentials, but such stimuli do not change the activity of central sensory neurons and thus are not detected. Thus stimulus threshold is defined as the weakest stimulus that can be reliably detected.
Environmental events that evoke sensory transduction can be mechanical, thermal, chemical, or other forms of energy. However, the types of information used by a
CHAPTER 5 Generation and Conduction of Action Potentials particular organism depend on its set of sensory receptors.
For example, humans cannot sense electrical or magnetic fields, but other animals can sense such stimuli. In particular, many fish have electroreceptors, and various fish and birds use the earth’s magnetic field to orient themselves during migration.
The transduction process varies with the type of environmental stimulus being detected.
Fig. 5.13 shows three examples of how stimuli can alter the membrane properties of the specific sensory receptors that transduce such stimuli 0246 (further details for each of these examples are given in other 36 72 108 m/s chapters).
Fig. 5.13A illustrates how a chemoreceptor, such A as that used for taste and smell, might respond when a chemical stimulant reacts with receptor molecules on the plasma membrane of the sensory receptor. Binding of the •Fig. 5.12 Thedistributionofaxons,bysizeandconductionvelocity,inamixed(muscle)nerve(A) andacutaneousnerve(B). Notetheincreasednumberofsmall-diameterfibersandtheabsenceofAα fibersinthecutaneousnerve.(FromHainesDE[ed].Fundamental Neuroscience for Basic and Clinical Applications. 3rded.Philadelphia:ChurchillLivingstone;2006.) chemical stimulant to the receptor molecule opens an ion channel, which enables the influx of an ionic current that depolarizes the sensory receptor cell. (This is similar to what is described for ligand-gated channels in
Chapter 6.)
Fig. 5.13B, the ion channel of a mechanoreceptor, such as those in the skin, opens in response to the application of a mechanical force along the membrane, and this allows an influx of current to depolarize the sensory receptor. In