Fig. 5.13C, the ion channel of a retinal photoreceptor cell (so-called because it responds to light) is open in the dark and closed when a photon is absorbed by pigment on an internal disc membrane. In this case, an influx of current occurs in the dark; the current ceases when light is applied. When the current stops, the photoreceptor hyperpolarizes. (Because capture of the photon is distant from the ion channel that it influences, this process must involve an intracellular “second messenger” mechanism.)
The nature of the receptor also can vary. In the simplest situation, a receptor is just a specialized portion of an axon,
Aα N/A8-1344-78ExtrafusalskeletalmusclefibersAγ N/A2-812-48IntrafusalmusclefibersBN/A1-36-18PreganglionicautonomicfibersCN/A0.2-20.5-2Postganglionicautonomicfibers
FromHainesDE(ed).Fundamental Neuroscience for Basic and Clinical Applications. 3rded.Philadelphia:ChurchillLivingstone;2006.N/A,notapplicable.
•Fig. 5.13 Models of Transducer Mechanisms in Three Types of Receptors. A, Chemoreceptor.B, Mechanoreceptor.C, Photoreceptor.
in which case the transduction of a stimulus into receptor potential and the translation of this potential into a spike train all take place in the same cell. For example, a mechanical stimulus, such as pressure on the skin of a finger, can distort the membrane of an axon that forms part of a mechanoreceptor, as shown in
Fig. 5.14A . This distortion causes inward current flow at the end of the axon and longitudinal and outward current flow along the neighboring parts of the axon. The outward current produces a depolarization (the receptor potential) that might exceed the threshold for an action potential (see
Fig. 5.14B ). If so, one or more action potentials are evoked and then travel along this primary afferent fiber to the CNS and thereby convey information about the mechanical stimulus.
In many other cases, the receptor is composed of more than one cell. In this situation, transduction occurs in one cell, but spikes are generated in other cells that are •Fig. 5.14 A, Currentflow(thin arrows) producedbystimulation(thick arrow) ofamechanoreceptoratthetipofanaxon.AnintracellularrecordingelectrodeisplacedatthefirstnodeofRanvier.B, ThereceptorpotentialproducedbythecurrentandanactionpotentialthatwouldbesuperimposedonthereceptorpotentialifitweretoexceedthresholdatthefirstnodeofRanvier.
synaptically connected to it (see ). For example, in the cochlea, the primary afferent fibers get synaptic input from mechanoreceptive hair cells. Sensory transduction in such sense organs can be more complex in this arrangement. In photoreceptors, moreover, the receptor potential is hyperpolarizing, as mentioned earlier, and interruption of the dark current is the signal event. Information about each of these mechanisms is discussed in
Chapter 8.
Although the mechanisms of sensory transduction vary between stimulus types, the end result is typically a receptor potential in either the receptor cell or the primary afferent neuron (i.e., the first neuron in a sensory pathway) that has a synapse with the receptor cell.