This section focuses on the actual function of the sensory receptor in translating environmental energy into action potentials, the fundamental units of information in the nervous system. A device that performs such a translation is called a transducer; sensory receptors are biological transducers. The sequence of electrical events in the sensory transduction process is shown in Figure 4.2.
The Generator Potential. The sensory receptor in this example is a mechanoreceptor. Deformation or deflection of the tip of the receptor gives rise to a series of action potentials in the sensory nerve fiber leading to the central nervous system (CNS). The stimulus (1) is applied at the tip of the receptor, and the deflection (2) is held constant (dotted lines). This deformation of the receptor causes a
portion of its cell membrane (shaded region ) to become more permeable to positive ions (especially sodium). The increased permeability of the membrane leads to a localized depolarization, called the generator potential. At the depolarized region, sodium ions enter the cell down their electrochemical gradient, causing a current to flow in the extracellular fluid. Because current is flowing into the cell at one place, it must flow out of the cell in another place. It does this at a region of the receptor membrane (4) called the impulse initiation region (or coding region) because here the flowing current causes the cell membrane to produce action potentials at a frequency related to the strength of the current caused by the stimulus. These currents, called local excitatory currents, provide the link between the formation of the generator potential and the excitation of the nerve fiber membrane.
In complex sensory organs that contain a great many individual receptors, the generator potential may be called a receptor potential, and it may arise from several sources within the organ. Often the receptor potential is given a special name related to the function of the receptor,- for example, in the ear it is called the cochlear microphonic, while an electroretinogram may be recorded from the eye. Note that in the eye the change in receptor membrane potential associated with the stimulus of light is a hyperpolar-ization, not a depolarization.
The production of the generator potential is of critical importance in the transduction process because it is the step in which information related to stimulus intensity and duration is transduced. The strength (intensity) of the stimulus applied (in Fig. 4.2, the amount of deflection) determines the size of the generator potential depolarization. Varying the intensity of the stimulation will correspondingly vary the generator potential, although the changes will not usually be directly proportional to the intensity. This is called a graded response, in contrast to the all-or-
none response of an action potential, and it causes a similar gradation of the strength of the local excitatory currents. These, in turn, determine the amount of depolarization produced in the impulse initiation region (4) of the receptor, and events in this region constitute the next important link in the process.
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