Human Physiology - William F. Ganong 2002

Physiology of Nerve and Muscle Cells
Initiation of Impulses in Sense Organs
Ionic Basis of Excitation

In some cases, mechanical distortion opens channels in the receptor membrane. There is evidence that such mechanoreceptors are linked to epithelial sodium channels (ENaCs; see Chapter 1). Consequently, the influx of Na⁺ triggers a receptor potential, and the number of open channels is likely proportional to the stimulus intensity. This mechanism is thought to operate in Pacinian corpuscles and the Hair Cells of the Auditory and Vestibular systems. In other receptors, such as rods and cones, responses rely on heterotrimeric G Proteins and Cyclic NUCLEOTIDES. In certain instances, these mechanisms remain unknown.

Adaptation

When a receptor is subjected to a sustained stimulus of constant strength, the frequency of action potentials in its sensory nerve gradually decreases over time. This phenomenon is known as adaptation or desensitization. The degree of adaptation varies depending on the sensory organ type (Fig. 5-4). Touch is a rapidly adapting sensation, and its receptors are termed phasic receptors. On the other hand, the carotid sinus, Muscle spindles, cold and pain receptors, as well as pulmonary stretch receptors, adapt very slowly and incompletely; these are referred to as tonic receptors. Pressure applied to a Pacinian corpuscle elicits a receptor potential that rapidly decays despite the continued pressure. If the Connective Tissue capsule is removed and the bare nerve ending is subjected to pressure, a receptor potential can still be induced, though it decays more slowly (see Fig. 5-2). The rapid decay observed in an intact corpuscle is likely due to the lateral Displacement of the connective tissue lamellae, which relieves the pressure on the nerve ending. However, this is not the sole factor in adaptation; a gradual decline in the number of action potentials generated over time still occurs under a constant stimulus even after the outer lamellae of the Pacinian corpuscle are removed. This decrease results from the accommodation of the sensory nerve fiber to the receptor potential.

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Fig. 5-3. Relationship between muscle length and receptor potential (top) and impulse frequency (bottom) in a crustacean stretch receptor. Squares and circles represent values from two different preparations. (Reproduced with permission from Terzuolo CA, Washizu Y: Relation between stimulus strength, generator potential, and impulse frequency in stretch receptor of Crustacea. J Neurophysiol 1962;25:56.)

The slow, incomplete adaptation of the muscle spindle is due to the fact that its receptor potential persists and decays very slowly during a constant stimulus. The slow adaptation of muscle spindles, carotid sinuses, pain, and cold receptors serves a clear biological purpose for the Organism. Muscle stretch plays a vital role in maintaining posture over extended periods. Furthermore, sensations of pain and cold are triggered by potentially harmful stimuli; if their receptors adapted rapidly, they would largely lose their warning function. Carotid and aortic receptors are continuously involved in Blood Pressure Regulation, and adaptation of these receptors would impair the precision of this regulatory system.

Fig. 5-4. Adaptation. The height of the curve in each case reflects the discharge frequency in sensory nerve fibers at various times after the onset of a constant stimulus. (Reproduced with permission from Adrian ED: Basis of Sensation, Christophers, 1928.)