Review of Medical Physiology - William F. Ganong 2002

Physiology of Nerve and Muscle Cells
Initiation of Impulses in Sense Organs
Electrical and Chemical Phenomena in Receptors

Anatomical Relationships

The question of how receptors convert energy into an Action Potential in sensory nerves has been extensively studied. Complex Sensory Organs, such as the organs of Vision, Hearing, equilibrium, and taste, feature distinct receptor Cells and synaptic connections between the receptors and afferent nerves. However, in most cutaneous sensory organs, receptors consist of specialized, histologically modified terminals of sensory nerve fibers.

Pacinian corpuscles, which function as tactile receptors, have been studied in detail. Due to their relatively large size and accessibility within the mesentery of experimental animals, they can be isolated, examined using microelectrodes, and subjected to microdissection. Each corpuscle consists of a straight, unmyelinated terminal of a sensory nerve fiber, 2 µm in diameter, surrounded by concentric layers of Connective Tissue, giving the organ the appearance of a tiny onion. The myelin Sheath of the sensory nerve begins inside the corpuscle. The first node of Ranvier is also located within, whereas the second is typically situated near the point where the nerve fiber exits the corpuscle (Fig. 5-2).

Generator Potentials

Recording electrodes can be placed at the point where the sensory nerve exits the Pacinian corpuscle, and the corpuscle can then be subjected to graded pressure. Applying a mild pressure elicits a non-propagated depolarization potential resembling an EPSP. This is known as a generator or receptor potential. As pressure increases, the receptor potential increases in amplitude. When the generator potential reaches approximately 10 mV, an action potential is generated in the sensory nerve. If the pressure is increased further, the generator potential becomes larger, and the sensory nerve fires repeatedly.

Water/144.html">Origin of the Generator Potential

Microdissection studies have demonstrated that detaching the connective tissue lamellae from the unmyelinated nerve terminal and removing them from the Pacinian corpuscle does not abolish the generator potential. If the first node of Ranvier is blocked by pressure or local anesthetics, the generator potential persists, whereas impulse conduction ceases (see Fig. 5-2). Conversely, if the sensory nerve is severed and the unmyelinated terminal is allowed to degenerate, no generator potential is produced. These and other experiments have established that the generator potential originates in the unmyelinated nerve terminal. It has also been proven that such nerve endings are the site of generator potential generation in Merkel discs. Thus, the receptor converts mechanical energy into an electrical response proportional to the stimulus intensity. The generator potential, in turn, depolarizes the sensory nerve at the level of the first node of Ranvier. Once the threshold of excitation is reached, an action potential is triggered, and the membrane repolarizes. If the generator potential is sufficiently large, the neuron fires again as soon as it repolarizes, continuing to fire as long as the generator potential remains high enough to bring the Membrane Potential of the node to the firing threshold. Consequently, the node translates the receptor's graded response into action potentials at a frequency proportional to the stimulus intensity.

Class="center">

Fig. 5-2. Generation of the generator potential in the unmyelinated nerve terminal inside a Pacinian corpuscle: 1 - electrical responses to pressure 1x (trace a), 2x (b), 3x (c), 4x (d). The strongest stimulus elicits an action potential in the sensory nerve (e); 2 - similar responses were recorded after removal of the connective tissue capsule, although the responses were more prolonged due to partially impaired adaptation; 3 - generator responses persisted, but no action potential was recorded when the first node of Ranvier was blocked by pressure or drugs (arrow); 4 - all responses disappeared when the sensory nerve was transected prior to the experiment and underwent degeneration

Similar generator potentials have been studied in Muscle spindles. The relationship between muscle length—which determines the intensity of the stimulus within the spindle—and the magnitude of the generator potential, as well as the relationship between muscle length and the frequency of action potentials in the afferent nerve fiber exiting the spindle, is illustrated in Fig. 5-3. Under normal conditions, action potential frequency is related to stimulus intensity by a power function (see below). Generator potentials also occur in other organs and are generally essential for initiating depolarization in the sensory nerve fibers leaving the organ.