Review of Medical Physiology - William F. Ganong 2002

Physiology of Nerve and Muscle Cells
Initiation of Impulses in Sense Organs
Coding of Sensory Information

There are differences in conduction velocity and other characteristics among sensory nerve fibers (see Chapter 2), yet Action potentials are similar across all nerves. For instance, action potentials in a nerve originating from a Touch receptor are identical to those in a nerve from a thermal receptor. Given this, the question arises as to why stimulating a touch receptor elicits a sensation of touch rather than warmth. Another question is how light or heavy touch can be communicated.

Doctrine of Specific Nerve Energies

The sensation elicited by impulses generated in a receptor depends on the specific region of the Brain they ultimately activate. Specific sensory pathways remain distinct all the way from the sense organ to the cortex. Consequently, when Neural Pathways from a particular sense organ are stimulated, the resulting sensation is the one for which that receptor is specialized, regardless of how or where along the sensory pathway the activity is induced. This principle, first formulated by Müller in 1835, came to be known by the rather cumbersome name of The Doctrine of specific nerve energies. For example, if a sensory nerve from a Pacinian corpuscle in the hand is stimulated by pressure near the elbow or by irritation from a tumor in the Brachial Plexus, the resulting sensation will be touch. Similarly, if a sufficiently fine electrode were inserted into the appropriate fibers of the dorsal columns of the Spinal Cord, the thalamus, or the postcentral gyrus of the Cerebral Cortex, the sensation evoked by the stimulation would be touch. This doctrine has occasionally been challenged, particularly by those who believed that pain results from the hyperstimulation of very diverse receptors. However, the overstimulation hypothesis has been largely discredited, and THE PRINCIPLE OF specific nerve energies stands as a cornerstone of sensory physiology.

Projection

Regardless of where a sensory tract is stimulated along its pathway to the cortex, the resulting conscious sensation is localized to the site of the receptor. This principle is known as the law of projection. Experiments involving stimulation of the cerebral cortex during neurosurgical Procedures on conscious patients illustrate this phenomenon. For example, if the cortical receptive area for impulses from the left hand is stimulated, the patient localizes the sensation in the left hand rather than in the HEAD. Another example is amputees. Some of these patients may complain of pain, often severe, and proprioceptive sensations in a limb that is no longer there (phantom limb). The severed nerve ends resulting from amputation frequently form tangled clusters of nerve fibers called neuromas. Pain can arise spontaneously in these neuromas or upon pressure. The impulses generated in this manner originate in nerve fibers that previously led from Sensory Organs in the amputated limb, and the perceived sensations are projected back to where the receptors used to be. Nevertheless, evidence indicates that the phantom limb phenomenon is also linked to plasticity within the Central Nervous System's Sensory systems (see Chapter 7).

Coding of Stimulus Intensity

There are two ways in which information regarding stimulus intensity is transmitted to the brain: by changing the frequency of action potentials generated by The activity of a specific receptor, and by altering the number of activated receptors.

It has long been established that the intensity of a sensation is proportional to the logarithm of the stimulus intensity (the Weber-Fechner law). However, it has since been discovered that this phenomenon is more accurately described by the formula

Class="center">R = KS_A,

where R represents sensation; S, stimulus intensity; and K and A are constants for any given modality of sensation. The frequency of action potentials generated by The stimulation of a sensory nerve fiber also relates to stimulus intensity as a power function. An example of such a relationship is shown in Fig. 5-3, where the exponent is approximately 1.0. Another example is depicted in Fig. 5-5, where the calculated exponent is 0.52. Modern evidence indicates that within the central nervous system, the relationship between stimulus and sensation versus stimulus intensity is primarily determined by The properties of the peripheral receptors themselves.

Sensory Units

The term "sensory unit" refers to a single sensory axon and all of its peripheral branches. The number of these branches varies, but they can be numerous, particularly in cutaneous sensory organs. The receptive field of a sensory unit is the area within which a stimulus elicits a response in that unit. In the cornea and adjacent sclera of the eye, the surface area supplied by a single sensory unit measures 50–200 mm². In general, areas supplied by one sensory unit overlap and interdigitate with areas supplied by neighboring units.

Fig. 5-5. Relationship between tactile stimulus (S) and Action Potential frequency in sensory nerve fibers (R). The data points correspond to individual values in cats, plotted on linear coordinates (left) and logarithmic coordinates (right). The equation describes a power-law relationship between R and S (reproduced with permission from Werner G, Mountcastle VB: Neural activity in mechanoreceptive cutaneous afferents. J Neurophysiol. 1965;28:359).

Recruitment of Sensory Units

As stimulus strength increases, it tends to spread over a broader area, generally activating sensory organs not only in direct contact but also in surrounding regions. Furthermore, weak stimuli activate receptors with the lowest thresholds, whereas strong stimuli engage receptors with higher thresholds. Some of the activated receptors belong to the same sensory unit, causing the impulse frequency within that unit to rise. However, due to the overlap and interdigitation of one unit with another, receptors of adjacent units are also stimulated, thereby recruiting more units into activity. In this manner, a greater number of afferent pathways are engaged, which the brain interprets as an increase in stimulus intensity.