Converting a receptor stimulus to a recognizable sensation is termed sensory coding. All sensory systems code for four elementary attributes of a stimulus: modality, location, intensity, and duration. Modality is the type of energy transmitted by the stimulus. Location is the site on the body or space where the stimulus originated. Intensity is signaled by the response amplitude or frequency of action potential generation. Duration refers to the time from start to end of a response in the receptor. These attributes of sensory coding are shown for the modality of touch in Figure1.
Fig1. Sensory systems encode four elementary attributes of stimuli: modality, location (receptive field), intensity, and duration (timing). A) The human hand has four types of mechanoreceptors; their combined activation produces the sensation of contact with an object. Selective activation of Merkel cells and Ruffini endings causes sensation of steady pressure; selective activation of Meissner and Pacinian corpuscles causes tingling and vibratory sensation. B) Location of a stimulus is encoded by spatial distribution of the population of receptors activated. A receptor fires only when the skin close to its sensory terminals is touched. These receptive fields of mechanoreceptors (shown as red areas on fingertips) differ in size and response to touch. Merkel cells and Meissner corpuscles provide the most precise localization as they have the smallest receptive fields and are most sensitive to pressure applied by a small probe. C) Stimulus intensity is signaled by firing rates of individual receptors; duration of stimulus is signaled by time course of firing. The spike trains indicate action potentials elicited by pressure from a small probe at the center of each receptive field. Meissner and Pacinian corpuscles adapt rapidly, the others adapt slowly. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. New York, NY: McGraw-Hill; 2000.)
When the nerve from a particular sensory receptor is stimulated, the sensation evoked is that for which the receptor is specialized no matter how or where along the nerve the activity is initiated. This principle, first enunciated by Johannes Müller in 1835, has been called the law of specific nerve energies. For example, if the sensory nerve from a Pacinian corpuscle in the hand is stimulated by pressure at the elbow or by irritation from a tumor in the brachial plexus, the sensation evoked is touch. The general principle of specific nerve energies remains one of the cornerstones of sensory physiology.
MODALITY
Humans have four basic classes of receptors based on their sensitivity to one predominant form of energy: mechanical, thermal, electromagnetic, or chemical. The particular form of energy to which a receptor is most sensitive is called its adequate stimulus. The adequate stimulus for the rods and cones in the eye, for example, is light (an example of electromagnetic energy). Receptors do respond to forms of energy other than their adequate stimuli, but the threshold for these nonspecific responses is much higher. Pressure on the eyeball will stimulate the rods and cones, for example, but the threshold of these receptors to pressure is much higher than the threshold of the pressure receptors in the skin.
LOCATION
The term sensory unit refers to a single sensory axon and all of its peripheral branches. These branches vary in number but may be numerous, especially in the cutaneous senses. The receptive field of a sensory unit is the spatial distribution from which a stimulus produces a response in that unit (Figure1). Representation of the senses in the skin is punctate. If the skin is carefully mapped, millimeter by millimeter, with a fine hair, a sensation of touch is evoked from spots over lying these touch receptors. None is evoked from the intervening areas. Similarly, temperature sensations and pain are produced by stimulation of the skin only over the spots where the receptors for these modalities are located. In the cornea and adjacent sclera of the eye, the surface area supplied by a single sensory unit is 50–200 mm2. The area supplied by one sensory unit usually overlaps and interdigitates with the areas supplied by others.
One of the most important mechanisms that enable localization of a stimulus site is lateral inhibition. Information from sensory neurons whose receptors are at the peripheral edge of the stimulus is inhibited compared to information from the sensory neurons at the center of the stimulus. Thus, lateral inhibition enhances the contrast between the center and periphery of a stimulated area and increases the ability of the brain to localize a sensory input. Lateral inhibition under lies two-point discrimination (see Clinical Box 1).
Clinical Box 1
INTENSITY
The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor. This is illustrated in Figure 2. As a greater pressure is applied to the skin, the receptor potential in the mechanoreceptor increases (not shown), and the frequency of the action potentials in a single axon transmitting information to the CNS is also increased. In addition to increasing the firing rate in a single axon, the greater intensity of stimulation also will recruit more receptors into the receptive field.
Fig2. Relationship between stimulus and impulse frequency in an afferent fiber. Action potentials in an afferent fiber from a mechanoreceptor of a single sensory unit increase in frequency as branches of the afferent neuron are stimulated by pressure of increasing magnitude. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vanders Human Physiology. 11th edition. New York, NY: McGraw-Hill; 2008.)
As the strength of a stimulus is increased, it tends to spread over a large area and generally not only activates the sense organs immediately in contact with it but also “recruits” those in the surrounding area. Furthermore, weak stimuli activate the receptors with the lowest thresholds, and stronger stimuli also activate those with higher thresholds. Some of the receptors activated are part of the same sensory unit, and impulse frequency in the unit therefore increases. Because of overlap and interdigitation of one unit with another, however, receptors of other units are also stimulated, and consequently more units fire. In this way, more afferent pathways are activated, which is interpreted in the brain as an increase in intensity of the sensation.
DURATION
If a stimulus of constant strength is maintained on a sensory receptor, the frequency of the action potentials in its sensory nerve declines over time. This phenomenon is known as receptor adaptation or desensitization. The degree to which adaptation occurs varies from one sense to another. Receptors can be classified into rapidly adapting (phasic) receptors and slowly adapting (tonic) receptors. This is illustrated for different types of touch receptors in Figure1. Meissner and Pacinian corpuscles are examples of rapidly adapting receptors, and Merkel cells and Ruffini endings are examples of slowly adapting receptors. Other examples of slowly adapting receptors are muscle spindles and nociceptors. Different types of sensory adaptation likely have some value to the individual.
Light touch would be distracting if it were persistent; and, conversely, slow adaptation of spindle input is needed to maintain posture. Similarly, input from nociceptors provides a warning that it would lose its value if it is adapted and disappeared.
NEUROLOGIC EXAM
The sensory component of a neurologic exam includes an assessment of various sensory modalities including touch, proprioception, vibratory sense, and pain. Cortical sensory function can be tested by placing familiar objects in a patient’s hands and asking him or her to identify it with the eyes closed. Clinical Box 1 describes some of the common assessments made in a neurologic exam.
