The Somatosensory System

Low-Threshold Mechanoreceptors

The skin is an important sensory organ, and not surprisingly, is richly innervated with a variety of somatosensory receptors and associated afferents. We first consider the afferent types related to fine or discriminatory touch sensations. These afferents are related to what are called low-threshold mechanoreceptors. Nociceptor and thermoceptor innervation will be considered separately in a later section of this chapter.

To study the responsiveness of tactile receptors, a small-diameter rod or wire is used to press on a localized region of skin. With this technique, two basic types of responses may be seen when recording sensory afferent fibers: fast-adapting (FA) and slow-adapting (SA) responses ( Fig. 7.2 ). They are present in similar quantities. FA fibers show a short burst of action potentials when the rod first pushes down on the skin, but then they will cease firing despite continued application of the rod. They may also burst at the cessation of the stimulus (i.e., when the rod is lifted off). In contrast, SA units will start firing action potentials (or increase their firing rate) at the onset of the stimulus and continue to fire until the stimulus ends.

Both the FA and SA afferent classes can be subdivided on the basis of other aspects of their receptive fields, where receptive field is defined as the region of skin from which stimuli can evoke a response (i.e., change the firing of the afferent axon). Type 1 units have small receptive fields with well-defined borders. Particularly for glabrous skin (i.e., hairless skin, such as on the palms of the hands and soles of the feet), the receptive field has a circular or ovoid shape within which there is relatively uniform and high sensitivity to stimuli that decreases sharply at the border ( Fig. 7.3 ). Type 1 units, particularly SA1 units, respond best to edges. That is, a larger response is elicited from them when the edge of a stimulus cuts through their receptive field than when the entire receptive field is indented by the stimulus.

Type 2 units have wider receptive fields with poorly defined borders and only a single point of maximal sensitivity, from which there is a gradual reduction in sensitivity with distance (see Fig. 7.3 ). By comparison, a type 1 unit’s receptive field typically covers about four papillary ridges in the fingertip, whereas a type 2 unit will have a receptive field that covers most, if not all, of a finger.

Receptive Field Properties

There are four main classes of low-threshold mechanosensitive afferents that have been identified physiologically (FA1, FA2, SA1, and SA2). Peripherally these axons may terminate either as free nerve endings, associated with a hair follicle, or within a specialized receptor structure made up of supporting cells.

For glabrous skin the four afferent classes have been associated with four specific types of histologically identified receptor structures whose locations and physical characteristics help explain the firing properties of these sensory afferents. FA1 afferents terminate in Meissner’s corpuscles, whereas SA1 afferents terminate in Merkel’s disks. In both cases the receptor is located relatively superficially, either in the basal epidermis (Merkel) or just below the epidermis (Meissner) (see Fig. 7.2 ). These receptors are small and oriented to detect stimuli pressing down on the skin surface just above them, thus allowing SA1 and FA1 afferents to have small receptive fields. For glabrous skin, SA2 afferents terminate in Ruffini endings and FA2 afferents end in Pacinian corpuscles. Both of these receptors lie deeper in the dermis and connective tissue and therefore are sensitive to stimuli applied over much larger territory. The capsules of both Pacinian and Meissner receptors act to filter out slowly changing or steady stimuli, thus making these afferents selectively sensitive to changing stimuli.

For hairy skin, the relationship between receptors and afferent classes is similar to that of glabrous skin. SA1 and SA2 fibers connect to Merkel and Ruffini endings. Pacinian corpuscles also underlie the properties of FA2 afferents; however, they are not found in hairy skin but instead are located in deep tissues surrounding muscles and blood vessels. There is not an exact analog to the FA1 afferents; rather, there are hair units, which are afferents whose free endings wrap around hair follicles (see Fig. 7.2 ). Each such hair unit connects with approximately 20 hairs to produce a large ovoid or irregularly shaped receptive field. These units are extremely sensitive to movement of even a single hair. There are also field units that respond to touch of the skin, but unlike FA1 units, they have large receptive fields.

Several psychophysical and neural coding questions can be related to the receptive field properties and sensitivities of the various categories of afferents. For example, is the threshold of perception of tactile stimuli due to the sensitivity of the peripheral receptors or to central processes? In fact, by using microneurography it is possible to show that a single spike in an FA1 afferent from the finger can be perceived, thus indicating that the receptors determine the sensitivity; however, for other skin regions, perception is more dependent on CNS factors, such as attention.

An important behavioral and clinical measure of somatosensory function is spatial acuity or two-point discrimination. Clinically, a doctor will apply two needle-like points simultaneously to the skin of a patient. The patient will generally perceive the points as two distinct stimuli, as long as they are farther apart than some threshold distance, which varies across the body. The best discrimination (shortest threshold distance) is at the fingertips. Type 1 units underlie spatial acuity, which is not surprising given the smaller receptive fields of type 1 units than type 2 units. Moreover, the threshold distance for a region of skin is most closely related to its density of type 1 units, because these units have similarly sized receptive fields throughout the glabrous skin. Their density, however, diminishes from fingertip to palm to forearm, and this decrease correlates with the rise in threshold distance. Note that this variation in innervation density also matches the overall sensitivity of different skin regions to cutaneous stimuli.

The relationship of the firing rates in the various afferent classes to perceived stimulus quality is another important factor that has been observed with microneurographic techniques. When a single SA fiber is stimulated with brief current pulses, such that each pulse triggers a spike, a sensation of steady pressure is felt at the receptive field area of that fiber. As pulse frequency is increased, a concurrent increase in pressure is perceived—the firing rate in SA fibers codes for the force of the tactile stimulus. As another example, when an FA fiber is repetitively stimulated, a sensation of tapping results first, and as the frequency of the stimulus is increased, the sensation turns to one of vibration. Interestingly, in neither case does the stimulus change its qualitative character (e.g., the perception of pain) as long as the stimulus activates only a particular fiber class. This is evidence that pain is a distinct submodality that uses a set of fibers distinct from those used by low-threshold mechanoreceptors.

These findings illustrate an important principle of sensory systems called labeled line. The idea is that the quality (i.e., modality) of a particular sensation results from being transmitted to the CNS by a specific set of afferents that have a distinct set of targets in the nervous system. Alterations in activity in these afferents will therefore change only quantitative aspects of the sensation. As will be seen in more detail later, the various somatosensory submodalities (i.e., information arising from FA and SA mechanoreceptors, proprioceptors, and nociceptors) appear to use relatively separate dedicated cell populations, even at relatively high levels of the CNS such as the thalamus and primary somatosensory cortex.