The conveyance of somatosensory submodalities to the central nervous system (CNS) is carried out by afferent fibers of different sizes, which is of great experimental importance. It should be noted that the compound action potential observed in a peripheral nerve consists of a series of peaks, indicating that the axon diameters in a nerve are grouped rather than uniformly distributed. Tactile sensations are primarily carried by large-diameter myelinated fibers in the Aβ class, while pain and temperature information are transmitted by small-diameter, lightly myelinated (Aδ) and unmyelinated (C) fibers. By selectively blocking or stimulating a specific class of axons based on their size, it becomes possible to study the different somatosensory submodalities in isolation.

The skin, being a significant sensory organ, is densely innervated by various types of afferents. In this discussion, we will focus on the afferent types associated with fine or discriminatory touch sensations, while the innervation related to nociceptors and thermoceptors will be addressed separately later on.

To examine the responsiveness of tactile receptors, a small-diameter rod or wire is utilized to apply pressure on a localized area of the skin. When recording sensory afferent fibers using this technique, two main types of responses can be observed: fast-adapting (FA) and slow-adapting (SA) responses (Fig. 7.2). Both types are present in similar quantities. FA fibers display a brief burst of action potentials when the rod first makes contact with the skin, but they cease firing even if the rod continues to be applied. They may also exhibit a burst of activity when the stimulus is removed (i.e., when the rod is lifted off). In contrast, SA units start firing action potentials (or increase their firing rate) at the onset of the stimulus and continue to do so until the stimulus is terminated.

Both FA and SA afferent classes can be further divided based on additional characteristics of their receptive fields, where a receptive field refers to the area of skin from which stimuli can elicit a response (i.e., alter the firing of the afferent axon). Type 1 units have small receptive fields with clearly defined borders. Particularly in glabrous skin (i.e., hairless skin, such as the palms of the hands and soles of the feet), the receptive field has a circular or ovoid shape within which stimuli evoke relatively uniform and high sensitivity, which sharply decreases at the border (Fig. 7.3).

Type 1 units, especially SA1 units, exhibit a stronger response to edges. This means that they elicit a larger response when the edge of a stimulus intersects their receptive field compared to when the entire receptive field is indented by the stimulus.

Type 2 units have broader receptive fields with poorly defined borders and only one point of maximal sensitivity, from which sensitivity gradually diminishes with distance (see Fig. 7.3). For comparison, a type 1 unit's receptive field typically covers approximately four papillary ridges in the fingertip, while a type 2 unit has a receptive field that encompasses most or all of a finger.

Consequently, four main classes of low-threshold mechanosensitive afferents have been physiologically identified (FA1, FA2, SA1, and SA2). These axons terminate either as free nerve endings, associated with a hair follicle, or within a specialized receptor structure composed of supporting cells. In the case of glabrous skin, the four afferent classes have been linked to four specific types of histologically identified receptor structures, which are situated at specific locations and possess distinct physical properties.
Of great importance experimentally, the afferent fibers that convey these somatosensory submodalities to the CNS are different sizes. Recall that the compound action potential recorded from a peripheral nerve (see 5.1 ) consists of a series of peaks, thus implying that the diameters of axons in a nerve are grouped rather than being uniformly distributed. Information about tactile sensations is carried primarily by large-diameter myelinated fibers in the Aβ class, whereas pain and temperature information travels via small-diameter, lightly myelinated (Aδ) and unmyelinated (C) fibers. It is possible to block or stimulate selectively a class of axons of particular size, thereby allowing study of the different somatosensory submodalities in isolation.
Innervation of the Skin
The skin is an important sensory organ and not surprisingly is richly innervated with a variety of 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 will 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 ). For comparison a type 1 unit’s receptive field typically will cover approximately four papillary ridges in the fingertip, whereas a type 2 unit will have a receptive field that covers most or all of a finger.
Thus, four main classes of low-threshold mechanosensitive afferents 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