Chapter 7 of the medical textbook focuses on the somatosensory system. It discusses the cutaneous mechanoreceptors and the response patterns of the associated afferent fibers. Figure 7.2 provides a schematic view of the arrangement of major mechanoreceptors in both glabrous (hairless) and hairy skin. It also shows the firing patterns of different low-threshold mechanosensitive afferent fibers that innervate the encapsulated receptors of the skin. The data used in Figure 7.2 is based on the work of Johansson RS and Vallbo ÅB. 

Figure 7.3 illustrates the receptive field characteristics for type 1 and type 2 sensory afferents. The plots in the top row show the threshold level of force needed to evoke a response as a function of the distance across the receptive field. The size of the receptive field is depicted on the hand below each plot. This data is also based on the work of Johansson RS and Vallbo ÅB.

The characteristics of the cutaneous mechanoreceptors, such as FA1 and SA1 afferents, explain the firing properties of these sensory afferents. Meissner's corpuscles are innervated by FA1 afferents, while Merkel's disks are innervated by SA1 afferents. Both receptors are located superficially, either in the basal epidermis (Merkel) or just below the epidermis (Meissner). They are small and oriented to detect stimuli pressing down on the skin surface above them, resulting in small receptive fields for SA1 and FA1 afferents. In the case of glabrous skin, SA2 afferents terminate in Ruffini endings and FA2 afferents end in Pacinian corpuscles. These receptors are deeper in the dermis and connective tissue, making them sensitive to stimuli applied over larger areas. The capsules of Pacinian and Meissner receptors act as filters for slowly changing or steady stimuli, enabling these afferents to selectively respond 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, respectively. Pacinian corpuscles also play a role in the properties of FA2 afferents, but they are not found in hairy skin. Instead, they are located in deep tissues surrounding muscles and blood vessels. There is no exact analogue to FA1 afferents in hairy skin. Instead, there are hair units where afferent free endings wrap around hair follicles. Each hair unit connects with approximately 20 hairs, resulting in a large ovoid or irregularly shaped receptive field. These units are extremely sensitive to even the slightest hair movement. Additionally, there are field units that respond to touch on the skin, but unlike FA1 units, they have large receptive fields.

The receptive field properties and sensitivities of the various categories of afferents raise several psychophysical and neural coding questions. For instance, is the threshold of perception of tactile stimuli determined by the sensitivity of peripheral receptors or central processes? Microneurography experiments have shown that a single spike in an FA1 afferent from the finger can be perceived, indicating that the receptors limit sensitivity. However, perception in other skin regions is more dependent on central factors like attention.
CHAPTER 7 The Somatosensory System •Fig. 7.2 Cutaneousmechanoreceptorsandtheresponsepatternsofassociatedafferentfibers.A,Schematicviewsofglabrous(hairless)andhairyskinshowingthearrangementofthevariousmajormechanoreceptors.B,Firingpatternsofthedifferentcutaneouslow-thresholdmechanosensitiveafferentfibersthatinnervatethevariousencapsulatedreceptorsoftheskin.(TracesinBarebasedondatafromJohanssonRS,VallboÅB.Trends Neurosci 1983;6:27.) •Fig. 7.3 Receptivefieldcharacteristicsfortype1andtype2sensoryafferents.Plotsinthetoprowshowthethresholdlevelofforceneededtoevokearesponseasafunctionofthedistanceacrossthereceptivefield.Receptivefieldsizeisshownonthehandbeloweachplot.(DatafromJohanssonRS,VallboÅB.Trends Neurosci 1983;6:27.) 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, the same as for glabrous skin. 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 analogue 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 will connect with about 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 limit the sensitivity; however, for other skin regions, perception is more dependent on central factors such as attention.