Spatial acuity or two-point discrimination is a crucial measure of somatosensory function in both behavioral and clinical settings. To assess this, doctors simultaneously apply two needle-like points to a patient's skin. The patient will perceive the points as distinct stimuli if they are farther apart than a certain threshold distance, which varies across the body. The highest discrimination (with the shortest threshold distance) occurs at the fingertips. Spatial acuity is primarily governed by type 1 units, which have smaller receptive fields compared to type 2 units. The threshold distance for a specific skin region is closely linked to the density of type 1 units. Notably, the density of these units decreases from the fingertips to the palm and forearm, correlating with an increase in threshold distance. This variation in innervation density aligns with the overall sensitivity of different skin regions to cutaneous stimuli.

Microneurographic techniques have been used to study the relationship between firing rates in different afferent classes and perceived stimulus quality. When a single SA fiber is stimulated with brief current pulses that trigger spikes, the sensation experienced is one of steady pressure at the fiber's receptive field area. As the pulse frequency increases, there is a corresponding increase in perceived pressure. Therefore, the firing rate in SA fibers encodes the force of the tactile stimulus. Similarly, repetitive stimulation of an FA fiber initially leads to a sensation of tapping, but as the stimulus frequency rises, it changes to a perception of vibration. Importantly, in both cases, the qualitative character of the stimulus remains the same and does not elicit a feeling of pain as long as it only activates a specific fiber class. This demonstrates that pain is a distinct submodality that involves a set of fibers separate from those utilized by low-threshold mechanoreceptors.

These findings illustrate the labeled line principle in sensory systems. According to this principle, the modality or quality of a particular sensation is conveyed to the central nervous system (CNS) by a specific set of afferents with distinct targets in the nervous system. Changes in activity within these afferents only impact quantitative aspects of the sensation. Even at higher levels of the CNS, such as the thalamus and primary somatosensory cortex, somatosensory submodalities (e.g., information from FA and SA mechanoreceptors, proprioceptors, and nociceptors) seem to utilize relatively separate cell populations.

During embryological development, axons of the peripheral nervous system (PNS) enter or exit the CNS through spinal roots or cranial nerves. The dorsal root of a given spinal segment consists entirely of the central processes of its associated dorsal root ganglion cells. The ventral root primarily contains motor axons, including α and γ motor neuron axons, and, at specific segmental levels, autonomic preganglionic axons. The pattern of innervation is established during embryological development, with each dorsal root ganglion supplying a specific cutaneous region known as a dermatome in adults. Although the development and rotation of the upper and lower extremities, as well as upright posture in humans, can lead to distortion of dermatomes, their sequence can be easily understood when depicted on the body of a quadrupedal individual. While a dermatome receives the densest innervation from the corresponding spinal cord segment, collateral fibers from adjacent spinal segments also supply the dermatome. Consequently, transecting a single dorsal root has minimal impact on sensory perception within the corresponding dermatome, requiring the interruption of several adjacent dorsal roots to induce anesthesia in a specific dermatome.
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, but their density falls off from fingertip to palm to forearm, and this fall-off 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 issue that has been addressed 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. Thus 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—for example, to a feeling of pain—so 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 the fact that it is conveyed 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.
Innervation of the Body
Axons of the peripheral nervous system (PNS) enter or leave the CNS through the spinal roots (or through cranial nerves). The dorsal root of a given spinal segment is composed entirely of the central processes of its associated dorsal root ganglion cells. The ventral root consists chiefly of motor axons, including α and γ motor neuron axons (see ), and at certain segmental levels, autonomic preganglionic axons (see
CHAPTER 7 The Somatosensory System
The pattern of innervation is determined during embryo-logical development. In adults a given dorsal root ganglion supplies a specific cutaneous region called a dermatome. Many dermatomes become distorted during development, chiefly because of rotation of the upper and lower extremities as they are formed, but also because humans maintain an upright posture. However, the sequence of dermatomes can readily be understood if depicted on the body of a person in a quadrupedal position (
Fig. 7.4).
Although a dermatome receives its densest innervation from the corresponding spinal cord segment, collaterals of afferent fibers from the adjacent spinal segments also supply the dermatome. Thus transection of a single dorsal root causes little sensory loss in the corresponding dermatome. Anesthesia of any given dermatome requires interruption of several adjacent dorsal roots.