Robert Schleip and Heike Jäger

Introduction

While the sense of proprioception is fairly well known to therapists working with fascia, interoception and its inclusion in fascial therapies may be a “new concept” for many. The concept is not so new: in the nineteenth century it was called coenesthesia: the neurological model of a mostly unconscious sense of the normal functioning of the body and its organs. Early German physiologists called it Gemeingefuehl or “common sensations” and differentiated them from the five senses of Sherrington’s early writings. Recently, however, the same concept has been intensely revived under the term interoception, and novel insights regarding the anatomical, physiological, and neurological details of this sensory system have led to an almost explosive increase of scientific attention and exploration.

Disorders such as anxiety, depression, or irritable bowel syndrome have subsequently been described as interoceptive disorders. Most notably, it has been proposed that the neural pathways associated with interoception may be considered as a potential correlate for consciousness (Craig 2009). The sensory receptors for interoception are free nerve endings, most of which are located in fascial tissues throughout the human body. It is helpful to understand that proprioception and interoception are organized differently in the human brain and that very different afferent pathways are involved in them.

What is interoception?

Previous concepts of interoception often focused on visceral sensations only. Current concepts describe interoception as a sense of the physiological condition of the body, which includes a much wider range of physiological sensations, including, for example, muscular effort, tickling, or vasomotor sensations (see Box 2.3.1). These sensations are triggered by stimulation of unmyelinated sensory nerve endings (free nerve endings) that project to the insular cortex rather than to the primary somatosensory cortex which is usually considered as the main target of proprioceptive sensations (Berlucchi & Aglioti 2010).

Feelings from these sensations not only have a sensory, but also an affective, motivational aspect and are always related to the homeostatic needs of the body. They are associated with behavioral motivations that are essential for the maintenance of physiological body integrity.

Sensual touch

A recent and surprising addition to the above list of interoceptive sensations is the sense of sensual or pleasant touch. This discovery was triggered by examinations of a unique patient lacking myelinated afferents in whom slow stroking of the skin with a soft brush triggered a faint and obscure sensation of pleasant touch (and general well-being), although the patient was unable to recognize any stroking direction. Functional magnetic imaging showed that this vague sensation was accompanied by a clear activation of the insular cortex, while no activation was seen in the primary somatosensory cortex. (Olausson et al. 2010).

Based on the innervation of primate skin and on subsequent studies with other patients it was concluded that the affected sensory receptors are unmyelinated C-fiber afferents with a low mechanical threshold, and that these endings are themselves connected with neural interoceptive pathways. Those afferents have a slow conduction velocity (0.5–1.0 s delay from stimulus to arrival in the brain). Since these receptor types have never been found in the palm of the hand despite numerous microneurographic recordings, it is assumed that they are present in hairy skin only and are absent in glabrous skin. It is concluded that human skin contains particular touch receptors which form a system for social touch that may underlie emotional, hormonal (for example oxytocin), and affiliative responses to caress-like, skin-to-skin contact between individuals (Fig. 2.3.1). The profound importance of such a system for human health and well-being has long been indicated (Montague 1971), at least since the classical study of Harlow (1958) with baby rhesus monkeys that express affection for a surrogate mother in response to tactile comfort.

A new phylogenetic development

The afferent neurons related to interoception terminate in lamina I, the most superficial layer of the dorsal horn of the spinal cord. This lamina projects strongly to the sympathetic cell columns of the thoracolumbar spinal cord, where the sympathetic preganglionic cells of the autonomic nervous system originate. From there they project to the main homeostatic integration sites in the brainstem. The latter include brainstem regions – such as the parabrachial nucleus – which are densely interconnected with the amygdala and hypothalamus. In addition, they project to the insular cortex.

Interestingly, this particular “lamina I spinothalamocortical pathway” is a comparatively recent phylogenetic acquisition of primates. It evolved from the afferent portion of the evolutionary ancient system that maintains the homeostatic integrity of the body. In mammals, the activity of lamina I neurons is integrated in the parabrachial nucleus; it is only from there that they are further projected to the insular cortex via the ventromedial thalamic nucleus (Craig 2009). In primates, however, there are direct projections from lamina I to thalamic regions from which they are further conveyed to the insular cortex (Fig. 2.3.2). In other words, primates possess a more direct route between the afferent region for interoceptive sensations in the spinal cord (lamina I) and their insular cortex. No comparable difference in terms of neuronal architecture between primates and other mammals has been observed regarding their processing of proprioception.

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