Somatosensory System: Overview
Somatosensation includes multiple senses: pain (nociception), temperature (thermoreception), touch, and the sense of our limb position in space (proprioception). Each submodality of somatosensation relies on different types of receptors embedded in the skin, muscle, and joints and involves different structures in the spinal cord and in the brain.
Pain is arguably one of the most vital senses as it signals when our body is liable to being damaged. There are many different types of receptors in the skin that signal a potentially harmful stimulus. Some receptors respond to intense mechanical deformations of the skin, others to extreme temperatures, and still others to different kinds of chemicals. Pain comprises a sensory discriminative component, which provides information about the location, duration, intensity, and quality of the pain; an affective one, which signals its unpleasantness; and a cognitive-evaluative one, which is associated with cognitive variables such as attention, which can modulate the sensory experience.
Our ability to sense whether an object is warm or cold relies on two types of thermoreceptive fibers – so-called “cold” and “warm” fibers – embedded in the skin. As their names suggest, “cold” fibers are activated when the skin is cooled and “warm” fibers are activated when the skin is warmed. In contrast to thermosensitive nociceptive fibers, which respond at extreme temperatures, thermoreceptive fibers only respond at intermediate, non-noxious temperatures (with the exception of the paradoxical response of some cold fibers to high temperatures).
The sense of touch plays a critical role in our ability to grasp and manipulate objects. Indeed, cutaneous signals provide information about the forces we exert on objects and whether these are slipping from our grasp. Without these signals, we would routinely crush or drop objects. Touch also plays an important role in emotional communication: We touch the people we care about and wish to be touched by them. Finally, our sense of touch plays a key role in embodiment, making our body feel as a part of us.
The skin is innervated by several types of mechanoreceptive afferents, each of which conveys different information about skin deformations (link: sensory innervation of the skin) and conveys different types of information about events impinging upon the skin. Merkel cells convey information about the shape of objects grasped in the hand (link: cutaneous mechanoreceptive afferents: neural coding of shape), Meissner corpuscles about motion of objects across the skin, and Pacinian corpuscles about surface texture (link: cutaneous mechanoreceptive afferents: neural coding of texture). Afferents produce highly repeatable and temporally patterned responses to skin stimulation (link: somatosensory neurons: spike timing), and models have been developed that predict with high accuracy the responses of somatosensory neurons to spatiotemporal skin deformations (link: mechanotransduction: models).
When we palpate an object, we obtain information about its shape (link: somatosensory cortex: neural coding of shape), its texture (link: cutaneous mechanoreceptive afferents: neural coding of texture), and its motion across the skin (link: somatosensory cortex: neural coding of motion).
Proprioception (link: proprioception) plays a critical role in guiding motor behavior. Individuals with intact motor systems but compromised proprioception have difficulty planning and executing movements, almost as if they had a motor impairment. Proprioception, like touch, is also important for our sense of embodiment.
There are several types of proprioceptive receptors, located in muscles, in the skin, and in joint capsules. Two types of muscle proprioceptors, muscle spindles and Golgi tendon organs, are thought to be the primary contributors to proprioception. One population of receptors in the skin is sensitive to skin stretch and can convey information about joint angle. Another type of proprioceptor, the joint capsule receptor, fires at the extreme ends of the joint’s range and may be involved in preventing overextension of the joint.
Proprioceptive and cutaneous signals are then processed in the dorsal column nuclei, then in the ventroposterior lateral nucleus in the thalamus, and then in primary and secondary somatosensory cortices (link: somatosensory cortex: organization).