NEUROPHYSIOLOGICAL STUDIES OF THE INDIVIDUAL sensory modalities were first conducted in the somatosensory system (Greek soma, the body), the system that transmits information coded by receptors distributed throughout the body. Charles Sherrington, one of the earliest investigators of these bodily senses, noted that the somatosensory system serves three major functions: proprioception, exteroception, and interoception.
Proprioception is the sense of oneself (Latin proprius, one’s own). Receptors in skeletal muscle, joint capsules, and the skin enable us to have conscious awareness of the posture and movements of our own body, particularly the four limbs and the head. Although one can move parts of the body without sensory feedback from proprioceptors, the movements are often clumsy, poorly coordinated, and inadequately adapted to complex tasks, particularly if visual guidance is absent.
Exteroception is the sense of direct interaction with the external world as it impacts the body. The principal mode of exteroception is the sense of touch, which includes sensations of contact, pressure, stroking, motion, and vibration, and is used to identify objects. Some touch involves an active motor component—stroking, tapping, grasping, or pressing—whereby a part of the body is moved against another surface or organism. The sensory and motor components of touch are intimately connected anatomically in the brain and are important in guiding behavior.
Exteroception also includes the thermal senses of heat and cold. Thermal sensations are important controllers of behavior and homeostatic mechanisms needed to maintain the body temperature near 37°C (98.6°F). Finally, exteroception includes the sense of pain, or nociception, a response to external events that damage or harm the body. Nociception is a prime motivator of actions necessary for survival, such as fight or flight.
The third component of somatic sensation, interoception, is the sense of the function of the major organ systems of the body and its internal state. The information conveyed by receptors in the viscera is crucial for regulating autonomic functions, particularly in the cardiovascular, respiratory, digestive, and renal systems, although most of the stimuli registered by these receptors do not lead to conscious sensations. Interoceptors are primarily chemoreceptors that monitor organ function through such indicators as blood gases and pH, and mechanoreceptors that sense tissue distention, which may be perceived as painful.
This diverse group of sensory functions may seem an unlikely combination to form a sensory system. We treat all of the somatic senses in one introductory chapter because they are mediated by one class of sensory neurons, the dorsal root ganglion (DRG) neurons. Somatosensory information from the skin, muscles, joint capsules, and viscera is conveyed by DRG neurons innervating the limbs and trunk or by trigeminal sensory neurons that innervate cranial structures (the face, lips, oral cavity, conjunctiva, and dura mater). These sensory neurons perform two major functions: the transduction and encoding of stimuli into electrical signals and the transmission of those signals ...