Pain and Temperature

Abstract

Our perception of the external world, state of attentiveness, assessment of body image, and regulation of movements are all dependent upon input from sensory systems. The sensory receptors that respond to a particular physical property or modality (e.g., temperature), together with tracts and nuclei that transmit this information to higher levels of the central nervous system (CNS), are referred to as a somatosensory system. There are four major general somatic modalities—the sensations of pain (signaling tissue damage or chemical irritation), temperature (warmth or cold), touch, (for recognition of size, shape, and texture), and proprioception (sense of static position and movements of the limbs, body, and head) (see also Chap. 10). Somatosensory receptors are located in the skin, muscles, joints, and viscera, a distribution that makes this system the largest and most varied of the sensory systems. Although primarily sensory, this system is also of importance in the control of coordinated movements by providing appropriate feedback to the somatic motor system about joint position, muscle tension, velocity of muscular contractions, and contact of the body with external surfaces (Chaps. 8 and 11). Sensations occur when stimuli interact with receptors (sensors). Sensory information then is transmitted cephalically as patterns of action potentials by individual neurons and by assemblies of neurons acting in consort. All sensory systems, regardless of modality, transmit information pertaining to the intensity, duration, and location of the stimuli that activate them. Every general sensory system processes input sequentially through: (1) primary afferent fibers, (2) relay nuclei located in the spinal cord, brainstem, and thalamus, (3) cerebral cortex. Each processing nucleus not only consolidates inputs from adjacent receptors but also integrates signals from inhibitory neurons and descending projections to transform and enhance the sensory information (Fig. 3.13).