19.1: What is a Sensory System?
Sensory systems detect stimuli—such as light and sound waves—and transduce them into neural signals that can be interpreted by the nervous system. In addition to external stimuli detected by the senses, some sensory systems detect internal stimuli—such as the proprioceptors in muscles and tendons that send feedback about limb position.
Sensory systems include the visual, auditory, gustatory (taste), olfactory (smell), somatosensory (touch, pain, temperature, and proprioception), and vestibular (balance, spatial orientation) systems.
All sensory systems have receptor cells that are specialized to detect a particular type of stimulus. For example, hair cells in the inner ear have cilia that move in the presence of sound waves, while olfactory receptor neurons in the nasal cavity have receptors that bind to odorant molecules.
The presence of an appropriate stimulus triggers electrochemical changes in the nervous system. This stimulus typically changes the membrane potential of a sensory neuron, triggering an action potential. The information is then transmitted from the sensory organ to the spinal cord and then the brain, or directly to the brain (as in the visual system).
The different types of sensory information—also called modalities—travel in different pathways through the central nervous system, but most are transmitted to the thalamus—a structure in the middle of the brain. From here, sensory information is typically sent to areas of the cerebral cortex dedicated to the analysis of specific modalities—for instance, primary visual cortex or primary auditory cortex.
These areas, in turn, send information to the association cortex, where sensory information is integrated with other types of information for higher-level analysis. Ultimately, the neural processing that occurs throughout these pathways and networks allows for accurate perception of sensory stimuli—such as the identity and location of objects in the visual world, or the understanding of speech.