The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017
Exteroception is the awareness of the body relative to the external environment. This includes information about its position in space (proprioception), the five main senses (auditory, visual, gustatory, olfactory, and somatosensory), pain (nociception), temperature (thermoception), and balance (equilibrioception). Counter to exteroception is interoception, which is responsible for visceral sensory information. Exteroception is incredibly important to survival as it is what allows humans to interact with their environment.
The auditory system is responsible for hearing and translates vibrations into sound. This occurs when vibrations hit the hair-like cilia within the ear and is amplified as it travels from the outer ear to the inner ear. Once there it interacts with the eardrum and is translated into an electrochemical signal and carried by the auditory nerve to the primary auditory cortex in the cerebral cortex. Errors in the auditory system result in partial or complete deafness.
Vision is created when light hits the rods and cones that reside within the retina in the eye. These rods and cones pick up different frequencies of light and send the information to the brain via the optic nerve. This information is processed as color and patterns in the thalamus. Once there it is integrated into a comprehensive image. Visual system errors can result in partial or complete blindness as well as poor vision either close or far.
Taste is conveyed by the gustatory system to the brain. Taste cell receptors on the tongue reside in the taste buds and convey information about the five main tastes: sour, sweet, salty, bitter, and umami. The signal for each taste is triggered differently and these signals are conveyed to the brain via two different nerves—the glossopharyngeal nerve and the facial nerve—depending on what region of the tongue the receptor is coming from. Tastes help individuals determine what is and is not suitable to eat. For example, bitterness indicates highly alkaline food, which often can be harmful or poisonous to the consumer. Additionally, sweet is an indicator of a high carbohydrate content, which is a very useful source of energy for most individuals.
The olfactory system is responsible for a sense of smell. Signals are conveyed by free nerve endings (cilia) within the nostril that convert chemical odorants into electrical signals that can be transduced to the brain. The trigeminal nerve conveys the information back to the olfactory bulb where the signal then proceeds to the olfactory cortex via the olfactory tract. Errors in the olfactory system can result in partial or complete loss of smell (anosmia), distorted smell (dysosmia), and phantom smells (phantosmia).
Touch is relayed through the somatosensory system. There are four main types of sensory receptors for touch: Meissner’s corpuscles (respond to moderate vibration and light touch), Pacinian corpuscles (respond to high vibrations and gross touch), Merkel nerve endings (respond to low vibrations and sustained touch), and Ruffini corpuscles (respond to sustained skin stretch—which helps maintain the pressure needed to hold objects). Information from these receptors is conveyed to the brain and mapped to the cortex based on the region it comes from. This mapping is called a homunculus. The somatosensory system also conveys information about temperature, pain, and contact with chemicals.
Nociception is the sensory system responsible for pain. “Free” nerve endings respond to noxious stimuli such as excessive heat, excessive cold, and some chemicals. The signals produced by these stimuli are conveyed down the spinothalamic pathway and the trigeminal pathway to the brain depending on the location of the stimuli. Both pathways have synapses at many levels along the spinal cord, brainstem, and cerebral cortex, allowing for both a fast and slow response to the stimulus. The synapses within the spinal cord and brainstem are why you pull your hand away from a hot stove before your brain realizes it was hot.
Thermoception is achieved by thermoreceptors that are nonspecific and react to relative changes. In humans there are two main types of thermoreceptors: those that respond to decreases in temperature (cold receptors) and those that respond to increases in temperature (heat receptors). The information picked up by these receptors is sent to the thalamus. Receptors for temperature are located in the skin, cornea, and urinary bladder.
Balance is conveyed by equilibrioception and is a complex integration of information from three other sensory systems: visual, proprioceptive, and vestibular. The vestibular system consists of specialized organs within the inner ear: the utricle, the saccule, and three semicircular canals. These organs provide information about vertical position (which way is down), linear movement, and rotational movement. Each of the organs is filled with fluid and it is the motion of the fluid that provides information. Information from the vestibular system is conveyed via the vestibulocochlear nerve to the brain.
Riannon C. Atwater
See also: Auditory System; Balance; Interoception; Nociception; Olfactory System; Proprioception; Somatosensory System; Taste System; Thermoreceptors; Visual System
Purves, Dale, George J. Augustine, David Fitzpatrick, William C. Hall, Anthony-Samuel LaMantia, James O. McNamara, & Leonard E. White. (2008). Neuroscience (4th ed.). Sunderland, MA: Sinauer Associates.