The first sensory system in animals was probably a chemical sensitivity of some type. Chemical senses are important in finding food, locating potential mates, and avoiding danger. Although most people would think that vision is the most important sense—in terms of survival value, chemical senses have far more to offer for most animals. This chapter reviews the chemical senses of taste and smell in humans.
Taste receptors are found on the tongue and in parts of the mouth (oral cavity). These receptors typically occur in groups of approximately fifty, termed taste buds. Taste buds located on the tongue are most often found in the papillae, which are the small protrusions located on the tongue. The average number of taste buds a person has is estimated to be at around 10,000. There is not a one-to-one correspondence between a taste bud and a neuron; the neurons responding to taste buds receive many inputs. You have probably heard at some time or another that there are four or five primary tastes. Typically these primary tastes consist of bitter, salty, sour, and sweet. In some cases meaty (umami) is included as a primary taste. Other newer models of taste include these five primary tastes plus metallic as a primary taste, making the number of basic tastes the tongue can detect as six. Other models extend the number further.
Can you taste water?
Dogs and pigs, and perhaps some other animals, can taste water, but people cannot. Humans do not actually taste water, even mineral waters. What humans taste are the chemicals or the impurities in the water.
While taste receptors interact with smell, this conventional view of taste purports that every taste that humans experience results from the combination of one or more of these primary tastes. One of the difficulties with this notion is that many tastes that people experience could not be formed by a combination of these five primary tastes. Nonetheless, this particular model continues to dominate the field regarding the experience of human taste as no empirically validated alternative models have yet been proposed.
Taste signals leave the mouth via three cranial nerves:
Cranial nerve VII, the facial nerve, carries information from the front of the tongue.
Cranial nerve IX, the glossopharyngeal nerve, carries information from the back of the tongue.
The vagus nerve, cranial nerve X, carries information from the back of the oral cavity.
Ageusia is the inability to taste, but it is rare because taste is transmitted by three neural pathways. Partial ageusia is more common, usually following ear damage on the same side of the head. The loss of taste is restricted to the anterior two-thirds of the tongue as cranial nerve VII carries taste information by passing through the middle ear before synapsing in the medulla.
All of these fibers travel to the medulla where they synapse on a structure called the solitary nucleus. These taste signals project to the thalamus. From the thalamus, neurons project to the primary gustatory cortex, which is near the section of the somatosensory cortex that represents the facial area. Some neurons from the thalamus also project to the secondary gustatory cortex, which is in the lateral fissure of the brain. The projections from the taste receptors in the tongue to the brain do not decussate (cross over) to the other side of the brain as projections for other senses do.
A supertaster experiences a much greater sensitivity and intensity in the sense of taste than the average person due to having far more papillae than average. These people tend to need less sugar and fat in their foods; however, due to their heightened taste for bitterness, they prefer salty foods. Supertasters are more likely to be females of Asian, South American, or African heritage.
The sense of taste is modulated by two satiety mechanisms. The first of these occurs in the brain, and some texts refer to this as a central mechanism. After you have been eating for a while, neurons that respond to both smell and taste in the orbitofrontal portions of the brain begin to fire more slowly. As a result, you begin to lose your preference for the item that you are eating. This process is called alliesthesia (changed taste). A more quickly working mechanism occurs in both the taste buds and olfactory (smell) receptors called sensory-specific satiety. Appetite is also regulated by hormones from the gastrointestinal tract that affect the brain in the hypothalamus and brain stem.
The receptor cells for smell (olfaction) are embedded in the olfactory mucosa, a layer of tissue covered with mucus in the upper part of the nose that also contains supporting cells and basal cells. The dendrites for the olfactory receptor cells are located in the nasal passages, and their axons pass through the cribriform plate, a porous portion of the skull. Unlike other senses, the neurons for olfaction do not synapse in the thalamus but instead enter the olfactory bulbs and synapse on neurons that project to the olfactory tracts located in the brain.
Bears have one of the most highly developed senses of smell, over 2,000 times more acute than a human’s. A bloodhound has a sense of smell about 300 times that of a human. However, snakes may have the advantage when it comes to smell, as they are able to collect molecules in the air with their tongues and transfer them back to their brain via an organ known as the Jacobson’s organ.
Olfaction is a very important sense, and other mammals have larger areas of the brain devoted to it. It is believed that bypassing the thalamus results in a time-saving mechanism basic to survival, as scent is important in enemy detection and the mating of many animals. In humans, it is estimated that there are about 10 million olfactory receptors and approximately 1,000 different receptor types. At this time researchers have not been able to determine if there is some type of organized distribution of all of these various olfactory receptors in the olfactory mucosa; however, it appears that all of the receptors project to the same area in the olfactory bulb.
Olfactory bulbs are made up of six layers of different types of neurons. Olfactory receptor axons terminate near the surface of the olfactory bulbs in groups called olfactory glomeruli. Each of these gets its input from many olfactory receptor cells. It appears as if the glomeruli are arranged in a systematic fashion because their layout is similar in related species and there is a mirror symmetry between left and right olfactory bulbs (the glomeruli that respond to a particular odor are located on the same site of each olfactory bulb). Olfactory receptor cells demonstrate a constant process of deterioration and replacement. New olfactory cells develop axons that grow and extend to the appropriate part of the olfactory bulb. This process of deterioration and regeneration occurs every few weeks.
The smells that people experience are made up of a mix of many different odors that produce hundreds of signatures across the olfactory bulb. The olfactory bulb has several main projections that go directly to areas of the cortex without going to the thalamus first. This makes the sense of smell unique among all of the other senses.
The olfactory bulb projects directly to the orbitofrontal cortex and then relays to the mediodorsal thalamus, which sends projections back to the orbitofrontal cortex. This projection allows for conscious attention and awareness to odors. The interconnection with the thalamus interacts with other sensory information to integrate the sense of smell with visual, auditory, and somatosensory input. For instance, when standing at the seashore, you see the ocean waves coming in; you hear the sound of the waves crashing on the shore; you feel brisk salty air; and you smell the combination of salt-fresh air and seaweed. These all make a lasting impression on you.
Projections to the orbitofrontal cortex pair smells from food with information from taste receptors to create perceptions of flavor. This is why food tastes bland when you have a cold. There are also approach/avoidance programs associated with this tract that work in unison with smell-related information from other tracts (“programs” are stored action patterns of behavior).
The olfactory bulb sends projections to an area of the cortex known as the piriform cortex (sometimes spelled pyriform cortex), an area of the medial (middle) temporal cortex. The piriform cortex is considered the primary olfactory cortex by many researchers. This area also appears to be involved in approach/avoidance behaviors associated with smells.
Dementia is a brain disorder that typically occurs in elderly people and most often initially presents itself as a loss of memory. The connection between the hippocampus (a brain structure important in creating new memories) and olfaction is so strong that specific tests for dementia have been developed that measure the loss of smell in an affected patient.
The olfactory bulb also projects to the entorhinal cortex, an area in the medial (middle) temporal lobe that is important in memory. The entorhinal cortex interfaces with the hippocampus. This projection allows for the memory of odors that were important to you. The olfactory bulb also has projections with the amygdala, an area of the brain that is important in emotional responses. The amygdala, positioned in front of the hippocampus, specializes in important memories with emotional overtones. Because certain smells have inherent emotional aspects to them, the amygdala allows for emotional reactions to important smells. A large percentage of smells evoke emotional reactions, such as the smell of freshly baked bread or the smell of skunk. The amygdala also connects to the hippocampus so that there is a brain circuit that is involved with memories that have emotional overtones, such as the smell of rotten food or odors associated with sex. Projections from the amygdala also go to the hypothalamus, an area of the brain that triggers the release of certain hormones.
In humans, the main role of the chemical senses of taste and smell is recognition of flavors. However, in other species of animals chemical senses also play an important role in regulating social interactions between members of the same species. Many animals release pheromones, which are chemicals that can influence the behavior of animals within the same species. For example, when a female dog is “in heat,” she releases pheromones that notify male dogs that she is primed for mating.
There has been some attention, especially in the business world, to the possibility that humans may also release pheromones. Some findings have supported this notion. For example, the menstrual cycles of women living together tend to become synchronized; the olfactory potential of women is at its most sensitive when they are ovulating or when they are pregnant; and some men can judge the stage of a woman’s menstrual cycle based on her vaginal odor. However, despite the huge commercial market for human pheromones, there is no direct empirical evidence that human odors serve as sexual attractants. However, certain smells do directly affect the behavior of humans.