2.4 Other Senses
Sensation and Perception
After Chapter 2.4, you will be able to:
· List the structures in the olfactory pathway
· Distinguish between the chemicals detected by the nose and mouth
· Recall the four main modalities of somatosensation
While vision and hearing are, by far, the most heavily tested senses on the MCAT, the other senses are still considered fair game on Test Day. These include the chemical senses of smell and taste; somatosensation, which includes all of the modalities of “touch”; and kinesthetic sense.
Smell is considered one of the chemical senses, which means that it responds to incoming chemicals from the outside world. Specifically, smell responds to volatile or aerosolized compounds. Olfactory chemoreceptors (olfactory nerves) are located in olfactory epithelium in the upper part of the nasal cavity. Chemical stimuli must bind to their respective chemoreceptors to cause a signal. There are a tremendous number of specific chemoreceptors, which allows us to recognize subtle differences in similar scents, such as lavender and jasmine.
Smell is an impressive motivator for behavior. Food aromas may make a person hungry, a familiar fragrance may remind a person of a significant other from years ago, and an unpleasant smell may signify that an unknown bottle contains a dangerous chemical rather than water. Smell is the only sense that does not pass through the thalamus, but rather travels—unfiltered—into higher-order brain centers.
Smell can also carry interpersonal information through the medium of pheromones, which are chemicals secreted by one animal, and which, once bonded with chemoreceptors, compel or urge another animal to behave in a specific way. Pheromones have debatable effects on humans, but play an enormous role in many animals’ social, foraging, and sexual behaviors.
As is true with all senses, there is a defined olfactory pathway to the brain. Odor molecules are inhaled into the nasal passages and then contact the olfactory nerves in the olfactory epithelium. These receptor cells are activated, sending signals to the olfactory bulb. These signals are then relayed via the olfactory tract to higher regions of the brain, including the limbic system.
As a sense, taste is often simpler than we imagine. There are five basic tastes: sweet, sour, salty, bitter, and umami (savory). Flavor is not synonymous with taste, but rather refers to the complex interplay between smell and taste, which can be affected by nonchemical stimuli like texture and the individual’s mood.
Tastes are detected by chemoreceptors, which are sensitive to dissolved compounds. Saltiness, for example, is a reaction to alkali metals, and is generally triggered by the sodium found in table salt. Sourness, on the other hand, is a reaction to acid, such as lemon juice or vinegar. Sweet, bitter, and savory flavors are also triggered by specific molecules binding to receptors. The receptors for taste are groups of cells called taste buds, which are found in little bumps on the tongue called papillae. Taste information travels from taste buds to the brainstem, and then ascends to the taste center in the thalamus before traveling to higher-order brain regions.
Somatosensation is often reduced to “touch” when listed as a sense, but is actually quite complex. Somatosensation is usually described as having four modalities: pressure, vibration, pain, and temperature. At least five different types of receptor receive tactile information, including:
· Pacinian corpuscles: respond to deep pressure and vibration
· Meissner corpuscles: respond to light touch
· Merkel cells (discs): respond to deep pressure and texture
· Ruffini endings: respond to stretch
· Free nerve endings: respond to pain and temperature
Pain and temperature actually use a different pathway than pressure and vibration through the spinal cord. This can be seen in Brown-Séquard syndrome, in which half of the spinal cord is severed. Patients lose pressure and vibration sense on the same side as the lesion, but lose pain and temperature sensation on the opposite side.
Transduction occurs in the receptors, which send the signal to the central nervous system where it eventually travels to the somatosensory cortex in the parietal lobe.
There are three additional concepts related to touch perception that are important to know: two-point thresholds, physiological zero, and gate theory of pain. A two-point threshold refers to the minimum distance necessary between two points of stimulation on the skin such that the points will be felt as two distinct stimuli. Below the two-point threshold, the two stimuli will be felt as one. The size of the two-point threshold depends on the density of nerves in the particular area of skin being tested.
Temperature is judged relative to physiological zero, or the normal temperature of the skin (between 86° and 97°F). Thus, an object feels “cold” because it is under physiological zero; an object feels “warm” because it is above physiological zero.
Pain perception is part of the somatosensory system and can result from signals sent from a variety of sensory receptors, most commonly nociceptors. Pain also relies on thresholds, which may vary greatly from person to person. For example, the temperature of water that is perceived to be “so hot it hurts” may vary by several degrees between individuals. The gate theory of pain proposes that a special “gating” mechanism can turn pain signals on or off, affecting whether or not we perceive pain. In this theory, the spinal cord is able to preferentially forward the signals from other touch modalities (pressure, temperature) to the brain, thus reducing the sensation of pain. Gate theory has been superseded by other theories, but is still a useful model for understanding how touch is processed at the spinal cord.
The gate theory of pain explains why rubbing an injury (like bumping your knee on a table) seems to reduce the pain of the injury.
Kinesthetic sense is also called proprioception and refers to the ability to tell where one’s body is in space. For example, even with your eyes closed, you could still describe the location and position of your hand. The receptors for proprioception, called proprioceptors, are found mostly in muscle and joints, and play critical roles in hand—eye coordination, balance, and mobility.
MCAT Concept Check 2.4:
Before you move on, assess your understanding of the material with these questions.
1. List the structures in the olfactory pathway, from where odor molecules enter the nose to where olfactory signals project in the brain.
2. Both smell and taste are sensitive to chemicals. What is different about the types of chemicals each one can sense?
3. What are the four main modalities of somatosensation?