Lesson 1.16 · Unit 1 · CED topic 1.6 (part 4)
Smell, taste, touch, pain, balance, and body position
Vision and hearing get most of the attention, but the other senses keep you fed, safe, and upright. This lesson covers the chemical senses (smell and taste), the skin senses of touch and pain, and the two body senses that tell you where you are and how you are moving: balance and the sense of body position. The exam asks the same kind of question about each: what stimulus is detected, what structure transduces it, where it goes in the brain, and what happens when the system is disrupted or two systems interact.
Learning objectives.
- 1.6.D. Explain how the structures and functions of the chemical sensory systems relate to behavior and mental processes.
- 1.6.E. Explain how the structures and functions of the touch sensory system relate to behavior and mental processes.
- 1.6.F. Explain how the structures and functions of the pain sensory system relate to behavior and mental processes.
- 1.6.G. Explain how the structures and functions that maintain balance (vestibular) and body movement (kinesthetic) relate to behavior and mental processes.
Essential knowledge.
- 1.6.D.1. Structures in the nose and brain process and/or transduce olfactory stimuli. Smell is the only sense not processed first in the thalamus of the brain. Pheromones produce chemical messages for the olfactory system.
- 1.6.D.2. Gustation is the sense of taste, and types of tastes include sweet, sour, salty, bitter, umami, and oleogustus.
- 1.6.D.3. Structures in the tongue, mouth, and brain process and/or transduce basic tastes. The number of taste receptors on the tongue is related to how sensitive people are to tastes, classifying them as supertasters, medium tasters, or nontasters.
- 1.6.D.4. The chemical senses interact to create the sensation of taste. Without the sense of smell, taste sensations are either muted or not experienced.
- 1.6.E.1. Structures within the skin and brain process and/or transduce touch stimuli. The sensation of “hot” is produced by the activation of warm and cold receptors in the skin. 45
- 1.6.F.1. Pain is processed both in the body and in the brain. Gate control theory is one attempt to describe the complexities of pain. Phantom limb sensation occurs when people who have lost limbs report sensation or pain where the limb used to be.
- 1.6.G.1. The vestibular sense controls balance and is primarily detected by the semicircular canals and structures in the brain.
- 1.6.G.2. Kinesthesis the sense of one’s body movement. Kinesthesis allows the body to move in coordinated ways without having to look at the various parts of the body as it moves. 46
Smell (olfaction)
Smell is detected by receptor cells in the upper part of the nasal cavity that respond to chemical molecules in the air. When the molecules bind to receptors, the receptors transduce them into neural signals. The signals go to the olfactory bulb at the base of the brain and from there directly to parts of the limbic system and the temporal lobes. Smell is the only sense that is not processed first in the thalamus, which helps explain why smells are tied so closely to emotion and memory: a scent can bring back a memory from childhood in a flash, because the pathway connects directly to the amygdala and hippocampus.
Animals also communicate by pheromones, chemicals released by one individual that influence the behavior of another of the same species. They are well established in insects and many mammals (for example, signaling readiness to mate), and they are a chemical message for the olfactory system. Whether pheromones affect human behavior is still debated.
Taste (gustation)
Gustation, the sense of taste, responds to chemicals dissolved in saliva. The basic tastes are sweet, sour, salty, bitter, umami (savory, as in meat and broth), and oleogustus (the taste of fatty acids). Receptor cells in taste buds, located in bumps on the tongue and in the mouth, transduce the chemicals, and signals travel to the thalamus and on to the brain's taste areas.
People differ in how sensitive they are, and the difference is related to the number of taste receptors on the tongue. Researchers classify people as supertasters (with the most taste buds, who find bitter foods and strong tastes intense), medium tasters, or nontasters (with the fewest, who find the same foods mild). Supertasters are more likely to dislike coffee and bitter vegetables.
Taste and smell interact. Much of what we call the "flavor" of food comes from smell, which is why food tastes bland when you have a cold. Without the sense of smell, taste sensations are either muted or not experienced.
Touch
The skin contains many kinds of receptors that respond to pressure, vibration, temperature, and tissue damage. Receptors in the skin transduce the stimulus, the signals travel through the spinal cord and the thalamus to the somatosensory cortex in the parietal lobe. More cortex is devoted to the most sensitive parts of the body, such as the fingertips and lips, than to less sensitive areas like the back. There are separate receptors for warmth and for cold. Interestingly, the sensation of "hot" is produced when both warm and cold receptors are activated at the same time, which explains why touching something very hot can briefly feel cold or painful at once.
Pain
Pain is a protective sense that warns of tissue damage. It is processed both in the body and in the brain. Nociceptors, receptors in the skin and organs that respond to damaging stimuli, send signals along nerve fibers to the spinal cord and brain. The brain's interpretation depends on more than the signal itself: mood, attention, expectations, and culture all affect how much pain a person feels.
Gate control theory, proposed by Ronald Melzack and Patrick Wall in 1965, is an attempt to describe this complexity. It says that the spinal cord contains a neurological "gate." Signals from small pain fibers open the gate and let pain messages travel to the brain, while signals from larger fibers carrying touch information, or messages from the brain itself, can close the gate and block pain messages. This explains why rubbing a bumped elbow reduces the pain, and why distraction, relaxation, and the release of endorphins (the body's natural pain relievers) can lessen it.
Phantom limb sensation occurs when people who have lost a limb still feel sensations, or even pain, where the limb used to be. It shows that sensation is constructed in the brain: the brain areas that once received input from the limb continue to be active.
V. S. Ramachandran treated patients who felt that their missing arm was clenched in a painful cramp. He placed a mirror in a box so that, when the patient looked into it, the reflection of the intact hand appeared where the missing hand would be. When the patient moved the intact hand and watched the reflection, the "phantom" hand seemed to move too, and some patients reported that the cramped feeling released and pain was reduced.
Reading the research: a small set of case studies. It suggests that the brain's expectations and vision can alter the pain signal, consistent with gate control ideas. Limitations are the small number of patients and the lack of a control group, so later controlled studies were needed to test how well the approach works.
Balance and body position
The vestibular sense controls balance and your sense of head position and movement. It is detected primarily by the semicircular canals in the inner ear (three fluid-filled loops oriented at right angles to one another, so that they sense rotation in any direction) and structures in the brain. When you spin and stop, the fluid keeps moving and makes you feel dizzy.
Kinesthesis is the sense of your body's movement and position. Receptors in the muscles, tendons, and joints send information about where your limbs are. Because of kinesthesis, you can touch your nose with your eyes closed and move in coordinated ways without having to look at your body parts. Together, the vestibular and kinesthetic senses allow you to walk and keep balance without conscious thought.
A woman smells fresh bread in a bakery and suddenly recalls her grandmother's kitchen in great detail. Explain with a fact about the brain.
Step 1, smell is detected by receptors in the nose and sent to the olfactory bulb.
Step 2, unlike the other senses, smell is not processed first in the thalamus. The olfactory information goes directly to the limbic system, including the amygdala and hippocampus, which are involved in emotion and memory.
Step 3, this direct connection explains why smells can trigger vivid emotional memories.
A student bangs her shin on a desk and immediately rubs it. The pain lessens. Later, she is absorbed in a computer game and does not notice a headache until the game ends. Explain both with gate control theory.
Step 1, rubbing stimulates large fibers that carry touch information. According to the theory, this activity closes the spinal "gate" and reduces the pain signals from the small pain fibers reaching the brain.
Step 2, being absorbed in the game involves the brain sending signals (attention and the release of endorphins) that can close the gate from above, so she notices the pain less.
Step 3, when the game ends and her attention shifts, the gate opens more and the pain is felt.
Step 4, this shows that pain is not only a matter of the body's signal but also of how the brain handles it.
Two friends try black coffee. One makes a face and calls it unbearably bitter. The other thinks it is mild. A researcher counts taste buds on each tongue and finds the first has far more. Classify each and explain.
Step 1, the first friend has many taste receptors, so is a supertaster who experiences strong tastes intensely.
Step 2, the second friend, with fewer, is probably a nontaster (or medium taster).
Step 3, the number of taste receptors is related to sensitivity, so the same stimulus produces different sensation. This also shows that individuals differ in sensation, not just in preferences.
- Smell: nasal receptors, olfactory bulb, limbic system; the only sense not first processed by the thalamus; pheromones are chemical messages.
- Taste: sweet, sour, salty, bitter, umami, oleogustus; supertasters, medium tasters, nontasters; taste and smell interact.
- Touch: skin receptors, somatosensory cortex; "hot" = warm and cold receptors together.
- Pain: gate control theory; phantom limb sensation.
- Vestibular sense (semicircular canals) = balance; kinesthesis = body movement and position.
- Why do smells trigger memories so powerfully?
Show answer
Smell is the only sense not first processed in the thalamus; olfactory signals go directly to the limbic system, including the amygdala and hippocampus, which handle emotion and memory.
- List the six basic tastes in the course.
Show answer
Sweet, sour, salty, bitter, umami, and oleogustus.
- Why does a person with a stuffy nose think food has little taste?
Show answer
Taste and smell interact: much of flavor comes from smell. Without it, taste sensations are muted.
- Explain how gate control theory accounts for rubbing a sore spot to relieve pain.
Show answer
Touch signals carried by larger fibers close the spinal "gate" to pain signals from the smaller pain fibers, so less pain reaches the brain.
- A person spins in a circle and then stops, feeling dizzy. Which sense is responsible and where are its receptors?
Show answer
The vestibular sense; the fluid in the semicircular canals of the inner ear continues to move after you stop, sending signals that the body is still rotating.
- Distinguish the vestibular sense from kinesthesis.
Show answer
The vestibular sense detects balance and head position and movement (mainly via the semicircular canals); kinesthesis detects the position and movement of body parts via receptors in muscles, tendons, and joints.