Behavioral Neuroscience VIII
How do we Taste? Or Smell?
Gustation & Olfaction: How to taste a taste, or smell a smell
Taste
Gustation is the fancy word for taste. Taste is typically broken down into 5 flavors, each flavor loosely correlating with a specific ion channel opening:
- Salty is due to Na+ channels (salt!)
- Sour is due to K+ channels
- Sweet due TR1 and TR2 channels
- Bitter (evolved to fight poisons) due to ~30 specialized TR2 channels
- Umami due to 2nd messenger responses to certain compounds
These flavors are sensed by the 6 – 10k taste buds we have on our tongue. Taste buds have a 6-day lifespan and have about 100 receptors per taste bud that all focus on one of the above flavors. Taste buds are different from other sensory cells we have looked at so far as the taste buds themselves fire action potentials to the connected receptors. When an action potential is created through transduction, the impulse travels up cranial nerves VII, IX, and X to the thalamus and then to the taste region of the cortex.
Smell
Olfaction is the fancy word for smell. Smell is sensed by thousands of odorant receptors. Odorant Receptors are chemically sensing receptors inside the nose that have many proteins that bind to very specific odorant molecules (smells). This binding activates cAMP—recall this is a second messenger protein—which in turn opens Na+ channels and causes an action potential.
When we breath in, air rushed over our olfactory epithelium, a 2cm^2 region within our nasal cavity where all our odorant receptors are located. Each breath exposes our odorant receptors to a large variety of chemicals/smells, causing a large variety of action potentials to fire. All these action potentials are consolidated by the Olfactory Bulb, a region right above our nasal cavity.
Unlike other senses, smell signals travel to several interesting places prior to the Thalamus. They travel to the Amygdala, center of emotion, as well as the Hypothalamus, center of hormones. This is why smells have a particularly strong association with specific emotions, feelings, states of mind, and memories.
Another interesting thing to note about smell – other animals have a pronounced vomeronasal organ (VNO) on the olfactory epithelium which detects pheromones. Pheromones are chemicals released by nearby animals to subconsciously communicate mood. It is still an open debate whether humans have a VNO or if we can detect pheromones.
Recap
- All 5 tastes are sensed by the taste buds on our tongue
- All smells are sensed by the odorant receptors inside our nose