The Other Senses Lab

Smell and taste are chemical senses: their receptors respond to molecules, not to light or sound waves.

Olfactory receptor cells sit high in the nasal cavity. There are millions of them, built from roughly 350–400 receptor types. A single scent activates a pattern across many types, which is how a limited set of receptors can tell apart thousands of smells.

Here’s the big one: smell is the only sense that is not processed first in the thalamus. Signals go to the olfactory bulb, then straight to the olfactory cortex and to emotion and memory areas (the amygdala and hippocampus). That’s why a smell can drag a whole memory back in an instant.

Pheromones are chemical signals detected by the olfactory system. They clearly shape behavior in many animals; evidence for human pheromones is much weaker and still debated.

Smell also adapts quickly (sensory adaptation), and losing it entirely is called anosmia.

The bumps on your tongue are papillae. They hold thousands of taste buds, and each taste bud holds dozens of taste receptor cells. Those cells are replaced every week or two, but the total number of taste buds drops as we age, which is one reason taste preferences change over a lifetime.

The AP list has six basic tastes, each one a survival signal: sweet (energy), salty (sodium), sour (acids, possibly spoiled food), bitter (possible toxins), umami (proteins), and oleogustus (fat).

How many taste receptors you have affects how intense taste feels. Supertasters have many and find bitter and spicy foods intense. Medium tasters are in the middle. Nontasters have fewer and experience weaker tastes.

Taste and smell interact to create flavor. Without smell, taste is muted or lost. This is sensory interaction: senses working together.

Receptors in your skin respond to pressure, warmth, and cold (plus pain, in Section 4). Other skin sensations are combinations of these. The strangest example: the sensation of “hot” comes from warm and cold receptors firing at the same time.

Touch signals go through the thalamus to the somatosensory cortex, a strip at the front of the parietal lobe. Processing is contralateral: the left side of your body is felt in the right hemisphere, and vice versa.

The somatosensory cortex doesn’t give every body part equal space. Sensitive areas like the lips and fingertips get huge amounts of cortex. Drawing a person with body parts sized by their cortex space gives you the homunculus: giant hands and lips, tiny back and legs.

Pain is processed in both the body and the brain. Pain receptors (nociceptors) detect potentially damaging stimulation, and signals travel up the spinal cord to the brain, using neurotransmitters like substance P. Your body’s own endorphins can dampen those signals.

Not all pain feels the same: fast, sharp pain travels on quicker fibers (A-delta), while slow, aching pain travels on slower ones (C fibers).

Gate control theory (Melzack and Wall) is one attempt to explain why the same injury can hurt more or less. It proposes a neural “gate” in the spinal cord. Pain signals open it. Competing signals close it: touch input (rubbing a bumped elbow) and messages from the brain (distraction, calm, endorphins). Messages like anxiety and focusing on the pain can open it wider.

Phantom limb sensation: many people who lose a limb still feel touch, itching, or pain where it used to be. The brain keeps generating sensations even without input from the limb. Mirror therapy, which lets the brain “see” the missing limb move, can ease phantom pain for some people.

The vestibular sense controls balance. It’s detected mainly by the semicircular canals in the inner ear: three fluid-filled loops set at right angles to each other, one for each direction your head can rotate. When your head turns, the fluid lags behind and bends tiny hair-like receptors. Next to them, the vestibular sacs sense straight-line motion and which way is up, like a bus braking or your head tilting. Vestibular signals work closely with the cerebellum, which coordinates balance and movement.

Kinesthesis is the sense of your body’s own movement and position. Receptors in your muscles, tendons, and joints let you move in coordinated ways without looking at your body parts.

Quick test: close your eyes and touch your nose. Kinesthesis tells you where your hand is. Now stand on one foot with your eyes closed. That wobble is your vestibular sense working without help from vision.

Sensory interaction: your senses constantly influence each other. You’ve already seen two examples: flavor depends on smell, and motion sickness often comes from a mismatch between what your eyes and your vestibular sense report. In the McGurk effect, watching someone’s lips form one sound changes the sound you think you hear.

Synesthesia goes further: stimulation of one sense automatically produces an experience in another. Some people see colors when they hear music, or taste words. It’s involuntary and consistent over time, and only a small minority of people have it.