Notes on Other Senses

While vision and hearing are the most tested senses on the MCAT, the other senses are still important. These include:

A chemical sense that responds to volatile or aerosolized compounds.
Olfactory chemoreceptors (olfactory nerves) are located in the olfactory epithelium in the upper nasal cavity.
Chemical stimuli must bind to chemoreceptors to cause a signal.
A vast number of chemoreceptors allow us to distinguish between similar scents.
Pheromones: Chemicals secreted by one animal that influence the behavior of another. Effects on humans are debatable but play a significant role in animal social and sexual behaviors.

Odor molecules are inhaled into the nasal passages.
They contact the olfactory nerves in the olfactory epithelium.
Receptor cells are activated, sending signals to the olfactory bulb.
Signals are relayed via the olfactory tract to higher brain regions, including the limbic system.

Five basic tastes: sweet, sour, salty, bitter, and umami (savory).
Flavor: Complex interplay between smell and taste, affected by non-chemical stimuli like texture and mood.
Detected by chemoreceptors sensitive to dissolved compounds.
- Saltiness: Reaction to alkali metals (e.g., sodium in table salt).
- Sourness: Reaction to acids (e.g., lemon juice, vinegar).
- Sweet, bitter, and savory: Triggered by specific molecules binding to receptors.

Taste receptors are grouped into taste buds, located on papillae (bumps) on the tongue.
Information travels from taste buds to the brainstem.
Ascends to the taste center in the thalamus.
Travels to higher-order brain regions.

Encompasses multiple modalities: pressure, vibration, pain, and temperature.
Five types of receptors:
- 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.
Transduction occurs in the receptors, sending signals to the central nervous system.
Signals travel to the somatosensory cortex in the parietal lobe.

Two-Point Threshold: 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 threshold, the two stimuli are felt as one.
Depends on the density of nerves in the area of skin.
Physiological Zero: Normal skin temperature, between $86$ and $97$ degrees Fahrenheit.
- Objects below this range feel cold.
- Objects above this range feel warm.
Pain Perception: Relies on nociceptors and pain thresholds, which vary between individuals.
Gate Theory of Pain: A gating mechanism in the spinal cord can turn pain signals on or off.
- The spinal cord preferentially forwards signals from other touch modalities (pressure, temperature) to the brain, reducing pain sensation.