Neurosensation: Smell and Taste

Match the question to the correct definition.

Where are the terminals of the sensory neuron dendrites located?

They are located at the olfactory epithelium and are branched out into multiple cilia.

Where are the cilia embedded in the epithelial layer?

They are embedded in the mucus layer of the olfactory epithelium, which helps in dissolving odour molecules for detection.

How is mucus created?

They are created by olfactory glands in the basal lamina.

Explain the purpose of olfactory receptors.

Olfactory receptors are G-protein-linked membrane receptors. The combination of most odorant molecules with their receptors activate G_olf.

Explain how G_olf activates adenylyl cyclase.

G_olf exchanges GDP to GTP to activate G_⍺olf, which then stimulates adenylyl cyclase to produce cAMP.

How does cAMP depolarize olfactory cells?

cAMP binds to and opens cation channels, allowing Ca²⁺ ions to enter the cell, leading to depolarization.

How could olfactory cells trigger action potentials?

If the EPSPs are strong enough, it can lead to a threshold depolarization, triggering action potentials that could travel along the axon to the bulb.

Once the olfactory neurons receive input from the primary olfactory neurons, how would signals reach the olfactory cortex?

The olfactory nerve transmits sensory information from the olfactory epithelium to the olfactory bulb, through the olfactory tract (bypassing the thalamus), and to the olfactory cortex for further processing and perception of smells.

Where is the olfactory cortex located?

The olfactory cortex is located between the temporal and frontal lobes of the brain.

Where could ascending tracts also lead to in the brain?

The ascending tracts could lead to the limbic system (cingulate gyrus, hippocampus, and amygdala), which is involved in emotion and memory, as well as higher cortical areas for more complex processing.

How do our odours call up emotional memories?

For us humans, it remains embedded in our brains. Processing odours in our limbic system creates deeply buried olfactory memories.

Why might sewer workers not notice anything objectionable or that people are unaware of their odour?

This phenomenon, known as olfactory adaptation or habituation, occurs when individuals become desensitized to persistent odors over time, leading them to not perceive them as strongly.

Give an example of adaptation and habituation in terms of odour sensitivity.

An example of adaptation is when a person becomes less aware of a strong perfume after being in a room with it for a while. Habituation is when someone who lives near a farm stops noticing the smell of manure.

Where is the VNO organ located?

It’s located in the nasal cavity.

The VNO may disappear in early fetal development in humans, but it’s still present in babies, even if they don’t use it. How do newborns tract their mother’s scent?

By using the main olfactory system, newborns track their mother's scent using their sensitivity to olfactory cues, which helps them recognize and bond with her.

How many receptor cells does each taste bud contain?

Each taste bud contains approximately 50 to 100 receptor cells.

What do sweet receptor cells detect?

Sweet receptor cells detect sugary substances, signaling the presence of carbohydrates and energy-rich nutrients.

What do umami receptors detect?

Umami receptors detect amino acids, specifically glutamate, which signals the presence of protein-rich foods.

What do bitter receptor cells detect?

Bitter receptor cells detect toxic compounds or bitter substances, often serving as a warning signal against potential poisons.

What do salty and sour receptor cells detect?

Salty receptor cells detect sodium ions, indicating the presence of salt, while sour receptor cells detect hydrogen ions from acids, signalling the presence of sour and spoiled foods.

Different taste buds in the areas of your tongue have…

slightly different regions of sensitivity (some areas have stronger or lower thresholds for certain taste qualities, enhancing flavour perception), such as sweet, salty, sour, bitter, and umami.

Give an example of thresholds influencing different taste modalities.

For instance, the tip of your tongue is most sensitive to sweet stimuli, while the sides are more responsive to sour.

Why is the type 1 cell the eldest daughter?

She is always salty because she reached puberty first. She is experiencing a lot of hormonal imbalance. She is upset that she doesn’t get to share ATP with her siblings.

Why is the type 2 cell the middle child?

She has a bit of a hormonal imbalance. Sometimes, she could be sweet towards her younger sister, but is often bitter and umami when she has to share her ATP food with her younger sister, Type 3.

Why is the type 3 cell the youngest?

She is always sour and feels left out because she is not as mature as her older sisters, often looking for attention.

Explain, step by step, the signal transduction of bitterness, sweetness, and umami.

The binding of specific ligands activates gustducin. Phospholipase C is activated and cleaved into IP₃ and DAG. IP₃ releases Ca²⁺ from the ER. Ca²⁺ opens TRPM5 channels. Cells are depolarized, and ATP is released.

Explain, step by step, the signal transduction of sourness.

The protons (H⁺) to sour taste receptors travel through ion channels, causing depolarization. Ca²⁺ voltage-gated calcium channels are opened, which allows calcium ions to enter the cell, further depolarizing it and releasing seretonin.

Explain the signal transduction of saltiness.

The sodium ions (Na⁺) from salt enter the taste receptor cells through ion channels, causing depolarization. This leads to the opening of voltage-gated calcium channels, allowing an influx of calcium ions, which triggers the release of neurotransmitters.

How could our experience of food depend on other sensors? Give an example.

Nerve endings of the walls of our mouth have TRP channels, sensitive to temperature and chemicals. Example: A hot pepper may feel spicy due to both its chemical composition and the heat sensed by these channels.

Do chemoreceptors in our stomach and intestines resemble receptors in the tongue? Provide an example.

Yes, chemoreceptors in the stomach and intestines resemble taste receptors on the tongue, as they both respond to chemical stimuli. For example, both types of receptors can detect glucose, which can influence hunger and digestive responses.

Explain how the sensation for the heat and coolness of food travels to the brain.

Cranial nerves 5, 8, 9, and 10 bring information to the brainstem, where they synapse in a region called the NST before being relayed to the thalamus and then to the gustatory cortex in the brain for processing.