Smell and Taste

Question 1

1. Smell and taste are forms of x
   a) x is evolutionarily old: bacteria use it to x; animals without brains use it to x
   b) Chemoreception may have evolved into x

Answer 1

1. Smell and taste are forms of chemoreception
   a) Chemoreception is evolutionarily old: bacteria use it to guide their movements; animals without brains use it to find food and mates.
   b) Chemoreception may have evolved into chemical synaptic communication

Question 2

1. The olfactory receptors are in x
   a) This epithelium lies at x, covering ~x cm2 in x. It contains ~x million receptor cells in total.
   b) The epithelium is x. No one knows why, but the x correlates with olfactory sensitivity: in us it is pale yellow, in cats a “dark mustard brown
2. Olfactory receptor cells have x in their membranes
   a) The genes for these receptor molecules form x— x genes, or x% of the genome— though only ~x are expressed in humans.
   b) When an odorant molecule binds its receptor, it activates a x, which increases x
   c) x channels open, x the receptor neurons and triggering an x that travels along the cell’s x to the x
3. The receptor cells are sensitive
4. Olfactory receptor cells have unusual properties. 3 properties
5. The receptor cells project to x which is x and lies on x
   b) x is called the olfactory nerve, or x
6. Many receptor cells x on each bulb neuron
   a) As with rods converging on ganglion cells, this arrangement x but x

Answer 2

1. The olfactory receptors are in the olfactory epithelium
   a) This epithelium lies at the top of the nasal cavity, covering ~3 cm2 in each of the 2 sides. It contains ~10 million receptor cells in total.
   b) The epithelium is pigmented. No one knows why, but the richness of its color correlates with olfactory sensitivity: in us it is pale yellow, in cats a “dark mustard brown
2. Olfactory receptor cells have G protein-coupled receptor molecules in their membranes
   a) The genes for these receptor molecules form the largest known gene family in vertebrates — 1000 genes, or ~3–5% of the genome— though only ~400 are expressed in humans.
   b) When an odorant molecule binds its receptor, it activates a G protein, Golf , which increases the local concentration of cAMP
   c) cAMP-gated cation channels open, depolarizing the receptor neurons and triggering an action potential that travels along the cell’s axon to the olfactory bulb.
3. The receptor cells are sensitive
4. Olfactory receptor cells have unusual properties
   a) They are pinocytotic, continually sipping in fluid and sending it along the nerves into the brain. We don’t know why.
   b) They are short-lived, degenerating after a month or 2, to be replaced by new ones from below.
   c) They send their axons into the brain through tiny holes in the cribriform ( sievelike”) plate — the bone at the base of the cranial“ cavity
5. The receptor cells project to the olfactory bulb
   a) The bulb is an extension of the cerebrum, and lies on the underside of the frontal lobes.
   b) The projection from the receptors to the bulb is called the olfactory nerve, or cranial nerve I
6. Many receptor cells converge on each bulb neuron
   a) As with rods converging on ganglion cells, this arrangement enhances sensitivity but discards spatial information

Question 3

1. Rodents and maybe humans have x
   x are x which affect x
   b) Rodents have an x in the nasal cavity called the x), which is involved in x
   c) In humans, the VNO x during fetal development, but we do respond to x

Answer 3

1. Rodents and maybe humans have pheromones
   a) Pheromones are chemicals released by an animal into the environment which affect the physiology or behavior of other
   members of its species.
   b) Rodents have an olfactory structure in the nasal cavity called the vomeronasal organ (VNO), which is involved in their behavioral
   responses to sex pheromones.
   c) In humans, the VNO disappears during fetal development, but we do respond to airborne chemical signals

Question 4

1. The bulb projects directly to x, bypassing x
2. Olfactory cortex is in the x and x lobes
3. The bulb also projects to x
   a) This is an x concerned with x and x. For early animals, x was tightly linked to smell: they used their noses to identify x.
   b) Our emotions are no longer x (e.g. we like money) but they are still handled by these x. Maybe that is why odors call up emotional memories
4. Olfaction adapts x

Answer 4

1. The bulb projects directly to olfactory cortex, bypassing thalamus
2. Olfactory cortex is in the frontal and temporal lobes
3. The bulb also projects to the limbic system
   a) This is an old group of brain regions concerned with motivation and emotion. For early animals, motivation was tightly linked to smell: they used their noses to identify food and poisons, mates and predators.
   b) Our emotions are no longer so smell-related (e.g. we like money) but they are still handled by these old olfactory areas. Maybe that is why odors call up emotional memories
4. Olfaction adapts slowly but completely

Question 5

1. Our main taste receptor cells are clustered x
   a) We have ~x taste buds, mainly where but also on the x, x and x. Babies have x. A taste bud lives only ~x days.
   b) Each taste bud contains ~x receptor cells, which are x cells (not x) arranged like x. They contact the x through x, the x
2. A typical taste bud contains at least x kinds of receptor cell
   a) Each kind of receptor cell detects x, and all x have clear biological roles:
   b) x and x receptor cells detect sugar (energy) and the amino acid x (indicating x), respectively.
   c) Bitter receptor cells detect x.
   d) Salty and sour receptor cells detect x and x — 2 important ions.
   d) The tongue may also have receptors for x
3. There are receptor cells of all 5 kinds all over the top of the tongue
   a) For instance, it is not true that sweetness is sensed only x.
   b) But different areas of the tongue do vary slightly in x

Answer 5

1. Our main taste receptor cells are clustered in taste buds
   a) We have ~5000 taste buds, mainly on the top of the tongue but also on the soft palate, epiglottis and upper esophagus. Babies have 10,000. A taste bud lives only ~10 days.
   b) Each taste bud contains ~100 receptor cells, which are epithelial cells (not neurons) arranged like petals. They contact the oral cavity through a small opening, the taste pore
2. A typical taste bud contains at least 5 kinds of receptor cell
   a) Each kind of receptor cell detects one flavor, and all 5 have clear biological roles:
   b) Sweet and umami receptor cells detect sugar (energy) and the amino acid glutamate (indicating protein), respectively.
   c) Bitter receptor cells detect poison.
   d) Salty and sour receptor cells detect Na and H — 2 important ions.
   d) The tongue may also have receptors for fatty acids
3. There are receptor cells of all 5 kinds all over the top of the tongue
   a) For instance, it is not true that sweetness is sensed only by the tip of the tongue.
   b) But different areas of the tongue do vary slightly in their thresholds
   for different flavors

Question 6

1. Taste receptor cells are grouped into x types
   a)Only type III cells form x, activating them with x.
   b) Type II cells release x, which acts on 2x
2. Different kinds of cell employ different
   x
   a) Cells for sweet, umami, and bitter have x coupled to a x called x, which activates x increasing x and triggering release of x.
   b) Detection of salt and sour involves x which are not linked with x

Answer 6

1. Taste receptor cells are grouped into 3 types
   a)Only type III cells form synapses with sensory neurons, activating them with serotonin.
   b) Type II cells release ATP, which acts on neurons and type IIIs
2. Different kinds of cell employ different
   membrane proteins
   a) Cells for sweet, umami, and bitter have receptor molecules coupled to a G protein called gustducin, which activates signal path-
   ways, increasing intracellular [Ca ] and triggering release of ATP.
   b) Detection of salt and sour involves ion channels which are not linked with G proteins.

Question 7

1. Our experience of food depends on:
   a) It depends on x, x, x, x, x, x, and x — if I tell you some lousy food is a delicacy then you like it better.
   b) Nerve endings in the walls of the mouth have x sensitive to x and x, e.g. x receptors respond to heat and to capsaicin in ; x channels respond to cold chilies
   and to menthol.
   c) Chemoreceptors in our stomach and intestines x; some of these receptors resemble x, e.g. for sweet and umami
2. Taste signals take several paths to the brain
   a) Receptor cells in the taste buds excite x, the x, x, and x nerves. These pathways synapse in x and x en route to the x.
   b) TRP receptors in the walls of the mouth excite x, the x.

Answer 7

1. Our experience of food depends on:
   a) It depends on smell, temperature, pain, texture, crunch, appearance, and cognition — if I tell you some lousy food is a delicacy then you like it better.
   b) Nerve endings in the walls of the mouth have TRP channels sensitive to temperature and chemicals, e.g. vanilloid receptors respond to heat and to capsaicin in ; TRPM8 channels respond to coldchilies
   and to menthol.
   c) Chemoreceptors in our stomach and intestines monitor their contents; some of these receptors resemble ones on the tongue, e.g. for sweet and umami
2. Taste signals take several paths to the brain
   a) Receptor cells in the taste buds excite fibers of cranial nerves VII, IX, and X, the facial, glossopharyngeal, and vagus nerves. These pathways synapse in medulla and thalamus en route to the cortex.
   b) TRP receptors in the walls of the mouth excite cranial nerve V, the trigeminal.