PSYC2274 Chapter Notes - Chapter 14: Cranial Nerves, Learning, Cribriform Plate
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- Detecting chemicals in the environment is crucial for survival.
- Thus these senses were the first to evolve.
- Olfaction → the sense of smell.
- Gustation → the sense of taste.
- Our sense of smell is critically involved in our experience of food.
- There are 2 routes through which we perceive odors:
- 1. Orthonasal olfaction → sniffing in and perceiving odors through our
nostrils.
- Occurs when we are smelling something that is outside of us.
- Odorant molecules travel through our nostrils and up our nose onto the
olfactory epithelium.
- 2. Retronasal olfaction → perceiving odors through you mouth while
breathing and chewing.
- Occurs when we are smelling something that is inside our mouth and is
what gives us the experience of flavor.
- Odorant molecules travel through our mouth and up from the back of our
mouth into our upper nasal cavity and onto the olfactory epithelium.
OLFACTORY PHYSIOLOGY
ODORS AND ODORANTS
- Odors → Olfactory sensation.
- Stimuli for odors = chemical compounds called odorants.
- But, not every chemical is an odorant → molecule that is defined by its
physicochemical characteristics.
- To me smelled, odorant molecules must be volatile (able to float through air), small, and
hydrophobic (repellent to water).
- However, many molecules that meet basic reqs still do not smell to us…evolutionarily,
we had no reason to detect these substances which are not dangerous in the
concentrations found in nature.
THE HUMAN OLFACTORY APPARATUS
- Human olfactory system is tacked onto an organ that serves another purpose.
- Primary function of the nose is to filter, warm, and humidify the air we breathe.
- Breath must rise past a narrow space called the olfactory cleft (back of nose where air
flows and where olfactory epithelium is located) and settle on the mucous membrane of
the olfactory epithelium (primary function is to detect odorants in inhaled air.
Located on both sides of upper portion of nasal cavity and olfactory clefts → contains
3 types of cells).
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2
- Our 2 nostrils take in different amounts of air → nasal dominance.
- Alternates between our nostrils throughout the day.
- Means that the 2 nostrils continually vary in their sensitivities to odorants as a
function of the amount of air inhaled.
- Olfactory epithelium is the “retina of the nose.”
- Each contains 3 types of cells:
- 1. Supporting cells → provide metabolic/ physical support for the
olfactory sensory neurons.
- 2. Basal cells → the precursor cells to olfactory sensory neurons.
- 3. Olfactory sensory neurons (OSNs) → main cell type. Small neurons
located beneath a mucous layer in the epithelium. The cilia on the
OSN dendrites contain the receptor sites for odorant molecules.
- OSNs are small neurons that have cilia (hairlike protrusions on dendrites of OSNs.
receptor sites for odorant molecules are on the cilia. These are the first structures
involved in olfactory signal transduction).
- Cilia are the OSNs dendrites and have odorant receptors (ORs) on their tips → region
on cilia of OSNs where odorant molecules bind.
- “One to one to one.”
- Each OSN expresses only 1 type of odorant receptor (OR), and all OSNs
expressing the same type of receptor project to the same glomerulus.
- Interaction between an odorant and the OR ultimately produces an action potential that is
transmitted to the olfactory bulb.
- To initiate an AP, about 7 or 8 odorant molecules must bind to a receptor.
- Takes about 40 of these nerve impulses for a smell sensation to be reported.
- We can detect an INFINITE amount of smells.
- Whatever the exact number of OSNs, we know that vision is the only sensory system that
has more sensory neurons than olfaction.
- Dogs have at least 100x more OSNs than humans, and a much higher proportion of
the dog’s brain is dedicated to olfaction → about 5% vs. 0.1% for humans.
- Humans can smell about the same number of scents, but dogs can sense odors at
concentrations nearly 100 million times lower than we can.
- Axons on the ends of OSNs opposite the cilia (dendrites) pass through the tiny sievelike
holes of the cribriform plate → a bony structure riddled with tiny holes at the level of
the eyebrows, which separates the nose from the brain.
- Axons from the OSNs pass through the tiny holes of the cribriform plate to enter
the brain.
- Anosmia → the total inability to smell, most often resulting from sinus illness or head
trauma.
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3
- All our OSNs die and regenerate about once every 28 days.
NEUROPHYSIOLOGY OF OLFACTION
- OSN axons pass through cribriform plate and bundle together to form the olfactory
nerve (cranial nerve I) → first pair of cranial nerves.
- Conducts impulses from the olfactory epithelia in the nose to the olfactory bulb.
- Olfactory bulb → blueberry-sized extension of the brain just above the nose, where
olfactory info is first processed.
- 2 olfactory bulbs, one in each brain hemisphere.
- The right bulb corresponds to the RIGHT nostril, and vice versa!!!!
- It is ipsilateral (referring to same side of body or brain).
- The olfactory bulb is where the sensory nerve endings gather together to form tiny
spheres called glomeruli.
- All neurons expressing a particular OR type send their axons to the same glomerulus pair
in the olfactory bulb.
- A specific odorant will activate a set of specific ORs and consequently produce a pattern
of glomerular activity that is unique to it.
- Complicated by the fact that each glomerulus may receive axons from several different
receptor types.
- ALSO, our personal experience can change the pattern of activity that is produced
by the glomeruli in the olfactory bulb.
- “Learned taste aversion.”
- There is no fixed code for odor perception. Our personal experience determines how an
odor will be processed by the olfactory system.
- Surrounding each glomerulus is a population of excitatory and inhibitory cells called
juxtaglomerular neurons → first layer of cells.
- Respond to a wider range of odorants than next layer of neurons…
- Tufted cells → next layer of cells.
- Respond to a wider range of odorants than next layer of neurons…
- Mitral cells → deepest layer of neurons in olfactory bulb.
- Responds only to a few specific odorants.
- This increasing fine tuning through neuronal layers is important for helping the brain
distinguish between similar odors.
- Deepest level is the granule cells → extensive network of inhibitory neurons which
integrate input from all the earlier projections and are thought to function as higher-
order feature detectors.
- Respond specifically to different odorants.
- Olfactory tract → where axons of the mitral and tufted cells of each bulb combine
and send odor info to primary olfactory cortex.
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Document Summary
Detecting chemicals in the environment is crucial for survival. Thus these senses were the first to evolve. Our sense of smell is critically involved in our experience of food. There are 2 routes through which we perceive odors: Orthonasal olfaction sniffing in and perceiving odors through our nostrils. Occurs when we are smelling something that is outside of us. Odorant molecules travel through our nostrils and up our nose onto the olfactory epithelium. Retronasal olfaction perceiving odors through you mouth while breathing and chewing. Occurs when we are smelling something that is inside our mouth and is what gives us the experience of flavor. Odorant molecules travel through our mouth and up from the back of our mouth into our upper nasal cavity and onto the olfactory epithelium. Stimuli for odors = chemical compounds called odorants. But, not every chemical is an odorant molecule that is defined by its physicochemical characteristics.
