Olfactory system Flashcards
What are the four olfactory bulb targets?
pyriform cortex
olfactory tubercle
amygdala
entorhinal cortext
What principle that also governs sensory modalities does the olfactory system abide by?
sensory stimuli (airborne chemicals) interact with receptors at the periphery and are transduced and encoded into electrical signals, which are relayed via synaptic transmission to higher-order centers
How does the number of olfactory receptors differ in mammals?
a dog has way more receptor neurons, proteins, and brain regions dedicated to olfactory than humans
How does olfactory perception differ with different odors?
sensitivity to odors
distinct brain regions for pleasant and unpleasant odors based on their chemical composition and concentration
What is anosmia?
the loss of the ability to detect one or more odors
can be congenital or acquired following chronic sinus infection or inflammation, traumatic head injury, exposure to toxins, aging, neurodegenerative conditions, chemotherapy, eating disorders, diabetes, and psychotic disorders (schizophrenia)
What are the physiological and behavioural responses to olfactory stimuli?
visceral motor responses to the aroma of appetizing food - salivation, increased gastric mobility
or to a noxious smell - gagging, vomiting
influence reproductive, endocrine functions, mother-child interactions, male and female specific odorants
What does the olfactory endothelium consist of?
a layer of olfactory receptor cells, supporting cells, and basal cells
What is the path of odorants to the CNS?
dissolve in the muscle layer and contact the cilia of olfactory cells
axons of olfactory cells penetrate the bony cribriform plate on the way to CNS
What is the odor transduction mechanism in the receptor cilia?
odorant binds to a specific odorant receptor proteins concentrated on the external surface of the olfactory cilia
each receptor protein has a unique structure and bind different odorants
What is G olf?
a special G-protein found only in olfactory receptor cells
What are the three steps of transduction in olfactory receptor neurons?
- odorants generate a slow receptor potential in the cilia
- the receptor potential propagates along the dendrite and triggers a series of action potentials within the soma of the olfactory receptor cell
- the action potentials propagate along the olfactory nerve axon
What are the three mechanisms we use to distinguish between odors?
- olfactory population coding: each odor is represented by the activity of a large population of neurons
- olfactory spatial maps: the neurons responsive to particular odors may be organized into spatial maps
- olfactory temporal coding: the timing of action potentials may be an essential code for odor discrimination
Explain broad tuning of single olfactory receptor neurons (population coding)
Each receptor cell expresses a single olfactory receptor protein and different cells are randomly scattered within a region of the epithelium
each olfactory receptor cell has single receptor protein expressed for a ‘preferred’ odor
Brain distinguishes each of the four odors individual odor by the combination of responses
What are the central olfactory pathways?
Olfactory receptor neurons send axons into the two olfactory bulbs
The input layer of each bulb contains spherical structures called glomeruli
The endings of primary olfactory axons converge and terminate on the dendrites of about 25-100 second-order olfactory neurons (mitral cells) within each glomerulus
Explain the convergence of olfactory neuron axons (spatial maps)
Olfactory receptor neurons expressing a particular receptor gene all send their axons to the same glomeruli
The axons of each neuron expressing only one particular receptor gene all converge onto the same glomerulus
So, the glomeruli within each bulb is an orderly map of the receptor cells in the epithelium expressing a particular receptor protein
– and, by implication, a map of odor information
What is zone-to-zone projection in spatial mapping?
Odorant receptors with highly homologous amino acid sequences tend to be localized in the same zone of the olfactory epithelium (zonal organization is preserved in olfactory bulb)
While many bulb neurons may be activated by one odor, the neurons’ positions form complex but reproducible spatial patterns
The smell of a particular odor is converted into a specific map within the ‘neural space’ of the bulb, and the form of the map depends of the nature and concentration of the odor
What is a spatial code?
A code in which information is conveyed by the relative positions of activated neurons
How are maps of neurol activation in the olfacotry bulb formed?
Different odors evoke different patterns of glomerular activation
the smell of a particular odor is converted into a specific map defined by the positions of active neurons within the “neural space” of the bulbs, and the form of the map depends on the odorant
Explain temporal coding in the olfactory system
Odor information may be encoded by the detailed timing of spikes within cells and between groups of cells as well as by the number, temporal pattern, rhythmicity, and cell-to-cell synchrony of spikes
The brain may analyze an odor not only by keeping track of which olfactory neurons fire but also by when they fire
How is olfactory bulb circuitry modified?
through inhibitory and activating mechanisms
granule cells act as inhibitory interneurons
What is odor molecule quality coded by?
combination of activated glomeruli
What two characteristic structural features to mitral cells tune for in specificity?
1: the overall stereochemical structure of the hydrocarbon chain
2: the type and position of the attached functional group
What is lateral inhibition?
using inhibitory interneurons to enhance contrast
*increases sensitivity
What results from mitral cell dendrodendritic reciprocal synapses with granule cells?
enhance the contrast between strongly activated and faintly activated glomeruli (lateral inhibition) and so sharpen the tuning specificity of individual mitral cells to odor molecules
- The second-order mitral cells may therefore be more sharply tuned to specific molecular features than olfactory sensory neurons are
Explain maps of neural activation of the olfacotry cortex
The segregation of information in the olfactory bulb is not maintained in the olfactory cortex
Individual cortical neurons are more broadly tuned to different odors than in the olfactory bulb and the neurons that respond to single odors are distributed throughout extended regions
- Differential activation of widely distributed ensembles of neurons
