smell 2 -olfactory pathways

Question 1

olfactory epithelium

Answer 1

• covers an area of 2.5 cm2
• pseudostratified columnar epithelium
• 3 cell types: olfactory cells, sustentacular cells (glial cells), basal cells (stem cells)
• Bowmans glands

Question 2

bowmans glands function

Answer 2

secrete a mucous fluid
- lubricates olfactory epithelium
- dissolve odorant-containing gases
- aid binding of odorant to receptor

Question 3

olfactory nerve

Answer 3

-olfactory cells are bipolar neurons
- dendrite expanded to form olfactory vesicle. possesses long cilia which contain receptors. extends to epithelial surface
- axons packaged into approx 20 bundles on each side of the nose - the olfactory nerve, which passes through cribriform plate of ethmoid

Question 4

smell detection

Answer 4

odorants enter nasal cavity as a gas or aerosol
1. dissolve in the mucous and bind to glycoproteins
2. presented to receptor proteins on cilia - approx 3000 different receptor proteins
3. stimulates depolarisation of olfactory neurone
4. olfactory neurone terminates in the olfactory bulb
5. spatial mapping of olfactory bulb - specific sites of termination for neurones that possess particular chemical receptors

Question 5

smell detection adaptation

Answer 5

continuous stimuli becomes unnoticed

Accommodation is the process by which sensory receptors gradually become less responsive to a constant stimulus over time. Swell bodies contribute to this process by allowing the nerve endings to adjust their sensitivity to the ongoing stimulus. When a constant pressure stimulus is applied, the swell bodies initially respond with a burst of action potentials. However, as the stimulus persists, the swell bodies adapt by reducing their rate of firing, resulting in a decreased perception of the stimulus intensity.

Question 6

when are olfactory cells replaced

Answer 6

olfactory cellsa re destroyed by wear and tear and lost by desquamation

they are replaced approx every 3 months

Question 7

olfactory bulb

Answer 7

• located on the orbital surface of the frontal lobe
• site of convergence of olfactory nerves with 2nd order neurones
• arranged in 5 layers

Question 8

layer 1 of olfactory bulb - nerve fibre layer

Answer 8

contains axons of primary olfactory neurones

Question 9

layer 2 of olfactory bulb - glomerular layer

Answer 9

contains approx 8000 glomeruli where olfactory axons terminate on the dendrites of tufted and mitral cells

Question 10

layer 3 of olfactory bulb - external plexiform layer

Answer 10

contains nerve cell bodies of tufted cells

Question 11

layer4 of olfactory bulb - mitral cell layer

Answer 11

contains nerve cell bodies of mitral cells

Question 12

layer 5 of olfactory bulb - granule layer

Answer 12

axons of tufted and mitral cells and inhibitory neurons

Question 13

olfactory tract

Answer 13

Sit underneath the brain and pass backwards
- contains the axons of the mitral and tufted cells and fibres from the contralateral olfactory bulb
- the tract runs posteriorly to the trigone where it divides into the medial and lateral olfactory stria
- the anterior olfactory nucleus is located along its length

Question 14

anterior olfactory nucleus

Answer 14

-3 small groups of nuclei located along the olfactory tract
- receive collateral branches of tufted and mitral cells
- 3rd orfer neurones arising here and pass through anterior commissure to contralateral olfactory bulb
- may be involved in setting sensitivity to different odours

Question 15

medial olfactory stria

Answer 15

• fibres in the medial stria enter the anterior commissure
• project to contralateral olfactory bulb
• may have an inhibitory action on the contralateral olfactory bulb
• enhances the more active bulb thereby providing directional cues to the source of the olfactory stimulus

Question 16

lateral olfactory stria

Answer 16

most of the axons of tufted and mitral cells pass in the lateral olfactory stria to the primary olfactory areas

Question 17

primary olfactory area

Answer 17

responsible for conscious awareness of olfactory stimuli

includes those areas that receive afferent fibres from the olfactory bulb; uncut, entorhinal area, limen insula and amygdaloid body
- these are collectively referred to as pyriform area

projections sent to the olfactory association area

Question 18

olfactory assocation cortex

Answer 18

monitors and interprets information from the primary olfactory cortex
- located in the entorhinal area

may also include the lateral part of the orbital surface of the frontal lobe

Question 19

other olfactory cortical projections - limbic system

Answer 19

involved in retention and recall of olfactory stimuli

Question 20

other olfactory cortical projections - autonomic NS via hypothalamus

Answer 20

superior and inferior salivatory nuclei -> saliva production
dorsal vagal nucleus -> increased peristalsis