Organisation of Receptors of Special Sensation

Question 1

The nasal cavity consits of two chambers separated by a nasal septum. Name the three histological regions present in each cavity and state the type of epithelium that lines each.

Answer 1

1. Nasal vestibule: it is lined by the epidermis of the skin i.e. stratified squamous keratinized epithelium [this region also contains hair follicles (nasal vibrissae) and sebaceous glands; olfactory cells are absent in this region]
2. The respiratory region: it is lined by respiratory mucosa i.e. pseudostratified ciliated columnar epithelium with goblet cells [beneath this lining is the lamina propria; olfactory cells are absent in this region]
3. The olfactory region: it is lined by olfactory mucosa: pseudostratified columnar ciliated epithelium without goblet cells. [The lamina propria underlying this epithelium is a bit different; it has numerous Bowman’s glands and olfactory nerve bundles.]
4. [Histological slide: respiratory mucosa]
   [Histological slide: respiratory mucosa (2)]
   [Histological slide: olfactory epithelium]
   [Histological slide: olfactory epithelium (2)]

Question 2

Name the three major cell types of the olfactory epithelium.

Answer 2

(a) Olfactory receptor cells (chemoreceptor cells)
(b) Sustentacular cells; they provide structural support to the epithelium
(c) Basal cells; regenerative stem cells
(d) [Diagram] [Histological slide]

Further notes:
The cilia of the olfactory epithelium are dendritic processes of the olfactory receptor cells, rather than from the supporting cells. These cilia are hence non-motile.

Question 3

Name three structures lined by the olfactory epithelium.

Answer 3

⚚ roof of the nose (cribriform plate and ethmoid bone)
⚚ upper nasal septum
⚚ superior nasal concha (turbinate)

Question 4

Briefly outline the olfactory relay pathway.

Answer 4

~ olfaction is detected by the chemoreceptor cells of the olfactory epithelium
~ multiple olfactory nervelets go through the cribriform plate of the ethmoid bone to the anterior cranial fossa
~ the nerves synapse with 2nd order neurons in the olfactory bulb, which then form the olfactory tracts
~ most fibres of the olfactory tracts project to the primary olfactory cortex in the piriform lobe
~ [Diagram 1] [Diagram 2]

Further notes:
Parts of the piriform lobe: uncus (Brodmann 34), entorhinal cortex (Brodmann 28) and lateral olfactory stria

Question 5

Briefly outline 4 differences between olfactory mucosa and ordinary respiratory mucosa.

Answer 5

1. The olfactory epithelium is thicker than respiratory epithelium.
2. The olfactory mucosa has olfactory nerve bundles in its lamina propria. There are no nerve bundles in the respiratory mucosa.
3. The respiratory epithelium has goblet cells while the olfactory epithelium has none.
4. The columnar cells of the respiratory epithelium bear motile cilia on their free surface while the columnar cells of the olfactory epithelium bear microvilli on their surfaces.
5. The nuclei of the cells of the respiratory epithelium lie at all levels of epithelium.

Question 6

What is the function of Bowman’s glands present in olfactory mucosa?

Answer 6

Their serous secretions dissolve and later wash out odoriferous substances after activating the receptors.

Question 7

What is the name of the V-shaped groove that divides the tongue into an anterior two thirds and a posterior one third?

Answer 7

sulcus terminalis
[Diagram: the topography of the tongue]

Question 8

Name four types of tongue papillae.

Answer 8

filiform, fungiform, foliate, circumvallate
[Diagram: the topography of the tongue papillae]

Further notes:
Tongue papillae are the little bumps on the top of the tongue that help grip food during chewing. They also contain taste buds.

Question 9

What kind of epithelium lines the tongue?

Answer 9

stratified squamous parakeratinized epithelium (patchy/incomplete keratinizaiton)

Question 10

Name the types of cells present in taste buds.

Answer 10

1. neuroepithelial taste receptor cells (gustatory cells)
2. sustentacular cells (supporting cells)
3. basal cells (regenerative stem cells)
4. parietal cells
5. [Diagram: taste buds] [Histological slide: taste buds]
6. [Diagram: regions of the tongue detecting different tastes]

Question 11

Briefly outline the taste relay pathway.

Answer 11

~ Gustatory stimuli are detected by the chemoreceptor cells in the taste buds (gustatory receptor cells).
~ Taste buds in anterior 2/3 relay nervous impulses through the chorda tympani nerve (CN VII), those in the posterior 1/3 relay via the glossopharyngeal nerve, those in the epiglottis relay via the vagus nerve.
~ Peripheral processes of first order neurons terminate in the brainstem at the gustatory nucleus.
~ 2nd order neurons decussate and project to the thalamus (ventral posteromedial nucleus)
~ 3rd order neurons project to the primary gustatory cortex (Brodmann 43) in the parietal lobe (parietal operculum) where the sense of taste is perceived

Question 12

What is the function of the von Ebner glands in the papillae of the tongue?

Answer 12

Their serous secretions wash away food materials and thereby permit the tastants dissolved in saliva to contact the taste cells through the taste pore, leading to their depolarization and hence, the reception of new taste stimuli.

Question 13

Here are some histological sides of the tongue. Click Answer to view them.

Answer 13

[Histological slide: taste buds]
[Histological slide: Circumvallate papilla with taste buds and von Ebner glands]

Question 14

The Cochlear Duct
When observed in histological sections, the cochlea appears to be divided into three spaces: ____(a)____. The cochlear duct, which contains endolymph, ends at the apex of the cochlea. The other two scalas contain ____(b)____ and are in reality one long tube, beginning at the ____(c)____ window and terminating at the ____(d)____. They communicate at the apex of the cochlea via an opening knowns as the ____(e)____.

The cochlear duct has the following histological structure. The ____(f)____ membrane consists of two layers of squamous epithelium, one delivered from the scala media and the other from the lining of the scala vestibuli. Cells of both layers are joined by means of extensive tight juncitons that help preserve the very high ionic gradients across the membrane. The ____(g)____ is an unusual vascularized epithelium located in teh lateral wall of the cochlear ducts. It consists of cells that have m any infoldings of their basal plasma membranes, where numerous mitochondria are located. These characteristics indicate that they are ion and water-transporting cells, and thus they are responsible for the characteristic ionic composition of endolymph. The structure of the internal ear that contains special auditory receptors is called the ____(h)____; it contains hair cells that respond to different sound frequencies. It rests on a thick layer of ground substances - the ____(i)____.

Answer 14

(a) scala vestibuli, scala media and scala tympani
(b) perilymph
(c) oval
(d) round
(e) helicotrema
(f) Vestibular (Reissners)
(g) stria vascularis
(h) Organ of Corti
(i) basilar membrane

[Histological slide][Diagram]

[2-minute video: Auditory Pathway]

Question 15

The Organ of Corti
On the ____(a)____ lies the organ of Corti. Which is built up by hair cells arranged in a complicated manner. It runs from the ____(b)____ to the ____(c)____ of the cochlea. There are no blood vessels in the Organ of Corti. A portion of it rests on the tympanic lip of the lamina spiralis ossea, but most of it lies on the basilar membrane. Between these parts is a free triangular space called the tunnel, which is delimited by two rows of supporting cells: the ____(d)____.

Answer 15

(a) basilar membrane
(b) oval window
(c) helicotrema
(d) inner pillar cells
[2-minute video: Auditory Pathway]

Question 16

The Retina
In addition to the three main types of cells i.e. photoreceptors, bipolar cells and ganglion cells, there are other types of cells that are distributed more diffusely in the layers of the retina. Give four examples of such cells.

Answer 16

1. Horizontal cells: they establish contact between the photoreceptor cells
2. Amacrine cells: they establish contact between ganglion cells
3. Astrocytes
4. Microglia
5. Muller cells [are functionally analogous to neuroglia in that they support, nourish and insulate the retinal neurons and fibres] (they extend from the outer limiting membrane to the inner limiting membrane)
6. [Diagram]

Question 17

State the 10 layers of the retina, from the innermost to the outermost.

Answer 17

1. pigment epithelium layer
2. rods and cones layer
3. outer limiting membrane
4. outer nuclear layer
5. outer plexiform layer
6. inner nuclear layer
7. inner plexiform layer
8. ganglion cell layer
9. nerve fibre layer
10. inner limiting membrane
11. [Diagram] [Histological slide]

Question 18

State the functions of the pigment epithelium of the retina.

Answer 18

(a) synthesis of melanin granules that absorb light
(b) phagocytosis of peeled apical parts of the photoreceptors
(c) ion transport and thereby maintenance of a potential between the two surfaces of the epithelial membrane
(d) esterification and transport of Vitamin A

Further notes:
Processes extending from the free surface of the cells of the retinal pigment epithelium interdigitate with the outer photosensitive segments of the rod and cone cells. The processes, which are filled with granules of melanin pigment, isolate individual photorecepors and enhance visual acuity. [Diagram]

Question 19

List the structures whose nuclei of cell bodies lie in the:
a) outer nuclear layer (2)
b) inner nuclear layer (4)

Answer 19

a) rods and cones
b) bipolar neurons, horizontal cells, amacrine cells, cells of Muller

Question 20

What is the difference between the ganglion layer of the fovea and the ganglion layer outside the fovea?

Answer 20

At the fovea centralis, one ganglion cell receives impulses from one bipolar cell in order to maintain the discrete relay from cone photoreceptors that is required for maximal acquity of the vision. Outside the fovea, the number of bipolar cells converging on one ganglion cell increases towards the periphery of the retina.
Note that cone cells are the only cells found at the fovea centralis.

Question 21

Which layer of the retina is the:
a) site of synapse between rods and cones with bipolar neurons?
b) site of synapse between 1st order and 2nd order neurons?

Answer 21

a) outer plexiform layer
b) inner plexiform layer

Question 22

Functional classification of cell types found in retina.

Answer 22

1. Photoreceptors - rods and cones
2. Conducting cells - bipolar and ganglion
3. Sustentacular - muller
4. Association cells - horizontal, amacrine