Chapter 17: The Special Senses
I. An Introduction to the Special Senses, p. 550
The five special senses are: olfaction
, and hearing
The olfactory organs
are located in the nasal cavity on either side of the nasal septum.
The olfactory epithelium
contains the olfactory receptors
, supporting cells, and basal
The lamina propria consists of areolar tissue, numerous blood vessels, nerves, and olfactory glands.
The surfaces of the olfactory organs are coated with the secretions of the olfactory glands
The olfactory receptors are highly modified neurons.
Olfactory reception involves detecting dissolved chemicals as they interact with odorant-binding
, p. 551
Axons leaving the olfactory epithelium collect into 20 or more bundles that penetrate the cribriform
plate of the ethmoid bone to reach the olfactory bulbs of the cerebrum where the first synapse occurs.
Axons leaving the olfactory bulb travel along the olfactory tract to reach the olfactory cortex, the
hypothalamus, and portions of the limbic system.
In olfaction, the arriving information reaches the information centers without first synapsing in the
The olfactory system can distinguish thousands of chemical stimuli. The CNS interprets smells by the
pattern of receptor activity.
Aging and Olfactory Sensitivity,
The olfactory receptor population shows considerable turnover. The number of olfactory receptors
declines with age.
III. Gustation, p. 552
are clustered in taste buds
The human tongue has three types of lingual papillae:
1. filiform papillae
: provide friction, do not contain taste buds
2. fungiform papillae
: contains five taste buds each
3. circumvallate papillae
: contain as many as 100 taste buds each
Each taste bud contains basal cells
, which appear to be stem cells, and gustatory cells
, which extend
through a narrow taste pore
A typical gustatory cell survives for only about 10 days before it is replaced.
The taste buds are monitored by cranial nerves that synapse within the solitary nucleus of the medulla
oblongata and then on to the thalamus and the primary sensory cortex.
The primary taste sensations
are sweet, salty, sour, and bitter.
Humans have two additional taste sensations:
: characteristic of beef and chicken broths and parmesan cheese. Detected by receptors
sensitive to amino acids, small peptides, and nucleotides.
2. water: detected by water receptors
in the pharynx.
Dissolved chemicals contacting the taste hairs bind to receptor proteins of the gustatory cell.