An easy-to-follow complete review of the special senses. Explains each concept in detail, step-by-step. This review guide will help you focus on what is important to learn for midterm and final exams.
THE SPECIAL SENSES
Special senses include:
Smell (olfaction)
Taste (gustation)
Hearing
Equilibrium and balance
Vision
OLFACTION
Olfactory epithelium is located in the most superior portion of the nasal cavity
Respond to odorants (chemicals) that must be dissolved in mucus covering the nasal
epithelium to stimulate olfactory receptor cells
o Odorants activate a second messenger system that produces cAMP, which opens
up sodium channels to depolarize the olfactory receptor cell
Olfactory receptor cells are bipolar neurons whose dendritic process are
chemoreceptors
o Replaced every 30-60 days
o Synapse with the olfactory bulbs (CNI) on the inferior surface of the brain
through penetration of the cribiform plate
Neurons in the olfactory tract are distributed to the olfactory cortex, hypothalamus and
limbic system
Olfactory information is identified by the olfactory cortex in the temporal lobe but may
also trigger emotional responses and memories as the info is processed by the
hypothalamus and the limbic system
, GUSTATION
Gustatory cells are the receptors for taste
o Clustered together in tastebuds found on the papillae of the tongue
o Replaced ever 7-10 days
o Each cell responds most effectively to one of the primary taste sensations
Sweet: sugar, saccharin, alcohols,
Sour: hydrogen ions (acid)
Salt: metal ions (inorganic salts), acids
Bitter: alkaloids
Umami: glutamate and aspartate (meat, cheese, MSG)
Gustatory cells contain synaptic vesicles that release neruotransmitters when
stimulated, which simulate sensory bipolar neurons supplying the tastebud
NT triggers action potential in the sensory neurons
o Anterior 2/3 – facial nerve CVII
o Poster 1/3 – glossopharyngeal nerve CIX
o Axons from these nerves go to the solitary nucleus in the MO = primary nucleus
for visceral sensations
o Second order neurons cross over to opposite side as they travel to the thalamus,
from which they are sent to the gustatory cortex in the insula
Salt and sour receptors open ion channels directly
Sweet, bitter and umami are indirect acting and create second messengers
Other senses contribute to taste as well, such as smell
HEARING AND EQUILIBRIUM
Sensory hair cells – mechanoreceptors found in the inner each structures that sense
hearing and equilibrium
Apical surface of the sensory cells are covered with stereocilia
External forces cause movement of the stereocilia which produces distortion of the cell
membrane
o This opens mechanically gated ion channels, resulting in NT release
ANATOMY OF THE EXTERNAL EAR
Auricle (pinna) – visible portion of the ear that collects and directs sound waves toward
the middle ear
External acoustic meatus – passageway in temporal bone
o Lined with ceruminous glands (modified apocrine sweat glands) and sebaceous
glands which together produce cerumen (helps trap foreign bodies and keep the
ear canal clean)
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