The Chemical Senses: Gustatory,
Visceral Afferents and Olfactory
Systems
I. Gustatory and Visceral Afferent Systems
A. Overview of the systems.
B. Cranial nerve branches and sensory organs for
taste and visceral afferent system.
C. Solitary nuclear complex and its projections for
gustatory system and visceral afferents.
D. Thalamic projections for gustatory system:
parvocellular portion of the VPM.
E. 1° cortical gustatory areas: Frontal operculum
and the anterior insular cortex.
II. Olfactory System
A. Overview and special features of this system – direct
projection with thalamic relay; - 1° cortex = allocortex.
B. 1° olfactory neurons in nasal mucosa.
C. Olfactory bulb.
D. 1° olfactory cortex constitutes 5 different brain areas:
1.
2.
3.
4.
5.
Anterior olfactory nucleus.
Amygdala.
Olfactory tubercle.
Piriform/periamygdaloid cortices.
Entorhinal cortex.
Olfaction
• Specialized neurons present in the
olfactory epithelium in the nose.
• They project cilia into a mucus
layer. The cilia are able to bind to
odorant molecules  the binding
triggers an AP which is
transmitted to the olfactory area of
the olfactory bulb  olfactory
cortex (lower frontal area and
limbic system of the brain
• Each olfactory receptor is
specialized for 1 odorant molecule
I. Gustatory and Visceral Afferent Systems
A. Overview of the systems.
Taste is mediated by 3 different cranial nerves:
facial (VII), glossapharyngeal (IX), and vagus
(X).
Visceral afferent system uses largely IX and X.
In the gustatory system, receptor
cells (“taste buds”) are separate
from the 1° afferent fibres.
This is an e.g. where the job of
transducing the info and
transmitting it are done by separate
cells.
1° afferent fibres from both systems
collect in the solitary tract in the
rostral medulla, and then terminate
in the solitary nucleus.
The gustatory portion of this nucleus
is more rostral than the cardiorespiratory
portion.
Taste
• Receptors for taste are
modified epithelial
cell present in taste
buds located on the
tongue, roof of the
mouth and pharynx
Ascending Gustatory Pathway
•Gustatory fibres ascend
ipsilaterally in the central
tegmental tract
Medial portion of VPN
1° cortex (frontal operculum
and anterior insular cortex) –
Mediates the discriminative
aspects of taste.
• Four primary types of taste receptors : sour,
salt, sweet and bitter (and a new one: umami)
• The binding of the receptor to a taste molecule
triggers the entry of calcium in the cell 
release of neurotransmitter in a synapse with a
neuron
Taste receptors
Neural pathway
• Taste impulses travel through nerves VII, IX
and X to a gustatory nucleus in the medulla
oblongata (cross over)  thalamus 
gustatory cortex located in the parietal lobe in
the mouth area.
Ascending Visceral
Afferent System
Ascending projections synapse
in the parabrachial nucleus of
the pons
VPM
Amygdala and hypothalamus –
For regulating visceral and food
intake.
I. Gustatory and Visceral Afferent Systems
B. Cranial nerve branches and sensory organs.
1. Taste (gustatory system)
Taste buds contain the receptor cells which transduce
soluble chemical stimuli into a neural signal.
These have synaptic contact with 1° afferent fibres.
On tongue: clustered in papillae
(Fig. 9-2B):
anterior 2/3  branch of VII (facial)
posterior 1/3  IX (glossopharyngeal)
Also
palate  VII
epiglottis, larynx  X
pharynx  IX
Papillae
I. Gustatory and Visceral Afferent Systems
B. Cranial nerve branches and sensory organs.
2. Visceral afferents
IX, X also have branches which serve arterial bp
receptors in carotid sinus, aortic arch.
X  respiratory structures, GIT (rostral to splenic
flexture) – pseudounipolar neurons, much like 1°
afferents for somatic sensory system.
Both systems come together in the ganglion located just
outside the brainstem (Fig. 9-3).
Fig. 9-3:
VII  geniculate
ganglion;
IX, X  inferior ganglion
(or petrosal, nodose) 
rostral medulla
C. Solitary Nuclear Complex
1.Gustatory separation within
bs  solitary tract 
surrounding solitary nucleus
(anterior portion)  ipsilateral
central tegmented tract 
thalamus.
2. Cardiorespiratory nucleus
(caudal portion of solitary complex.
Receives a variety of inputs from
CV, respiratory, and GIT systems
“internal state”
descending
(controls HR, bp,
gut motility,
secretions, etc.)
Ascending parabrachial
n. within pons  amy,
hypo {integrates visceral
info with autonomic functions}
D. Ventral medial (parvocellular)
nucleus of thalamus
E. Primary Gustatory Cortex (Fig. 9-6)
Frontal operculum and
anterior insular cortex:
Modalities of touch and
taste from the tongue
are represented separately
II. Olfactory System
A. Overview + special features of nerve = cranial
nerve I:
1. Info goes to 1° cortex with thalamic relay.
2. 1° olfactory cortex = phylogenetically older
allocortex.
3. Projections to 5 separate cortical regions, rather
than to just 1 (all allocortex) (Fig. 9-7, below):
- less developed in humans, compared to some
other species.
- less precise topographic organization than
other systems we have reviewed.
II. Olfactory System
B. 1° olfactory neurons in nasal mucosa (olfactory
mucosa) in superior nasal concha contains
bipolar neurons which are chemosensitive 
small fasciculus  cribiform plate of ethmoid
bone  olfactory nerve)
-Note risk of anosmia with head trauma (shearing of
these fibres.
Receptor cells have olfactory cilia (like many other
sensory systems), containing the machinery for
receiving chemical stimuli (multiple types of
transmembrane receptors recognize molecular
characteristics of oderants).
Fig. 9-9
II. Olfactory System
C. Olfactory bulb – 1st CNS relay for olfactory input:
- on ventral surface of brain
- neurons organized into discrete layers
Fig. 9-7
Fig. 9-8. Input to neurons with discrete morphological units, called
Glomeruli (surrounded by glial sheath) – limits the spread of
neurotransmitters, neurons, mitral and tufted cells (olfactory tract)
Other neurons receiving inputs: periglomular cells (input directly from
olfactory nerve) and granule cells (receives excitatory input from
mitral cells). Both of these cells are inhibitory interneurons, providing
feedback.
II. Olfactory System
Projections (Fig. 9-9):
Olfactory tract bifurcates into lateral and
medial olfactory striae.
Axons from other brain regions projecting to
olfactory bulb synapse with the medial.
Axons from the olfactory bulb itself  lateral.
One 1° projection (cortical region) is seen
directly caudal to these on basal forebrain:
olfactory tubercle (location in a region
called the anterior perforated substance)
D. Primary olfactory cortical areas
(Fig. 9-7)
Most areas are alocortex (more specifically, 3-layered paleocortex –
archicortex associated with hippocampus)
D. Primary olfactory cortical areas
1.
2.
3.
4.
5.
Anterior olfactory nucleus – modulates info processing in the
olfactory bulb.
Amygdala – in ant temporal lobe  for self-regulation of various
behaviors (e.g., feeding (endocrine), reproductive behaviors), via
projection to hypothalamus.
Olfactory tubercle – part of basal forebrain projections to and from
olfactory bulb – play a role in regulating emotion.
Piriform + periamygdaloid cortices – ant temporal lobe (shaped like a
pear): olfactory perception as in:
- input in internal processing of odors.
- projects to frontal neocortical areas for olfactory discrimination (in
part, through thalamus).
Rostral entorhinal cortex – on parahippocampal gyrus.
- projects to hippocampus
- modulates the association of odors with long-term memories.
Fig. 9-11