2. Index
Ascending sensory Introduction 3
pathways
Sensory receptors 4
Done by: Mina Fouad
Classification of receptors 4
Somatic Sensory Receptors 6
Special sensory receptors 10
Sensory pathways 12
Spinal Cord Organization 12
Reticular Formation 14
Anterolateral system 16
(ALS)
References The dorsal column–medial 22
lemniscal (DCML) pathway
Neuroscience, 2nd edition by Dale Purves,
George J Augustine, David Fitzpatrick, Lawrence C
Katz, Anthony-Samuel LaMantia, James O McNa- The somatosensory pathways 24
to the cerebellum
mara, and S Mark Williams
Principles of medical physiology by sabyasachi
sircar Trigeminal pathways 27
Richer color experience in observers with multiple
photopigment opsin genes Visual Pathway 31
KIMBERLY A. JAMESON and SUSAN M. HIGHNOTE
University of California at San Diego, La Jolla, Cali-
fornia. Auditory Pathways 36
A Textbook of Neuroanatomy
Maria A. Patestas , Leslie P. Gartner
Vestibular pathway 42
Color Atlas of Neuroscience (Neuroanatomy and
Neurophysiology) Ben Greenstein, Adam Green-
stein Olfactory pathway 44
The Human Nervous System
Structure and Function Sixth Edition Gustatory pathway 46
Charles R. Noback, Norman L. Strominger
[ Ascending sensory pathways ]
Review on neuroanatomy of ascending sensory pathways.
2

3. Introduction
The sensory system protects a person by detecting changes in the environment. An
environmental change becomes a stimulus when it initiates a nerve impulse, which then travels
to the central nervous system (CNS) by way of a sensory (afferent) neuron. A stimulus becomes a
sensation (something we experience) only when a specialized area of the cerebral cortex
interprets the nerve impulse it generates. Many stimuli arrive from the external environment
and are detected at or near the body surface. Others, such as stimuli from the viscera, originate
internally and help to maintain homeostasis.
Classification of sensation:
sensations
special visceral somatic
sensations sensations sensations
-vision.
-hearing. Cortical Deep Superficial
-smell. sensations sensations sensations
-taste.
-tactile localization. -vibration. -Pain.
-2 point -joint sense.
discrimination. -muscle sense.
-temperature.
-stereognosis.
-nerve sense. -touch.
-graphosthesia.
-perceptual rivalry.
3

4. Sensory receptors
What are sensory receptors?
A sensory receptor is a part of a sensory neuron or cell that receives information from a
stimulus in the internal or external environment of an organism and relates it to nervous
system.
Classification of receptors:
By complexity:
1. Free nerve endings are dendrites whose terminal ends have little or no physical speciali-
zation.
2. Encapsulated nerve endings are dendrites whose terminal ends are enclosed
in a capsule of connective tissue.
3. Sense organs (such as the eyes and ears) consist of sensory neurons with
receptors for the special senses (vision, hearing, smell, taste, and equilibrium)
together with connective, epithelial, or other tissues.
By location:
1. Exteroceptors occur at or near the surface of the skin and are sensitive to
stimuli occurring outside or on the surface of the body. These receptors in-
clude those for tactile sensations, such as touch, pain, and temperature, as
well as those for vision, hearing, smell, and taste.
2. Interoceptors (visceroceptors) respond to stimuli occurring in the body from
visceral organs and blood vessels. These receptors are the sensory neurons
associated with the autonomic nervous system.
3. Proprioceptors respond to stimuli occurring in skeletal muscles, tendons, li-
gaments, and joints. These receptors collect information concerning body po-
sition and the physical conditions of these locations.
By type of stimulus detected:
1. Mechanoreceptors touch, pressure, vibrations, stretch.
2. Thermoreceptors sensitive to temperature changes.
3. Photoreceptors - retina of the eye.
4. Chemoreceptors- respond to chemicals in solution, molecules smelled or
tasted changes in blood chemistry.
5. Nociceptors - respond to potentially damaging stimuli that result in pain.
Virtually all receptors function as nociceptors at one time or another. (Excessive heat,
cold, pressure and chemicals released at site of inflammation)
4

5. Exteroceptors proprioceptors Interoceptors
General Special General
Photoreceptors
rods &cones
Mechanoreceptor
Mechanoreceptor Mechanoreceptor
hair cells in cochlea baroreceptors
Superficial Deep Chemorecptors Chemorecptors
olfactory & gustatory glucoreceptors
slowly adapting slowly adapting
merkel's disc ruffini osmoreceptors
rapidly adapting rapidly adapting
meissener pacinian
Thermoreceptors
General special
Nociceptors
Mechanoreceptor Mechanoreceptor
hair cells in semicircular canals&
Golgi tendon otolith organs
muscle spindle
joint capsule
Sensory nerve endings in
the skin.
5

6. Somatic Sensory
Receptors
Receptor Anatomical Associated Location Rate of Threshold
type characteristics axons & function adaptation of
activation
Minimally C(paleospinothalamic -All skin
specialized tract)
nerve -Free nerve end-
endings. ings can detect
Diameter: Myelin: Velocity: temperature, me-
0.2- No 0.5-2.0 chanical stimuli
1.5 µm m/s
(touch, pressure,
stretch) or pain slow high
(nociception).
Thus, different
Aδ(Neospinothalamic free nerve end-
Tract) ings work as
Free nerve endings
(FNE) thermoreceptors,
Diameter Myelin: Velocity:
:1-5 µm Thin 3–30 m/s cutaneous me-
chanoreceptors
and nociceptors.
In other words,
they express po-
lymodality.
Encapsulated Aβ -They are distri-
between buted throughout
dermal Diame- Myelin: Velocity: the skin, but con-
papillae ter: centrated in fin-
Yes 33–75 gertips, palms,
6-12 µm m/s soles, lips, ton- Rapid Low
gue, face and the
skin of the male
and female genit-
als.
Meissner corpuscle
- Touch, pres-
sure,
low- frequency
vibrations (30–50
Hz) that occur
when textured
objects are
moved across the
skin.
6

7. Encapsulated; Aβ -Subcutaneous
onion like tissue, interos-
covering Diame- Myelin: Velocity: seous mem-
ter: branes, viscera
Yes 33–75
6-12 µm m/s - Deep pressure, Rapid Low
vibration (high
frequencies).
Pacinian corpuscles
Encapsulated; Aβ - All skin, hair
associated follicles
with peptide- Diame- Myelin: Velocity:
releasing cells. ter: - Touch
Yes 33–75
6-12 µm m/s
Slow Low
Merkel disc
Encapsulated; Aβ
oriented along
stretch lines Diame- Myelin: Velocity: -All skin.
ter:
Yes 33–75
- Stretching of Slow Low
6-12 µm m/s
skin.*
Ruffini Endings
Encapsulated Aβ -Lips,
tongue,
and genitals.
Diame- Myelin: Velocity:
ter:
Yes 33–75 -Responds to
6-12 µm m/s
pressure.*
KRAUSE
CORPUSCLE
7

8. Aβ - Wraps around
hair follicle.
Diame- Myelin: Velocity:
ter:
Yes 33–75
6-12 µm m/s - Responds to
hair displace- Rapid
ment.
Hair Follicle
Ending
Highly specia- Type Ib -Tendons.
lized. -Aα
- 13-20 µm
- 80–120 m/s
-myelinated - Muscle ten-
sion
Slow Low
Golgi tendon or-
gans
Highly specia- -Muscles.
lized.
-Type Ia 1ry Respond to the - Muscle length.
Muscle -Aα rate of change
spindle - 13-20 µm in muscle
Both slow
- 80–120 length, as well
m/s to change in and rap-
-myelinated length id Low
Type II 2ry Respond only
(Aβ) to changes in
length
Joint receptors Minimally Joints
specialized Rapid Low
Joint position
8

9.  classifying axons according to their conduction velocity. Three main categories were discerned, called A, B,
and C. A comprises the largest and fastest axons, C the smallest and slowest. Mechanorecep-
tor axons generally fall into category A. The A group is further broken down into subgroups designated α
(the fastest), β, and δ (the slowest). To make matters even more confusing, muscle afferent axons are
usually classified into four additional groups—I (the fastest), II, III, and IV (the slowest)—with subgroups
designated by lowercase roman letters!
 Touch, pressure & vibration are different form of the same sensation, pressure is felt when
force applied on the skin is sufficient to reach the deep receptors whereas touch is felt
when force is insufficient to reach the deep receptors. Vibration is rhythmic variation in
pressure, whether the tactile receptor senses pressure or vibration depends on whether the
receptor is rapidly adapting or slowly adapting. The higher the adaption rate of receptor
the higher vibration frequencies it can detect.
 *= Skin thermoreceptors (hot and cold receptors) detect changes in environmental temper-
ature. Some scientists believe that Ruffini's corpuscles (hot) and Krause's end bodies (cold)
act as skin thermoreceptors. Other scientists are convinced that the receptors are naked
nerve endings and that Ruffini's corpuscles and Krause's end bodies are mechanoreceptors.
9

10. Special Sensory Receptors
Special sensory receptors are distinct receptor cells. They are either localized within complex
sensory organs such as the eyes and ears, or within epithelial structures such as the taste buds
and olfactory epithelium.
Receptor Location and function Comment
Photo receptors Rod cell Cones are less sensitive to light
than the rod cells in the retin but
allow the perception of color. They
Location Retina are also able to perceive finer de-
tail. Because humans usually have
Function Low light three kinds of cones which have
different response curves and thus
photoreceptor respond to variation in color in dif-
ferent ways, they have trichromat-
cones ic vision. Being color blind can
change this, and there have been
Location Retina reports of people with four types
of cones, giving them tetrachro-
Function Bright matic vision.
light photoreceptor
perception of color
Hair cells in organ of Hair cells are located within
corti the organ of Corti on a thin
basilar membrane in the coch-
lea of the inner ear.
They amplify sound waves and
transduce auditory informa-
tion to the Brain Stem.
10

11. Equilibrium
Ampulla found in the semicircular canals
In each ampulla is a small ele-
vation called a crista. Each cris-
for Dynamic equilibrium ta is made up of hair cells.
Saccule : is responsible for
Maculae
vertical acceleration
Saccule Utricle: Is responsible for
horizontal acceleration
Utricle
Taste buds concentrated on the upper sur- There are five primary taste sensa-
face of the tongue. tions:
 salty
 sour
 sweet
detect the flavor of substances  bitter
 umami
A single taste bud contains 50–
100 taste cells representing all 5
taste sensations (so the classic
textbook pictures showing sepa-
rate taste areas on the tongue
are wrong)
Olfactory receptor neuron
Location olfactory epithelium in
the nose
Bipolar sensory receptor
Function Detect traces of chemi-
cals in inhaled air (sense
of smell)
11

12. Sensory pathways
Anatomically, the ascending sensory systems consist of three distinct pathways:
1- The anterolateral system (ALS)
relays predominantly pain and temperature sensation, as well as nondiscriminative (crude or poorly
localized) touch.
2- The dorsal column–medial lemniscal (DCML) pathway
relays discriminative (fine) tactile sense, vibratory sense, and position sense.
3- The somatosensory pathways to the cerebellum
relay primarily proprioceptive (but also some pain and pressure) information.
Spinal Cord Organization:
The spinal cord is composed of a column of gray
matter surrounded by a sheath of white matter.
Gray matter is composed of neurons, their
processes, and neuroglia. It is the large number of
nerve cell bodies that is responsible for the
grayish appearance of the gray matter. White
matter is composed of myelinated and
unmyelinated processes of neurons, neuroglia,
and blood vessels, and it is the white coloration of
the myelin that gives white matter its name.
The white matter consists of the ascending and descending pathways or tracts. The white matter has
been arbitrarily divided into three main sections, namely the dorsal, lateral, and ventral funiculi. The
white matter of the cord is organized into pathways that separate the transmission of different
sensations.
12

13. All sensory information enters the spinal cord through the dorsal roots. Where the dorsal root fibers
enter the spinal cord at the dorsal root entry zone, these separate into two divisions, the medial and
lateral divisions.
The medial division fibers are of relatively larger diameter than those in the lateral division (alpha-beta
fibers); these transmit information of discriminative touch, pressure, vibration, and conscious
proprioception originating from spinal levels C2 through S5.
The lateral division of the dorsal root contains lightly myelinated delta fiber and unmyelinated axons C
fiber of small diameter. These transmit pain, temperature and crude touch sensation from the body.
The gray matter composed of neurons, their processes, and neuroglia, is subdivided into the ventral,
dorsal, and lateral columns. Although the gray matter is completely surrounded by white matter, the
dorsal horn approaches the limit of the spinal cord and is separated from the dorsolateral sulcus by a
small bundle of nerve fibers, known as the dorsolateral tract (of Lissauer).The gray matter of the spinal
cord can be organized into 9 layers plus the region surrounding the central canal, named Rexed
laminae I–X, after the Swedish neuroanatomist who mapped out their distribution.
Rexed Extent Neuronal Column Function
lamina group
Marginal Dorsal receives afferent fibers carrying pain, temperature, and light
I C1–S5 zone gray touch sensations. It also contributes fibers for the lateral and
nucleus ventral spinothalamic tracts.
C1–S5 Substantia gelati- Dorsal It relays pain, temperature and mechanical (light touch) in-
II nosa of Rolando gray formation.
III, C1–S5 Nucleus Dorsal receives pain, light touch, and temperature sensations and
IV proprius gray provides input to the lateral and ventral spinothalamic
,(V..?) tracts.
VI C1–S5 ----- Dorsal This deepest layer of the dorsal horn contains neurons that
gray respond to mechanical signals from joints and skin.
VII C8–L3 Nucleus dorsalis Dorsal receive synapses from proprioceptive fibers, which bring
(Clarke’s column) gray information from Golgi tendon organs and muscle spin-
dles. Some of the axons of these large nerve cell bodies tra-
vel in the dorsal spinocerebellar tracts
Lateral contains preganglionic sympathetic neurons.
T1–L2
Lateral nucleus gray
(or L3)
Sacral Lateral These preganglionic neurons of the sacral outflow of the
S2–S4 parasympathetic nuc- gray parasympathetic nervous system
leus (Onufrowicz)
13

14. VIII C1–S5 -------------- Ventral
gray
IX C1–S5 motor neuron Ventral subdivided into three groups: medial, central, and lateral
groups gray groups
X C1–S5 -Gray commissure Peri- This represents the small neurons around the central canal.
central
-Substantia canal
gelatinosa
centralis
Rexed classification is useful since it is related
more accurately to function than the
previous classification scheme which
was based on major nuclear groups .
Laminae I to IV, in general, are concerned with
exteroceptive sensation.
laminae V: Lamina VII are concerned primarily
with proprioceptive sensations.
laminae VIII-IX comprise the ventral horn and
contain mainly motor neurons.
Lamina X surrounds the central canal and
contains neuroglia.
Reticular Formation:
The reticular formation consists of interconnected circuits of neurons in the tegmentum of the
brain stem, the lateral hypothalamic area, and the medial, intralaminar, and reticular nuclei of
the thalamus.
More than 100 nuclei scattered throughout the tegmentum of the midbrain, pons, and medulla
have been identified as being part of the brainstem reticular formation Although the nuclei of the
reticular formation have a number of diverse functions, they are classified according to the
following four general functions:
1 -The regulation of the level of consciousness, and ultimately cortical alertness.
2 -The control of somatic motor movements.
3 -The regulation of visceral motor or autonomic functions.
4 -The control of sensory transmission.
14

15. Anatomically, the reticular formation is divided into four longitudinal zones (columns) on the basis
of their mediolateral location in
the brainstem.The zones of the
reticular formation are:
- The unpaired median zone:
also known as the median column,
midline raphe ,
The neurons of the median zone that
project to higher brain centers are
associated with sleep
-The paired paramedian zone:
Via their connections with the cerebral
cortex, cerebellum, vestibular nuclei, and
spinal cord, the nuclei of the paramedian
zone function in feedback systems
associated with intricate movements.
-The paired medial zone:
The neurons of the medial zone influence
the ANS, level of arousal, and motor
control of the axial and proximal limb
musculature
- The paired lateral zone: The lateral zone
receives sensory information, integrates
it, and then relays it to the medial zone.
The medial zone then mediates the modulation of sensory afferent input and maintenance of alertness.
Some authors consider the median and paramedian zones to be one zone.
15

16. Anterolateral system
(ALS)
The anterolateral system (ALS) transmits nociceptive, thermal, and nondiscriminatory (crude) touch
information into higher brain centers.
Crude touch Pain from the body temperature
Receptor Free nerve endings, Aδ and C Free nerve Aδ and C Free nerve
Merkel’s discs, Fiber endings Fiber endings
peritrichial nerve endings
1st Peripheral Receive the sensation from Thinly myelinated Aδ (fast- Lightly myelinated Aδ
the receptors. conducting) fibers, which fibers cold stimuli
order process
relay sharp, short-term,
neuron
well-localized pain C fibers warm stimuli
Or
Unmyelinated C (slow-
conducting) fibers which
relay dull, persistent, poorly
localized pain
located in a dorsal root gan-
Cell body located in a dorsal root located in a dorsal root glion.
ganglion. ganglion.
enter the spinal cord at the enter the spinal cord at the
enter the spinal cord at the dorsal root entry zone, via dorsal root entry zone, via
dorsal root entry zone, via the lateral division of the the lateral division of the
the dorsal roots of the dorsal roots of the spinal dorsal roots of the spinal
spinal nerves, and upon nerves, and upon nerves, and upon
entry collectively form the entry collectively form the entry collectively form the
dorsolateral fasciculus dorsolateral fasciculus dorsolateral fasciculus
(tract of Lissauer), These (tract of Lissauer), These (tract of Lissauer), These
Centeral
central processes bifurcate central processes bifurcate central processes bifurcate
into short ascending and into short ascending and into short ascending and
process
descending branches. descending branches. descending branches.
These branches either as- These branches either as- These branches either as-
cend or descend one to cend or descend one to cend or descend one to
three spinal cord levels three spinal cord levels three spinal cord levels
within this tract, to termi- within this tract, to termi- within this tract, to termi-
nate in their target lami- nate in their target laminae nate in their target lami-
nae of the dorsal horn, of the dorsal horn, where nae of the dorsal horn,
where they synapse with they synapse with where they synapse with
second order neurons (or second order neurons (or second order neurons (or
with interneurons). with interneurons). with interneurons).
16

17. 2nd order neuron
The cell bodies of the second order neurons reside in the dorsal horn of the spinal cord
Recent findings indicate that the axons of these second order neurons course in either the direct
(spinothalamic) or indirect (spinoreticular) pathways of the ALS, or as three sets of fibers (the re-
maining components of the ALS): the spinomesencephalic, spinotectal, or spinohypothalamic fibers.
Direct pathway of the anterolateral system:
Type Aδ fibers of first order neurons synapse primarily with second order neurons in lamina I (post-
eromarginal nucleus) and lamina V (reticular nuc-
leus) of the spinal cord gray matter. However,
many first order neurons synapse with spinal cord
interneurons that are associated with reflex motor
activity. The axons of the second order neurons
flow across the midline to the contralateral side of
the spinal cord in the anterior white commissure,
forming the spinothalamic tract which continues up
in the brainstem as spinal lemniscus to end in :
contralateral ventral posterior lateral nucleus of
the thalamus.(P.L.V.N.T )
It also sends some projections to the ventral post-
erior inferior (VPI), and the intralaminar nuclei of
the thalamus. It also sends collaterals to the reti-
cular formation.
Since the spinothalamic tract (direct pathway) is phylogenetically a newer pathway, it is referred to
as neospinothalamic pathway.
Spinothalamic tract actually consists of two anatomically distinct tracts: the lateral spinothalamic
tract (located in the lateral funiculus) and the very small anterior spinothalamic tract (located in the
anterior funiculus). Earlier studies indicated that the lateral spinothalamic tract transmitted only no-
ciceptive and thermal input, whereas the anterior spinothalamic tract transmitted only nondiscri-
minative (crude) touch. Recent studies however, support the finding that both the anterior and later-
al spinothalamic tracts (as well as the other component fibers of the ALS: spinoreticular, spinome-
sencephalic, spinotectal, and spinohypothalamic), transmit nociceptive, thermal, and nondiscrimina-
tive(crude) tactile signals to higher brain centers.
17

18. Indirect pathway of the anterolateral system:
Type C fibers of first order neurons terminate on interneurons in laminae II (substantia gelatinosa)
and III of the dorsal horn. Axons of these interneurons synapse with second order neurons in lami-
nae V–VIII. Many of the axons of these second order neurons ascend ipsilaterally, however a small
number of axons sweep to the opposite side of the spinal cord in the anterior white commissure.
These axons form the more prominent ipsilateral and smaller contralateral spinoreticular tracts. The
spinoreticular tracts transmit nociceptive, thermal, and nondiscriminatory (crude) touch signals from
the spinal cord to the thalamus indirectly, by forming multiple synapses in the reticular formation
prior to their thalamic projections. Since the spinoreticular tract (indirect pathway) is phylogenetically
an older pathway, it is referred to as the paleospinothalamic pathway.
Other component fibers of the anterolateral system:
The spinomesencephalic fibers terminate in the periaqueductal gray matter and the midbrain raphe
nuclei, both of which are believed to give rise to fibers that modulate nociceptive transmission and
are thus collectively referred toas the “descending pain-inhibiting system”.
Furthermore, some spinomesencephalic fibers terminate in the parabrachial nucleus, which sends
fibers to the amygdala—a component of the limbic system associated with the processing of emo-
tions. Via their connections to the limbic system, the spinomesencephalic fibers play a role in the
emotional component of pain.
The spinotectal fibers terminate mainly in the deep layers of the superior colliculus. The superior
colliculi have the reflex function of turning the upper body, head, and eyes in the direction of a pain-
ful stimulus.
The spinohypothalamic fibers ascend to the hypothalamus where they synapse with neurons that
give rise to the hypothalamospinal tract. This pathway is associated with the autonomic and reflex
responses (i.e., endocrine and cardiovascular) to nociception.
3rd order neuron Cell bodies of third order neurons are housed in: the ventral posterior lateral, the ventral posterior
inferior, and the intralaminar thalamic nuclei
The ventral posterior lateral nucleus gives rise to fibers that course in the posterior limb of the inter-
nal capsule and in the corona radiata to terminate in the postcentral gyrus (primary somatosensory
cortex, S-I) of the parietal lobe of the cerebral cortex. Additionally, the ventral posterior lateral
18

19. nucleus also sends some direct projections to the secondary somatosensory cortex, S-II
The ventral posterior inferior nucleus projects mostly to the secondary somatosensory cortex (S-II),
although some of its fibers terminate in the primary somatosensory cortex
(S-I).
The intralaminar nuclei send fibers to the striatum (the caudate nucleus and the putamen), the S-I
and S-II, as well as to the cingulate gyrus and the prefrontal cortex.
Visceral pain:
Visceral pain is characterized as diffuse and poorly localized, and is often “referred to” and felt in
another somatic structure distant or near the source of visceral pain. Nociceptive signals from the
viscera generally follow the same pathway as signals arising from somatic structures.
General visceral afferent nociceptive information from visceral structures of the trunk is carried
mostly by type C, Aδ, or Aβ fibers. The peripheral terminals of these fibers are associated with
Pacinian corpuscles that respond to excessive stretching of the intestinal wall, a lesion in the wall of
the gastrointestinal tract, or to smooth muscle spasm. The cell bodies of these sensory
(pseudounipolar), first order neurons are housed in the dorsal root ganglia, and theircentral
processes carry the information, via the dorsolateral fasciculus (tract of Lissauer), to the dorsal
horn and lateral gray matter of the spinal cord. Here, these central processes synapse with second
order neurons as well as with neurons associated with reflex activities. The axons of the second
order neurons join the anterolateral system to relay nociceptive signals from visceral structures to
the reticular formation and the thalamus. Fibers from the reticular formation project to the
intralaminar nuclei of the thalamus, which in turn project to the cerebral cortex and the
hypothalamus. Visceral pain signals relayed to the primary somatosensory cortex may be
associated with referred pain to a somatic structure. In addition to projections to the
somatosensory cortex, recent studies indicate that nociceptive signals are also relayed to the
anterior cingulate and anterior insular cortices, two cortical areas implicated in the processing of
visceral pain.
19

20. Spinothalamic tract pathway
20

21. Summary of ALS
Receptors (free nerve endings) Peripheral processes of pseudounipolar neurons
Cell bodies of type Aδ and type C pseudounipolar neurons (first order neurons) in dorsal root gangllia
Central processes of pseudounipolar neurons
collect to form
Lateral division of dorsal root of spinal nerves
enter spinal cord, at dorsal root entry zone and course in the
Dorsolateral fasciculus (tract of Lissauer( as ascending and descending branches
Direct pathway of the ALS Indirect pathway of the
signals from Aδ fibers ALS signals from C fibers
(tract of Lissauer) as as-
cending and descending
(substantia gelatinosa, lamina II)
branches
Laminae I and V Laminae II – IV
and lamina III of dorsal horn
of dorsal horn of dorsal horn
synapse with
Interneurons
Second order neurons Interneurons
form
Spinothalamic tract )neospino- Motoneurons Second order neurons
thalamic pathway(
decussate in Reflexes Spinoreticular tract)paleospino-
Anterior white commissure thalamic pathway(
terminate in
Some fibers decussate in ante-
P.L.V.N.T V.P.I Intralaminar Collaterals to
rior white commissure
nuclei reticular
formation Many fibers ascend ipsilaterally
Posterior limb of the
internal capsule
Striatum,S-I, S-II, cingu-
Reticular formation
late gyrus ,prefrontal
Corona radiata cortex
Intralaminar nuclei of the
S1 S2
21 thalamus, hypothalamus,
limbic cortex
S2

22. The dorsal column–medial lemniscal
(DCML) pathway
It relays discriminative (fine) tactile sense, vibratory sense, and position sense.
Touch, pressure & vibration are different form of the same sensation, pressure is felt when force applied on the
skin is sufficient to reach the deep receptors whereas touch is felt when force is insufficient to reach the deep
receptors. Vibration is rhythmic variation in pressure, whether the tactile receptor senses pressure or vibration
depends on whether the receptor is rapidly adapting or slowly adapting. The higher the adaption rate of receptor
the higher vibration frequencies it can detect.
Receptor • Free nerve endings responding to touch, pressure, and proprioception in the skin, muscles, and
joint capsules.
• tactile (Merkel’s) discs responding to touch and pressure
in the skin;
• peritrichial endings stimulated by touch of the hair follicles;
• Meissner’s corpuscles activated by touch of the skin; and
• Pacinian corpuscles stimulated by touch, pressure, vibration,and proprioception in the deep layers
of the skin, and in visceral structures.
st
1 Peripheral These peripheral processes are medium-size type Aβ and large-size type Aα fibers.
order process
neuron
Cell body
Cell bodies are located in the dorsal root ganglia.
Enter the spinal cord at the dorsal root entry zone via the medial division of the dorsal roots of the
spinal nerves. Upon entry into the posterior funiculus of the spinal cord, the afferent fibers bifurcate
into long ascending and short descending fibers.
The long ascending and short descending fibers give rise to collateral branches that may synapse
Centeral
process with several distinct cell groups of the dorsal horn interneurons and with ventral horn motoneurons.
These fibers collectively form the dorsal column pathways, either the fasciculus gracilis or the fasci-
culus cuneatus, depending on the level of the spinal cord in which they enter.
below level T6-gracilis
include the lower thoracic, lumbar, and sacral levels that bring information from the lower limb and
lower half of the trunk
at level T6 and above cuneate
bring information from the upper thoracic and cervical levels, that is from the upper half of the trunk
and upper limb
22

23. 2nd order neuron The first order fibers terminating in the nucleus gracilis and nucleus cuneatus in the medulla syn-
apse with second order neurons whose cell bodies are housed in these nuclei The fibers of the
second order neurons form the internal arcuate fibers as they curve ventromedially to the opposite
side. These fibers ascend as the medial lemniscus in the brain stem to synapse with third order
neurons in the posterior lateral ventral nucleus of the thalamus.(P.L.V.N.T)
rd
3 order neuron The posterior lateral ventral nucleus of the thalamus. (P.L.V.N.T) houses the cell bodies of the third
order neurons of the DCML pathway. The fibers arising from the thalamus ascend in the posterior
limb of the internal capsule and the corona radiate to terminate in the primary somatosensory cortex
of the postcentral gyrus
23

24. The somatosensory pathways to the cerebellum
Most of the proprioceptive information does not reach conscious levels, and instead is transmitted directly
to the cerebellum via the ascending somatosensory cerebellar pathways without projecting to the
thalamus or the cerebral cortex. These pathways, which process subconscious proprioception from
muscles, tendons, and joints, are two-neuron pathways, consisting of first order and second order
neurons.
The pathways include:
- dorsal (posterior) spinocerebellar tract.
- The cuneocerebellar tract.
- The ventral (anterior) spinocerebellar tract.
- The rostral spinocerebellar tract.
Dorsal Cuneocerebellar Ventral Rostral
spinocerebellar tract. spinocerebellar spinocerebellar
tract. tract. tract.
1st Peripheral (pseudounipolar neurons) whose cell bodies are housed in the dorsal root ganglia
order process &
send their peripheral processes to the skin, muscles, tendons, and joints. Here they
Cell body
neuron
perceive proprioceptive information, which is then transmitted to the spinal cord by
their central processes.
These central processes ascend in the fasciculus transmit sensory input to synapse with 2nd
join the medial division of cuneatus and terminate laminae V–VII of the order neurons
the dorsal roots of the in the external lumbar, sacral, and. whose cell bodies
Centeral
spinal nerves to synapse in (accessory) cuneate coccygeal spinal cord reside in lamina VII
process
the nucleus dorsalis(Clark’s nucleus—the nucleus levels, where they of the dorsal horn
column, lamina VII of spinal dorsalis of Clark terminate and synapse
cord levels C8 to L2,3) at homologue at cervical with 2nd order neurons.
their level of entry.Sensory levels above C8
information transmitted by
spinal nerves entering
below Clark’s column is
relayed to the caudal
extent of the nucleus
dorsalis (L2,3) by
ascending in the fasciculus
gracilis.
24

25. 2nd order neuron Clark’s column houses the The axons of the 2nd The axons of these 2nd The fibers of the 2nd
cell bodies of 2nd order order neurons, whose order neurons, known as neurons form the
neurons whose axons form cell bodies are housed spinal border cells, form primarily uncrossed
the dorsal spinocerebellar in the accessory the ventral (anterior) spi- rostral spinocerebel-
tract, which ascends ipsila- cuneate nucleus, form nocerebellar tract, which lar tract, the head
terally in the lateral funicu- the cuneocerebellar decussates in the anterior and upper limb
lus of the spinal cord. tract. This tract is re- white comissure and as- counterpart of the
When this tract reaches the ferred to as the neck cends in the lateral funi- ventral
brainstem it joins the and upper limb counter- culus of the spinal cord to spinocerebellar tract.
restiform body (of the infe- part of the dorsal spino- the medulla. At pontine These fibers join the
rior cerebellar pduncle), cerebellar tract. Fibers levels these fibers join restiform body (of
and then passes into the of the cuneocerebellar the superior the inferior
vermis of the cerebellum. tract join the restiform cerebellar peduncle to cerebellar peduncle)
body (of the inferior pass into the vermis of to enter the
cerebellar peduncle) the cerebellum. These cerebellum.
and then enter the fibers then decussate Additionally, some
anterior lobe of the ce- again to their actual side fibers pass into the
rebellum ipsilaterally. of origin within the cerebellum via the
cerebellum. superior cerebellar
peduncle.
Function 1-Relays proprioceptive 1-Relays proprioceptive 1-Relays proprioceptive 1-Relays propriocep-
input from the ipsilateral information from input from the ipsilateral tive information from
trunk and lower limb the ipsilateral neck and trunk and lower limb the ipsilateral head
2-Coordination of move- upper limb 2-Coordination of and upper limb
ments of the lower limb 2-Movement of head movements of the lower 2-Movement of head
muscles and upper limb limb muscles and upper limb
3-Posture maintenance 3-Posture maintenance
25

26. The dorsalspinocerebellar
tract
and
The cuneocerebellar tract
the ventral spinocerebellar
tract and
the rostral spinocerebellar
tract
26

27. Face sensation
(trigeminal sensory
pathway)
The trigeminal nerve, the largest of the cranial nerves, provides the major general sensory innervation to
part of the scalp, most of the dura mater, the conjuctiva and cornea of the eye, the face, nasal cavities,
paranasal sinuses, palate, temporomandibular joint, lower jaw, oral cavity, and teeth.
The trigeminal sensory pathway, which transmits touch, nociception, and thermal sensation, consists of a
three neuron sequence (first, second, and third order neurons) from the periphery to the cerebral cortex
respectively.
First order neuron:
Cell bodies are housed in the trigeminal ganglion.
The peripheral processes radiating from the trigeminal ganglion gather to form three separate nerves, the
three divisions of the trigeminal nerve whose peripheral endings terminate in sensory receptors of the
orofacial region.
Nearly half of the sensory fibers in the trigeminal nerve are Aβ myelinated discriminatory touch fibers. The
remaining half of the sensory fibers in the trigeminal nerve is similar to the Aδ and C nociceptive and
temperature fibers of the spinal nerves.
Dvisions: ophthalmic , maxillary , and mandibular .
The central processes of these neurons enter the pons and terminate in the trigeminal nuclei where they
establish synaptic contacts with second order neurons housed in these nuclei.
2nd order neuron:
The trigeminal nuclei, with the exception of the mesencephalic nucleus, contain second order neurons as
well as interneurons.
Trigeminal Sensory nuclei:
• Main (chief, principal) nucleus: Is located in the midpons. It is homologous to the nucleus gracilis and
nucleus cuneatus. It is associated with the transmission of mechanoreceptor information for discriminatory
(fine) tactile and pressure sense.
• Mesencephalic nucleus of the trigeminal: is unique, since it is a true “sensory ganglion” (and not a
nucleus). During development, neural crest cells are believed to become embedded within the CNS,
27

28. instead of becoming part of the peripheral nervous system, as other sensory ganglia. This nucleus houses
the cell bodies of sensory (first order) pseudounipolar neurons, thus there are no synapses in the
mesencephalic nucleus. The peripheral large-diameter myelinated processes of these neurons convey
general proprioception input from the muscles innervated by the trigeminal nerve (and the extraocular
muscles, as well as from the periodontal ligament of the teeth. Pseudounipolar neurons of the
mesencephalic nucleus transmit general proprioception input to the main sensory and motor nuclei of the
trigeminal and reticular formation to mediate reflex responses.
• Spinal nucleus of the trigeminal: is the largest nucleus consists of three subnuclei
Subnucleus oralis: It is associated with the transmission of discriminative (fine) tactile sense from the
orofacial region.
Subnucleus interpolaris: is also associated with the transmission of tactile sense, as well as dental pain.
Subnucleus caudalis: is associated with the transmission of nociception and thermal sensations from the
head.
28

29. The trigeminal pathway for touch and pressure:
-As the central processes of pseudounipolar (first order) neurons enter the pons, they bifurcate into:
Short ascending fibers which synapse in the main sensory nucleus
Long descending fibers which terminate and synapse mainly in the subnucleus oralis and less
frequently in the subnucleus interpolaris
-Fibers from the main sensory nucleus:
Some 2nd order fibers from the main sensory nucleus cross the midline and join the ventral trigeminal
lemniscus to ascend and terminate in the contralateral VPM nucleus of the thalamus.
Other second order fibers from the main sensory nucleus do not cross. They form the dorsal trigeminal
lemniscus, and then ascend and terminate in the ipsilateral VPM nucleus of the thalamus.
-Fibers terminating in the subnucleus oralis or interpolaris synapse with second order neurons whose
fibers cross the midline and ascend in the ventral trigeminal lemniscus to the contralateral VPM nucleus of
the thalamus.
29

30. Pain and thermal pathway:
- As the central processes of pseudounipolar neurons enter the pons, they descend in the spinal tract of
the trigeminal and most of them synapse in the subnucleus caudalis.
Most of the second order fibers from the subnucleus caudalis cross the midline and join the contralateral
ventral trigeminal lemniscus, whereas others join the ipsilateral ventral trigeminal lemniscus. All the fibers
ascend to the VPM nucleus of the thalamus.
3rd order neuron:
the ventral posterior medial (VPM) nucleus of the
thalamus. The third order neurons then relay
sensory information to the postcentral gyrus of the
cerebral cortex for further processing.
30

31. VISUAL PATHWAY
The visual pathway consists of photoreceptors, first order and second order neurons residing in the
retina, and third order neurons in the lateral geniculate nucleus of the thalamus
Photoreceptors
Incoming light rays impinging on the retina cause the retinal photoreceptor cells (modified neurons), the
rods and cones, to become hyperpolarized. The photoreceptors then stop releasing neurotransmitters
and the bipolar cells (first order neurons) are no longer inhibited.
Bipolar cells
)first order neurons(
Bipolar cells (first order neurons) are no longer inhibited, and fire. The bipolar cells along with the
interneurons, the horizontal and amacrine cells, process, integrate, and modulate visual input. The
bipolar cells relay this sensory input to the ganglion cells (second order neurons) of the retina.
Ganglion cells
(second order neurons)
possess nonmyelinated that course on the inner surface of the retina collect at Optic disc. Axons pierce
sclera in lamina cribrosa to emerge from the back of the bulb of the eye. At this point, the axons become
myelinated and they form a large bundle, the optic nerve (CN II).
Optic nerve
31

32. The optic nerves of the right and left sides join superior to the body of the sphenoid bone in the middle
cranial fossa to form optic chiasma.
To form
Optic chiasma
where partial decussation of the optic nerve fibers (axons) of the two sides occurs. All ganglion cell axons
arising from the temporal half of the retina course in the lateral aspect of the optic chiasma without
decussating, to join the optic tract of the same side. All ganglion cell axons arising from the nasal half of
the retina decussate at the optic chiasma, and enter the optic tract of the opposite side, to join the
temporal fibers. Thus, each optic tract consists of ganglion cell axons arising from both eyes (the
ipsilateral temporal half and the contralateral nasal half of the retina).
Optic tract
it courses around the cerebral peduncle to end and relay visual information primarily in the lateral
geniculate nucleus (LGN) of the thalamus, which processes visual input. The optic nerve also ends and
relays visual information in:
(i) The superior colliculus, a mesencephalic relay nucleus for vision having an important function in
somatic motor reflexes.
(ii) The pretectal area, which mediates autonomic reflexes such as the control of pupillary constriction
and lens accommodation.
(iii) The hypothalamus, which has an important function in circadian rhythms (day–night) and the
reproductive cycle.
Lateral geniculate
nucleus (3rd order N.)
32

33. The LGN houses the cell bodies of third order neurons of the visual pathway.The LGN is a laminated
structure consisting of six distinct layers that are readily dentifiable in a horizontal section. Although each
LGN receives information from the contralateral visual hemifield, each of its layers receives input from
only one eye.
Layers 1, 4, and 6 receive ganglion cell axons arising from the contralateral retina.
Layers 2, 3, and 5 receive ganglion cell axons arising from the ipsilateral retina.
Layers 1 and 2 consist of large neurons and are therefore referred to as the magnocellular layers; they
receive information from ganglion cells that are sensitive to movement and contrast but are insensitive to
color.
Layers 3–6 consist of small neurons and are referred to as the parvocellular layers; they receive
information from the ganglion cells responding to color and form.
33

34. Axons of third order neurons originating from the LGN form the geniculocalcarine tract (optic radiations,
thalamocortical projections)
Geniculocalcarine
Lract
Join the
Internal capsule
Retrolenticular portion Sublenticular portion
Cuneate gyrus Lingual gyrus
Primary visual cortex
2ry visual cortex
Tertiary visual cortex
34

35. 35

36. AUDITORY PATHWAYS
Sound waves transmitted via the Auricle (pinna) and external auditory meatus (canal) to the tympanic
membrane (eardrum) causing it to vibrate, vibrations transmitted via the Malleus (which is attached to
the tympanic membrane) Incus (which articulates with the malleus and stapes) Stapes
causing it to oscillate, oscillating footplate attaches to the membrane of the oval window causing it to
oscillate and in turn agitate the perilymph of the scala vestibule perilymph waves agitate the
vestibular (Reissner's membrane) which begins to oscillate generating waves in the Endolymph of the
scala media (cochlear duct) endolymph waves cause the basilar membrane (which supports the
organ of Corti) to oscillate stimulating the Hair receptor cells which convert mechanical energy into
electrical energy .
Hair receptor cells
stimulating the Peripheral processes (dendrites) of the bipolar (first order) neurons whose cell bodies are
housed in the cochlear (spiral) ganglion .
Cochlear (spiral) ganglion
1st order N.
Impulses are transmitted to the central processes (axons) of the bipolar (first order) neurons which form
the root of the cochlear nerve axons leave the inner ear via the Internal auditory meatus (canal) to enter
the posterior cranial fossa then pierce the brainstem at the pontomedullary angle of the brainstem to
terminate in the cochlear nuclei including:
-The ventral cochlear nucleus is subdivided into a posteroventral cochlear nucleus and an anteroventral
cochlear nucleus.
- The dorsal cochlear nucleus
Cochlear nuclei
2nd order N.
36

37. Second order fibers arising from:
1-anteroventral cochlear nucleus: Either
-Ascend ipsilaterally to the medial and lateral superior olivary nuclei.
-or decussate forming ventral acoustic striae to:
• The medial nucleus of the trapezoid body, which in turn
projects to the lateral superior olivary nucleus.
• The medial superior olivary nucleus.
• The dorsal nucleus of the lateral lemniscus and the inferior
colliculus (by ascending in the contralateral lateral lemniscus)
2-the posteroventral cochlear nucleus:
form the intermediate acoustic stria. These fibers subsequently join the ipsilateral and contralateral lateral
lemniscus to ascend to, and terminate in, the ventral nucleus of the lateral lemniscus and the inferior
colliculus, bilaterally
37

38. 3-the dorsal cochlear nucleus
form the dorsal acoustic stria, which decussates. These fibers join the contralateral lateral lemniscus to
ascend to, and terminate in, the inferior colliculus.
2nd neuron fibers and termination
anteroventral cochlear nucleus:
ipsilateraHy medial and lateral superior olivary nuclei.
decussate ventral acoustic striae  dorsal nucleus of the lateral lemniscus and the inferior colliculus
medial superior olivary nucleus
lateral superior olivary nuclei via medial nucleus of trapezoid
posteroventral cochlear nucleus:
 intermediate acoustic stria ipsilateral and contralateral the ventral nucleus of the lateral lemniscus and the inferior
colliculus.
Dorsal cochlear nucleus:
 dorsal acoustic striadecussate  the inferior colliculus.
38

39. Superior olivary nuclei
3rd order N.
The main nuclei of this complex are the medial superior olivary nucleus and the lateral superior olivary
nucleus, both of which receive second order fiber terminals from the cochlear nuclei and have an
important function in sound localization in the following manner:
The medial superior olivary nucleus processes auditory input by comparing the amount of time it takes for
a sound to reach each ear.
The lateral superior olivary nucleus processes auditory input by comparing the intensity (volume) of a
sound arriving at each ear.
the fibers arising from the medial superior olivary nucleus join the ipsilateral lateral lemniscus, whereas
those that arise from the lateral superior olivary nucleus join the ipsilateral and contralateral lateral
lemniscus that terminate in the dorsal nucleus of the lateral lemniscus and in the superior colliculus.
Inferior colliculus.
It receives afferents ascending in the lateral lemniscus from the cochlear nuclei, the superior olivary
nuclear complex, and the nuclei of the lateral lemniscus. The inferior colliculus also receives afferents
from the contralateral inferior colliculus.
The inferior colliculus gives rise to fibers end in the ipsilateral medial geniculate nucleus, a thalamic relay
station of the auditory system. The inferior colliculus also projects to the contralateral medial geniculate
nucleus and the superior colliculus (which is involved in visual reflexes).
Medial geniculate nucleus
Fibers arising in the medial geniculate nucleus form the auditory radiations that join the sublenticular
portion of the posterior limb of the internal capsule to terminate in the primary auditory cortex.
Primary auditory cortex
39

40. Hair receptor cells
Cochlear (spiral) ganglion
1st order N.
anteroventral cochlear posteroventral cochlear Dorsal cochlear
nucleus cells nucleus cells nucleus cells
Medial
Lateral superior olivary Medial superior olivary
nucleus of
trapezoid
nucleus nucleus
dorsal nucleus of the ventral nucleus of the
lateral lemniscus lateral lemniscus
Inferior colliculus
Ipsilateraly
decussate
Medial geniculate nucleus
40

41. Lateral lemniscus contains the following fibers:
1 -Second order fibers arising from the contralateral anteroventral cochlear nucleus (which do not
synapse in the superior olivary complex) that terminate in the dorsal nucleus of the lateral lemniscus and
the inferior colliculus.
2- Second order fibers arising from the ipsilateral and contralateral posteroventral cochlear nucleus that
terminate in the ventral nucleus of the lateral lemniscus and in the inferior colliculus.
3 -Second order fibers arising from the contralateral dorsal cochlear nucleus that terminate in the ventral
nucleus of the lateral lemniscus and in the inferior colliculus.
4- Third order fibers originating from the superior olivary nuclear complex (the fibers arising from the
medial superior olivary nucleus join the ipsilateral lateral lemniscus, whereas those that arise from the
lateral superior olivary nucleus join the ipsilateral and contralateral lateral lemniscus) that terminate in the
dorsal nucleus of the lateral lemniscus and in the superior colliculus.
5- Fibers arising from the dorsal and ventral nuclei of the lateral lemniscus that project to the ipsilateral
inferior colliculus.
41

42. Vestibular pathway
First order neuron:
The cell bodies of the sensory first order bipolar neurons of the vestibular nerve reside within the
vestibular ganglion of Scarpa.
Their peripheral processes terminate in special receptors, the cristae in the ampullae of the semicircular
ducts and the maculae of the utricle and saccule.
The central processes of these neurons enter the brainstem to synapse not only in the vestibular nuclear
complex, where they synapse with second order neurons of the vestibular pathway, but also in the
cerebellum. Some first order vestibular fibers, however, do not terminate in the vestibular nuclei, but take
an alternate route by going around them, joining the juxtarestiform body in the inferior cerebellar
peduncle and terminating directly in the ipsilateral flocculonodular lobe of the cerebellum.
The vestibular nerve is unique since it is the only cranial nerve that sends the central processes of some
of its first order neurons to synapse directly in the cerebellum.
Second order neuron:
Vestibular nuclear complex: the vestibular nuclear complex is composed of four vestibular nuclei:
1 The superior (Bechterew’s) vestibular nucleus.
2 The medial (Schwalbe’s) vestibular nucleus.
3 The lateral (Deiter’s) vestibular nucleus.
4 The inferior (spinal, descending) vestibular nucleus.
The superior and medial vestibular nuclei receive the first order neuron terminals relaying sensory input
from the cristae ampullares of the semicircular canals. Following the reception of this sensory input, these
nuclei then relay it via two structures:
1 The medial longitudinal fasciculus (MLF) to the extraocular muscle nuclei to elicit compensatory ocular
movements triggered by movements of the head.
2 The medial vestibulospinal tract to the cervical spinal cord to elicit suitable head movements.
The lateral vestibular nucleus receives vestibular sensory input mainly from the maculae of the utricle, but
may also receive input from the saccule and semicircular canals. This nucleus projects via the lateral
vestibulospinal tract to motoneurons or interneurons at all spinal cord levels to make postural
adjustments.
42

43. The inferior vestibular nucleus receives vestibular sensory input from the semicircular canals as well as
the utricle. Most of the first order vestibular fibers terminate in this nucleus. It projects to the reticular
formation and the cerebellum.
3rd orden neuron:
The superior and lateral vestibular nuclei give rise to
second order fibers that join the MLF bilaterally to
ascend to the ventral posterior lateral and ventral
posterior inferior nuclei of the thalamus.
The thalamus gives rise to third order fibers that
terminate in the primary vestibular cortex (Brodmann’s
area 3a) in the parietal lobe, located next to the primary
motor area (Brodmann’s area 4).
43

44. Olfactory System
Pathways
The sense of smell is mediated by the olfactory system. This is the detection of airborne chemicals by
specialized receptors in the olfactory mucosa.
The olfactory system is completely neural, since the receptors are modified neurons that transduce and
transmit olfactory inputs to the brain via the olfactory bulb, the lateral olfactory tract, and from there to the
olfactory cortex.
The olfactory system is unique among the senses, in that receptors project directly to cortex; the other
senses relay through the thalamus.
Each olfactory receptor cell gives rise to an unmyelinated centrally directed axon. They are the slowest
impulse-conducting axons of the central nervous system (CNS). The axons of these bipolar cells
converge and assemble to form 15–20 bundles (fascicles)—the olfactory fila. The olfactory fila course
superiorly, traversing the sieve-like perforations of the cribriform plate of the ethmoid bone of the skull to
terminate in the ventral surface of the ipsilateral olfactory bulb.
The olfactory bulb is part of the forebrain, situated on its ventral surface in the olfactory sulcus, and
attached to it by the olfactory tract. The olfactory tract consists mainly of fibers of the anterior olfactory
nucleus, the lateral olfactory tract, and the anterior limb of the anterior commissure. This tract carries
many centrifugal fibers from the brain to the olfactory bulb.
The lateral olfactory tract (LOT), which transmits olfactory inputs to the brain, gives off collaterals to the
limbic system, to the olfactory cortex, and to the anterior olfactory nucleus. The anterior olfactory nucleus
projects mainly to both the olfactory bulbs and to its contralateral partner. The axons of the LOT travel
caudally as the lateral olfactory stria; these synapse in the piriform cortex, a major component of the
olfactory cortex, and the olfactory tubercle. The LOT projects further caudally to the anterior cortical
amygdaloid nucleus, the lateral entorhinal cortex and the periamygdaloid cortex, which is part of the
piriform cortex that overlies the amygdala.
The main areas of the olfactory cortex are the anterior cortical amygdaloid nucleus, anterior olfactory
nucleus, lateral entorhinal cortex, periamygdaloid nucleus, piriform cortex, and olfactory tubercle. All
these areas have reciprocal intrinsic connections. The main intrinsic connections stem from the anterior
olfactory nucleus, lateral entorhinal cortex, and piriform cortex. The olfactory cortex is phylogenetically
44

45. identified as paleocortex, because most of it contains three cell layers, while neocortex has six layers of
cells.
The olfactory cortex projects to several other extrinsic areas. These are the olfactory bulb, which receives
fibers from all areas of the olfactory cortex except the olfactory tubercle; to the hippocampus from the
lateral entorhinal cortex, and to the lateral hypothalamus, mainly from the piriform cortex and anterior
olfactory nucleus. The connections to the hippocampus mediate olfactory contribution to memory and
learning. The connections to the hypothalamus mediate feeding behavior and perhaps emotional
responses such as food-evoked rage responses.
45

46. Gustatory Pathway
The gustatory (taste) system makes possible the phenomenon of flavor perception. Modalities of taste
are sensed by taste buds in the oropharyngeal mucosa, which detects chemicals that are dissolved in the
saliva. The information is transmitted by afferent conduction to the CNS, where the modality is
recognized. Taste buds are modified oral mucosa cells, which transduce the chemical modality into an
electrical impulse; this impulse travels through first-order neurons along one of more of the cranial nerves
VII, IX, and X to the solitary nucleus. From there, second-order neurons project to the thalamus Third -
order neurons project to diencephalic areas involved in appetite control, food intake, and fluid and ion
balance. From the thalamus, fibers project to the orbitofrontal and insular cortex.
46