2

Ascending sensory

pathways

Done by: Mina Fouad

Introduction 3

Sensory receptors 4

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)

The dorsal column–medial 22

lemniscal (DCML) pathway

The somatosensory pathways 24

to the cerebellum

Trigeminal pathways 27

Visual Pathway 31

Auditory Pathways 36

Vestibular pathway 42

Olfactory pathway 44

Gustatory pathway 46

[ Ascending sensory pathways ] Review on neuroanatomy of ascending sensory pathways.

Index

Neuroscience, 2nd edition by Dale Purves,

George J Augustine, David Fitzpatrick, Lawrence C

Katz, Anthony-Samuel LaMantia, James O McNa-

mara, and S Mark Williams

Principles of medical physiology by sabyasachi

sircar

Richer color experience in observers with multiple

photopigment opsin genes

KIMBERLY A. JAMESON and SUSAN M. HIGHNOTE

University of California at San Diego, La Jolla, Cali-

fornia.

A Textbook of Neuroanatomy

Maria A. Patestas , Leslie P. Gartner

Color Atlas of Neuroscience (Neuroanatomy and

Neurophysiology) Ben Greenstein, Adam Green-

stein

The Human Nervous System

Structure and Function Sixth Edition

Charles R. Noback, Norman L. Strominger

References

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

somatic sensations

Superficial sensations

-Pain. -temperature.-touch.

Deep sensations

-vibration.-joint sense.-muscle sense.-nerve sense.

Cortical sensations

-tactile localization.-2 point discrimination.-stereognosis.-graphosthesia.-perceptual rivalry.

visceral sensations

special sensations

-vision.-hearing.-smell.-taste.

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)

5

Sensory nerve endings in

the skin.

Mechanoreceptor

Superficial Deep

slowly adapting slowly adapting

merkel's disc ruffini

rapidly adapting rapidly adapting

meissener pacinian

Thermoreceptors

Nociceptors

Photoreceptors rods &cones

Mechanoreceptor

hair cells in cochlea

Chemorecptors olfactory & gustatory

Mechanoreceptor

Golgi tendon

muscle spindle

joint capsule

Mechanoreceptor

hair cells in semicircular canals&

otolith organs

Mechanoreceptor

baroreceptors

Chemorecptors

glucoreceptors

osmoreceptors

Exteroceptors proprioceptors Interoceptors

General Special

General special

General

6

Receptor type

type

Anatomical characteristics

Associated axons

Location & function

Rate of adaptation

Threshold of activation

Free nerve endings

(FNE)

Minimally specialized nerve endings.

C(paleospinothalamic

tract)

Diameter: 0.2-1.5 µm

Myelin: No

Velocity: 0.5-2.0 m/s

-All skin -Free nerve end-ings can detect temperature, me-chanical stimuli (touch, pressure, stretch) or pain (nociception). Thus, different free nerve end-ings work as thermoreceptors, cutaneous me-chanoreceptors and nociceptors. In other words, they express po-lymodality.

slow

high

Aδ(Neospinothalamic

Tract)

Diameter :1-5 µm

Myelin: Thin

Velocity: 3–30 m/s

Meissner corpuscle

Encapsulated between dermal papillae

Aβ

Diame-ter:

6-12 µm

Myelin:

Yes

Velocity:

33–75 m/s

-They are distri-buted throughout the skin, but con-centrated in fin-gertips, palms, soles, lips, ton-gue, face and the skin of the male and female genit-als. - Touch, pres-sure, low- frequency vibrations (30–50 Hz) that occur when textured objects are moved across the skin.

Rapid

Low

Somatic Sensory

Receptors

1-sensory receptors

7

Pacinian corpuscles

Encapsulated; onion like covering

Aβ

Diame-ter:

6-12 µm

Myelin:

Yes

Velocity:

33–75 m/s

-Subcutaneous tissue, interos-seous mem-branes, viscera - Deep pressure, vibration (high frequencies).

Rapid

Low

Merkel disc

Encapsulated; associated with peptide- releasing cells.

Aβ

Diame-ter:

6-12 µm

Myelin:

Yes

Velocity:

33–75 m/s

- All skin, hair follicles - Touch

Slow

Low

Ruffini Endings

Encapsulated; oriented along stretch lines

Aβ

Diame-ter:

6-12 µm

Myelin:

Yes

Velocity:

33–75 m/s

-All skin.

- Stretching of

skin.*

Slow

Low

KRAUSE

CORPUSCLE

Encapsulated Aβ

Diame-ter:

6-12 µm

Myelin:

Yes

Velocity:

33–75 m/s

-Lips, tongue,

and genitals. -Responds to

pressure.*

8

Hair Follicle

Ending

Aβ

Diame-ter:

6-12 µm

Myelin:

Yes

Velocity:

33–75 m/s

- Wraps around hair follicle.

- Responds to hair displace-ment.

Rapid

Golgi tendon or-

gans

Highly specia-lized.

Type Ib

-Aα

- 13-20 µm

- 80–120 m/s

-myelinated

-Tendons. - Muscle ten-sion

Slow

Low

Muscle

spindle

Highly specia-

lized. -Type Ia

-Aα

- 13-20 µm

- 80–120

m/s

-myelinated

1ry Respond to the

rate of change

in muscle

length, as well

to change in

length

Type II

(Aβ)

2ry Respond only

to changes in

length

-Muscles. - Muscle length.

Both slow and rap-id

Low

Joint receptors Minimally specialized

Joints Joint position

Rapid

Low

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.

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.

Ree Receptor Location and function Comment

Photo receptors

Rod cell

Location Retina

Function Low light

photoreceptor

cones

Location Retina

Function Bright

light photoreceptor

perception of color

Cones are less sensitive to light than the rod cells in the retin but allow the perception of color. They are also able to perceive finer de-tail. Because humans usually have three kinds of cones which have different response curves and thus respond to variation in color in dif-ferent ways, they have trichromat-ic vision. Being color blind can change this, and there have been reports of people with four types of cones, giving them tetrachro-matic vision.

Hair cells in organ of corti

Hair cells are located within 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.

11

Equilibrium Ampulla

Maculae

Saccule Utricle

found in the semicircular canals for Dynamic equilibrium

Saccule : is responsible for vertical acceleration Utricle: Is responsible for horizontal acceleration

In each ampulla is a small ele-vation called a crista. Each cris-ta is made up of hair cells.

Taste buds concentrated on the upper sur-face of the tongue. detect the flavor of substances

There are five primary taste sensa-tions:

salty

sour

sweet

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

Function Detect traces of chemi-

cals in inhaled air (sense

of smell)

Bipolar sensory receptor

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.

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 lamina

Extent Neuronal group

Column Function

I

C1–S5 Marginal zone nucleus

Dorsal gray

receives afferent fibers carrying pain, temperature, and light touch sensations. It also contributes fibers for the lateral and ventral spinothalamic tracts.

II

C1–S5 Substantia gelati-nosa of Rolando

Dorsal gray

It relays pain, temperature and mechanical (light touch) in-formation.

III, IV

,(V..?)

C1–S5 Nucleus proprius

Dorsal gray

receives pain, light touch, and temperature sensations and provides input to the lateral and ventral spinothalamic tracts.

VI C1–S5 ----- Dorsal gray

This deepest layer of the dorsal horn contains neurons that respond to mechanical signals from joints and skin.

VII C8–L3

T1–L2 (or L3)

S2–S4

Nucleus dorsalis (Clarke’s column) Lateral nucleus Sacral parasympathetic nuc-leus (Onufrowicz)

Dorsal gray Lateral gray Lateral gray

receive synapses from proprioceptive fibers, which bring 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

contains preganglionic sympathetic neurons.

These preganglionic neurons of the sacral outflow of the parasympathetic nervous system

14

VIII C1–S5 -------------- Ventral gray

IX C1–S5 motor neuron groups

Ventral gray

subdivided into three groups: medial, central, and lateral groups

X C1–S5 -Gray commissure -Substantia gelatinosa centralis

Peri- central canal

This represents the small neurons around the central canal.

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.

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.

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.

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, Merkel’s discs, peritrichial nerve endings

Aδ and C Free nerve

Fiber endings Aδ and C Free nerve

Fiber endings

1st order neuron

Receive the sensation from the receptors. located in a dorsal root ganglion. enter the spinal cord at the dorsal root entry zone, via the dorsal roots of the spinal nerves, and upon entry collectively form the dorsolateral fasciculus (tract of Lissauer), These central processes bifurcate into short ascending and descending branches. These branches either as-cend or descend one to three spinal cord levels within this tract, to termi-nate in their target lami-nae of the dorsal horn, where they synapse with second order neurons (or with interneurons).

Thinly myelinated Aδ (fast-

conducting) fibers, which relay sharp, short-term, well-localized pain Or Unmyelinated C (slow-conducting) fibers which relay dull, persistent, poorly localized pain located in a dorsal root ganglion. enter the spinal cord at the dorsal root entry zone, via the lateral division of the dorsal roots of the spinal nerves, and upon entry collectively form the dorsolateral fasciculus (tract of Lissauer), These central processes bifurcate into short ascending and descending branches. These branches either as-cend or descend one to three spinal cord levels within this tract, to termi-nate in their target laminae of the dorsal horn, where they synapse with second order neurons (or with interneurons).

Lightly myelinated Aδ fibers cold stimuli C fibers warm stimuli located in a dorsal root gan-glion. enter the spinal cord at the dorsal root entry zone, via the lateral division of the dorsal roots of the spinal nerves, and upon entry collectively form the dorsolateral fasciculus (tract of Lissauer), These central processes bifurcate into short ascending and descending branches. These branches either as-cend or descend one to three spinal cord levels within this tract, to termi-nate in their target lami-nae of the dorsal horn, where they synapse with second order neurons (or with interneurons).

Peripheral

process

Cell body

Centeral

process

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.

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

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.

20

Spinothalamic tract pathway

21

collect to form

enter spinal cord, at dorsal root entry zone and course in the synapse with form decussate in

terminate in

Striatum,S-I, S-II, cingu-

late gyrus ,prefrontal

cortex

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

Lateral division of dorsal root of spinal nerves

Dorsolateral fasciculus (tract of Lissauer( as ascending and descending branches

Direct pathway of the ALS

signals from Aδ fibers

Indirect pathway of the

ALS signals from C fibers

(tract of Lissauer ) as as-

cending and descending

branches (substantia gelatinosa, lamina II)

and lamina III of dorsal horn

Interneurons

Second order neurons

Spinoreticular tract)paleospino-

thalamic pathway(

Some fibers decussate in ante-

rior white commissure

Many fibers ascend ipsilaterally

Intralaminar nuclei of the

thalamus, hypothalamus,

limbic cortex

Reticular formation

Laminae I and V

of dorsal horn

Interneurons

Laminae II – IV

of dorsal horn

Motoneurons

Reflexes

Second order neurons

Spinothalamic tract )neospino-

thalamic pathway(

Anterior white commissure

P.L.V.N.T V.P.I Intralaminar

nuclei

Collaterals to

reticular

formation Posterior limb of the

internal capsule

Corona radiata

S1 S2

S2

Summary of ALS

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.

1st order neuron

These peripheral processes are medium-size type Aβ and large-size type Aα fibers.

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

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

Peripheral

process

Cell body

Centeral process

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)

3rd 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

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

spinocerebellar

tract.

Cuneocerebellar

tract.

Ventral

spinocerebellar

tract.

Rostral

spinocerebellar

tract.

1st order neuron

(pseudounipolar neurons) whose cell bodies are housed in the dorsal root ganglia

send their peripheral processes to the skin, muscles, tendons, and joints. Here they

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

spinal nerves to synapse in (accessory) cuneate coccygeal spinal cord reside in lamina VII

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.

Peripheral

process &

Cell body

Centeral process

25

2nd order neuron Clark’s column houses the

cell bodies of 2nd order

neurons whose axons form

the dorsal spinocerebellar

tract, which ascends ipsila-

terally in the lateral funicu-

lus of the spinal cord.

When this tract reaches the

brainstem it joins the

restiform body (of the infe-

rior cerebellar pduncle),

and then passes into the

vermis of the cerebellum.

The axons of the 2nd

order neurons, whose

cell bodies are housed

in the accessory

cuneate nucleus, form

the cuneocerebellar

tract. This tract is re-

ferred to as the neck

and upper limb counter-

part of the dorsal spino-

cerebellar tract. Fibers

of the cuneocerebellar

tract join the restiform

body (of the inferior

cerebellar peduncle)

and then enter the

anterior lobe of the ce-

rebellum ipsilaterally.

The axons of these 2nd

order neurons, known as

spinal border cells, form

the ventral (anterior) spi-

nocerebellar tract, which

decussates in the anterior

white comissure and as-

cends in the lateral funi-

culus of the spinal cord to

the medulla. At pontine

levels these fibers join

the superior

cerebellar peduncle to

pass into the vermis of

the cerebellum. These

fibers then decussate

again to their actual side

of origin within the

cerebellum.

The fibers of the 2nd

neurons form the

primarily uncrossed

rostral spinocerebel-

lar tract, the head

and upper limb

counterpart of the

ventral

spinocerebellar tract.

These fibers join the

restiform body (of

the inferior

cerebellar peduncle)

to enter the

cerebellum.

Additionally, some

fibers pass into the

cerebellum via the

superior cerebellar

peduncle.

Function 1-Relays proprioceptive

input from the ipsilateral

trunk and lower limb

2-Coordination of move-

ments of the lower limb

muscles

3-Posture maintenance

1-Relays proprioceptive

information from

the ipsilateral neck and

upper limb

2-Movement of head

and upper limb

1-Relays proprioceptive

input from the ipsilateral

trunk and lower limb

2-Coordination of

movements of the lower

limb muscles

3-Posture maintenance

1-Relays propriocep-

tive information from

the ipsilateral head

and upper limb

2-Movement of head

and upper limb

26

The dorsalspinocerebellar tract and The cuneocerebellar tract

the ventral spinocerebellar

tract and

the rostral spinocerebellar

tract

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,

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.

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.

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.

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

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) 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.

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).

Photoreceptors

Bipolar cells

(first order neurons)

Ganglion cells

(second order neurons)

Optic nerve

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

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).

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.

Optic chiasma

Optic tract

Lateral geniculate

nucleus (3rd order N.)

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.

34

Axons of third order neurons originating from the LGN form the geniculocalcarine tract (optic radiations,

thalamocortical projections)

Join the

Geniculocalcarine

Lract

Internal capsule

Retrolenticular portion Sublenticular portion

Cuneate gyrus Lingual gyrus

Primary visual cortex

2ry visual cortex

Tertiary visual cortex

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 .

stimulating the Peripheral processes (dendrites) of the bipolar (first order) neurons whose cell bodies are

housed in the cochlear (spiral) ganglion .

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

Hair receptor cells

Cochlear (spiral) ganglion

1st order N.

Cochlear nuclei

2nd order N.

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

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.

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 striadecussate the inferior colliculus.

2nd neuron fibers and termination

39

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.

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).

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.

Superior olivary nuclei

3rd order N.

Inferior colliculus.

Medial geniculate nucleus

Primary auditory cortex

40

Hair receptor cells

Cochlear (spiral) ganglion

1st order N.

anteroventral cochlear

nucleus cells

posteroventral cochlear

nucleus cells

Dorsal cochlear

nucleus cells

Lateral superior olivary

nucleus

Medial superior olivary

nucleus

ventral nucleus of the

lateral lemniscus

Inferior colliculus

Medial geniculate nucleus

Medial

nucleus of

trapezoid

Ipsilateraly

decussate

dorsal nucleus of the

lateral lemniscus

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.

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.

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).

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

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.

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.