Neuroanatomy lecture

1. Special Sensory pathways
DR. Mohammed Mahmoud Mosaed

2. Special Senses
•Taste
•Smell
•Vision
•Hearing/Balance

3. Taste (gustatory) pathway
• 1. Receptors Taste buds on tongue, lips, palatal arch
and soft palate. Each “bud” contains several cell types
in microvilli (taste hairs) that project through taste
pore.
• Gustatory receptor cells communicate with cranial
nerve axon endings to transmit sensation to brain.
• Cranial Nerves of taste
• Anterior 2/3 tongue: chorda tympani→ Facial nerve
• Posterior 1/3 tongue: Glossopharyngeal nerve
• Most posterior part of the tongue: Vagus nerve

5. Taste (gustatory)pathway
• 1. Receptors: Taste buds on tongue, lips, palatal arch and
soft palate
• 2.The first order neuron in the pathway is the geniculate
ganglion of the facial nerve and inferior ganglia of the
glossopharyngeal and vagus nerves
• 3. The second order neuron is the nucleus solitarius and
its upper part enlarged and called the gustatory nucleus.
The axons of the cells of the nucleus cross to the opposite
side and ascend to end in the posteromedial ventral
nucleus of the thalamus
• 4. The third order neuron is the posteromedial ventral
nucleus of the thalamus.
• The axons of the cells pass through the sensory radiation
to the gustatory area in the superior wall of the posterior
ramus of the lateral sulcus.

7. Olfactory pathways
• Olfactory Receptors.
• The olfactory receptors are embedded in mucous
membrane of the upper part of the nasal cavity above the
superior concha. The fine central processes of bipolar
receptor nerve cells form the olfactory nerve fibers.
• Bundles of these nerve fibers pass through the openings
of the cribriform plate of the ethmoid bone to enter the
olfactory bulb
• Olfactory Bulb.
• This ovoid structure have several types of nerve cells; the
mitral cells, tufted cells and granular cells. The incoming
olfactory nerve fibers synapse with the dendrites of the
mitral cells and form synaptic glomeruli. The olfactory
bulb, in addition, receives axons from the contralateral
olfactory bulb through the olfactory tract.

8. • Olfactory Tract. This narrow band of white matter runs from the
posterior end of the olfactory bulb under the inferior surface of the frontal
lobe of the brain.
• It consists of the central axons of the mitral and tufted cells of the bulb
and some fibers from the opposite olfactory bulb.
• As the olfactory tract reaches the anterior perforated substance, it divides
into medial and lateral olfactory striae.
• The lateral stria carries the axons to the olfactory area of the cerebral
cortex.
• The medial olfactory stria carries the fibers that cross the median plane in
the anterior commissure to pass to the olfactory bulb of the opposite side.
• Anterior commissure. This is a small commissure that connects the
two halves of the olfactory system.
• Olfactory Cortex. includes the following regions;
• Primary olfactory cortex: uncus, limen insulae (apical region of the insula)
and corticomedial part of the amygdaloid body.
• Secondary olfactory cortex is the entorhinal area (anterior part of the
parahipocampal gyrus that lies behind the uncus
• The olfactory cortex collectively called piriform lobe
• Note the olfactory cortex is the one area of cortex that receives direct
sensory input without an inter posed thalamic connection.

10. Visual Pathway
• The visual pathway includes the interneurones of
the retina, retinal ganglion cells whose axons
project via the optic nerve, chiasma, and optic tract
to the lateral geniculate nucleus (LGN) and
neurones within the LGN which project via the optic
radiation to the primary visual cortex.
• Axons of ganglion cells of the retina → optic nerve
→ optic chiasm (partial decussation) → optic tract
→ lateral geniculate nucleus (thalamic relay
nucleus for vision) → corona radiata (optic
radiation) → primary visual cortex

11. The retina
• Receptors are the rod (cylindrical processes) and cons
(conical processes) of the retina
• The axons of the cells synapse with the dendrites of the
bipolar cells
• Bipolar cells are the first order neuron in this pathway
• The axons of bipolar cells synapse with the dendrites of
ganglion cells
• The ganglion cells is the second order neuron in this
pathway, the axons of the ganglion cells forming the optic
nerve
• The retina can be divided by a horizontal line bisecting the
fovea into 2 halves; temporal and nasal halves
• The fibers from the nasal half cross to the opposite side.
While the fibers from the temporal half pass through the
optic chiasma without crossing

13. Optic Nerve
• The unmyelinated axons of ganglion cells collect at the
optic disk and exit from the eye, about 3 or 4 mm to the
nasal side of its center, as the optic nerve .
• They pierce the sclera in a region called the lamina
cribrosa. Here they acquire myelin sheaths and forming
the optic nerve.
• The optic nerve is actually a tract of the CNS and has
meningeal coverings, the subarachnoid space around the
optic nerve communicates with subarachnoid space
generally.
• The optic nerve leaves the orbital cavity through the optic
canal and unites with the optic nerve of the opposite side
to form the optic chiasma.

14. The optic chiasma
• The optic chiasma is a flattened bundle of nerve fibers
situated at the junction of the anterior wall and floor of the
third ventricle just anterior to the infundibular stalk .
• The superior surface is attached to the lamina terminalis,
and inferiorly, it is related to the hypophysis cerebri, from
which it is separated by the diaphragma sellae.
• The anterolateral corners of the chiasma are continuous with
the optic nerves, and the posterolateral corners are
continuous with the optic tracts
• All fibers from the nasal half of each retina cross to the
contralateral optic tract.
• All fibers from the temporal half of each retina pass through
the lateral portions of the chiasm without crossing and enter
the ipsilateral optic tract.

15. Optic Tract
• The optic tract emerges from the optic chiasma and
passes posterolaterally around the cerebral peduncle.
• Each optic tract contains:
1. The fibres arising in the temporal retina of the
ipsilateral eye (same side);
2. The fibres of the nasal retina of the contralateral eye
(opposite side)
• Most of the fibers now terminate by synapsing with
nerve cells in the lateral geniculate body. A few of the
fibers pass to the pretectal nucleus and the superior
colliculus of the midbrain and are concerned with
light reflexes

16. Lateral Geniculate Body
• The lateral geniculate body is a small, oval
swelling projecting from the pulvinar of the
thalamus.
• It consists of six layers of cells, on which synapse
the axons of the optic tract. The axons of the
nerve cells within the geniculate body leave it to
form the optic radiation
• The fibers of the optic radiation are the axons of
the nerve cells of the lateral geniculate body that
passes posteriorly through the retrolenticular part
of the internal capsule and terminates in the
visual cortex

17. Visual cortex
• The visual cortex (area 17) occupies the upper and
lower lips of the calcarine sulcus on the medial
surface of the cerebral hemisphere.
• The visual association cortex (areas 18 and 19) is
responsible for recognition of objects and
perception of color

19. Neurons of the Visual Pathway
• Four neurons conduct visual impulses to the visual
cortex:
• (1) rods and cones, which are specialized receptor
neurons in the retina;
• (2) bipolar neurons, which connect the rods and
cones to the ganglion cells;
• (3) ganglion cells, whose axons pass to the lateral
geniculate body
• (4) neurons of the lateral geniculate body, whose
axons pass to the cerebral cortex.

20. Binocular Vision
• In binocular vision, the right and left fields of vision are
projected on portions of both retinae .
• The image of an object in the right field of vision is
projected on the nasal half of the right retina and the
temporal half of the left retina.
• In the optic chiasma, the axons from these two retinal
halves are combined to form the left optic tract.
• The lateral geniculate body neurons now project the
complete right field of vision on the visual cortex of the left
hemisphere and the left visual field on the visual cortex of
the right hemisphere. The lower retinal quadrants (upper
field of vision) project on the lower wall of the calcarine
sulcus, while the upper retinal quadrants (lower field of
vision) project on the upper wall of the sulcus.
• Note also that the macula lutea is represented on the
posterior part of area 17, and the periphery of the retina is
represented anteriorly.

22. Pupillary Light Reflex
• Light directed into one eye cause both pupils to constrict.
• The response of the pupil of the illuminated eye is called the
direct pupillary light reflex.
• That of the other eye is called the consensual pupillary light
reflex.
• Impulse from the retina → Optic tract axons (afferent limb) →
lateral root of the optic tract → lateral geniculate body →
superior brachium → pretectal nucleus → Edinger-Westphal
nucleus (bilaterally) → inferior division of the oculomotor nerve
(as preganglionic parasympathetic fibres) → Ciliary ganglion →
postganglionic parasympathetic fibres (short ciliary nerves) →
Sphincter pupillae muscle
• Both pupils constrict in the consensual light reflex because the
pretectal nucleus sends fibers to the parasympathetic nuclei on
both sides of the midbrain

24. Near Reflex (accommodation reflex)
When visual attention is directed to a nearby object 3 things happen in a
reflex manner:
• Convergence of eyes, so that the image of the object falls on both
foveae;
• Contraction of the ciliary muscle and a resultant thickening of the lens,
so the image of the object is in focus on the retina;
• pupillary constriction, which improves the optical performance of the
eye by reducing certain types of aberration and by increasing its depth
of focus.
• Normal visual pathway → primary visual cortex → visual association
cortex --> superior colliculus and/or pretectal area or (visual eye field
in the medial side of the frontal lobe through the superior longitudinal
bundle) → Corticobulbar fibers → Edinger-Westphal nucleus → ciliary
ganglion → short ciliary nerves → sphincter pupillae muscle and ciliary
muscle (contraction of the ciliary muscle increase the thickness of the
lens) with convergance of both eyes by the contraction of medial
rectus in both side

26. Corneal Reflex
• Light touching of the cornea or conjunctiva results
in blinking of the eyelids.
• Afferent impulses from the cornea or conjunctiva
travel through the ophthalmic division of the
trigeminal nerve to the sensory nucleus of the
trigeminal nerve.
• Internuncial neurons connect with the motor
nucleus of the facial nerve on both sides through
the medial longitudinal fasciculus. The facial nerve
and its branches supply the orbicularis oculi
muscle, which causes closure of the eyelids.

27. • Visual Body Reflexes
• The automatic movement of the eyes, head, and
neck toward the source of the visual stimulus, and
the protective closing of the eyes and even the
raising of the arm for protection are reflex actions
that involve the following reflex arcs.
• The visual impulses follow the optic nerves, optic
chiasma, and optic tracts to the superior colliculi.
Here, the impulses are relayed to the tectospinal
and tectobulbar (tectonuclear) tracts and to the
neurons of the anterior gray columns of the spinal
cord and cranial motor nuclei.

28. Pupillary Skin Reflex
• The pupil will dilate if the skin is painfully stimulated
by pinching. The afferent sensory fibers are believed
to have connections with the efferent preganglionic
sympathetic neurons in the lateral gray columns of
the first and second thoracic segments of the spinal
cord. The white rami communicantes of these
segments pass to the sympathetic trunk, and the
preganglionic fibers ascend to the superior cervical
sympathetic ganglion. The postganglionic fibers pass
through the internal carotid plexus and the long
ciliary nerves to the dilator pupillae muscle of the
iris.

29. Auditory pathway
• Receptors: hair cells of organ of Coti
• The cell bodies of the auditory primary afferents
are located in the spiral ganglion of the cochlea.
• The fibers of the cochlear nerve are the central
processes of nerve cells located in the spiral
ganglion of the cochlea
• The nerve enters the brainstem at the
pontomedullary junction.
• The fibers of the cochlear nerve bifurcate and
sends one branch to: The dorsal cochlear nucleus
and other branch to the ventral cochlear nucleus
• Both nuclei lie at the dorsal and lateral aspects of
the inferior cerebellar peduncle respectively.

30. • The axons of the cells of both nuclei cross to the
opposite side in the caudal part of the pons
forming trapizoid body. Some fibers which
involved in sound localization relay in the
superior olivary nucleus.
• The fibers of trapizoid body forming the lateral
lemniscus. This is the major ascending auditory
pathway of the brainstem.
• A smaller number of fibers from the cochlear
nuclei do not cross the midline. They instead join
the ipsilateral lateral lemniscus.
• Thus, each lateral lemniscus carries some
information from both ears.

31. • Nearly all fibers of the lateral lemniscus terminate in the
inferior colliculus.
• The inferior colliculus then gives rise to the inferior
brachium. This terminates in the medial geniculate
nucleus.
• Fibres from the medial geniculate nucleus project to the
primary auditory cortex (Brodman's area 41 & 42 or the
transverse gyri of Heschl).
• This is located in the upper surface of the superior
temporal gyrus hidden in the lateral sulcus.
• A few of these ascending auditory fibers, representing
the contralateral ear, may proceed directly to the medial
geniculate nucleus without stopping at the inferior
colliculus.

33. Descending Auditory Pathways
• Descending fibers originating in the auditory
cortex and in other nuclei in the auditory
pathway accompany the ascending pathway.
These fibers are bilateral and end on nerve cells
at different levels of the auditory pathway and on
the hair cells of the organ of Corti. It is believed
that these fibers serve as a feedback mechanism
and inhibit the reception of sound.
• They may also have a role in the process of
auditory sharpening, suppressing some signals
and enhancing others.

34. Vestibular Pathways
• Receptors;
• Cristae ampularis; 3 neuroepithelial receptors in the
ampulla of the semicircular ducts on each side
• Macula utriculi receptors in the utricle
• Macula sacculi receptors in the sacule
• The 1st order vestibular afferents arise in bipolar cells of
vestibular (Scarpa's) ganglion, which is in the distal portion
of the internal auditory meatus.
• The axons travel in the vestibular portion of the 8th cranial
nerve and enter the brain stem at the pontomedullary
junction. A few of the vestibular afferents go directly to the
cerebellum through the inferior cerebellar peduncle. The
cerebellum coordinates the movements that maintain
balance.

35. • The 2nd order neuron is the vestibular nuclei: the
inferior, medial, lateral (Deiter’s) and superior
vestibular nuclei. Ascending fibers from the
vestibular nuclei cross to the opposite side to end in
the posteromedial ventral nucleus of the thalamus
• the posteromedial ventral nucleus of the thalamus
is the 3rd order neuron the axons of these cells pass
through the optic radiations to reach the vestibular
area which lies behind the lower part of sensory
area, opposite the sensory area of the face

37. Other connections of vestibular nuclei
• The main projections from these nuclei are to the spinal cord (controlling head
and body position), to the three, extraocular motor nuclei (III, IV, VI,
controlling eye movements), to the thalamus (VPI, eventually reaching the
cortex and conscious perception of movement and gravity), and to the
cerebellum (coordinating postural adjustments).
• The main descending tracts are the lateral vestibulospinal tract from the
lateral vestibular nucleus and the medial vestibulospinal tract from the medial
vestibular nucleus.
• The lateral vestibular tract starts in the lateral vestibular nucleus and descends
the length of the spinal cord on the same side. This pathway helps us walk
upright.
• The medial vestibular tract starts in the medial vestibular nucleus and extends
bilaterally through mid-thoracic levels of the spinal cord in the MLF. This tract
affects head movements and helps integrate head and eye movements.
• In summary, remember that the lateral vestibulo-spinal tract is ipsilateral and
long; the medial vestibulo-spinal tract is bilateral but shorter.
• The main ascending tracts are from the superior and medial vestibular nuclei to
the extraocular muscles through the medial longitudinal fasciculus (MLF).