Special Sensory pathways

DR. Mohammed Mahmoud Mosaed

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

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

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.

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.

• 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.

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

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

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.

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.

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

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

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

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.

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.

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

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

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.

• 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.

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.

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.

• 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.

• 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.

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.

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.

• 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

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).