Olfactory PathwayOlfactory Pathway

Dr. Zeenat Zaidi

Olfactory PathwayOlfactory Pathway The receptors located in an The receptors located in an

area, covering about 5 area, covering about 5 cubic centimeters, in the cubic centimeters, in the superior part of the nosesuperior part of the nose. .

There are about 10 million There are about 10 million to 20 million receptor cells to 20 million receptor cells for smell.for smell.

Each receptor is a Each receptor is a specialized, ciliated specialized, ciliated neuron. neuron.

Their axons form Their axons form numerous small fasciculi, numerous small fasciculi, the the olfactory nervesolfactory nerves, that , that pass through tiny openings pass through tiny openings in the in the cribriform platecribriform plate of of the ethmoid bone to reach the ethmoid bone to reach the cranial cavity.the cranial cavity.

Olfactory PathwayOlfactory Pathway In the cranial cavity, In the cranial cavity,

the olfactory neurons the olfactory neurons synapse with neurons synapse with neurons in the in the olfactory bulbolfactory bulb. .

The olfactory bulbs The olfactory bulbs (right and left) are (right and left) are club-like structures club-like structures that contain that contain interneuronsinterneurons and and large mitral cells.large mitral cells.

The axons of the The axons of the mitral cells form the mitral cells form the olfactory tractolfactory tract, which , which passes backward on passes backward on the basal surface of the basal surface of frontal lobe.frontal lobe.

Just before reaching the level of the optic chiasma, the Just before reaching the level of the optic chiasma, the tract divides into tract divides into laterallateral, , intermediateintermediate and and medial striamedial stria

Fibers in the medial stria cross in the midline (in

anterior commissure) and terminate in the opposite olfactory bulb

Small intermediate stria terminates into the anterior perforated substance

Fibers in the lateral stria terminate in the primary olfactory

cortex of the uncus

Olfactory Pathway cont’dOlfactory Pathway cont’d Adjacent to Adjacent to

the uncus, the uncus, the anterior the anterior part of the part of the parahippo-parahippo-campal campal gyrus, or gyrus, or entorhinal entorhinal areaarea, , constitutes constitutes the the olfactory olfactory association association cortexcortex

The primary and association cortices are also collectively

referred to as the pyriform cortex

The olfactory projection is unique in that it consists The olfactory projection is unique in that it consists of a sequence of of a sequence of two neuronstwo neurons between the between the sensory receptors and cerebral cortexsensory receptors and cerebral cortex, and , and does does not project via the thalamusnot project via the thalamus

The neurons from the olfactory bulb take the The neurons from the olfactory bulb take the impulses to many areas of the brain, specifically to impulses to many areas of the brain, specifically to those responsible for those responsible for memorymemory (amygdala) and (amygdala) and emotionsemotions (limbic system). This is the reason why (limbic system). This is the reason why smell can evoke memories of incidences that smell can evoke memories of incidences that occurred many years ago.occurred many years ago.

Unlike most other neurons, the olfactory neurons Unlike most other neurons, the olfactory neurons are constantly replaced every few weeks. The are constantly replaced every few weeks. The renewal is regulated by special growth factors. renewal is regulated by special growth factors.

Clinical AnatomyClinical Anatomy

Damage to the olfactory nerves lead to Damage to the olfactory nerves lead to anosmiaanosmia The delicate olfactory nerves are prone to The delicate olfactory nerves are prone to

damage by damage by head injuryhead injury (e.g., whiplash) where (e.g., whiplash) where the head is jolted in an anteroposterior the head is jolted in an anteroposterior direction. Here, the brain tends to move direction. Here, the brain tends to move forward/backward, thus damaging the neurons forward/backward, thus damaging the neurons entering vertically through the cribriform plate.entering vertically through the cribriform plate.

Tumors of meningesTumors of meninges may invade the olfactory may invade the olfactory nerves nerves

Vestibulo-Cochlear Vestibulo-Cochlear PathwaysPathways

Dr. Zeenat Zaidi

Vestibular PathwayVestibular Pathway

Vestibular PathwayVestibular Pathway The receptors are the The receptors are the hair cellshair cells

located in the membranous located in the membranous labyrinthlabyrinth

Primary Primary afferent neuronafferent neuron make make dendritic contact with hair dendritic contact with hair cells. Their cell bodies are cells. Their cell bodies are located in the located in the vestibular vestibular ganglionganglion. Their central . Their central processes:processes: Mostly end up in the Mostly end up in the

vestibular nucleivestibular nuclei (lateral, (lateral, medial, inferior and lateral) medial, inferior and lateral)

Some fibers go to the Some fibers go to the cerebellumcerebellum through the through the inferior cerebellar peduncleinferior cerebellar peduncle

The The efferentsefferents from the from the vestibular nuclei project vestibular nuclei project to:to:

Ipsilateral Ipsilateral flocculonodular lobe of flocculonodular lobe of cerebellum through cerebellum through inferior cerebellar inferior cerebellar pedunclepeduncle

Motor nuclei of cranial Motor nuclei of cranial nerves through medial nerves through medial longitudinal fasciculuslongitudinal fasciculus

Spinal cord as lateral & Spinal cord as lateral & medial vestibulospinal medial vestibulospinal tracts.tracts.

Bilaterally to ventral Bilaterally to ventral posterior nucleus of posterior nucleus of thalamus, which in turn thalamus, which in turn project to the cerebral project to the cerebral cortex. cortex.

Vestibular CortexVestibular Cortex

The cortical region responsible for conscious awareness of vestibular sensation is

uncertain but is probably:adjacent to head area of the sensory cortex

in the parietal lobe or

adjacent to the auditory cortex in the temporal lobe

Medial Longitudinal FasciculusMedial Longitudinal Fasciculus Extends through out the brain Extends through out the brain

stemstem Continues into the spinal cord Continues into the spinal cord

as the medial vestibulospinal as the medial vestibulospinal tracttract

Projects Projects bilaterallybilaterally Has two components:Has two components:

The The ascending componentascending component establishes connections with establishes connections with the nuclei of the 12the nuclei of the 12thth, 6, 6thth, 4, 4thth, , & 3& 3rdrd cranial nerves for the cranial nerves for the coordination of head and eye coordination of head and eye movementsmovements

The The descending componentdescending component extends into the spinal cord extends into the spinal cord as the medial vestibulospinal as the medial vestibulospinal tract tract

Vestibulospinal TractsVestibulospinal Tracts Vestibulospinal fibers Vestibulospinal fibers

influence the activity of influence the activity of spinal motor neurons spinal motor neurons concerned with the concerned with the control of control of body posture and balancebody posture and balance

Two tracts: Two tracts: lateral & mediallateral & medial Lateral Lateral arises from lateral arises from lateral

vestibular (Deiter’s) nucleus, vestibular (Deiter’s) nucleus, descends descends ipsilaterallyipsilaterally

MedialMedial is the descending part is the descending part of the medial longitudinal of the medial longitudinal fasciculus, projects fasciculus, projects bilaterallybilaterally

Cochlear (Auditory) Cochlear (Auditory) PathwayPathway

We really don't hear We really don't hear with our ears - we with our ears - we hear with our hear with our brains! brains!

Sound vibrations from Sound vibrations from the outside world are the outside world are conveyed through this conveyed through this system until they system until they reach the brain, and reach the brain, and we hear the sound in we hear the sound in the cortexthe cortex

Cochlear (Auditory) PathwayCochlear (Auditory) Pathway Multisynaptic pathwayMultisynaptic pathway The receptors are the The receptors are the

hair cellshair cells of the of the organ organ of Cortiof Corti

Primary afferentPrimary afferent neurons make neurons make dendritic contact with dendritic contact with hair cells. Their cell hair cells. Their cell bodies are located in bodies are located in the the spiral ganglionspiral ganglion. . Their central Their central processes terminate processes terminate in the dorsal and in the dorsal and ventral ventral cochlear nucleicochlear nuclei

22ndnd order neurons order neurons ascend ascend into the pons, where:into the pons, where: Some fibers run Some fibers run

ipsilaterallyipsilaterally and terminate and terminate in the in the superior olivary superior olivary nucleusnucleus

Some fibers Some fibers crosscross the the midline in midline in trapezoid bodytrapezoid body and terminate in the and terminate in the nucleus of trapezoid nucleus of trapezoid bodybody or in the or in the contralateral contralateral superior superior olivary nucleusolivary nucleus

From the superior olivary From the superior olivary nuclei, ascending fibers nuclei, ascending fibers comprise the comprise the lateral lateral lemniscuslemniscus, which runs , which runs through tegmentum of pons through tegmentum of pons and terminate in the and terminate in the inferior inferior colliculuscolliculus of the mdibrain of the mdibrain

Some axons within lateral Some axons within lateral lemniscus terminate in lemniscus terminate in small small nucleus of the lateral nucleus of the lateral lemniscuslemniscus

The inferior colliculus The inferior colliculus project to project to medial geniculate medial geniculate nucleusnucleus of thalamus of thalamus

The axons originating in The axons originating in the medial geniculate the medial geniculate nucleus (nucleus (auditory radiationauditory radiation) ) pass through pass through sublentiform sublentiform part of the internal capsulepart of the internal capsule to the to the primary auditory primary auditory cortexcortex ( (Brodmann’s areas Brodmann’s areas 41, 4241, 42) located in the ) located in the dorsal surface of the dorsal surface of the superior temporal gyrussuperior temporal gyrus (Heschl’s gyri)(Heschl’s gyri)

The region of temporal The region of temporal lobe surrounding the lobe surrounding the primary auditory cortex primary auditory cortex is known as the is known as the auditory association auditory association cortexcortex or or Wernick’s Wernick’s area area ((Brodmann’s areas Brodmann’s areas 22)22)

Wernick’s area is Wernick’s area is related to processing of related to processing of language by the brainlanguage by the brain

Cochlear (Auditory) Pathway Cochlear (Auditory) Pathway cont’dcont’d

The tonotopic pattern The tonotopic pattern in the auditory area is in the auditory area is such that fibers for such that fibers for sounds of sounds of low low frequencyfrequency end in the end in the anterolateral partanterolateral part, , whereas fibers for whereas fibers for sounds of sounds of high high frequencyfrequency go to go to posteromedial partposteromedial part

Superior olivary nucleusSuperior olivary nucleus sends olivocochlear fibers sends olivocochlear fibers to end in organ of Corti through the to end in organ of Corti through the vestibulocochlear nerve. These fibers are inhibitory vestibulocochlear nerve. These fibers are inhibitory in function and serve to modulate transmission to in function and serve to modulate transmission to the cochlear nerve the cochlear nerve

Superior olivary nucleus & the nucleus of the Superior olivary nucleus & the nucleus of the lateral lemniscus lateral lemniscus establish reflex connections with establish reflex connections with motor neurons of trigeminal and facial motor nuclei motor neurons of trigeminal and facial motor nuclei mediating contraction of mediating contraction of tensor tympanitensor tympani and and stapediusstapedius muscles in response to loud noise muscles in response to loud noise

Inferior colliculi Inferior colliculi establish reflex connections with establish reflex connections with motor neurons in the cervical spinal segments motor neurons in the cervical spinal segments (tectospinal tract)(tectospinal tract) for the movement of head and for the movement of head and neck in response to auditory stimulation neck in response to auditory stimulation

Clinical NotesClinical Notes Disturbnce of vestibular nerve functionsDisturbnce of vestibular nerve functions

VertigoVertigo NystagmusNystagmus

Disturbnce of cochlear nerve functionsDisturbnce of cochlear nerve functions Deafness and tinnitisDeafness and tinnitis

The representation of cochlea is essentially The representation of cochlea is essentially bilateralbilateral at all at all levels rostral to the cochlear nucleilevels rostral to the cochlear nuclei

Lesions anywhere along the pathway usually have no Lesions anywhere along the pathway usually have no obvious effect on hearing. obvious effect on hearing.

Deafness is essentially only caused by damage to the Deafness is essentially only caused by damage to the middle ear, cochlea, or auditory nerve. middle ear, cochlea, or auditory nerve.

Acoustic neuroma: a benign tumour of 8th nerve leads to compression of the nerve leading to attacks of

dizziness, and profound deafness and ataxia