Olfactory Pathway Dr. Zeenat Zaidi

Olfactory Pathway The receptors located in an area, covering about 5 cubic centimeters, in the superior part of the nose. There are about 10 million to 20 million receptor cells for smell. Each receptor is a specialized, ciliated neuron. Their axons form numerous small fasciculi, the olfactory nerves, that pass through tiny openings in the cribriform plate of the ethmoid bone to reach the cranial cavity.

Olfactory Pathway In the cranial cavity, the olfactory neurons synapse with neurons in the olfactory bulb. The olfactory bulbs (right and left) are club-like structures that contain interneurons and large mitral cells. The axons of the mitral cells form the olfactory tract, which passes backward on the basal surface of frontal lobe.

Just before reaching the level of the optic chiasma, the tract divides into lateral, intermediate and medial 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’d Adjacent to the uncus, the anterior part of the parahippo-campal gyrus, or entorhinal area, constitutes the olfactory association cortex The primary and association cortices are also collectively referred to as the pyriform cortex

The olfactory projection is unique in that it consists of a sequence of two neurons between the sensory receptors and cerebral cortex, and does not project via the thalamus The neurons from the olfactory bulb take the impulses to many areas of the brain, specifically to those responsible for memory (amygdala) and emotions (limbic system). This is the reason why smell can evoke memories of incidences that occurred many years ago. Unlike most other neurons, the olfactory neurons are constantly replaced every few weeks. The renewal is regulated by special growth factors.

Clinical Anatomy Damage to the olfactory nerves lead to anosmia The delicate olfactory nerves are prone to damage by head injury (e.g., whiplash) where the head is jolted in an anteroposterior direction. Here, the brain tends to move forward/backward, thus damaging the neurons entering vertically through the cribriform plate. Tumors of meninges may invade the olfactory nerves

Vestibulo-Cochlear Pathways Dr. Zeenat Zaidi

Vestibular Pathway

Vestibular Pathway The receptors are the hair cells located in the membranous labyrinth Primary afferent neuron make dendritic contact with hair cells. Their cell bodies are located in the vestibular ganglion. Their central processes: Mostly end up in the vestibular nuclei (lateral, medial, inferior and lateral) Some fibers go to the cerebellum through the inferior cerebellar peduncle

The efferents from the vestibular nuclei project to: Ipsilateral flocculonodular lobe of cerebellum through inferior cerebellar peduncle Motor nuclei of cranial nerves through medial longitudinal fasciculus Spinal cord as lateral & medial vestibulospinal tracts. Bilaterally to ventral posterior nucleus of thalamus, which in turn project to the cerebral cortex.

adjacent to the auditory cortex in the temporal lobe Vestibular 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 Fasciculus Extends through out the brain stem Continues into the spinal cord as the medial vestibulospinal tract Projects bilaterally Has two components: The ascending component establishes connections with the nuclei of the 12th, 6th, 4th, & 3rd cranial nerves for the coordination of head and eye movements The descending component extends into the spinal cord as the medial vestibulospinal tract

Vestibulospinal Tracts Vestibulospinal fibers influence the activity of spinal motor neurons concerned with the control of body posture and balance Two tracts: lateral & medial Lateral arises from lateral vestibular (Deiter’s) nucleus, descends ipsilaterally Medial is the descending part of the medial longitudinal fasciculus, projects bilaterally

Cochlear (Auditory) Pathway

We really don't hear with our ears - we hear with our brains! Sound vibrations from the outside world are conveyed through this system until they reach the brain, and we hear the sound in the cortex

Cochlear (Auditory) Pathway Multisynaptic pathway The receptors are the hair cells of the organ of Corti Primary afferent neurons make dendritic contact with hair cells. Their cell bodies are located in the spiral ganglion. Their central processes terminate in the dorsal and ventral cochlear nuclei

2nd order neurons ascend into the pons, where: Some fibers run ipsilaterally and terminate in the superior olivary nucleus Some fibers cross the midline in trapezoid body and terminate in the nucleus of trapezoid body or in the contralateral superior olivary nucleus From the superior olivary nuclei, ascending fibers comprise the lateral lemniscus, which runs through tegmentum of pons and terminate in the inferior colliculus of the mdibrain

Some axons within lateral lemniscus terminate in small nucleus of the lateral lemniscus The inferior colliculus project to medial geniculate nucleus of thalamus The axons originating in the medial geniculate nucleus (auditory radiation) pass through sublentiform part of the internal capsule to the primary auditory cortex (Brodmann’s areas 41, 42) located in the dorsal surface of the superior temporal gyrus (Heschl’s gyri)

The region of temporal lobe surrounding the primary auditory cortex is known as the auditory association cortex or Wernick’s area (Brodmann’s areas 22) Wernick’s area is related to processing of language by the brain

Cochlear (Auditory) Pathway cont’d The tonotopic pattern in the auditory area is such that fibers for sounds of low frequency end in the anterolateral part, whereas fibers for sounds of high frequency go to posteromedial part

Superior olivary nucleus sends olivocochlear fibers to end in organ of Corti through the vestibulocochlear nerve. These fibers are inhibitory in function and serve to modulate transmission to the cochlear nerve Superior olivary nucleus & the nucleus of the lateral lemniscus establish reflex connections with motor neurons of trigeminal and facial motor nuclei mediating contraction of tensor tympani and stapedius muscles in response to loud noise Inferior colliculi establish reflex connections with motor neurons in the cervical spinal segments (tectospinal tract) for the movement of head and neck in response to auditory stimulation

Clinical Notes Disturbnce of vestibular nerve functions Vertigo Nystagmus Disturbnce of cochlear nerve functions Deafness and tinnitis The representation of cochlea is essentially bilateral at all levels rostral to the cochlear nuclei Lesions anywhere along the pathway usually have no obvious effect on hearing. Deafness is essentially only caused by damage to the 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

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