Olfactory pathway

1 -first neuron:

It is formed of the olfactory bipolar cells placed in the mucosa of the roof of the nose and upper part of the nasal cavity . The axon of these cells form 12 -20 olfactory nerve fibers ( non –myelinated ) which pass through cribriform plate of ethmoid to synapse around

the cells of the olfactory bulb.

2 -Second neuron:

It is formed of the cells of the olfactory bulb. ( interneurones and large mitral cells) Axons from the mitral cells ( myelinated nerve fibers ) form the olfactory tract . The olfactory tract passes backwards on the basal surface of the frontal lobe. On reaching the level of optic chiasma , most olfactory tract fibers form the lateral olfactory stria. These fibers pass into

the depth of lateral fissure .

They terminate in the primary olfactory cortex of the uncus. Also, in the amygdala which is responsible for conscious appreciation of

sense of smell .

The uncus is present on inferomedial aspect of the temporal lobe .

Adjacent to the uncus is the entorhinal area (the anterior part of the parahippocampal gyrus) which constitutes the olfactory association

cortex .

The primary and association areas are referred as the pyriform cortex.

The medial root crosses the midline through the anterior commissure and joins the uncrossed lateral root of the opposite side . So, it connects the olfactory centers of the 2 cerebral hemisphere together .

N.B. The olfactory projection is consists only from 2 neurones between the sensory receptors and cerebral cortex. Also, it does not project via the thalamus.

In a depression on either side of the crista galli lie the cribriform plates of the ethmoid which accommodate the olfactory bulbs.

Anosmia

Follows damage to the olfactory nerves . There is loss in the sense of smell and also of the flavour of foods.

It can occur when tumours of the meninges ( meningiomas ) invade the olfactory nerves .

Or follows head trauma.

N.B. Elementary aspects of taste e.g. sweet ,salt , bitter and

sour are preserved.

Hearing pathway 1 -first neurone:

The cell bodies of the cochlear nerve fibers lie within the cochlea and are called spiral

ganglion.

The dendrites carry hearing sensation from hair cells of the organ of Corti in the inner ear

( cochlea ) .

The axons form the cochlear nerve which enters the brain stem at the junction of the pons and medulla oblongata to end around the cells of the ventral and dorsal cochlear nuclei in sections at the pontomedullary junction.

2 -second neurone:

It is formed by the cells of the cochlear nuclei. Few axons ascend in the lateral lemniscus of the same side. Most of the axons form the trapezoid body which crosses the midline of the caudal pons to reach the opposite side where it is joined by the few fibers from the cochlear nuclei of the same side and both groups of fibers ascend as lateral lemniscus of the opposite side.

Therefore each lateral lemniscus contains cochlear fibers and carries hearing stimuli from both ears mostly from the

opposite ear .

Each lateral lemniscus ascends to midbrain where the fibers end in 2 site:

A. Some fibers end directly by synapsing around the cells of the medial geniculate body , third order neuron ( lower center

of hearing ) (metathalamus ) .

B. Other fibers end directly by synapsing around the cells of the inferior colliculus which send their axons through inferior brachium quadrigeminum to the medial geniculate body , third neuron.

The final step in the pathway consists of axons pass through the internal capsule to the primary auditory cortex of the temporal lobe in Heschl’s gyri which are situated on the dorsal surface of the superior temporal gyrus and are largely

hidden within the lateral fissure.

The region of the temporal lobe surrounding the primary auditory cortex is called auditory association cortex or Wernicke’s area . In the dominant hemisphere , it used for understanding of the spoken word and has important connections with other language

areas .

N.B.Some cochlear fibers may synapse around the cells of

1 -superior olive 2- nucleus of trapezoid body 3- nucleus of lateral lemniscus

The superior olivary nucleus and the nucleus of the lateral lemniscus establish reflex connections with motor neurons of the trigeminal and facial motor nuclei , mediating contraction of the tensor tympani and stapedius muscles in response to loud noise .

Throughout the ascending auditory projection there exists a so – called tonotopical representation of the cochlea which is analogous to the somatotopic

At the cortical level , the organs of hearing are bilaterally represented so that unilateral lesions of the primary auditory cortex cause partial deafness.

Acoustic neuroma

It is a benign tumour of 8th nerve . Attacks of dizziness with deafness occur .

Unilateral or bilateral acoustic neuromas occur in the inherited disease neurofibromatosis.

Vestibular pathway

Function : it carries coordinating stimuli to the eyes ,neck, and body to maintain equilibrium according to the

position and movements of the head .

First neurone : Their cell bodies are located in the vestibular ganglion in the internal auditory meatus. The dendrites of these cells carry stimuli from the hair cells in the 3 semicircular canals ( crista ampularis ) , utricle and saccule

( maculae ) of membranous labyrinth.

The axons form the vestibular nerve which enters the brain stem at the pontomedullary junction ends at 5 places a. cerebellum ( direct vestibulo-cerebellar tract ) though inferior cerebellar peduncle

B. 4 vestibular nuclei located close together beneath the lateral part of the floor of the 4th ventricle ( inferior in the medulla –lateral at the pontomedullary junction – medial &

superior in the pons ).

Second neurone:

The axons of vestibular nuclei form 5 tracts that reach :

1-one tract to the cerebellum:

Indirect vestibulo-cerebellar tract:

It is formed by the axons of the superior and lateral vestibular nuclei which pass through the inferior cerebellar peduncle to the flocculonodular lobe of the cerebellum . It is

concerned with the control of equilibrium.

N.B. The vestibular nuclei receive input from the labyrinthine and cerebellum. ( Fastigial Vestibular Pathway ) . The axons end in the lateral vestibular nucleus on both sides. Also, some Purkinje cell axons project directly to lateral vestibular nucleus.

2 -One ascending tract to 3, 4 and 6 cranial nerves

Vestibulo-Ocular tract:

mainly the axons of the superior & medial nuclei join the medial longitudinal bundle to reach 3,4 and 6 cranial nerve nuclei . It is concerned with coordination of head and eye movements.

1 -stimulation of the right vestibular nuclei causes conjugate movement of both eyes to the left

side .

2 -Vestibular reflexes help to make the eyes able to remain fixed on stationary objects during

movement of the head & body.

3 -When the head is turned slightly to the right side , the eyes turn to the left side with an equal distance so that the field of vision remains the same.

The 3rd and 4th tracts are two descending tracts:

The lateral and medial vestibulo-spinal tracts:

The function of these tracts is to increase the tone of the extensor muscles of the limbs to support the body against gravity and to maintain upright posture.

Lateral vestibulospinal tract is formed by the axons of the lateral vestibular nucleus ( Deiter’s nucleus ) . The fibers descend ipsilaterally in the ventral funiculus. It mediates excitatory influences upon extensor motor neurons .

Medial vestibulospinal tract is formed by the axons of the medial ,lateral and inferior nuclei. The fibers descend ipsilaterally in the medial longitudinal fasciculus and is located adjacent to ventral median fissure.

Both tracts descend directly to end around the anterior horn cells of the spinal cord.

5-Vestibulo - Reticular Connection

The axons of the vestibular nuclei reach the vomiting and vasomotor centers in the reticular formation.

Other fibers ascend to contralateral thalamus ( ventral posterior nucleus ) which project to cerebral cortex . It is probably adjacent to the head area of the sensory cortex or adjacent to the auditory

cortex in the temporal lobe .