Download - Anatomy of olfactory system
Oldest sensory modalities in the phylogenetic history of mammals
Olfactory system detects food and
influences social and sexual behavior
3 cell types, basal, supporting, and olfactory receptor cells
Basal cells are stem cells that give rise to
the olfactory receptor cells continuous turnover unique to the
olfactory system receptor cells are actually bipolar neurons,
each possessing a thin dendritic rod that contains specialized cilia extending from the olfactory vesicle and a long central process that forms the fila olfactoria
Table 14.3 (1 of 12)
olfactory nerve (I)
Olfactory bulb
Olfactory tract
Optic nerve (II)
Optic chiasma
Optic tract
Oculomotor nerve (III)
Trochlear nerve (IV)
Trigeminal nerve (V)
Abducens nerve (VI)
Cerebellum
Medulla
small unmyelinated axons of the olfactory receptor cells form the fine fibers of the first cranial nerve
travel centrally toward the ipsilateral olfactory bulb to make contact with the second-order neurons
cribriform plate of the ethmoid bone, separated at the midline by the crista galli, contains multiple small foramina through which the olfactory nerve fibers, or fila olfactoria, traverse
lies inferior to the basal frontal lobe The layers (from outside toward the
center of the bulb) are differentiated as follows:• Glomerular layer • External plexiform layer • Mitral cell layer • Internal plexiform layer • Granule cell layer
Medial fibers of the tract contact the anterior olfactory nucleus and the septal area.
Some fibers project to the contralateral olfactory bulb via the anterior commissure.
Lateral fibers contact third-order neurons in the primary olfactory cortex (prepyriform and entorhinal areas) directly
Third-order neurons send projections to the dorsomedial nucleus of the thalamus, the basal forebrain, and the limbic system
Pyriform lobe includes the olfactory tract, the uncus, and the anterior part of the parahippocampal gyrus is known as the entorhinal area the secondary olfactory cortex
The prepyriform and the periamygdaloid areas of the temporal lobe represent the primary olfactory cortex.
Communications with superior and inferior salivary nuclei are important in reflex salivation .
Patient with olfactory impairment c/o loss of taste [every thing feels like mud.]
Loss of smell sensation –patient miss olfactory danger signals.
olfactory deficits - two types 1-conductive 2-sensorineural 1-interfernce of an odour in contact 2-dysfunction of receptor/central
connections
ensure nasal passages are open stimuli should be non irritant examine each nostril separately
occluding the other one with eyes closed
ask the patient whether smells something or not if yes identify the substance
repeat the test in other nostril
the side that might be abnormal should be examined first
perception of an odour –continuty of olfactory pathway
identification of an odour – intact cortical functions
Anosmia - Absence of smell sensation
Hyposmia - Decreased sensation Dysosmia - Defect in sense of smell Cacosmia - Sensation of a bad or
foul smell Parosmia - Sensation of smell in the
absence of appropriate stimulus
URTI Head trauma Nasal and sinus disease Idiopathic
• These Four are most Common Causes
Others- Olfactory groove meningiomaFrontal Lobe tumorSellar /Parasellar tumorVitamin deficiency (B6,B12,A)
Parkinsons DiseaseKallmans SynddromePsychiatric conditionsCadmium ToxicityCocaine Abuse
Kallmann syndrome (ie, hypogonadism with anosmia)
Foster Kennedy syndrome (ie, papilledema, unilateral anosmia, and optic atrophy usually associated with an olfactory groove meningioma).
Cranio cerebral Trauma – Results in damage of olfactory nerve at the cribriform plate of ethmoid
Purely Motor Nerve Origin - Two nuclei from the medulla
lies beneath the floor of the fourth ventricle under the medial aspect of the hypoglossal trigone
Axons emerge in the sulcus between the pyramid and the inferior olive as many rootlets
Table 14.3 (12 of 12)
Runs inferior to the tongue - innervates the tongue muscles
Facial nerve (VII)
Vestibulocochlearnerve (VIII)
Glossopharyngeal nerve (IX)Vagus nerve (X)
Accessory nerve (XI)
Hypoglossal nerve (XII)
rootlets of this nerve are collected into two bundles, which perforate the dura mater separately, opposite the hypoglossal canal in the occipital bone
The nerve descends almost vertically to a point corresponding with the angle of the mandible
It descends through the neck at the level of the angle of mandible, here it lies beneath the carotid sheath
It runs forward above the hyoid bone and divides into number of fibers supplying the tongue
Branches of the hypoglossal nerve are• Meningial• Descending • thyrohyoid• Muscular
Its branches of communication are, Vagus. First and second cervical nerves. Sympathetic. Lingual
Tounge has extrinsic – paired musclesGenioglossusStyloglossusHyoglossusChondroglossus
Intrinsic muscles – Superior and inferior longitudinales, transversus, verticalis
Extrinsic mucles help in Protrusion and retraction of the tounge, move the tounge up and down
Intrinsic muscles help in Change in Length, width and curvature of the dorsal surface, and turn the non protruded tip from side to side
The centre that regulates the tounge movements – lower portion of precentral gyrus near and within the sylvian fissure
The supranuclear fibres run in corticobulbar tract through the genu of the internal capsule and through the cerebral peduncle
The supranuclear pathway is crossed
Examination consists of evaluating1) Strength2)Bulk3)Dextirity of the tongue
One has to look for• Weakness• Atrophy• Abnormal movements• Impairment of rapid movements
Patient is asked to move tongue in and out ,up and down , side to side
Best by assessing the strength against pressed cheek
In unilateral weakness, the tounge is deviated towards the same side
In bilateral weakness, tounge movements may be impaired
Symptoms-• Unilateral weakness- Speech and
swallowing are little affected• Bilateral weakness – First stage of
swallowing is impaired• Difficulty in articulation and respiratory
difficulty• Supranuclear, Nuclear, infranuclear lesions
Supranulear Lesions• Causes weakness but no atrophy• Lesions are seen in cotrex, corticobulbar tract,
internal capsule, cerebral peduncle and pons Pseudobulbar Palsy
• Due to bilateral UMN disease causing bilateral weakness
• Hemispheric lesion causes apraxia of tounge movements
Extrapyramidal disorders causes slowing of tounge movements
Nuclear and Infra nuclear lesions – causes weakness + Atrophy
In addition to above, Fasciculations are seen in Progressive nuclear disease
The disorders include neoplasms, vascular lesions, Motor neuron diseases, Granulomas etc.
Infra nuclear lesions• Intracranial• Extracranial
Intracranial lesions same as nuclear lesions and other causes includes meningitis, SAH, Trauma
Extracranial causes- penetrating wounds, carotid aneurysms, infections