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