At the End of Lecture the Student Will Able To

OLFACTORY NERVE

Learning Objectives

At the end of lecture the student will able to:

Define the olfactory nerve.

Describe the olfactory epithelium.

Discuss the olfactory pathway.

Discuss the applied anatomy of the nerve.

INTRODUCTION

Oldest sensory modality

1st Cranial nerve.

Sensory

Detects odor & influence social/sexual behavior.

OLFACTORY EPITHELIUM

Located in upper part of nasal cavity

3 cell types-

§  Basal cells

§  Supporting cell

§  Olfactory receptor cells

Olfactory receptor cells:

Bipolar cells with peripheral and central processes

OLFACTORY NERVE

Unmylinated axons

Pass through cribriform plate

Enter the olfactory bulb

Synapse with mitral cells.

Olfactory Nerves (Cranial Nerve I):

It arise from the olfactory receptor nerve cells in olfactory mucous membrane in upper part of nasal.

Olfactory receptor cells scattered among supporting cells.

Each receptor cell consists of a small bipolar nerve cell with a coarse peripheral process that passes to mucous membrane and a fine central process.

From peripheral process short cilia arise(olfactory hairs) which project into mucus covering mucous membrane. These cilia react to odors in air and stimulate the olfactory cells.

Fine central processes form olfactory nerve fibers. Bundles of these fibers pass through cribriform plate of ethmoid bone to enter the olfactory bulb. The olfactory nerve fibers are unmyelinated and are covered with Schwann cells.

OLFACTORY BULB

Ovoid structure possesses several types of nerve cells, the largest is the mitral cell.

Location- inferior to basal frontal lobe.

Highly organized.

Layers :

§  Glomerular layer

§  External plexiform layer

§  Mitral cell layer

§  Internal plexiform layer

§  Granular cell layer.

Arrangment of nerve cells in Olfactory Bulb:

It is ovoid structure with several types of nerve cells largest of which is the mitral cell.

Incoming olfactory fibers synapse with dendrites of mitral cells and form rounded areas known as synaptic glomeruli.

Smaller nerve cells, called tufted cells and granular cells, also synapse with mitral cells.

Olfactory bulb, in addition, receives axons from contralateral olfactory bulb through olfactory tract.

Olfactory Tract:

It is narrow band of white matter runs from posterior end of olfactory bulb beneath inferior surface of frontal lobe.

It consists of central axons of mitral and tufted cells of bulb and some centrifugal fibers from the opposite olfactory bulb.

At anterior perforated substance the tract divides into medial and lateral olfactory striae.

Lateral stria carries axons to olfactory area of cerebral cortex (periamygdaloid and prepiriform areas).

Medial olfactory stria carries fibers that cross median plane in anterior commissure to pass to olfactory bulb of opposite side.

In contrast to all other sensory pathways olfactory pathway has only two neurons and reaches cerebral cortex without synapsing thalamus.

Primary olfactory cortex sends nerve fibers to many other centers within brain to establish connections for emotional and autonomic responses to olfactory sensations.

OLFACTORY TRACT & CENTRAL CONECTIONS

Lateral stria-

prepiriform cortex,

Iry cortex

Periamygdaloid Nu.

Entorhinal complex (uncus)- IIry cortex

APPLIED ANATOMY

Bilateral anosmia, :--caused by the disease of olfactory mucosa—common cold or allergic rhinitis.

Unilateral anosmia: caused by the diseased affecting the olfactory nerves, bulb or tract.

Fracture of anterior Cranial Fossa: involvement of the cibriform plate of the ethmoid could tear the olfactory nerves.

Cerebral tumor—the meningiomas of anterior cranial fossa can produce anosmia—by pressing the olfactoru bulb or tract.