Olfactory nerve definition:
Special characteristic of olfactory nerve :
- It is the shortest cranial nerve
- It is one of the nerves that do not join with the brain stem.
- It has the ability to regenerate.
- It is unmyelinated.
Olfactory nerve location:
Olfactory receptor cells :
The olfactory receptor cells are scattered among supporting cells.
Structure of olfactory receptor cells:
Each olfactory receptor cell consists of a small bipolar nerve cell with a coarse peripheral process that passes to the surface of the membrane and a fine central process. From the coarse peripheral process, a number of short cilia arise, called olfactory hairs, which project into the mucus covering the surface of the mucous membrane. The projecting hairs react to odors in the air and stimulate the olfactory cells.
The fine central processes form the olfactory nerve fibers. Bundles of these nerve fibers pass through the opening of the cribriform plate of the ethmoid bone to enter the olfactory bulb. The olfactory nerve fibers are unmyelinated and are covered with Schwann cells.
Olfactory bulb :
It is an ovoid structure which possesses several types of nerve cells the largest of which is called the mitral cells.
The incoming olfactory nerve fibers synapse with the dendrites of the mitral cells and form rounded areas known as synaptic glomeruli.
Smaller nerve cells tufted cells and granular cells also synapse with the mitral cells.
The olfactory bulb, in addition, receives axons from the contralateral olfactory bulb through the olfactory tract.
It is a narrow band of white matter which runs from the posterior end of the olfactory bulb beneath the inferior surface of the frontal lobe of the brain. It consists of the central axon of the mitral and tufted cells of the bulb and some centrifugal fibers from the opposite olfactory bulb.
As the olfactory tract reaches the anterior perforated substance, it divides into medial and lateral olfactory striae. The lateral stria carries the axons to the olfactory area of the cerebral cortex, called the periamygloid and periform areas. The medial olfactory stria carries the fibers that cross the median plane in the anterior commissure to pass to the olfactory bulb of the opposite side.
The periamygloid and perpiriform areas of the cerebral cortex are often known as the primary olfactory cortex. The entorhinal area of the parahippocampal gyrus, which receives numerous connections from the primary olfactory cortex is called the secondary olfactory cortex. These areas of the cortex are responsible for the appreciation of olfactory sensation. It should be kept in mind that in contrast to all other sensory pathways the olfactory afferent pathway has only two neurons and reaches the cerebral cortex without synapsing in one of the thalamic nuclei.
The primary olfactory cortex sends nerve fibers to many other centers within the brain to establish connections for emotional and autonomic responses to olfactory sensations.
Function of olfactory nerve :
Sense of smell is the function of the olfactory cranial nerve.
Embryology of olfactory nerve :
Embryologically olfactory nerve is derived from the olfactory placode which is a thickening of the ectoderm layer . It also gives rise to the glial cells which support the olfactory nerve.
Clinical testing of olfactory nerve:
The olfactory nerve is clinically tested by asking the patient any changes their sense of smell or any changes in their food taste.
Examination of the olfactory nerve is via testing the nostrils and asking the person to identify a specific smell.