Lecture 4 - Olfaction and CN 7

Which nervous system is CN7 part of?

Sensory, motor, and parasympathetic

What is the sensory role of CN 7?

-Taste from rostral 2/3 of tongue

-Sensation from nasal cavity and inner ear

What is the motor role of CN 7?

-Muscles of facial expression

-Orbicularis oris and oculi, caudal belly of digastricus m.

What is the parasympathetic role of CN 7?

-Innervates lacrimal, nasal, and palatine glands

-Innervates mandibular and sublingual salivary glands

Explain the pathway of the CN 7 motor efferents.

1. Cell bodies originate in the facial nucleus of the brainstem

2. Enters internal acoustic meatus of the petrosal portion of the temporal bone

3. Travels in the facial canal

4. Exits in the stylomastoid foramen

What nerve is CN 7 closely related to? What is a potential consequence of thus?

-CN 8

-Inner ear/vestibular problems may also have facial paralysis

How do we test the functionality of CN 7?

-Palpebral reflex: touch inner and outer corners of eyelids

-Menace response: look for signs of blinking

-Muscles of facial expression: pinch lip and look for retraction

-Examine for deviation of nose

-Touch inner ears to see ear flick response

What does the palpebral reflex test?

-Inner corner: ophthalmic branch of CN 5 motor, CN 7 sensory

-Outer corner: maxillary branch of CN 5 sensory

What are common causes of damage to CN 7?

Infection of middle ear, viral infection, inflammation, tumor, trauma, idiopathic

What are clinical signs of damage to CN 7?

-Facial paralysis

-Ear drooping/impaired movement

-Inability to close eye/wide open

-Dry eye due to lack of blinking/tear production

-Lip paralysis

-Deviation of nose to contralateral side

-Face drooping

-Saliva dripping

-Food collection in cheek

What is a common problem with CN 7 in horses?

If in lateral recumbency buccal branch of facial n. may be compressed (innervates lip and nose)

What is a common problem with CN 7 in cattle?

-Stanchion may affect palpebral branches of auriculopalpebral nerve

-Eye lid paralysis

What are the bipolar olfactory neurons?

-First order neurons

-Receptor cells located in olfactory epithelium

-Nonmyelinated axons of these neurons gather into bundles that collectively form CN 1

Where does the olfactory nerve project to?

Penetrate the cribriform plate of the ethmoid bone and synapse on glomeruli in the olfactory bulb

What are second order neurons?

-Originate in olfactory bulb

-Brush and mitral cell axons from glomeruli project in olfactory tract and stria and synapse on piriform cortex (conscious pathway)

How is the olfactory tract unique?

It does not need to go through the thalamus before reaching the cortex

What is the primary olfactory cortex?

-Piriform cortex

-Conscious sense of smell

What are the 2 projection types of the olfactory nerve?

-Piriform cortex and then to other cortex areas (conscious perception)

-Reflexive or unconscious reaction to smell

What are the pathways of reflexive/unconscious sense of smell?

-Amygdala: emotional qualities of smell

-Hippocampus: storage and retrieval of memories

-Hypothalamus: regulation of homeostasis, circadian rhythm, reproduction

What is the vomeronasal organ?

-Special olfactory organ for pheromone detection

-Very sensitive, detects and responds to very low concentrations

The vomeronasal axons project to...

-Accessory olfactory bulb

-Amygdala and hypothalamus

What are clinical implications of CN 1?

-Lesions are rare

-Hard to test

-Chronic rhinitis is inflammation of the nasal mucosa, may affect smell

-Cats will not eat if they can't smell