Cranial Nerve Function and Testing
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List all the cranial nerves
Olfactory, optic, oculomotor, trochlear, trigeminal, abducent, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, hypoglossal
What is the function of the olfactory nerve?
Conscious perception of smell
How do we test the olfactory nerve?
Response of animal to aromatic substance whilst blindfolded. (Hide food in you hand and you will quickly be able to tell if the animal can sniff it out or not)
Describe the path of the optic nerve
Retina, optic chiasm, optic tract, lateral geniculate nucleus, up to the thalamus
What is the function of the oculomotor nerve?
It innervates the ipsilateral dorsal, ventral and medial recti muscls, and the ventral oblique muscle. It is also the efferent arm of the PLR.
What are the clinical signs of oculomotor dysfunction?
Ventrolateral strabismus and dilated unresponsive pupil
How do we test the oculomotor nerve?
Eye position and PLR
Describe a normal response to the PLR
Animal is placed in a fairly dark room and a light is shone into their eye. This should result in constriction of both eyes.
How can you use the PLR to determine between an abnormality of the eye/oculomotor nerve
- Abnormality in the eye (e.g. blind in one eye) - light will not get beyond the first part of the PLR pathway so you will get no constriction in either eye. But when you shine light into the non-blind eye it will travel all the way to the oculomotor nerve and cause constriction of the pupil.
- Abnormality with the oculomotor nerve - you will get a response in the opposite eye but not the side of damage, and when you shine a light in the functioning eye you will also get a response only in that one
What is the function of the trochlear nerve?
Innervates the contralateral dorsal oblique muscle
What are the clinical signs of trochlear nerve dysfunction?
Dorsolateral strabismus of contralateral eye, best evaluated by examining the fundus
What is the function of the trigemnial nerve?
Sensory innervation of the face (and motor innervation of the masticatory muscles)
What are the clinical signs of trigeminal nerve dysfunction?
Masticatory muscle atrophy, decreased jaw tone, loss of facial sensation
How do we test the trigeminal nerve?
Size and symmetry of muscles, jaw tone, corneal reflex, palpebral reflex, nasal stimulation
What is the function of the abducent nerve?
Innervates ipsilateral lateral rectus, and retractor bulbi muscle
What are the clinical signs of abducent nerve dysfunction?
Ipsilateral convergent strabismus, loss of physiological nystagmus, inability to retract eyeball
How do we test the abducent nerve?
Corneal reflex and assess vestibular ocular reflex
What is the function of the facial nerve?
Motor function to muscles of facial expression, sensory innervation to the rostral 2/3 of the tongue, parasympathetic components innervates lacrimal, mandibular and sublingual salivary glands
What part of the ear does the facial nerve course through?
The middle ear
What are the clinical signs of facial nerve dysfunction?
Ipsilateral drooping of the ear and lip, absence of blinking, kerratoconjuncitivitis sicca if the lesion is between the medulla and middle ear, lip is retracted
How do we test the facial nerve?
- Palpebral reflex
- Menace response
- Lip pinch
- Schirmer tear test
What is the function of the vestibulocochlear nerve?
Hearing via the cochlear nerve and vestibular function which adapts the position of the eye and body with respect to position of the head
What are the clinical signs of vestibulocochlear nerve dysfunction?
Head tilt, falling/rolling, nystagmus, strabismus, ataxia, deafness
How do we test the vestibulocochlear nerve?
- Evaluation of body and head posture
- Vestibulo-ocular reflex 'physiological nystagmus'
- BAER test for hearing
What are the functions of the glossopharyngeal and vagus nerves?
Innervates pharynx and palate, parasympathetic component of glossopharyngeal innervates parotid and zygomatic salivary glands, vagus controls motor function of larynx, pharynx and oesophagus,
What are the clinical signs of glossopharyngeal and vagus dysfunction?
Dysphagia, absent gag reflex, inspiratory dyspnoea, dysphonia, megaoesophagus
How do we test the glossopharyngeal/vagus nerves?
What is the function of the accessory nerve?
Motor innervation to trapezius and part of sternocephalicus and brachiocephalicus muscles
What are the clinical signs of accessory nerve dysfunction?
Deviation of the neck
What is the function of the hypoglossal nerve?
Motor innervation to the tongue
What are the clinical signs of hypoglossal nerve dysfunction?
Atrophy or deviation of tongue, problems with prehension and deglutition
How do we test the hypoglossal nerve?
Tongue tone and position
What afferent/efferent nerves do the following test: palpebral reflex, corneal reflex, menace response, pinching the face?
- Palpebral reflex - trigeminal nerve and facial nerve
- Corneal reflex - trigeminal nerve and facial nerve
- Menace response - optic nerve and facial nerve
- Pinching the face - trigeminal nerve and facial nerve
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