Cranial nerves I-IV
The cranial nerves do for the head and neck what spinal nerves do for the rest of the body. In this lesson, we’ll begin covering the cranial nerves by discussing the first four.
1) Watch the following videos about cranial nerves I through IV:
2) Next, watch this video about a reflex involving cranial nerves II and III:
3) Finally, answer these questions to test your understanding:
1. A patient presents with an upward deviation of the right eye and has adopted a compensatory head tilt to reduce double vision. Damage to which cranial nerve is the most likely cause of these symptoms?
- A. optic nerve (CN II)
- B. trochlear nerve (CN IV)
- C. trigeminal nerve (CN V)
- D. abducens nerve (CN VI)
Show Answer
Correct answer: B
Because the superior oblique muscle helps to move the eye downwards, when the trochlear nerve is damaged the eye tends to deviate upwards since there is no opposing force coming from the superior oblique. This can result in diplopia, or double-vision. Some patients will adopt a head tilt as a compensatory mechanism to better align the eyes and reduce the diplopia.
2. A patient presents with her right eye deviated laterally (toward the side of her head), a drooping right eyelid, and an inability to move her right eye up or down. Which cranial nerve has likely been impacted in this patient?
- A. right facial nerve
- B. left abducens nerve
- C. right oculomotor nerve
- D. left trochlear nerve
Show Answer
Correct answer: C
These symptoms are a clear sign of oculomotor nerve palsy, which involves lateral deviation of the eye, ptosis (drooping of the eyelid), impaired vertical movement of the affected eye, mydriasis (constantly dilated pupil), and diplopia (double vision).
3. The olfactory nerve projects directly to which brain structure?
- A. hippocampus
- B. olfactory bulb
- C. thalamus
- D. amygdala
Show Answer
Correct answer: B
The axons of olfactory receptor neurons in the olfactory epithelium synapse in the olfactory bulb, which is the first relay in the brain for processing smell information. From there, signals are sent to higher brain areas, including the olfactory cortex and other limbic structures, for perception and emotional responses to odors.
4. Which cranial nerve is primarily responsible for carrying visual information from the retina to the rest of the brain?
- A. CN II - optic nerve
- B. CN III - oculomotor nerve
- C. CN IV - trochlear nerve
- D. CN VI - abducens nerve
Show Answer
Correct answer: A
The optic nerve is a sensory nerve responsible for transmitting information about vision to the brain.
5. A physician shines a light into a patient’s left eye only. The left pupil constricts, but the right pupil does not. Based on these findings, which cranial nerve is most likely impaired?
- A. there is no obvious impairment
- B. the left optic nerve
- C. the right oculomotor nerve
- D. the right optic nerve
Show Answer
Correct answer: C
The pupillary light reflex is used to assess the function of the optic and oculomotor nerves and related structures. If light shined in the left eye causes only the left pupil to constrict, this indicates the sensory signal was received (since the left pupil constricted), but it may suggest a problem with the right oculomotor nerve (because the right pupil didn't constrict).
