04.06 Cranial Nerves

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Included In This Lesson

Study Tools

Outline

Overview

  1. Peripheral Nervous System
    1. Cranial nerves
      1. Supply head, neck, and shoulders
      2. Either sensory, motor, or mixed
    2. Spinal nerves and their ganglia
      1. Supply the rest of the body

Nursing Points

General

  1. Cranial nerves
    1. I – Olfactory nerve
      1. Sensory
      2. Smell
    2. II – Optic
      1. Sensory
      2. Vision
      3. Optic chiasma
        1. Nerve fibers cross over from optic nerve to eyes
    3. III – Oculomotor
      1. Motor
      2. Eye movement
        1. Extrinsic
          1. Moving eyes around in socket
        2. Intrinsic
          1. Moving lens and pupils
    4. IV – Trochlear
      1. Motor
      2. Extrinsic eye muscles
    5. V – Trigeminal
      1. Mixed
        1. Sensory – head, neck, teeth, gums
        2. Motor – throat muscles, swallowing – some salivary control – mastication
    6. VI – Abducens
      1. Motor
      2. Some extrinsic eye muscles
    7. VII – Facial
      1. Mixed
        1. Sensory – innervates anterior ⅔ of taste buds
        2. Motor – muscles of facial expression
    8. VIII – Vestibulocochlear
      1. Sensory – two branches
        1. Auditory nerve (AKA “acoustic” or “cochlear”)
          1. Innervates organ of corti in inner ear
          2. Hearing
        2. Vestibular nerve
          1. Innervates semicircular ducts, utricle, saccule
          2. Balance
    9. IX – Glossopharyngeal
      1. Mixed
        1. Sensory – sensations from throat – posterior ⅓ taste buds – monitors BP
        2. Motor – swallowing, vocal cords for sound production
    10. X – Vagus
      1. Medulla → neck → thoracic cavity → abdominal cavity
      2. Mixed
        1. Sensory – monitors BP – sensations from respiratory and digestive tracts
        2. Motor
          1. Slows heart rate (SA node)
          2. Stimulates glands for secretion
            1. Gastric
            2. Pancreatic
          3. Peristalsis in digestive tract
    11. XI – Accessory
      1. Motor
      2. Rotate, flex, and extend neck and shoulders
    12. XII – Hypoglossal
      1. Motor
      2. Movement of tongue

Assessment

  1. See Health Assessment Course (Neuro lesson) for how to assess each cranial nerve

References
Betts, J.G., et al. (2017). _Anatomy and physiology_. Houston, TX: OpenStax, Rice University. Retrieved from https://openstax.org/details/books/anatomy-and-physiology?Book%20details

Transcript

In this lesson we’re going to look at the different cranial nerves and their functions.

The cranial nerves are part of the peripheral nervous system. They extend off the base of the brain, as you can see here, and supply the head, neck, and shoulders. These nerves could either be sensory – meaning their purpose is to receive signals and bring them to the brain for interpretation – or motor – meaning they help produce some sort of action. Or they could have both sensory and motor pathways, in which case we call them mixed. There are twelve cranial nerves you need to know, and you need to know which number they are. In order, they are olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal. OOOTTAFVGVAH. There are quite a few mnemonics out there to help you – I like “Oh, Oh, Oh, to touch and feel very good velvet, AH”. We’ve attached a few to this lesson as well. Now I’m going to run quickly through each one of these and what their functions are. If you want to see details and demonstrations on how to assess each one, make sure you check out the Health Assessment course!

First is olfactory, it’s a sensory nerve, and it’s main function is smell. Usually we test by having the patient identify a common smell, like an alcohol pad.

Cranial nerve 2 is the optic nerve. It’s also sensory and is for vision. This is the nerve that takes what we see and sends that signal to the brain so we can understand it. We usually test this with a visual acuity chart like a Snellen chart.

Cranial nerve 3 is oculomotor. It is what moves the internal muscles of the eye – so the pupil and the lens – and it helps us focus. We test it by shining a light in the eyes to see if the pupil constricts like it should.

Now I’ve grouped cranial nerves 4 and 6 together because they have the same function. The trochlear and abducens nerve are both motor nerves responsible for extrinsic eye movement. That means our ability to move our eyes around in our head. We test it by having the patient follow our finger and see if they can do that with smooth, coordinated movements.

Cranial nerve 5 is the trigeminal nerve. It is mixed – both sensory and motor. It’s responsible for sensation in the head, neck, and gums, as well as motor movements for chewing and swallowing. We test by palpating the face to make sure it feels the same on both sides, and by having the patient clench their teeth together to feel the muscles evenly on both sides.

Cranial nerve 7 is the facial nerve. It’s also mixed and, as you probably guessed, has to do with the face! The sensory aspects have to do with parts of the taste buds, and motor is all of our facial movements. So we’ll have the patient make a bunch of different faces and make sure everything moves symmetrically.

Cranial nerve 8 is vestibulocochlear. It’s sensory and it has two branches. The vestibular branch helps with balance and the cochlear or auditory branch is our hearing nerve. We test this in quite a few ways, including tests with tuning forks called the Weber and Rinne tests. You can see more about these in the EENT assessment in the Health Assessment course.

Next is cranial nerve 9, which is the glossopharyngeal nerve. It is also mixed with some sensory of the taste buds and motor movements involved in swallowing. So we can see if the patient can swallow safely without coughing or choking.

Cranial nerve 10 is the vagus nerve. Now, the sensations and motor effects of the vagus nerve are mostly internal so there aren’t really solid ways we can test it. It does have some motor innervation in the pharynx or the back of the throat, so you can test by seeing the uvula rise when the patient says ‘Ah”. But the rest is internal. The vagus nerve senses changes in blood pressure as well as gets sensation from internal organs like in the digestive system. In terms of motor responses – again it goes internal – we see changes in heart rate from the SA node as well as effects on peristalsis in the digestive system. So the vagus nerve has a HUGE impact in the internal workings of our body.

Almost there – number 11 is the accessory nerve. It provides a motor supply to the neck and shoulders – so it’s all about movement of the neck and shoulders. So we have the patient shrug their shoulders and turn their head side to side against resistance.

Last one! Cranial nerve 12 is the hypoglossal nerve. It is also motor and involves tongue movement. So you can have them stick their tongue out and move it side to side and make sure the movement is symmetrical and smooth.
Okay, let’s recap. The cranial nerves are part of the peripheral nervous system and supply the head, neck, and shoulders. They are either sensory, motor, or both, depending on their purpose. There are 12 of them – and you need to learn them, so use whatever mnemonic works best for you! I like Oh, Oh, Oh, To Touch And Feel Very Good Velvet, AH! They each have specific functions and ways to assess them – make sure you check out the Health Assessment course for how to do that.

So that’s it for the A&P of the Cranial Nerves. Check out all of the resources attached to this lesson. Now, go out and be your best self today. And, as always, happy nursing!