Cranial Nerves
1. CN I - Olfactory nerve (sensory)
The olfactory nerve, which is cranial nerve number one (CN I), consists of numerous thin axons ascending a very short distance from olfactory receptor neurons in the nose up through the cribriform plate to synapse with neurons in the olfactory bulb. Therefore, the olfactory bulb (number 1 in the Cranial Nerves diagram), is the termination site for the olfactory nerve (CN I). Since the olfactory bulb is located at the anterior end of the olfactory tract, it is this tract we see extending back from the olfactory bulb to near number 2 (the optic nerve), not the olfactory nerve. With regard to function, the olfactory nerve is a sensory nerve central to the role of smell.
- 1.CN I - Olfactory nerve (sensory)
- 2.CN II - Optic nerve (sensory)
- 3.CN III - Oculomotor nerve (motor)
- 4.CN IV - Trochlear nerve (motor)
- 5.CN V - Trigeminal nerve (mixed)
- 6.CN VI - Abducens nerve (motor)
- 7.CN VII - Facial nerve (mixed)
- 8.CN VIII - Vestibulocochlear nerve (sensory)
- 9.CN IX - Glossopharyngeal nerve (mixed)
- 10.CN X - Vagus nerve (mixed)
- 11.CN XI - Accessory nerve (motor)
- 12.CN XII - Hypoglossal nerve (motor)
- 13.Midbrain
- 14.Pons
- 15.Medulla
- 16.Cerebellum
Of the twelve cranial nerves, eight are surprisingly easy to learn, leaving only four that require further work. They are all explained here and in the accompanying sections (Sensory, Motor, Mixed).
There are three cranial nerves with only sensory function – CN I linked to nose and smell, CN II linked to eyes and vision, and CN VIII linked to the inner ear and hearing, balance and movement.
There are five cranial nerves with only motor function – CNs III, IV and VI move the eye and eyelid muscles, CN XI moves the head via neck and shoulder muscles, and CN XII moves the tongue muscles.
That leaves the four more complex cranial nerves, which have both sensory and motor roles – CN V, CN VII, CN IX and CN X. We will examine them further in the accompanying Mixed section.
As you build your knowledge of the cranial nerves, remember that they come in pairs – they are duplicated on each side of the brain and brainstem.
TIP: Remembering the cranial nerves
An important early step is to learn the name of each cranial nerve.
Mnemonics are a useful and popular way to remember important and detailed information. Here, we suggest using a typical mnemonic that indicates the first initial of each cranial nerve, beginning with CN I. This has the benefit of prompting the name of each cranial nerve according to their order and their initial letter: ‘On Occasion Our Trusty Truck Acts Funny, Very Good Vehicle Any How’.
The logic behind the name of each cranial nerve is also useful. This is because the meaning of words such as oculomotor (eye mover), trochlea (pulley), trigeminal (triplets or three parts), abducens (abductor), vestibulocochlear (balance and hearing apparatus), glossopharyngeal (tongue and throat), vagus (wanderer) and hypoglossal (under the tongue) provides clues regarding the structure or function of these cranial nerves.
