Cranial Nerve Organization and Modalities

This section is meant to follow up on the cranial osteology with a look at the nerves and vessels that pass through the foramina of the skull. While it does not contain all of the material you will need to know about the twelve cranial nerves, it is a very solid introduction. Although we will mention the vessels in the context of their corresponding foramina, we will not discuss the details of cranial blood flow here (we’ll deal with that in lecture and in lab). 

First we will give a very general overview of the cranial nerves, their targets and their modalities (don’t worry, the word ‘modality’ as it is used here is described. We will then consider the foramina of the skull you are responsible for knowing, and what cranial nerves and vessels pass through them. 

As you are likely aware, each of the twelve pairs of cranial nerves has both a name and a number. Typically, the names make sense (for example, the optic nerve carries sensory information back from the retina of the eye), and the numbers more or less flow from the anterior to posterior (or rostral to caudal) end of the brain (I to XII). 

As the cranial nerves move away from the brain, the courses the individual nerves take may be quite variable. Some nerves, like the olfactory (I) and optic (II) move straight to their targets (the nose and eye, respectively) with little or no diversion. Others, exemplified by the trigeminal nerve (V) split repeatedly into multiple named nerves and exit the skull through multiple foramina. In fact, the meanderings of the trigeminal nerve are so wide that it is divided into three parts (V1, V2, and V3) to help us keep track of it. Of course there is also the Queen of wandering nerves, the Vagus (X), which provides sensory and motor innervation to structures from the pharynx all the way to the colon. 

In addition to knowing the name and number of each cranial nerve, and where it passes through the skull, you will need to know what modality(ies) is/are present in that nerve. You actually already know what we’re referring to when we say modality, even though we don’t typically refer to it by that particular word. 

The term modality simply refers to what types of nerve fibers are in a nerve; ‘motor’ is a modality, and ‘sensory’ is a modality. Most nerves, like spinal nerves, have more than one modality. Spinal nerves have both motor and sensory modalities. 

We can get more specific with the modalities than just motor and sensory. There are three motor modalities, and three sensory modalities. 

Motor modalities: 

1. Somatic motor is motor input to skeletal muscles. All spinal nerves, and some cranial nerves, contain somatic motor. 

2. Visceral (autonomic) motor is motor input from the autonomic nervous system and so it goes to smooth muscle, cardiac muscle, and glands. All spinal nerves contain some visceral motor, which goes out to the sweat glands, vascular smooth muscle, and arrector pili of the outer tube. Some cranial nerves contain visceral motor, and in the case of the cranial nerves the specific type of visceral motor is parasympathetic. 

3. Branchial motor is a specific time of somatic motor that goes to the skeletal muscle that is derived from the pharyngeal (gill) arches and is limited to the face and neck. The only difference between branchial motor and somatic motor is the embryology of the muscle; there are no other functional or anatomical differences between these two types. Branchial motor fibers are only in the cranial nerves. 

Sensory modalities: 

1. Somatic sensory is the general sensory input we get from our skin. It ascends through the spinal cord to the thalamus and then to the cortex of the parietal lobe. All spinal nerves contain somatic sensory paths. Some cranial nerves also contain somatic sensory paths (such as the somatic sensation from the face carried in CN V). 

2. Visceral sensory is the sensory information that is coming from our guts. These are stretch (pressure) receptors that carry information that is interpreted as blood pressure, chemoreceptors that monitor the pH of the stomach and intestinal contents as well as the oxygen and carbon dioxide levels of blood, etc. Both spinal and cranial nerves contain visceral sensory fibers. 

3. Special sensory is the sensory input for modalities that are only associated with the head and are found nowhere else in the body; sight, smell, hearing, balance, taste. Therefore, special sensory is found only in specific cranial nerves. 

HELPFUL HINTS

There are many mnemonics out there to help your remember the order of the cranial nerves, including a bunch that start with “Oh, Ohh, Ohhh...” and continue on to things that generally aren’t appropriate to include here. Unfortunately, those mnemonics are the generally best. Try Google. In the meantime, here are a few tips: 

The olfactory nerve (CN I) is responsible for your sense of smell. Remember, you have ONE NOSE. The optic nerve’s (CN II) function is sense of sight. Conveniently, you have TWO EYES. For CN VI (abducens), think of a SIX pack of Abs. The Vagus (CN X) has a wide range of targets; ‘vagus’ is rooted in Latin and means wandering (think vagabond). 

Many of the other names will be easier to remember once we discuss function. They seem funny now but they’re actually pretty descriptive. 

We will go over specific modalities next, but the table also lists the general modalities for each nerve (Motor, Sensory or Both).  A helpful mnemonic is: Some Say Marry Money But My Brother Says Big Brains Matter More.