| Introduction
Cranial Nerves |
The exit points from the brain for the cranial nerves can roughly be grouped into two columns (don't include the olfactory and optic systems into this generalization). On the left hand side of the image below, you'll see that cranial nerves III (oculomotor), VI (abducens) and XII (hypoglossal) are found close to the midline, right above the ventral (motor) root of the first cervical segment of the spinal cord. In fact, the hypoglossal will probably be seen as a series of rootlets between the pyramid and olive. The abducens will exit just at the base of the pons, and the oculomotor will exit just in front of the pons (it actually exits from the midbrain, but that's hard to see at this point). The trochlear (IV) actually exits at about the same distance from the midline, but it's the one nerve that leaves from the dorsal surface of the brainstem. Although you don't want to make too many generalizations when discussing neuroanatomy, you'll find that all these nerves have in common the fact that they are primarily motor neurons.
On the right, you'll find nerves that exit from nearly the lateral surface of the brainstem. Even if you can't find them on your specimen (just like the specimen pictured above), you need to know at least where you would expect to see these nerves exit the brainstem. These include V (trigeminal), VII (facial), VIII (vestibulo-cochlear), IX (glossopharyngeal), X (vagus) and X! (accessory). You might find it difficult to distinguish between VII and VIII (both exit the brain deep between the pons and cerebellum) and IX and X (they'll probably look like a series of rootlets exiting the medulla just lateral to the olive). Nerve XI (you really can't see it in the picture above) continues as a series of rootlets below the vagus nerve. These nerves have both major sensory as well as motor components. Later on in the course (in about two weeks), you'll come to understand the whys and wherefores of this organization. The olfactory and optic nerves form a class unto themselves. In fact, it's doubtful that you'll ever see an olfactory nerve (these are tiny fibers that arise from the olfactory epithelium and pass through the cribiform plate to terminate in the olfactory bulb). What you'll see on the ventral surface of the brain is the olfactory tract running from the olfactory bulb. In Gross Anatomy, you will soon study (or have already studied) the peripheral distribution of these nerves. Keep these distributions in mind as we study these nerves and their nuclei of origin in the CNS.
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