Page 532 - Fundamentals of anatomy physiology

on the primary function, and a cranial nerve can have important
secondary functions. Three examples are worth noting:
1. The olfactory receptors, the visual receptors, and the recep-
tors of the internal ear are innervated by cranial nerves that
are dedicated almost entirely to carrying special sensory
information. In contrast, the sensation of taste, considered to
be one of the special senses, is carried by axons that form only
a small part of large cranial nerves that have other primary
functions.
2. As elsewhere in the PNS, a nerve containing tens of thousands
of motor fibers that lead to a skeletal muscle also contains
sensory fibers from muscle spindles and Golgi tendon organs
(proprioceptive sensory nerve endings) in that muscle. We as-
sume that these sensory fibers are present but ignore them in
classifying the nerve.
3. Regardless of their other functions, several cranial nerves
(III, VII, IX, and X) distribute autonomic fibers to peripheral
ganglia, just as spinal nerves deliver them to ganglia along
the spinal cord. We will note the presence of small numbers of
autonomic fibers (and will discuss them further in Chapter 16)
but ignore them in the classification of the nerve.
Cranial nerves are clinically important, in part because they
can provide clues to underlying CNS problems. Clinicians assess
cranial nerve function with a number of standardized tests.
&T i p s T r i c k s
Two useful mnemonics for remembering the names of the cra-
nial nerves in order are “Oh Oh Oh, To Touch And Feel Very
Green Vegetables, Ah Heaven!” and “Oh, Once One Takes The
Anatomy Final, Very Good Vacations Are Heavenly!”
The Olfactory Nerves (I)
Primary function: Special sensory (smell)
Origin: Receptors of olfactory epithelium
Pass through: Olfactory foramina in cribriform plate of ethmoid
pp. 239, 244
Destination: Olfactory bulbs
The first pair of cranial nerves carries special sensory information
responsible for the sense of smell (Figure 14–20). The olfactory
receptors are specialized neurons in the epithelium covering
the roof of the nasal cavity, the superior nasal conchae, and the
Figure 14–20 The Olfactory Nerve. Left olfactory
bulb (termination
Olfactory tract of olfactory nerve)
(to olfactory cortex
Olfactory
of cerebrum) nerve (N I)
Olfactory
nerve fibers
Cribriform plate Olfactory
of ethmoid bone epithelium
superior parts of the nasal septum. Axons from these sensory
neurons collect to form 20 or more bundles that penetrate the
cribriform plate of the ethmoid bone. These bundles are compo-
nents of the olfactory nerves (I). Almost at once these bundles
enter the olfactory bulbs, neural masses on either side of the
crista galli. The olfactory afferents synapse within the olfactory
bulbs. The axons of the postsynaptic neurons proceed to the
cerebrum along the slender olfactory tracts (Figures 14–19 and
14–20).
Because the olfactory tracts look like typical peripheral nerves,
anatomists about a century ago misidentified these tracts as the
first cranial nerve. Later studies demonstrated that the olfactory
tracts and bulbs are part of the cerebrum, but by then the num-
bering system was already firmly established. Anatomists were
left with a forest of tiny olfactory nerve bundles lumped together
as cranial nerve I.
The olfactory nerves are the only cranial nerves attached
directly to the cerebrum. The rest originate or terminate within
nuclei of the diencephalon or brain stem, and the ascending sen-
sory information synapses in the thalamus before reaching the
cerebrum.
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